Pathophysiology curriculum

Early injury is a fixable energy and water problem: the cell swells, and liver cells may collect fat droplets, yet both can clear if the insult ends. The point of no return comes when mitochondria cannot recover and membranes break. Contents then leak out, such as troponin, a heart muscle protein rather than an enzyme. A clearly rising marker therefore usually means cells have been lost, not merely threatened.

Low oxygen alone leaves blood flowing, so glucose still arrives, lactate still leaves, and cells can keep making some energy without oxygen. Cut the flow and all of that stops, so energy runs out and acid builds up together. That is why a blocked artery usually injures tissue sooner than a comparable oxygen shortage from anemia or altitude, where flow continues.

Without enough ATP, the sodium-potassium pump slows, so sodium stays inside and water follows, swelling the cell. Anaerobic metabolism then adds lactic acid. Calcium pumps also falter, and calcium flooding in switches on enzymes that digest membranes, proteins, and DNA. Swelling is the early, reversible sign; calcium-driven enzyme damage pushes a cell toward death.

These unstable molecules form as normal by-products of oxygen use and readily damage nearby molecules. Hitting lipids breaks membranes, hitting proteins disables enzymes, and hitting DNA causes mutations. Cells normally neutralize them with enzymes and antioxidants such as glutathione. Injury comes from imbalance, as in inflammation, radiation, or reperfusion, so production relative to defenses is what matters.

Returning flow is still the goal, but starved tissue can be hurt further when oxygen and calcium rush back. Damaged mitochondria turn the new oxygen into reactive molecules, calcium overloads cells, and arriving white cells add inflammation. This explains why a reopened coronary artery can bring a burst of rhythm problems. It is not a reason to delay reopening a vessel.

Each change lets a cell survive a lasting demand. Bigger cells are hypertrophy, as in a heart pumping against high pressure. More cells are hyperplasia, as in breast tissue during pregnancy. Shrinking is atrophy, after lost use or blood supply. Swapping one mature cell type for another is metaplasia, as in a smoker's airway, and it can sometimes precede precancerous change.

When injury overwhelms a cell, it swells, bursts, and spills contents that provoke inflammation, often harming neighbors. The programmed route is orderly and uses energy: the cell shrinks and fragments, and phagocytes clear the pieces before they raise an alarm. It removes unneeded or damaged cells during development and normal turnover, so not every cell death signals disease.

Mediators widen arterioles, so extra blood brings color and warmth. Leaky vessels let fluid into tissue, producing edema. Pain comes from prostaglandins and bradykinin sensitizing nerves, plus pressure from the fluid. Lost function follows from pain and edema together. Tie each sign to its event and you can predict it: a deep infection may hurt and swell with little visible color change.

After at most a brief constriction, arterioles widen and flow rises. Small veins then leak protein-rich fluid, which thickens the remaining blood and slows it. Slower flow lets white cells drift to the vessel edge, called margination. They roll, stick, and squeeze between endothelial cells, called diapedesis, following chemical signals. An adhesion defect therefore brings high white counts with few cells at the wound.

Think of each one by what it does. From mast cells comes a quick signal that widens vessels and makes them leak, which is histamine. Prostaglandins add pain and fever, while leukotrienes constrict airways and attract cells. Cytokines such as TNF direct the response and cause body-wide effects. Complement tags microbes, draws in cells, and can punch holes in them.

The fluid's look hints at what produced it. Thin, low-protein fluid is serous, as in a blister. Fibrinous exudate holds clotting protein and signals more vessel leak, as in the rub over an inflamed pericardium. Purulent exudate, or pus, is dead neutrophils and debris, typical of pus-forming bacteria. Hemorrhagic exudate means vessels were badly damaged, so red cells escaped too.

When the cause cannot be cleared, neutrophils give way to macrophages and lymphocytes, and tissue is destroyed and repaired at the same time, leaving fibrosis. A granuloma is the extreme version: macrophages cluster to contain something they cannot digest, as in tuberculosis. Contained does not mean gone, so weakened immunity can let the organism reactivate.

Cytokines released at the site travel in blood and change distant organs. In the brain they reset temperature upward. In marrow they speed white-cell release, raising the count. In the liver they drive proteins such as C-reactive protein and fibrinogen. A rising C-reactive protein therefore shows inflammation is present somewhere but does not say where or why.

Repair moves in overlapping phases: a clot stops bleeding, white cells clean up, new tissue and vessels fill the gap, then collagen strengthens the scar. A wound reopening before it has enough strength is dehiscence. Excess scar growing beyond the wound edge is a keloid. Scar shortening that limits movement is contracture. Infection, poor blood supply, and malnutrition commonly stall healing.

The first response arrives immediately and recognizes broad danger patterns, but it cannot tell one strain from another. The second takes longer because specific B and T cells must activate and multiply, and some then stay on as memory cells. Innate cells hand off by presenting antigen and signaling trouble. Memory is why a repeat exposure, or a vaccine, can stop a pathogen before it settles in.

Each class fits where and when it works. IgM appears first in an infection and activates complement well. IgG comes later, lasts, coats pathogens for ingestion, and crosses the placenta. IgA guards mucosal surfaces such as the gut. IgE arms mast cells against parasites and allergens. IgD sits mainly on B cells. A high IgM therefore suggests recent infection, while IgG can reflect past exposure.

Cells marked CD4 are the coordinators: they secrete signals that rouse macrophages against intracellular microbes and B cells against parasites. Cells marked CD8 kill virus-infected and cancer cells directly. HIV depletes T cells, which opens the door to infections a healthy immune system would control. Losing this arm shifts risk toward organisms that live inside cells.

A first exposure makes IgE that coats mast cells and basophils without causing symptoms. On re-exposure, the allergen cross-links that IgE and the cells release histamine and other mediators within minutes. Small vessels dilate and leak, and airways narrow. Throughout the body this is anaphylaxis, and shock can follow. A mild past reaction does not promise a mild next one.

IgG or IgM binds an antigen sitting on a cell or tissue surface, then usually marks that cell for complement or phagocytes to destroy. An ABO-mismatched transfusion is the classic case: preformed antibodies attack donor red cells right away. Autoimmune hemolytic anemia and immune thrombocytopenia work the same way. Contrast type III, where the antigen floats free.

Antibody and soluble antigen form clumps in blood. Those not cleared settle in small vessels, kidney filters, and joint linings, where they activate complement and draw in neutrophils. The damage appears where the clumps lodge, not where the antigen came from. Lupus and serum sickness are classic examples, which is why rash, arthritis, and kidney injury can show up together.

No antibody is involved: previously sensitized T cells recognize the antigen, release cytokines, and recruit macrophages, or kill target cells directly. Recruiting takes time, so the reaction peaks a day or more later. Poison ivy and the tuberculin skin test both show this delay, and T-cell attack on a graft works the same way. A rash a day or two after contact, in someone already sensitized, points here rather than to IgE.

Developing T and B cells that react strongly to self are normally deleted, which is central tolerance. Survivors are kept quiet by anergy, a non-reactive state, and regulatory T cells, which is peripheral tolerance. Disease usually involves genes plus a trigger. An infection can display a molecule resembling self, called molecular mimicry, and injury can expose hidden antigens, as in the eye after trauma.

Lupus is the model: its autoantibodies bind targets shared by most cells rather than one organ's, so joints, skin, kidneys, heart, and blood can all be involved. Illness tends to come in unpredictable flares with quieter remissions between them. A quiet period therefore does not mean the disease has gone, and two people with lupus can look quite different.

Because one tissue is under attack, the symptoms follow from what that tissue makes. In type 1 diabetes, immune attack removes the pancreatic beta cells, so insulin is lost. In Hashimoto's disease, thyroid damage often leaves the gland short of hormone, though early on it can rarely release an excess. Predict the picture by asking what the lost tissue normally does.

Each inherited gap leaves a recognizable pattern. Without antibodies, expect recurrent bacterial sinus and lung infections, often starting after maternal antibody fades in infancy. T-cell defects bring viruses, fungi, and intracellular organisms, and severe combined disease shows up early in life. Phagocyte defects cause abscesses. Complement gaps favor encapsulated bacteria such as Neisseria.

Most immune weakness seen in practice is acquired rather than inherited. Chemotherapy and immunosuppressive drugs reduce cells or their activity, blood cancers crowd out normal immune cells, and HIV destroys T cells. Malnutrition, diabetes, and kidney disease impair defenses more quietly. These states may be temporary, so risk can lift when counts recover or a drug stops.

Timing points to the mechanism. Within minutes to hours, the fast form means the recipient already has antibodies against the graft. Weeks to months later, the acute form comes mainly from T cells reacting to donor HLA proteins. The chronic form scars vessels slowly over years. Graft-versus-host disease runs the other way: donor immune cells attack the recipient.

Spread needs every link: the place an agent normally lives, a way out of it, a way of traveling, a way into the next person, and someone who can be infected. A carrier with no symptoms can still be the reservoir. Control works by breaking a link, so isolating an infected person interrupts transmission, and vaccination protects the host at the end of the chain.

An organism does harm by attaching, hiding, spreading, and surviving. Surface proteins stick it to host cells, a capsule blocks phagocytes from ingesting it, and enzymes such as hyaluronidase help it move through tissue. Some bacteria survive inside macrophages, as the tuberculosis organism does. Encapsulated organisms are a particular danger after spleen removal, since the spleen helps clear them.

Exotoxins are proteins secreted mostly by living bacteria, each with a precise target, as in the muscle spasms of tetanus or the watery diarrhea of cholera. Endotoxin is a lipopolysaccharide in the outer membrane of gram-negative bacteria, and it triggers broad cytokine release, fever, and shock. A general septic picture does not prove endotoxin: gram-positive sepsis and toxic shock syndrome can look alike.

Viruses harm tissue in three separate ways. Some replicate and burst the host cell. Some disturb genes that control growth, pushing cells toward cancer, as human papillomavirus can. In others, the virus does limited harm itself and the immune attack on infected cells causes the injury, as in much hepatitis B liver damage. That is how someone can worsen as immunity rises.

Bacteria settling on a catheter or prosthetic valve surround themselves with a sticky coating and slow their metabolism. Phagocytes and antibodies penetrate poorly, and antibiotics that target active growth lose effect. Cells can also break off and seed the bloodstream repeatedly. That is why an infected device often has to be removed even when the organism tests as sensitive in the lab.

A drug kills susceptible microbes and leaves resistant survivors to multiply, so the resistant type comes to dominate. Resistance arises through mutation during rapid replication, and bacteria can also pass resistance genes to one another. Even appropriate use applies this pressure, and inappropriate use speeds it up. The practical result is an infection that no longer responds to the usual drug.

These organisms rarely cause disease in a healthy person. Low T cells allow Pneumocystis pneumonia and cryptococcal disease. Few neutrophils invite Aspergillus and gram-negative bacteria. Broken skin or a catheter lets skin flora and Candida in. Broad antibiotics can clear normal flora and let Clostridioides difficile overgrow. Naming the defect tells you which organism to watch for.

A noncancerous tumor usually grows slowly, resembles its tissue of origin, often has a capsule, and pushes rather than invades. A cancerous one tends to grow faster, looks less like its origin, and invades nearby tissue and spreads. Invasion is the decisive feature. Benign still does not mean safe: a tumor inside the skull can threaten life simply because of where it grows.

These traits explain how a cell escapes normal controls. It proliferates without outside growth cues and overrides both stop signals and the instruction to undergo apoptosis, or programmed death. It also keeps dividing past the limit normal cells reach, often by maintaining its telomeres, the protective tips of chromosomes. Most cancers need several such changes, which is one reason risk climbs with age.

Proto-oncogenes normally drive growth; altered or overactive, they stay switched on and push division when it is not needed. Suppressor genes normally slow or stop division, so trouble follows when they are inactivated. A third group, DNA repair genes, lets further mutations pile up when damaged. Beginners expect one culprit, but cancer usually builds as several changes accumulate.

Normally the cycle pauses so a cell can repair damaged DNA or, if it cannot, die by apoptosis. TP53, a gene often mutated in cancer, makes a key protein that responds to that damage. When these controls fail, the cell copies its errors into daughter cells, and each round adds mutations. That growing instability is why tumors change over time and why cells within one tumor can differ.

A pathologist grades a tumor by comparing its cells with normal tissue. Well-differentiated cells resemble their tissue of origin and usually grow and spread slowly. Poorly differentiated or undifferentiated cells, the extreme being called anaplasia, look least normal and tend to act more aggressively. Do not confuse this with stage: grade is about the cells, stage about size and spread.

New vessels are what let a solid mass keep enlarging. The tumor releases signals such as vascular endothelial growth factor, or VEGF, that coax endothelial cells to grow toward it. Those vessels bring oxygen and nutrients and help cancer cells move through the body. Drugs called angiogenesis inhibitors act on this vessel growth, so their effect on the tumor is indirect.

The immune system can recognize and destroy abnormal cells, so a growing tumor must get around it. Some display fewer of the antigens T cells look for. Others exploit checkpoints, the brakes that normally prevent excess immune attack, for example by making PD-L1 to switch T cells off. Regulatory cells and inhibitory cytokines nearby also dampen the attack.

A carcinoma must first break out of its own layer of epithelium. Cells lose adhesion molecules such as E-cadherin, then release enzymes that dissolve the basement membrane, the thin sheet beneath them. That breach is the line between carcinoma in situ, which stays in place, and invasive cancer, which can reach vessels and travel.

Carcinomas often travel first through lymph to nearby nodes, while sarcomas more often go through the blood. Blood-borne cells often lodge in the first capillary bed they meet, which is why colon cancer so often reaches the liver through the portal vein. Seeding means shedding across a body cavity, as ovarian cancer does in the peritoneum. The route tells you where to look first.

The TNM system records three facts: T scores how big the primary tumor is and how far it has grown, N whether nearby nodes hold cancer, and M whether it has reached distant sites. Together they give an overall stage, with stage IV meaning distant disease. Later progression does not rewrite the label given at diagnosis. Each axis is scored on its own, so a small tumor can still be advanced.

These effects do not come from the tumor's physical presence or from metastasis. Some tumors secrete hormones they have no normal reason to make, as small cell lung cancer can do with ACTH or ADH. Others provoke antibodies that cross-react with normal tissue, often nerves. The syndrome may appear before the cancer is known, so an odd hormone or nerve finding can be the first clue.

Unlike simple starvation, this loss takes muscle as well as fat, because tumor and immune cytokines change how the body handles energy. Appetite falls, breakdown rises, and weight can drop even in someone eating reasonably well. Beginners often assume more food will fix it. It also reduces strength and tolerance of treatment, so it matters beyond the number on a scale.

All three routes change genes that control cell growth. Chemicals such as those in tobacco smoke damage DNA, and ionizing radiation breaks it. Some viruses, such as high-risk human papillomavirus, bring in genes that disrupt growth control. Because further mutations must accumulate, cancer often appears years or decades after exposure, which makes it hard to trace a tumor to its original cause.

With a dominant trait, one altered copy is enough, so an affected person typically has an affected parent, though a new variant can appear with no family history. With a recessive trait, both copies must be altered, so parents are usually unaffected carriers and the condition may not show in every generation. Huntington's disease fits the first and cystic fibrosis the second.

With only one X chromosome, a male needs just one altered copy to have the condition. A female has a second X that usually compensates, and she rarely inherits two altered copies, so the condition shows up mostly in males. Hemophilia shows the pattern. The key pedigree clue is that a father cannot pass an X-linked trait to his son, because he gives his son a Y.

This cell-division error happens when chromosomes fail to separate, so an egg or sperm ends up with one extra or one missing. Three copies of a chromosome is trisomy, as in Down syndrome, and one copy is monosomy, as in Turner syndrome. These are usually random events rather than inherited. For Down syndrome, the chance rises as the mother gets older.

A lost piece of chromosome is a deletion, and a piece moved onto another chromosome is a translocation. When that swap leaves the total genetic material unchanged, it is called balanced. Even a small loss can remove many genes at once, as in 22q11.2 deletion syndrome. Effects turn on how large the change is and where it falls, not simply on whether a change exists.

Some three-letter DNA repeats are unstable and can lengthen from parent to child, so the condition appears earlier and more severely, as in Huntington's disease and myotonic dystrophy. Imprinting is different: some genes are active only when inherited from one parent. That is how errors in one region of chromosome 15 can cause either Prader-Willi or Angelman syndrome.

Common conditions such as type 2 diabetes and heart disease come from many genes plus lifestyle and exposures, not one variant. Picture liability accumulating until it crosses a threshold: below it no disease, above it disease appears. These conditions cluster in families without a clear inheritance pattern, partly because relatives also share habits and surroundings.

The same agent can do very different things depending on when in pregnancy it acts, because each organ has a sensitive window of formation. Exposure while organs are forming tends to cause major structural defects. Later exposure more often affects growth or function. Thalidomide is the classic case, where the limb defects tracked the timing of the drug. Amount and fetal genes also matter.

Fluid leaves capillaries under pressure and is pulled back by plasma proteins, with lymphatics draining the rest. Each failure has its own picture: raised venous pressure in heart failure, low albumin in liver disease or nephrotic syndrome, leaky vessels in inflammation, and blocked lymphatics after node removal. Ask which force changed before assuming every swollen leg has one cause.

Opening arterioles bring extra oxygen-rich blood, so tissue looks red and warm, as in exercising muscle or inflammation. When venous outflow backs up instead, deoxygenated blood pools and tissue looks blue and swollen. Because a backed-up bed gets little fresh flow, long-standing congestion can cause damage and fibrosis, as seen in the liver in right-sided heart failure.

Clots form when blood slows, when a vessel lining is damaged, or when blood clots too readily, a trio called Virchow's triad. Immobility and bed rest slow flow, surgery and injury damage veins, and inherited clotting disorders or cancer raise clotting tendency. Risk adds up, so count the factors: a cancer patient on bed rest after surgery has all three.

A clot does not stay as it formed. It can grow along the vessel or break off as an embolus. The body's clot-dissolving system, fibrinolysis, can clear it, especially when it is young. An older clot can organize, as ingrowing cells turn it into scar tissue, and new channels can then form through it. This is why clot-dissolving drugs work best early.

Most emboli are dislodged clots, often from deep leg veins reaching the lungs. Fat can enter blood after long-bone fractures, bringing breathing trouble, confusion, and a pinpoint rash after a delay. Gas can enter through open veins, or form as bubbles during rapid decompression, and block small vessels or the heart. Amniotic fluid entering maternal blood during delivery can cause sudden collapse and abnormal clotting.

Pale infarcts happen in solid organs supplied by end arteries, such as the heart, kidney, and spleen, where little blood reaches the dead area. Bloody ones occur where blood can still seep in, as in the lung with its dual supply, or in loose tissue after venous blockage, such as the testis. Collateral vessels also decide how much tissue dies after a blockage.

The body defends pressure first. Sympathetic activation speeds the heart and constricts skin and gut vessels, so the pulse climbs and skin turns cool while pressure stays near normal. Breathing quickens, urine falls, and anxiety or confusion appears as loss grows. Falling pressure is therefore a late sign. For the exact class cutoffs, use the figures from your current training.

Delivery of oxygen to tissue is what fails, not a particular number. Organs can be starved of blood while tightened vessels hold pressure near normal, and a person's usual pressure also changes what counts as low. Look for signs of poor flow such as confusion, cool skin, rapid pulse, low urine output, and rising lactate. Waiting for pressure to drop means recognizing it late.

Bleeding, burns, or severe vomiting and diarrhea leave less blood returning to the heart. Less filling means less blood ejected with each beat. Sympathetic signals raise heart rate and clamp vessels, holding pressure for a time, so skin turns cool and pale with a fast, weak pulse. Because the defect is volume, neck veins tend to look flat rather than distended.

A failing ventricle cannot empty well, so blood backs up behind it while too little goes forward. A serious heart attack is the most common cause. Because filling pressures are high, the lungs can congest and neck veins may distend, unlike hypovolemic shock. Compensating vasoconstriction also makes the weak heart push against more resistance, and extra fluid can worsen the picture.

The blood is present, but widened vessels leave too little effective volume. Sepsis and anaphylaxis add leaky capillaries, so fluid shifts out of circulation. Early sepsis may bring warm, flushed skin despite poor organ flow, which misleads beginners. Neurogenic shock comes from lost sympathetic tone after spinal cord injury, so the pulse may be slow rather than fast.

The heart and volume may be fine, yet something physically blocks flow. Fluid in the pericardial sac squeezes the heart so it cannot fill. Trapped air under tension raises chest pressure and cuts venous return. A large clot in the lungs blocks the right ventricle's outflow. Neck veins often bulge as pressure falls, and improvement depends on relieving the blockage, not on supporting the pump.

Early on, a faster pulse and tightened vessels hold pressure, and this is the stage most likely to reverse. As compensation fails, cells turn to anaerobic metabolism, acid builds up, and capillaries leak. In the refractory stage, organ damage no longer responds well to treatment. Kidneys, lungs, liver, gut, and brain can all fail, and one failing organ strains the others.

Two questions sort most anemias. The count of young red cells, or reticulocytes, asks whether marrow is responding: low points to underproduction, high points to loss or destruction. Mean cell volume then narrows the list: small cells suggest iron deficiency or thalassemia, large cells suggest B12 or folate lack. A normal young-cell count can still be inadequate when anemia is severe.

Stores run out before the blood changes. Ferritin, the storage form, falls first, then iron delivery falls, and only later do new red cells turn small and pale because they hold too little hemoglobin. Blood loss is a leading cause in adults, so new iron deficiency in an older adult raises the question of gut bleeding rather than diet alone.

Both vitamins are needed to build DNA, so when they run short, red-cell precursors grow but cannot divide properly, leaving large, fragile cells. B12 also supports normal nerve function, so its lack can bring numbness, trouble walking, and memory changes. Giving folate alone may improve the blood count while nerve damage continues, which is why B12 status has to be checked.

Inflammatory signals raise hepcidin, a liver hormone that traps iron inside macrophages and blocks gut absorption. Stores look adequate, with normal or high ferritin, but the marrow cannot get the iron. Reduced erythropoietin and shorter red-cell life add to it. Ferritin helps separate this from iron deficiency, though it also rises with inflammation, so a normal value is not the whole answer.

When macrophages in the spleen and liver remove red cells, jaundice and an enlarged spleen are typical. When cells burst inside vessels, hemoglobin spills into plasma and urine, which can darken it. Either way, unconjugated bilirubin and LDH rise, and haptoglobin, a protein that binds free hemoglobin, falls. Young red cells rise as marrow responds.

A gene change produces hemoglobin S, which stiffens into long chains when oxygen falls. Cells turn crescent-shaped and rigid, jam small vessels, and break apart early. The blockage causes pain crises, stroke, and organ injury, while early breakdown causes anemia. Repeated damage to the spleen leaves people open to encapsulated bacteria. Low oxygen and dehydration are classic triggers.

In polycythemia vera, a marrow disorder often carrying a JAK2 mutation overproduces red cells regardless of need, so erythropoietin, the hormone that drives them, is low. Secondary forms arise from chronic low oxygen, as in lung disease or high altitude, or from tumors making the hormone, so it is high. Either way blood thickens and flows sluggishly, but raised clot risk is best established in the primary form.

Neutrophils usually climb in bacterial infection, often with immature band forms. Lymphocytes climb in many viral infections. Eosinophils point toward parasites and allergy, and monocytes suggest chronic infection such as tuberculosis. These patterns are clues rather than rules: severe infection can lower counts, and stress or steroids alone can raise neutrophils.

Marrow cancers usually form no solid tumor. Abnormal white cells pile up and displace normal ones, so low red cells, platelets, and working white cells lead to fatigue, bleeding, and infection. Lymphomas instead collect mainly in nodes and lymph channels and can involve other organs. Leukemias are labeled acute or chronic by how fast they progress.

Redness, swelling, and pus depend largely on neutrophils arriving at the site. Without them, pneumonia may show little on a chest film and a skin infection may look mild, and fever may be the only sign. For someone on chemotherapy, that makes fever an emergency rather than a minor finding. A normal-looking wound is not reassurance when the count is very low.

Asking where the platelets went sorts the causes. The marrow may make too few, as with aplastic anemia, leukemia, or chemotherapy. They may be destroyed or used up, as with immune attack or widespread clotting. An enlarged spleen can hold too many. Massive fluid or blood replacement can dilute them. Bleeding shows as petechiae and purpura, small skin hemorrhages.

Screening tests map to pathways. The PT reflects the extrinsic pathway through factor VII, so warfarin or low vitamin K prolongs it early. The aPTT reflects the intrinsic pathway, so hemophilia A or B, lacking factor VIII or IX, prolongs it. A factor deficiency usually causes deep joint and muscle bleeds, while platelet problems cause skin and mucosal bleeding.

An underlying trigger, most often sepsis, drives clotting throughout small vessels, and those clots can block flow and damage organs. That runaway clotting spends the supply of platelets and factors, so bleeding follows, from wounds and gums or internally. It is always secondary to something else, so the cause has to be sought. Severe injury and pregnancy complications can also set it off.

These variants tilt the balance toward clotting, often silently until a second factor appears. Factor V Leiden is the most common: the altered factor V resists shutdown by activated protein C, so clotting runs longer. Surgery, pregnancy, injury, and estrogen use can unmask it. Risk is higher with two copies than one, and a positive gene test shows raised risk rather than a clot.

Healthy arterial lining releases nitric oxide, which relaxes vessel muscle and discourages clotting and inflammation. Smoking, high blood pressure, diabetes, and high cholesterol impair this well before plaque is visible. The result is a vessel that constricts when it should dilate and lets white cells stick. That explains why risk factors matter before any narrowing shows.

Damage to an artery wall lets LDL cholesterol enter and oxidize, which draws in monocytes. These become macrophages that eat lipid and turn into foam cells, forming early fatty streaks. Smooth-muscle cells then migrate in and lay down collagen, building a cap over the lipid core. This starts early in life and grows silently, so symptoms appear late.

Slow narrowing usually causes predictable angina, felt when demand outruns a limited supply. An acute event is different: a plaque breaks open or its surface wears away, a clot forms on it, and the clot can block the artery. This can happen with a small plaque that barely narrowed the vessel. So how much a plaque narrows an artery does not by itself tell you how dangerous it is.

Pressure equals output times resistance, so anything raising either raises pressure. In this form, no single cause is found. Kidneys may retain more sodium, the sympathetic system may add tone, and the renin-angiotensin-aldosterone system, or RAAS, can constrict vessels and hold salt and water. Stiffer arteries add to it with age, and drug classes target these different steps.

Suspect an identifiable cause when pressure is severe, starts at an unusual age, or resists treatment. Kidney disease and narrowed renal arteries activate the renin-angiotensin system, and adrenal tumors release aldosterone or catecholamines. Thyroid disease, sleep apnea, certain drugs, and narrowing of the aorta also play a part. Treating the underlying problem can lower pressure.

Sustained force thickens and damages arterial walls and makes the heart work harder. The left ventricle thickens and can fail. Brain arteries can block or rupture, causing stroke and dementia. Kidney vessels scar, lowering filtration and raising pressure further. Retinal vessels narrow and leak, and the eye is where these changes can be seen directly. Damage often builds without symptoms.

Weakened wall layers let an artery balloon, and this often grows silently until it becomes large or bursts. A tear in the inner layer instead lets blood force its way between layers, so it tends to announce itself with sudden severe chest or back pain. Blood in the false channel can then block branches and cut off organs. High blood pressure is a leading risk for both.

Heart muscle already extracts most of the oxygen it receives at rest, so extra need must be met with extra flow. Left coronary arteries fill mainly during diastole, when the muscle relaxes. A fast heart rate therefore hurts twice, raising demand while shortening filling time. Wall tension rises with a dilated chamber or high pressure, which adds to demand.

Predictable chest pain comes when a narrowed artery cannot raise flow as the heart works harder, and it settles with rest or medicine. An acute syndrome usually means a plaque has broken open and a clot has formed, so pain may be new, stronger, longer, or at rest and may not settle. A change in the usual pattern is the warning that stable disease may have become unstable.

Blocked flow first causes injury that can still reverse if blood returns quickly. Over minutes to hours, cells begin to die, with death spreading from the inner wall outward. Inflammation and macrophages then clear the dead muscle, and the area stays weak for a time before a firm scar forms. Scar cannot contract, so faster reopening preserves more pumping muscle.

Stroke volume is the blood ejected per beat. The stretch from filling is preload, reduced by bleeding or a stiff ventricle. The resistance the heart pumps against is afterload, raised by hypertension or aortic stenosis. The muscle's own strength is contractility, which falls after infarction or with cardiomyopathy. Name which one failed and the treatment logic follows.

A weak left ventricle that cannot pump enough out is heart failure with reduced ejection fraction, often after coronary disease or a heart attack. A stiff ventricle that cannot relax and fill is heart failure with preserved ejection fraction, often after long hypertension. Both cause breathlessness and fluid buildup, so symptoms alone will not separate them.

The left ventricle serves the body, so when it fails, pressure backs into the lungs and causes breathlessness, often worse lying flat. The right ventricle serves the lungs, so its failure backs blood into body veins and swells the legs and abdomen. Right-sided failure most often develops as a result of left-sided failure, because raised lung pressures strain the right ventricle.

When output falls, the body reads it as blood loss and activates the sympathetic system and the renin-angiotensin-aldosterone system. That raises heart rate, tightens vessels, and retains salt and water, which supports pressure in the short term. Over time these signals add workload and fluid and damage cells, and the heart remodels into a larger, weaker shape.

A narrowed outflow valve makes the ventricle pump against high pressure, so its wall thickens inward, called concentric hypertrophy. A leaky valve sends blood back and forth, so the chamber receives extra volume and enlarges, called eccentric hypertrophy. Compensation can hold for years, so symptoms may arrive late. The chamber just upstream of the faulty valve takes the strain first.

Each type impairs pumping, filling, or both, in its own way. An enlarged, weak ventricle pumps poorly: that is the dilated type. With thick muscle, outflow can be blocked and many people have no symptoms; this type often runs in families. Stiff chambers that keep a normal wall thickness resist filling: that is the restrictive type. Do not assume every type means a weak squeeze.

Inflamed heart muscle, often after an infection, can bring heart failure or rhythm problems. Inflammation of the sac typically gives a sharp pain that is worse on breathing in and relieved by leaning forward. Fluid can also collect in the sac, called an effusion. A large effusion compresses the heart and limits its filling, which is tamponade, and that can progress to shock.

Microbes circulating in blood can settle on the heart's inner lining, most often the valves, and especially where that lining is already abnormal. They can arrive from gum or urinary infections, surgery, or IV drug use. The growths, called vegetations, can damage a valve and cause regurgitation. Fragments that shed can lodge downstream, where they obstruct vessels or seed new infection.

High up, at the AV node, the heart usually keeps a steadier, relatively faster backup rhythm, and causes are often reversible, so people may tolerate it. Lower down, below the node, only a slow and unreliable ventricular backup remains, with fainting and a real risk of standstill. Complete heart block means no atrial signals reach the ventricles, wherever it sits, so the site matters.

A normal impulse dies after activating the heart, because the tissue behind it cannot fire again yet. Reentry needs two paths that differ: one blocks the impulse in one direction while the other conducts slowly enough that tissue has recovered when the impulse returns. It then loops, driving a fast rhythm. Scar after a heart attack often supplies this setup for ventricular tachycardia.

Obstruction narrows the airways, so air enters but leaves slowly and some stays trapped, and lung volumes tend to run normal or high. Restriction limits how far the lungs can expand, so volumes shrink, yet what does get in empties without delay. Spirometry separates them: the fraction of the full breath blown out in the first second falls in obstruction but holds up in restriction.

Sensitive airways overreact to exposures most people tolerate, such as pollen or mold, and narrow by three routes: the lining swells, mucus collects, and the surrounding muscle tightens. The narrowing largely eases between attacks or with treatment, which is the reversible part. Over time repeated inflammation can thicken the airway walls, a change called remodeling, and some of that narrowing may no longer reverse.

The walls between air sacs give the lung its elastic spring, so destroying them removes the force that pushes air out and shrinks the surface for gas exchange. Those walls also tether small airways open. Without them, the airways narrow during exhalation and trap air. The chest over-inflates, so the trouble is emptying the lung, not filling it.

Repeated irritation, usually from smoke, inflames the airway lining and enlarges the glands that make mucus. Damaged cilia, the tiny hairs that sweep mucus upward, cannot keep up, so thick mucus stays until it is coughed out, and it narrows the tubes. The diagnosis rests on a long-lasting productive cough, not on imaging. Many people also have some emphysema.

A first injury or poor clearance leaves mucus sitting in the airways, where bacteria multiply. Infection brings inflammation, inflammation damages the walls, and the stretched, scarred tubes clear mucus even worse, so each turn feeds the next. In cystic fibrosis a defective CFTR protein disturbs salt and water transport, so mucus holds too little water and turns thick, which starts the same cycle.

Scar thickens and stiffens the tissue around the air sacs, so the lung takes more work to inflate and breathing turns rapid and shallow. Oxygen must also cross a thicker barrier, so saturation may fall first with exercise. The airways are not narrowed, which means the air that moves flows out easily. That is the contrast with COPD: small volumes, but no trouble emptying.

The lung tissue can be healthy and still not expand when something outside it limits the bellows. A rigid or curved chest wall, fluid or scar in the pleura, heavy abdominal and chest fat, and weak breathing muscles from nerve or muscle disease all do this. Neuromuscular cases also weaken the cough, so retained secretions and rising carbon dioxide become the danger rather than scar.

Every lung unit needs air and blood in balance. In diseased lung, some units get too little air for the blood passing them, and that blood leaves under-oxygenated. Healthy units cannot make up the difference because their blood is already nearly full. Extra oxygen helps because it still reaches partly ventilated units, so a good response points away from a true shunt.

A shunt is blood that crosses the lung without meeting air, as through a collapsed or fluid-filled region. Added oxygen never reaches it, so saturation improves little when the shunt is large. Dead space is the reverse: air reaches units with no blood flow, as beyond a pulmonary clot, so that air does no gas exchange. Think of them as the two extremes of mismatch.

Nearly all oxygen in blood rides on hemoglobin, and the curve relating oxygen pressure to saturation is S-shaped. The flat top keeps saturation high despite modest pressure falls, while the steep part lets tissues unload oxygen. Acidity, high carbon dioxide, and fever shift the curve right, which favors release. Alkalosis, cold, and carbon monoxide shift it left, so hemoglobin holds on tighter.

A clot, usually from a deep leg vein, lodges in a pulmonary artery. The lung beyond it still gets air but no blood, which is dead space, and blood diverted to other regions overloads them and causes mismatch and low oxygen. A large clot also raises the resistance the right ventricle must pump against, so it can stretch and fail suddenly, and cardiac output falls with it.

Low oxygen makes lung arteries constrict, the opposite of arteries elsewhere. That reflex helps when one small region is poorly ventilated, but lung disease or sleep apnea makes it widespread and lasting, and the vessel walls thicken. The thin right ventricle must then pump against high pressure, so it enlarges and can fail. Right heart failure from lung disease is called cor pulmonale.

Damage to the alveolar wall lets capillaries leak protein-rich fluid into the air sacs, and the trigger can be local, such as pneumonia or smoke, or distant, such as sepsis. Inflammation also degrades surfactant, which normally holds the sacs open. Flooded, collapsed units behave like a shunt, so oxygen stays low despite supplemental oxygen. The fluid does not come from heart failure, and some lungs later scar and stiffen.

Failure can show as low oxygen, high carbon dioxide, or both, and each points at a different site. Fluid in the air sacs, as in pneumonia or ARDS, mainly blocks oxygen from crossing. Weak breathing muscles or a brain that stops responding, as in opioid overdose, mean too little air moves, so carbon dioxide builds up. Saturation can look acceptable, especially on supplemental oxygen, while carbon dioxide climbs.

Filtration pressure depends on two gates: the afferent arteriole bringing blood in and the efferent arteriole letting it out. When perfusion falls, the afferent opens and the efferent tightens, which holds pressure in the glomerulus. Drugs can remove this rescue. NSAIDs block the afferent widening and ACE inhibitors block the efferent tightening, so a dehydrated patient on both can lose filtration abruptly.

Immune injury inflames the glomerular wall, so red cells leak through and urine may look cola-colored. Filtration falls, so salt and water are retained, and hypertension and swelling follow. Protein leak is usually modest compared with nephrotic disease. Post-streptococcal glomerulonephritis and IgA nephropathy are classic examples. Think blood and pressure here, and think heavy protein loss for nephrotic disease.

When the glomerular filter leaks, large amounts of protein spill into urine and blood albumin falls. Fluid then collects in the eyelids and legs, and blood cholesterol and other fats rise. Losing other proteins has its own cost: immunoglobulin loss raises infection risk, and clots become more likely. Many different diseases produce these same findings, so the syndrome is a pattern rather than a diagnosis.

Tubule cells work hard and use a lot of oxygen, so they are vulnerable to low blood flow and to toxins such as some antibiotics and the pigment from muscle breakdown. When they die or slough, the kidney loses its ability to concentrate urine and reabsorb sodium. Urine stays dilute and salty even when the body is short of fluid, which helps distinguish this from prerenal injury.

Prerenal injury is low blood flow to a healthy kidney, so it saves sodium and makes scant, concentrated urine. Intrarenal injury damages the kidney itself: in its commonest form, acute tubular necrosis, tubules leak sodium and the sediment often shows casts. Postrenal injury is blocked outflow, found with imaging or a full bladder. Creatinine rises in all three, so it flags the problem but cannot name the site.

As nephrons are lost, the survivors enlarge and raise their own filtration to keep total function up, so blood tests can look stable for years. That extra pressure and flow injures the remaining filters, which scar and drop out in turn, so decline can continue even after the first cause is controlled. Blood pressure control and reducing protein in the urine aim at this loop.

Failing kidneys clear phosphorus poorly and make too little calcitriol, the active form of vitamin D, so calcium balance slips. The parathyroid glands respond with more hormone, which draws calcium out of bone. The bone disease that follows, with fragile and deformed bone, is renal osteodystrophy, and vessels can calcify and harden too. Retained wastes add the poor appetite and nausea of advanced disease.

Osmolality counts all dissolved particles, but only particles that cannot freely cross a cell membrane pull water. Sodium does this, and so does glucose when insulin is lacking, so both are effective osmoles. Urea crosses easily, so high urea raises osmolality without shrinking cells. That distinction is tonicity, and it explains why two solutions with the same osmolality can act differently on cells.

A sodium reading tells you about water relative to sodium, not how much sodium the body holds. Total body salt sets the size of the extracellular space, and you judge that at the bedside from blood pressure, pulse, and edema. A patient can have low sodium and be dry, normal, or swollen. The common error is treating a low number as a salt shortage without assessing volume first.

Water follows tonicity across brain cell membranes. Low sodium swells cells, causing headache, confusion, and seizures, while high sodium shrinks them, causing irritability and stupor. Over days the brain adapts by shedding or gaining solutes, so rapid correction drives water the wrong way. Correcting chronic low sodium too fast can damage myelin, and correcting chronic high sodium too fast can cause cerebral edema.

Potassium sits mostly inside cells, and the gradient across the membrane sets resting potential. High blood potassium narrows that gradient, so cells first fire more easily and then lose the ability to fire, producing weakness and dangerous rhythms. Low potassium makes heart cells irritable and muscles weak. Insulin, acid-base shifts, and kidney function all move the level, so a reading needs context.

Parathyroid hormone raises blood calcium by releasing it from bone, saving it in the kidney, and activating vitamin D. It also makes the kidney dump phosphate. Active vitamin D increases gut absorption of both minerals. Calcium also stabilizes nerve and muscle membranes, so low levels cause tingling and spasm, while high levels cause sluggish muscles, constipation, and confusion.

Compensation moves the other system in the direction that restores the ratio of bicarbonate to carbon dioxide. In metabolic acidosis the lungs blow off carbon dioxide within minutes to hours. In respiratory acidosis the kidneys retain bicarbonate, which takes days. Compensation rarely brings pH fully back to normal, so a normal pH with two abnormal values suggests a second disorder.

The gap is sodium minus the sum of chloride and bicarbonate, and it estimates unmeasured anions in the blood. When an added acid such as lactic acid or a ketoacid uses up bicarbonate, its anion widens the gap. When bicarbonate is lost directly, as with severe diarrhea, chloride rises to replace it and the gap stays normal. Low albumin can hide a raised gap, since albumin is itself an unmeasured anion.

When the lower esophageal sphincter weakens or relaxes at the wrong time, stomach contents repeatedly bathe the esophagus, whose squamous lining is not built for acid. In some people that lining converts to a more intestine-like type, a switch called metaplasia, and this is Barrett's esophagus. Dysplasia, or abnormal cells, can then arise in the changed tissue and raises cancer risk, although most people with Barrett's never develop cancer.

An ulcer forms when acid and pepsin outrun the defenses of the stomach or duodenal lining. Helicobacter pylori infection and NSAIDs such as aspirin or ibuprofen are the two leading causes: the bacterium inflames the mucosa, and NSAIDs reduce the prostaglandins that maintain it. An ulcer can stay silent until a complication appears, such as bleeding or perforation through the wall.

Sort the failure by step. Poor digestion, as with too few pancreatic enzymes, leaves fat undigested and stools greasy, with fat-soluble vitamins lost. Lost surface, as when celiac disease flattens the lining of the upper small bowel, often shows as iron and folate deficiency. Lost transport at a specific site, such as the last part of the small bowel, which absorbs vitamin B12, gives a targeted deficiency.

Crohn's disease can strike anywhere from mouth to anus, in patches, and the inflammation runs through the full wall, so it forms fistulas, abscesses, and strictures. Ulcerative colitis stays in the colon, starts at the rectum, spreads continuously, and affects the inner lining, so bleeding dominates. Toxic megacolon is a classic severe complication. Both raise colorectal cancer risk when the colon has been inflamed for years.

Gas and swallowed fluid collect above the blockage and stretch the bowel, which causes cramping pain and vomiting. The stretched wall absorbs poorly and secretes more, so fluid leaves the circulation into the gut, which is third spacing, and dehydration follows. If pressure or a twist cuts off the blood supply, the wall can die and perforate. Constant pain and fever suggest this strangulation.

When blood flow to the gut fails, the ischemic bowel causes severe, poorly localized pain, while the lining of the belly wall is not yet inflamed, so the exam can look nearly normal. That mismatch is the warning. Tenderness and shock come later, once the full bowel wall is dying. Think of it in an older patient with atrial fibrillation or known vascular disease.

Blood entering high in the tract, from the esophagus through the duodenum, is exposed to acid and digestion, so vomit can look like coffee grounds and stool turns black and tarry. Blood from the lower colon or anus arrives little changed and looks bright red. A very brisk upper bleed can also pass red, because it moves through too fast, and it may come with shock.

Old red cells yield unconjugated bilirubin, which the liver conjugates and excretes in bile. Heavy hemolysis overwhelms the liver and raises the unconjugated fraction, which is bound to albumin and so stays out of the urine. Liver cell damage can raise both. A blocked duct backs up conjugated bilirubin, which is water soluble, so urine darkens, and little reaches the gut, so stools go pale.

Viruses, alcohol, drugs, and autoimmune attack begin differently, but each kills liver cells, and those cells release enzymes such as ALT and AST into the blood. Some viruses are cleared after an acute illness, while hepatitis B and C can persist for years. Ongoing inflammation is often the larger danger, because continued cell death and repair drive scarring, though a severe first hit can cause acute liver failure.

Liver injury can heal with some scar. With repeated injury, scar replaces working tissue and surviving cells regrow in clumps called nodules. Two problems result: less functioning liver, and scar that obstructs blood on its way through the organ. Symptoms are often absent until damage is severe, and the scarring of established cirrhosis is permanent.

Blood from the gut and spleen reaches the liver through the portal vein. In cirrhosis, scar slows that flow and pressure builds behind it. Blood then detours through collateral veins, so those in the esophagus and stomach swell into varices, which can rupture and bleed heavily. Back-pressure also enlarges the spleen and contributes to fluid buildup in the abdomen and legs.

High portal pressure pushes fluid out of liver and gut vessels into the abdomen. Low albumin, from a failing liver, weakens the force that holds fluid in the blood. Meanwhile the arteries in the gut widen, the kidneys sense an underfilled circulation, and they retain sodium and water, so the fluid keeps coming. That kidney response is why sodium intake matters in this disease.

The liver normally clears ammonia and other nitrogen compounds made in the gut. With liver failure, or blood bypassing the liver through collateral veins, these reach the brain and impair function, from subtle sleep and attention changes to coma. GI bleeding, constipation, infection, and dehydration often set off an episode, so looking for the trigger is part of the reasoning.

Digestive enzymes are meant to switch on in the intestine. When they activate early inside the pancreas, the gland starts digesting itself, which causes inflammation and upper abdominal pain that can reach the back. A gallstone lodged where the bile and pancreatic ducts drain is the most common acute cause, and heavy alcohol use is another. Repeated attacks can leave lasting damage, known as chronic pancreatitis.

Follow the pair: the target gland's hormone and the pituitary hormone that drives it. Primary disease is in the gland, so in hypothyroidism thyroid hormone is low and TSH is high because the brake is off. Secondary disease is in the pituitary and tertiary disease in the hypothalamus, and either one leaves the pair low, so telling those two apart takes more testing. Reading one value alone is the usual mistake.

A pituitary adenoma, usually benign, causes trouble two ways. The hormone acts on the body: growth hormone drives the liver to make IGF-I, which enlarges hands, feet, and organs and disturbs glucose and lipids, while excess prolactin lowers sex hormones and can cause infertility. The mass acts locally: a large tumor can press on the optic nerves or chiasm and affect vision.

The pituitary drives other glands through trophic hormones, which keep a target gland working. Lose them and the thyroid, adrenals, and gonads can fall off together, so the picture is fatigue, cold intolerance, and lost periods or libido rather than one clear deficit. Because adrenal failure can be dangerous, cortisol replacement is taught as the priority before thyroid hormone.

ADH tells the kidney to hold water. Too little, or a kidney that ignores it, is diabetes insipidus: large volumes of dilute urine, thirst, and rising sodium if intake falls behind. Too much is SIADH, the syndrome of inappropriate ADH: concentrated urine, retained water, and low sodium without edema. Diabetes insipidus has normal glucose, so it is unrelated to diabetes mellitus despite the name.

Thyroid hormone sets the pace of metabolism in nearly every organ, so a shortage slows the whole body. That logic explains the fatigue, weight gain, cold intolerance, and high cholesterol. Hashimoto's disease, an immune attack on the gland, is the most common cause in the US. Onset is slow and can go unnoticed for years, and the rare severe form, myxedema coma, is an emergency.

Graves' disease is driven by an autoantibody, thyroid-stimulating immunoglobulin, that mimics TSH and keeps the gland switched on. Overactive nodules reach the same result by producing hormone on their own. Either way, body functions speed up, with tremor and a rapid heartbeat, and TSH falls because the brake is working. An irregular heart rhythm is a key risk.

Parathyroid hormone raises blood calcium by pulling it from bone and holding on to it in the kidney. Excess, usually from an overactive gland, can thin bone and cause kidney stones, and the high calcium dulls nerve and muscle. Deficiency, often after neck surgery, lets calcium fall, so membranes fire too easily, giving tingling, muscle spasm, and sometimes seizures.

Long-term high cortisol, most often from glucocorticoid medicines, raises glucose and blood pressure, thins skin and bone, and shifts fat to the trunk. Adrenal insufficiency is the opposite: fatigue, weakness, weight loss, and low blood pressure. Stopping long-term steroids suddenly can cause it, because the suppressed glands are slow to restart. Adrenal crisis under stress means shock and is an emergency.

Aldosterone makes the kidney keep sodium and lose potassium, so excess from an adrenal adenoma or hyperplasia gives high blood pressure, sometimes with low potassium. Pheochromocytoma is a tumor, usually in the adrenal gland, that secretes catecholamines, causing bursts of high pressure, headache, sweating, and palpitations. Suspect either when hypertension is severe or starts young.

The immune system destroys beta cells, the pancreatic cells that make insulin. With little or no insulin, glucose cannot enter most cells, so blood glucose climbs while those cells lack energy. Fat breakdown takes over as the fuel source, and the ketones it yields can build up into diabetic ketoacidosis, which may be how the disease first shows itself, especially in children.

Muscle, fat, and liver cells respond poorly to insulin, because signaling inside the cell is impaired more than the hormone is missing. The pancreas makes more to compensate, so insulin levels run high while glucose may stay near normal for years. That is why insulin resistance can be silent. Once the pancreas can no longer keep up, glucose rises.

Type 2 disease begins as a race: the pancreas makes extra insulin to overcome resistance, and glucose stays near normal while it can. Over time the beta cells make less and respond more slowly to meals, so glucose rises even if resistance has not changed. This is why treatment often has to step up over the years, which reflects the disease advancing rather than a failure to comply.

Both have high glucose and heavy fluid loss through the urine. Ketoacidosis occurs when insulin is too scarce to restrain fat breakdown, so ketones pour out and acidosis follows, with rapid deep breathing. In the hyperosmolar state, usually type 2, a little insulin remains, enough to restrain ketone formation but not glucose, so glucose and dehydration climb higher and confusion dominates.

Cells in the retina, kidney glomeruli, and nerves take up glucose without needing insulin, so they are exposed to whatever the blood holds. Excess glucose binds to proteins, forming advanced glycation products, and it is shunted through the sorbitol pathway, which stresses cells. Small vessel walls thicken and leak. The results are retinopathy, kidney disease, and neuropathy.

Chronic high glucose injures artery walls and the autonomic nerves serving the heart, and it often travels with hypertension and high LDL cholesterol, which feeds plaque. The result is more heart attack and stroke. Nerve damage adds a trap: an infarction can occur with little or no chest pain, so subtle symptoms deserve attention in a patient with diabetes.

Abdominal fat releases free fatty acids, which blunt the body's response to insulin. Insulin resistance and those fatty acids push LDL up and HDL down, and resistance also raises triglycerides and blood pressure. Fat tissue adds inflammatory signals that worsen the same problem. That shared root is why these conditions cluster and why the combination raises heart disease risk.

Brain arterioles widen when pressure falls and tighten when it rises, so blood flow stays steady across a wide range of blood pressure. Below that range flow falls passively and the brain becomes ischemic. Above it, vessels are forced open and fluid leaks, causing swelling. Injury can impair this reflex, so after a head injury brain flow may simply follow systemic pressure.

The adult skull cannot stretch, and it holds brain, blood, and spinal fluid. When a mass or swelling adds volume, spinal fluid and venous blood shift out to make room, and pressure stays near normal at first. Once that reserve is used, small added volumes make pressure rise steeply. Brain perfusion depends on the gap between arterial and intracranial pressure, so high pressure starves the tissue.

Rising pressure pushes brain tissue past rigid folds and openings, and what it squeezes predicts the signs. Uncal herniation presses the third nerve, giving a dilated pupil on the same side, then weakness and falling consciousness. Central downward shift compresses the brainstem. Slow pulse, high blood pressure, and irregular breathing together are Cushing's triad, which is a late sign.

When an artery blocks, tissue with the least collateral flow dies first and forms the core. Around it lies the penumbra, where cells have stopped working yet stay alive because some blood still arrives. Those cells can recover if flow returns and will die if it does not, so the rim shrinks with time. This is the physiology behind treating stroke as an emergency.

A bleed harms by a different route than a blocked artery. Blood in the skull acts as a mass that raises pressure and displaces structures, often with sudden severe headache, vomiting, and falling consciousness. Blood breakdown products also irritate tissue and vessels, so arteries near a subarachnoid bleed can spasm days later. Long-standing hypertension and aneurysms are classic causes.

Primary injury happens at the moment of impact: bleeding, bruising, and torn nerve fibers. Secondary injury follows over hours to days, as the blood-brain barrier breaks down, the brain swells, and contusions keep bleeding. Swelling in a closed skull raises pressure and chokes off perfusion, and low blood pressure or low oxygen adds to the harm. Primary damage is already done, so care aims at limiting the secondary wave.

Neurons normally fire in a controlled pattern, with signals that excite and signals that inhibit in balance. During a seizure many fire together, far faster than normal. Focal seizures begin in one part of the brain, so the symptoms reflect what that area does. Generalized seizures engage both hemispheres almost from the start. A focal seizure can also spread to both sides, so a brief odd sensation may come first.

Two abnormal protein deposits define the disease: amyloid plaques between neurons and tau tangles inside them. Synapses fail and neurons die, beginning in memory structures such as the hippocampus, which is why new learning goes first and old memories last longer. Damage begins years before symptoms and later spreads across the cortex, with brain shrinkage visible on imaging.

Cells in the substantia nigra make dopamine, the messenger the motor circuit needs for fluid, deliberate movement. Many are already lost by the time movement symptoms appear, so slowness, stiffness, and resting tremor surface well into a long process. Damaged cells often hold Lewy bodies, clumps of the protein alpha-synuclein. Loss of norepinephrine cells is thought to explain some non-movement problems, such as fatigue.

The huntingtin gene carries an expanded run of the DNA triplet CAG, and the resulting abnormal protein injures neurons, with the striatum especially vulnerable. Inheritance is dominant, so each child of an affected parent has a 50 percent chance. The repeat can lengthen when passed on, especially through the father, and longer repeats go with earlier onset. Earlier disease in later generations is called anticipation.

The immune system attacks myelin, the insulating coating on nerve fibers in the brain and spinal cord, leaving hardened patches called plaques. Signals then travel poorly, and the symptoms depend on where each plaque sits. In the common relapsing form, attacks recur with recovery between them, and new lesions appear at different times and places. Axons and gray matter are damaged too, so disability can build.

Days to weeks after an infection, often of the airway or gut, the immune response can turn on peripheral nerves, usually damaging myelin. Weakness typically starts in the feet and climbs symmetrically over days to a few weeks. The danger is reaching the breathing muscles, and blood pressure and heart rate can also become unstable. Campylobacter infection is a classic trigger.

At the junction, a nerve releases acetylcholine, which binds receptors on muscle. In myasthenia gravis, antibodies block or destroy receptors, so weakness worsens with use and improves with rest, often starting in eye and face muscles. Botulinum toxin instead stops acetylcholine release, producing descending paralysis that does not vary with effort. Both can reach the breathing muscles.

The farthest nerve endings usually fail first, so symptoms start in the feet and can later move upward. That is the length-dependent pattern, and diabetes is the leading cause in the US. Nerve conduction tests help sort the damage into axon loss or injury to myelin, the insulating sheath, as in CIDP, an immune attack on it. Knowing which narrows the list of causes.

Bone is always being rebuilt. Osteoclasts dissolve old bone and osteoblasts lay down new matrix in the same spot, and the two are normally linked so mass stays steady. When resorption outpaces formation, as after menopause when estrogen falls, bone is lost. Uncoupled means the balance has shifted, not that one cell type stopped working.

Resorption outpaces formation, so bone mass and structure decline, yet what remains is normally mineralized, which is the contrast with osteomalacia. There are usually no symptoms until something breaks, classically the hip, a vertebra, or the wrist. A fracture after a minor fall, one that healthy bone would survive, is the clue that points here.

The protein framework of bone is laid down normally but fails to harden, usually because low vitamin D limits the calcium and phosphate available. In children the same failure at growing bones is called rickets. Bones are soft and can bend, and aches and muscle weakness are common. Osteoporosis has too little normal bone, while this disease has bone that is under-mineralized.

A fracture bleeds and forms a hematoma, which brings inflammatory cells and growth signals. Cells then build a soft callus of cartilage and fibrous tissue, which hardens into woven bone, the hard callus. Over months, remodeling restores normal shape and strength. Too much motion at the site, poor blood supply, infection, smoking, and poor nutrition can stall the process.

Joint cartilage wears and fissures, and the bone at the joint margin responds with outgrowths called osteophytes, or bone spurs. Because the problem is mechanical, pain tends to come with movement and ease with rest, and stiffness after rest is brief. Disease stays in the joints, which separates it from rheumatoid arthritis. Extra weight and past joint injury add risk.

Autoimmune inflammation targets the synovium, the joint lining, then erodes the tissue covering the bone ends. Morning stiffness is typical, and involvement tends to be symmetric, hitting the same joints on both sides. Inflammation can also reach organs such as the eyes, lungs, and heart, and the immune changes can start years before symptoms. That systemic reach is a main contrast with osteoarthritis.

Urate builds up and forms needle-shaped crystals in a joint, which immune cells attack, causing a sudden hot, painful flare, classically in the big toe. Rhabdomyolysis is different: injured muscle breaks open and spills potassium and myoglobin into the blood. Myoglobin can injure kidney tubules, and urine may look dark, so the danger is acute kidney injury plus high potassium.

Intact skin retains water, blocks microbes, and regulates heat through sweating and blood flow. A large wound removes all three together, so evaporation dehydrates the person, heat escapes, and bacteria reach open tissue. That is why a severe burn or widespread skin failure is a whole-body problem and not just a surface one, and why such a patient can be chilled as well as dry and infected.

Pressure over a bony site squeezes capillaries shut, and tissue dies once that lasts long enough. Shear, the sliding of skin over deeper layers, kinks vessels, and moisture softens skin. Muscle tolerates low blood flow worse than skin does, so damage can be deeper than the surface shows, and a small skin defect may hide a large wound. Immobile, poorly perfused people are at highest risk.

Depth decides how the skin recovers. A superficial burn heals from surviving skin cells, while a full-thickness burn destroys them and the wound needs grafting or closes slowly with scar. Extent decides the whole-body effect. A large burn makes capillaries leak throughout the body, so plasma shifts into tissues, and shock can occur without any visible blood loss.

Bacteria on a wound are normal, and colonization alone usually does not stop healing. Infection begins when their numbers or virulence overwhelm local defenses, bringing spreading redness, warmth, pain, or pus. Some form biofilm, a slimy community that shields them from antibiotics and immune cells and can stall healing. Poor blood flow, dead tissue, and high glucose tip the balance toward infection.

Eczema pairs a leaky outer skin barrier with an overactive immune response, which lets irritants in and water out, so skin becomes dry and itchy, and scratching worsens it. Psoriasis is driven by overactive immune cells that speed skin cell turnover from about a month to days, which piles up thick patches with silvery scale. Neither is contagious, and both involve genes plus triggers.

Ultraviolet light damages DNA in skin cells, and sun exposure with a fair complexion raises risk. Basal and squamous cell carcinomas arise from the keratin-making cells of the epidermis, are far more common, and usually stay local, though squamous cell spreads more often. Melanoma arises from melanocytes, the pigment cells. It is far rarer but tends to invade and metastasize, which is why it causes most skin cancer deaths.

Ovulation depends on signals among the hypothalamus, pituitary, and ovary. Stress, low weight, thyroid disease, high prolactin, or PCOS can disrupt them. Without ovulation no corpus luteum forms and progesterone stays low, so any estrogen present keeps building the uterine lining with no signal to stabilize it. The lining then sheds unpredictably and can bleed heavily, so irregular bleeding often points to missed ovulation.

Tissue similar to the uterine lining grows outside the uterus, most often in the pelvis, and responds to the same monthly hormones. It can bleed with no outlet, which causes inflammation and scar tissue called adhesions. Backward menstrual flow is one proposed origin, though no single theory explains every case. Pain and infertility are the main problems, and pain does not reliably track lesion size.

Excess androgens interfere with the brain's control of ovulation and arrest developing follicles, which pile up in the ovary, small and immature, rather than releasing an egg. Insulin resistance is common too, and the compensating rise in insulin stimulates more androgen, so each problem feeds the other and the loop is hard to interrupt. The underlying cause is still unknown, with genes and environment both involved.

The prostate wraps the urethra just below the bladder, and enlarging tissue can press inward on that channel. The bladder wall must generate more pressure to force urine past the narrowing and may eventually weaken and leave urine behind, which is urinary retention. Gland size predicts symptoms poorly: how tightly the urethra is squeezed matters more than how big the gland has grown.

In normal pregnancy, placental cells remodel the mother's spiral arteries into wide, low-resistance channels. When this falls short, the placenta is under-perfused and is thought to release factors that injure the mother's blood vessel lining, producing hypertension and often protein in the urine. The exact cause remains unknown. Abruption is early separation of the placenta, with bleeding behind it.

Placental hormones progressively blunt insulin action, which keeps more glucose available for the fetus. Most pregnant people raise their insulin output enough to cover this. When the pancreas cannot keep up, gestational diabetes results. It often causes no symptoms or only mild ones such as thirst, which is why it is found by routine testing in pregnancy. Genetics and body weight add risk.