Human Biology curriculum

Each plane names the section it cuts. A sagittal plane runs front to back and splits right from left; the median plane is the sagittal plane through the midline. A coronal, or frontal, plane runs side to side and splits front from back, and a transverse plane is horizontal and splits upper from lower. The reference posture, the anatomical position, has the palms facing forward.

Superior, or cranial, points toward the head and inferior, or caudal, away from it. Anterior (ventral) is the front and posterior (dorsal) the back; medial is toward the midline and lateral away from it. Proximal and distal are judged along a limb from where it joins the trunk, so the knee is proximal to the toes. Every one of these terms is relative: it places a structure against another structure.

The dorsal cavity holds the brain in the cranial cavity and the spinal cord in the vertebral canal. The larger ventral cavity is split by the diaphragm into the thoracic cavity above and the abdominopelvic cavity below. Serous membranes line these closed cavities and cover their organs; their thin serous fluid lets organs slide without friction, and the pleura around the lungs is one example.

Each level is built from the one below. Atoms bond into molecules; similar cells and the material between them form a tissue; several kinds of tissue arranged for one job form an organ; and organs working together form a system. The stomach shows the pattern, combining muscle, connective, epithelial and nervous tissue in a single organ, and ten major systems make up the body.

Anatomy studies structure: the cells, tissues and organs of the body and how they are arranged. Physiology studies how a living body works. The two questions are linked because structure limits function. A very thin cell is poorly suited to protection, and bone cells are not built to conduct nerve impulses, so a part's build sets the limits of its job.

Homeostasis keeps the internal environment relatively constant, not fixed. In negative feedback a sensor detects a change, a control center compares it with a set point, and effectors push the variable back, as when low blood calcium makes the parathyroid glands release hormone. Positive feedback works the other way: each change drives a bigger change, so it builds rather than stabilizes.

Life processes alone do not keep you alive; you also need water, oxygen, nutrients, heat and pressure from the environment. Imaging shows structure without opening the body. An X-ray image uses low-dose radiation, CT links an X-ray machine to a computer to build 3-D views from many angles, MRI uses a strong magnet and radio waves, and ultrasound turns echoes of sound waves into pictures.

Carbon, hydrogen and oxygen are core elements of life. An atom has a nucleus of protons and neutrons with electrons around it, and isotopes of one element share a proton count but differ in neutrons. A covalent bond shares electrons between atoms; an ionic bond holds together ions charged by gaining or losing electrons; hydrogen bonds attract separate molecules or different parts of one molecule.

Water dissolves more substances than any other liquid, which is what lets cells put nutrients and minerals to use. It carries food molecules and wastes in the blood, cushions the brain and spinal cord, lubricates joints, and sheds heat through sweating. Water is also a reactant: hydrolysis uses water to split a compound into smaller parts.

An acid gives off hydrogen ions in water and a base accepts them. On the 0 to 14 pH scale, 7 is neutral, below 7 is acidic and above 7 is basic. Arterial blood stays in a narrow band near 7.35 to 7.45. The carbonic acid and bicarbonate buffer soaks up acid made by metabolism, the lungs and kidneys regulate the balance, and drifting outside the band is acidosis or alkalosis.

Electrolytes are minerals that break up into charged ions in body water: sodium, potassium, calcium, chloride, phosphate and others. Their charges let them steer water balance and blood acidity, move nutrients into cells and wastes out, and keep nerves, muscles and the heart working. Sodium is key to keeping body fluids in balance, and bicarbonate is the chief buffer in blood.

Carbohydrates are made only of carbon, hydrogen and oxygen; simple sugars such as glucose link into long polysaccharide chains, and glycogen is the form in which the body stores glucose. Lipids are the molecules that do not dissolve in water. Fats store energy for the long term, phospholipids build cell membranes, and steroids such as cholesterol serve as building blocks for many hormones.

A protein is one or more chains built from 20 amino acids, folded into a shape set by their side chains, with water-fearing ones tucked inside. Most enzymes are proteins that speed a reaction by lowering the energy it needs, and only the right substrate fits the active site. Cells switch enzymes on and off by adding or removing phosphate groups, and one wrong amino acid can leave an enzyme misshapen.

DNA and RNA are chains of nucleotides, each a sugar, a phosphate and a base. DNA is a double helix with deoxyribose backbones, A pairing with T and C with G. RNA uses ribose, swaps uracil for thymine and is usually single-stranded. ATP stores energy in the bonds holding its three phosphates; the third phosphate is added to ADP and removed again as the cell spends and rebuilds it.

The plasma membrane is a phospholipid bilayer: water-loving heads face out and water-fearing tails face each other, with cholesterol tuning its stiffness. Small uncharged molecules such as oxygen slip straight through, but ions and large molecules need protein channels and carriers. Other membrane proteins act as receptors, and sugars attached to them work as identity markers.

The nucleus is wrapped in a double nuclear membrane whose pores let only selected molecules in and out. Inside, DNA winds around histone proteins as chromatin, packed small enough to fit, and the nucleolus assembles ribosomes. Before a cell divides it duplicates all of its DNA, so both new cells inherit a full set of chromosomes, 46 in a human body cell.

In diffusion a solute spreads from higher to lower concentration, and osmosis is water doing the same across a selectively permeable membrane. Small uncharged molecules cross the lipid bilayer directly, while ions and large molecules need channel proteins. Tonicity compares solute levels: a red blood cell shrinks in a hypertonic solution, holds steady in an isotonic one, and swells and can burst in a hypotonic one.

Active transport moves a substance against its gradient, from low to high concentration, and needs a carrier and energy. The sodium-potassium pump spends one ATP to move three sodium ions out and two potassium ions in. The gradients it builds then power other carriers, such as transporters that run on the sodium gradient. Vesicles move bulk cargo: endocytosis brings it in, exocytosis sends it out.

A cell spends most of its cycle in interphase, growing and copying its chromosomes. Mitosis then separates the copies into two identical nuclei, and cytokinesis divides the cytoplasm. Checkpoints can pause the cycle when DNA is damaged or not fully copied, and a cell damaged beyond repair is removed by apoptosis. When that removal fails, uncontrolled division can lead to a tumor.

Ribosomes assemble proteins, many on the rough endoplasmic reticulum, which processes them; smooth ER makes lipids and breaks down toxins. The Golgi matures proteins and lipids and ships them out, lysosomes use enzymes to break down worn-out parts, and mitochondria turn food energy into ATP. The cytoskeleton of actin filaments, intermediate filaments and microtubules gives the cell shape and moves things inside it.

Transcription copies a gene's DNA into messenger RNA in the nucleus. Introns are cut out and exons joined before the mRNA moves to the cytoplasm. In translation a ribosome reads the mRNA three bases at a time; each codon calls for one amino acid, delivered by a matching transfer RNA. Of the 64 codons, 61 specify amino acids and 3 are stop signals that end the chain.

Mitosis gives daughter cells identical genomes, so specialization depends on which genes are switched on and how strongly. Stem cells can renew themselves or give rise to specialized cells. Pluripotent stem cells, from early embryos or reprogrammed adult cells, can become any cell of the adult body, while adult stem cells mostly replace the cell types of their own tissue.

Epithelial tissue forms barriers and linings, connective tissue supports and binds, muscle tissue moves the body, and nervous tissue carries messages. Organs mix them; the stomach contains all four. The tissues also differ in their cell-to-matrix balance: epithelial cells are packed tight with almost no matrix, while connective tissue is mostly matrix with relatively few cells.

Epithelium is named by cell shape, squamous, cuboidal or columnar, and by whether it has one layer or several. Each cell has a free surface, and on the opposite side the sheet is anchored by a basement membrane to underlying connective tissue. Simple columnar epithelium lines the stomach and intestines, pseudostratified columnar lines parts of the airway, and transitional epithelium can stretch.

Muscle tissue is made of long, slender fibers that contract using actin and myosin. Skeletal fibers are striated, multinucleated and voluntary; smooth muscle cells are spindle-shaped, unstriated and involuntary; cardiac fibers branch, carry one nucleus and join at intercalated disks. Nervous tissue pairs neurons, which generate and conduct impulses, with glial cells that bind, insulate and protect them.

Glandular epithelium is specialized to make and secrete substances, and its glands are sorted by where the product goes. Endocrine glands, such as the thyroid and pituitary, release hormones straight into the blood. Exocrine glands send sweat, tears, saliva, milk or digestive juices through a duct or opening to a body surface. A duct is therefore the quick way to tell the two apart.

Connective tissue has abundant matrix and relatively few cells, the most common being fibroblasts, macrophages and mast cells. Fibroblasts secrete the collagen that frames tissues and they help heal wounds. In its loose, adipose, dense fibrous and elastic forms, connective tissue binds structures together, supports organs, stores fat and helps repair damage.

Cartilage is tough yet flexible. Hyaline cartilage caps the ends of bones in movable joints and forms the costal cartilages that join ribs to the sternum, fibrocartilage forms the pad inside the pubic symphysis, and cartilage also shapes the nose, ears and larynx. Bone and blood are classed as connective tissues too.

Mucous membranes line passages that open to the outside, serous membranes line closed cavities, and synovial membranes line movable joints without any epithelium. Injury sets off inflammation, with redness, swelling, heat and pain. Repair then takes one of two routes: regeneration restores the lost cells, or, where normal tissue is destroyed, fibroblasts lay down fibrous scar tissue, which is fibrosis.

New keratinocytes come from the stratum basale, the deepest layer, which rests on the basement membrane. As they move out through the stratum spinosum and stratum granulosum they stop dividing and keratinize, and in the stratum corneum they are flat dead cells set in lipid, the barrier that is shed and replaced. Thick skin on the palms and soles has a fifth layer, the stratum lucidum, below the corneum.

Beneath the epidermis, the dermis holds connective tissue, blood vessels, nerves, hair follicles, and oil and sweat glands, arranged as a thin papillary layer over a thick reticular layer. Epidermis and dermis meet at a basement membrane, and if the proteins binding them fail the skin tears and blisters easily. Subcutaneous tissue lies below the dermis.

Melanocytes in the deepest layer of the epidermis make melanin, the pigment that colors skin and eyes and helps shield against ultraviolet light. Sun exposure makes melanocytes produce more, which darkens the skin. When melanocytes are destroyed, as in vitiligo, patches of skin and the hair growing in them lose their color.

Hair grows from hair follicles, structures in the skin lined by keratinocytes, and is built largely of keratin. Each follicle connects to a sebaceous gland whose oily sebum empties onto the skin through the pore. Because the hair depends on its follicle, an immune attack on follicles, as in alopecia areata, causes hair loss.

Keratin is the main structural protein of nails as well as hair and outer skin, so changes in keratin genes can thicken nails. Sebaceous glands release oily sebum into hair follicles to keep skin from drying out. Sweat glands are exocrine glands that open onto the surface, and waxes such as earwax protect the delicate skin of the ear canal.

Skin is the body's largest organ. It shields against heat, sunlight, injury and infection, helps control body temperature through sweating, and stores water, fat and vitamin D. Skin exposed to sunshine can also make vitamin D. Sensory receptors in the skin detect touch and injury, starting signals that the nervous system acts on.

Burns are graded by how deep they go and how much area they cover. A first-degree burn damages only the epidermis, a second-degree burn also reaches the dermis, and a third-degree burn damages or destroys both layers; higher grades extend into fat and beyond. Every burn sets off an inflammatory response, which usually protects but in severe burns can overreact and deepen the injury.

Bone supports the body, protects soft organs such as the brain, heart and lungs, and works with muscles as levers for movement. It stores calcium and other minerals, and its marrow makes blood cells; in adults much of the marrow becomes yellow marrow that stores fat. Bone also releases factors that soft organs such as the kidney need.

Bones are classed by shape. Long bones such as the femur are longer than they are wide. Short bones of the wrist and ankle are roughly cube-shaped and mostly spongy bone, flat bones like most of the cranium are thin and curved, and irregular bones such as vertebrae fit none of those groups. Holes and projections on a bone mark passages for vessels, joints and tendon attachments.

A long bone has a shaft, the diaphysis, around a medullary cavity, with an epiphysis at each end. Periosteum covers the outside and endosteum lines the cavity. In children a plate of cartilage between epiphysis and diaphysis lets the bone lengthen; once growth stops it turns to bone and leaves only a thin epiphyseal line.

Intramembranous ossification turns sheets of connective tissue straight into bone and forms some flat skull bones. Most bones instead form by endochondral ossification, which replaces a hyaline cartilage model. Bones lengthen at the epiphyseal plate until it ossifies, between adolescence and the early twenties, but can thicken for life as the periosteum adds bone outside and the endosteum removes it inside.

Compact bone is built from tightly packed osteons: a central canal carrying blood vessels, ringed by layers of matrix, with osteocytes in small spaces linked by tiny channels. Spongy bone is lighter, made of plates called trabeculae around marrow-filled cavities. Its trabeculae look haphazard but follow the lines of stress, and they can realign when the direction of stress changes.

Bone is living tissue that is constantly remodeled: osteoclasts resorb old bone and osteoblasts lay down new collagen that mineralizes. Osteocytes steer the balance, depending on whether bone is loaded by exercise or unloaded by inactivity. Children add more than they remove and young adults stay roughly even. After a fracture, a callus of new bone forms at the break.

Three cells run bone: osteoblasts build it, osteoclasts break it down, and osteocytes, long-lived cells inside the matrix, direct the other two. The matrix pairs collagen, which gives the flexibility that lets bone resist breaking, with calcium phosphate mineral, which makes it hard. With too little collagen or faulty collagen, as in osteogenesis imperfecta, bones break easily.

When blood calcium falls, the parathyroid glands release parathyroid hormone, which frees calcium from bone, helps the intestine absorb it and makes the kidneys return it to the blood rather than lose it in urine. Calcitonin from the thyroid works the other way when calcium runs high. Vitamin D is needed to absorb calcium from food in the first place.

The adult skeleton usually has 206 named bones in two divisions. The 80 bones of the axial skeleton form the body's vertical axis: the skull, the middle-ear ossicles, the hyoid, the vertebral column, the sternum and the ribs. The 126 bones of the appendicular skeleton are the limbs plus the girdles that attach them to that axis.

Eight cranial bones enclose the brain: one frontal, two parietal, two temporal, one occipital, one sphenoid and one ethmoid. Most are flat bones, and because they are fused into the cranium they leave the brain less exposed to injury. An opening at the base of the skull is where the spinal cord joins the brain.

Fourteen facial bones shape the eye sockets, nose, cheeks and jaw: paired maxillae, zygomatic, nasal, palatine, lacrimal and inferior nasal concha bones, plus a single vomer and a single mandible. With the eight cranial bones and the six middle-ear ossicles they make up the 28 bones counted as the skull.

Sutures are immovable joints where skull bones meet across a thin layer of fibrous tissue. In a baby the skull is a set of bony plates held at sutures that stay open so the skull can grow along with the brain. If sutures join too early, a condition called craniosynostosis, the skull's shape and the pressure on the brain can be affected.

The column has 7 cervical, 12 thoracic and 5 lumbar vertebrae, then the sacrum and coccyx. Each vertebra encircles the spinal canal that carries the cord, nerves leave through openings between neighboring vertebrae, and facet joints link them at the back. Intervertebral discs act as shock absorbers and spacers, and the joints they form allow only slight movement.

A healthy spine has normal curves, yet seen from behind it looks straight; a sideways S- or C-shaped curve is scoliosis. Vertebral fractures in osteoporosis can bend the upper back into a stooped, hunched kyphosis. Below the lumbar vertebrae, the sacrum, five fused vertebrae, forms part of the pelvis, and the coccyx of three to five fused bones ends the column.

The thoracic cage is the sternum and 12 pairs of ribs, and it helps guard the heart and lungs. Ribs 1 to 7 are true ribs, each reaching the sternum through its own costal cartilage; ribs 8 to 10 are false ribs that join the cartilage above instead; ribs 11 and 12 float, with no front attachment. External intercostal muscles between the ribs lift them as you breathe in.

Each pectoral girdle is a clavicle and a scapula. The clavicle, or collarbone, joins the breastbone to the shoulder blade, and the triangular scapula joins the clavicle to the upper arm bone. Muscle tendons crossing the shoulder are a major source of its stability.

The arm has a single bone, the humerus, which meets the scapula at the shoulder; the forearm has two, the radius and the ulna. All three are long bones, longer than they are wide, and in children they lengthen at growth plates near their ends until growth finishes during adolescence.

Each wrist has eight carpals, short and roughly cube-shaped, followed by five metacarpals in the palm and fourteen phalanges in the fingers. Ligament and carpal bones form the carpal tunnel at the base of the hand, a rigid passage shared by the median nerve and the tendons that bend the fingers, so swelling inside it can squeeze the nerve.

The pelvic girdle is the two hip bones, also called coxal or innominate bones. They meet at the pubic symphysis, a slightly movable joint with a fibrocartilage pad, and the sacrum closes the pelvis at the back. The pelvis carries the weight of the upper body, aids balance and walking, and shields the bladder, reproductive organs, colon and rectum.

The femur is the long bone of the thigh, and its rounded head fits into the hip socket. Below the knee, the tibia, or shinbone, is the larger of the two leg bones and the fibula the other, while the patella sits on the front of the knee. These large lower-limb bones carry the trunk whenever you stand.

Each foot has seven tarsals, five metatarsals and fourteen phalanges. The tarsals of the ankle, like the carpals of the wrist, are short bones: roughly cube-shaped and mostly spongy bone under a thin shell of compact bone. The ankle is one of the joints most prone to sprains, when a fall or blow stresses it out of position and stretches or tears its ligaments.

By how much they move, joints are immovable (synarthroses), slightly movable (amphiarthroses) or freely movable (diarthroses). Movement tracks what links the bones: a thin layer of fibrous tissue in skull sutures, hyaline cartilage or fibrocartilage in the slightly movable joints, and a fluid-filled cavity lined by synovial membrane in the free ones, which is why those are also called synovial joints.

In a cartilaginous joint the bones are joined by cartilage and move only slightly. Costal cartilages of hyaline cartilage tie the ribs to the sternum, while the pubic symphysis has a pad of fibrocartilage between its two bones. The joints between vertebrae and their intervertebral discs belong to this slightly movable group as well.

In a synovial joint, hyaline articular cartilage caps the bone ends across a joint cavity. A dense fibrous capsule encloses the joint: its outer ligaments hold the bones together and its inner synovial membrane secretes the synovial fluid that lubricates the cartilage. The same membrane lines bursae, small fluid-filled cushions between bone and moving tissues, and tendon sheaths.

Synovial joints are sorted by the shape of their meeting surfaces, which sets the movement allowed. A hinge such as the elbow or knee only bends and straightens, a pivot at the top of the neck turns one bone around another, and plane joints between carpal bones just glide. Condyloid joints at the wrist and the saddle joint of the thumb add side-to-side motion, and a ball-and-socket joint such as the hip can also rotate.

Flexion and extension are opposite movements at a joint: the hamstrings flex the leg at the knee while the quadriceps femoris straighten it. At the ankle, the tibialis anterior dorsiflexes the foot and the gastrocnemius and soleus plantar flex it. Many muscle names record a movement: an abductor abducts, an adductor adducts, and a levator lifts.

In the hip, the femoral head sits deep in the acetabulum of the pelvis. At the knee and shoulder, muscle tendons crossing the joint are a major source of stability, and the knee also has a meniscus. Ligaments join bone to bone to steady every joint, so a fall that wrenches a joint out of position can stretch or tear them, which is a sprain.

Skeletal muscle, attached to bone, is striated, has many nuclei per fiber and is under voluntary control. Smooth muscle in the walls of hollow organs such as blood vessels and the gut has spindle-shaped cells with one nucleus and no striations, and it contracts slowly. Cardiac muscle is striated like skeletal muscle but has one nucleus per cell, and like smooth muscle it works involuntarily.

A skeletal muscle is wrapped in nested connective tissue: epimysium around the whole muscle, perimysium around each fascicle, or bundle of fibers, and endomysium around each fiber, a single long cell. Inside a fiber, rod-like myofibrils fill most of the space, each a chain of sarcomeres whose thick and thin myofilaments slide past each other to make force. The wrappings carry nerves and vessels and run on into the tendon.

A sarcomere runs from one Z disc to the next. The dark A band at its center spans the myosin thick filaments, the lighter I bands toward each Z disc hold actin thin filaments and titin but no myosin, and the H zone in the middle of the A band holds thick filaments alone. On the thin filaments, tropomyosin blocks the sites myosin binds until calcium changes the shape of troponin and shifts it aside.

At the neuromuscular junction, a motor nerve ending releases acetylcholine, which binds receptors on the muscle fiber and starts contraction. Most muscle fibers have just one junction. Enzymes then break acetylcholine down, which is why drugs that slow that breakdown can strengthen weak transmission, and why antibodies that destroy the receptors, as in myasthenia gravis, cause weakness.

An action potential spreads along the fiber's membrane and inward down the transverse tubules. There, voltage-sensing dihydropyridine receptors open ryanodine receptor channels in the sarcoplasmic reticulum, releasing a burst of calcium that binds troponin C on the thin filaments and switches on contraction. Relaxation needs the SERCA pump to move calcium back into the reticulum against its gradient.

Contraction runs as a cross-bridge cycle: myosin heads attach to actin filaments, pull in a power stroke, then detach and go again, with each turn driven by ATP hydrolysis. ATP is needed for letting go as well as for pulling. After death, when ATP is used up and calcium stays high, cross-bridges stay locked on and the muscles stiffen in rigor mortis.

A muscle's ATP pool is small, so phosphocreatine, glycolysis and oxidative phosphorylation keep refilling it. Without oxygen, one glucose yields only two ATP by fermentation, against about 32 from cellular respiration. Fatigue is thought to begin as phosphate builds up, with acid adding to it later; both blunt the response to calcium. Type II fibers handle brief, high-force work.

Cardiac fibers branch, carry one central nucleus, are striated and are joined by intercalated disks; they contract involuntarily, strongly and rhythmically. Smooth muscle contracts slowly and can hold a steady baseline tone, as in the airways. Rising calcium saturates calmodulin, which activates myosin light-chain kinase; phosphorylating myosin then drives contraction, and a phosphatase reverses it.

Most muscle names describe a feature, often several at once: size (maximus, minimus), shape (deltoid, trapezius), fiber direction (rectus, oblique), location (pectoralis, gluteus), number of heads (biceps, triceps), attachments (sternocleidomastoid) or action (flexor, levator). Decode the name and you already know a good deal about the muscle.

Facial muscles such as the frontalis, orbicularis oris, buccinator and zygomaticus move the face to show surprise, disgust or anger, making them a channel of nonverbal communication. Four pairs of chewing muscles, among them the temporalis and masseter, all attach to the mandible and are some of the strongest in the body. The sternocleidomastoid and trapezius are the most obvious superficial neck muscles.

The erector spinae, a large mass on each side of the spine from sacrum to skull, extends the vertebral column and keeps you upright. The dome-shaped diaphragm separates thorax from abdomen, and the external intercostals lift the ribs as you breathe in. The abdominal wall has no bony support, only four layered muscle pairs, and two muscular sheets close the pelvic outlet.

Upper-limb muscles work in tiers. The trapezius and serratus anterior move the scapula. The pectoralis major, latissimus dorsi, deltoid and rotator cuff attach to the humerus and move the arm. The biceps brachii, brachialis, brachioradialis and triceps brachii move the forearm, and some 20 forearm muscles drive most wrist, hand and finger movements.

A muscle usually spans a joint and is tied by tendons to a bone at each end. One bone stays relatively fixed while the other moves, and names such as sternocleidomastoid record both attachments. Muscles also work in opposing pairs: the hamstrings are antagonists of the quadriceps femoris, and the tibialis anterior opposes the gastrocnemius and soleus.

Muscles that move the thigh arise on the pelvic girdle and insert on the femur: the iliopsoas flexes the thigh and the medial group adducts it. In the thigh, the quadriceps femoris straightens the knee and the hamstrings flex it. In the leg, the tibialis anterior dorsiflexes the foot while the gastrocnemius and soleus plantar flex it.

There is one nervous system, divided for description. The central nervous system is the brain and spinal cord; the peripheral system is the nerves and ganglia outside them. Its afferent division carries sensory input in, the CNS integrates it, and the efferent division carries motor output to muscles and glands, splitting into somatic (skeletal muscle) and autonomic (heart, smooth muscle and glands).

A neuron has three main parts. Branching dendrites receive signals, the cell body holds the nucleus and keeps the cell running, and a single axon carries impulses away. A typical action potential starts at the axon initial segment, and the axon ends in fine branches whose swollen tips form synaptic endings.

Neurons are classed by the direction they carry signals. Sensory, or afferent, neurons bring impulses from receptors to the central nervous system; motor, or efferent, neurons carry commands out to muscles and glands; and interneurons, which lie wholly inside the CNS, link the two. Each class holds hundreds of types that differ in how they send and receive messages.

Glial cells support neurons rather than carry impulses; in some brain regions they outnumber neurons, and they can divide. Star-shaped astrocytes hold neurons in place, oligodendrocytes form myelin in the brain and spinal cord, Schwann cells insulate peripheral nerves, ependymal cells line the fluid-filled spaces of the brain and cord, and microglia are another glial type.

Myelin is a fatty, insulating wrap around many axons, made by Schwann cells in peripheral nerves and by oligodendrocytes in the brain and spinal cord. It is interrupted at nodes of Ranvier, where sodium channels cluster, so the impulse is regenerated from node to node in fast saltatory conduction. Myelinated fibers are what make white matter white.

At rest a neuron holds a voltage across its membrane, set by unequal ion concentrations. The sodium-potassium pump spends ATP to export three sodium ions and import two potassium ions per cycle, and leak channels make the membrane most permeable to potassium, so potassium's gradient dominates the resting potential. If the pump falters, potassium builds up outside, the resting potential drifts less negative and the cell grows more excitable.

An action potential is all-or-none. Once the membrane is pushed to threshold, usually at the axon initial segment, voltage-gated sodium channels open and depolarize it; they then inactivate while potassium current repolarizes the membrane. The sodium channels must recover from inactivation before the next spike, and a brief afterhyperpolarization speeds that recovery.

When an impulse reaches the axon ending, vesicles release neurotransmitter into the synaptic cleft. The transmitter crosses, binds receptors on the next cell and changes it: excitatory transmitters such as glutamate push it toward firing, while inhibitory ones such as GABA damp its activity. The signal ends when enzymes break the transmitter down or the releasing neuron takes it back up.

The diencephalon sits deep in the brain, nearly surrounded by the cerebral hemispheres. The thalamus makes up most of it and relays incoming sensory signals to the cortex, with smell the one exception. The hypothalamus below it regulates body temperature, hunger and thirst, which makes it central to homeostasis. The small pineal gland of the epithalamus makes melatonin and is tied to puberty and daily rhythms.

Three meninges wrap the brain and spinal cord: tough outer dura mater, the web-like arachnoid, and delicate pia mater bound to the surface. Cerebrospinal fluid, made by the choroid plexus in the ventricles, fills the subarachnoid space, cushions the brain, nourishes cells and removes waste, then drains into the blood. The blood-brain barrier's packed cells screen what reaches the brain.

The spinal cord runs from the foramen magnum to about the first lumbar vertebra, and below that a bundle of nerve roots forms the cauda equina. Its 31 segments each give rise to a pair of mixed spinal nerves joined to the cord by a dorsal root, whose ganglion holds the sensory cell bodies, and a ventral root. Ascending tracts carry sensation up and descending tracts carry motor commands down.

The brain and spinal cord begin as the neural tube, formed from a special set of cells that folds and closes by about the third to fourth week of pregnancy. The top of the tube becomes the brain and the rest the spinal cord. The mature brain is organized as forebrain, midbrain and hindbrain, and a tube that fails to close causes neural tube defects such as anencephaly.

Each hemisphere has five lobes. The frontal lobes plan and reason, with the motor cortex along their back edge, and just behind it the somatosensory cortex at the front of each parietal lobe receives touch, temperature and movement information. The occipital lobes process vision, the temporal lobes receive hearing and help form memories, and the insula lies hidden deep in the lateral sulcus.

The cortex is a thin sheet of gray matter coating the cerebrum, gray because its nerve cells lack the insulation that whitens deeper tissue, and its folds add surface for processing. A deep fissure splits the two hemispheres, which talk through the corpus callosum. Most signals cross, so each hemisphere mainly runs the opposite side of the body, and the two are not mirror images in function.

Below the cortex lie paired structures that shape movement and memory. The basal ganglia, clusters of nerve cells around the thalamus, initiate and integrate movements; when dopamine-producing neurons there die, the tremor and slowness of Parkinson's disease follow. The hippocampus works as a memory indexer, sending memories to the cortex for long-term storage and retrieving them when needed.

The brainstem, midbrain over pons over medulla, links brain and spinal cord and runs vital functions such as breathing, heart rate and blood pressure; every ascending and descending fiber passes through the medulla. The cerebellum, behind and below the cerebrum, coordinates movement and balance and is engaged in learned movements such as a piano piece or a tennis swing.

Sensory receptors detect changes, called stimuli, and turn them into nerve impulses. Some monitor the outside world, such as temperature, light and sound; others watch the internal environment, tracking pressure, pH, carbon dioxide and electrolyte levels. Many are specialized for one kind of stimulus, like the eye's photoreceptors for light and the inner ear's hair cells for motion.

Touch, temperature and pain begin at receptors and nerve endings in the skin, and their signals travel through the spinal cord to the somatosensory cortex at the front of each parietal lobe. Pain caused by tissue damage or inflammation is called nociceptive pain. A spinal reflex can pull your hand from a thorn before the brain becomes aware of the pain.

Light is bent by the dome-shaped cornea, enters through the pupil, whose size the iris controls, and is focused by the lens onto the retina. There photoreceptors turn light into electrical signals: rods work in dim light, cones detect color in bright light. The optic nerve carries the signals to the brain, where the occipital lobes process them. A refractive error is an eye shape that misfocuses light.

Sound travels down the ear canal and vibrates the eardrum. Three middle-ear bones, the malleus, incus and stapes, amplify the vibration and pass it to the fluid-filled cochlea, where a traveling wave rides the basilar membrane. Hair cells near the wide end sense high pitches and those toward the center low pitches, and bending their stereocilia opens channels that make the auditory nerve's signal.

The vestibular system of the inner ear senses two different things. Its three fluid-filled semicircular canals, set roughly at right angles, detect head rotation: moving fluid flexes a gelatin-like cupula like a sail and bends the hair cells beneath it. The utricle and saccule sense head position relative to gravity and straight-line motion, using dense calcium carbonate grains that tug on their hair cells.

Taste cells in taste buds on the tongue, the roof of the mouth and the throat respond to five basic qualities: sweet, sour, bitter, salty and umami. They are scattered, not sorted into tongue zones. Smell relies on olfactory sensory neurons high in the nose that connect directly to the brain. Much of flavor is smell, which is why a blocked nose makes food taste bland.

Sensory signals enter the spinal cord through the dorsal roots and climb to the brain in ascending tracts, all of which pass through the medulla. Nearly every pathway between brain and body crosses to the other side along the way, so each hemisphere mainly receives sensation from, and controls, the opposite side of the body; a stroke on one side affects the other side.

Voluntary movement starts with upper motor neurons in the brain, which signal lower motor neurons in the brainstem and spinal cord that drive the muscles. Losing lower motor neurons leaves muscles weak, twitching and wasted, while cutting off their input from upper motor neurons causes stiffness and overactive reflexes. Spinal reflexes such as withdrawal bypass the brain, which makes them fast.

Autonomic output reaches its targets through a two-neuron chain. A preganglionic neuron synapses in a ganglion on a postganglionic neuron, which reaches the smooth muscle, cardiac muscle or gland. Sympathetic ganglia sit near the spinal cord; parasympathetic ganglia sit near or inside the target, so their preganglionic fibers run long.

Sympathetic preganglionic cell bodies sit in the thoracic and upper lumbar cord, which is why the outflow is called thoracolumbar. Their axons may synapse in the paravertebral chain, travel up or down it to other levels, or pass on to prevertebral ganglia. The adrenal medulla is the shortcut: it takes preganglionic input and releases epinephrine straight into the blood.

Parasympathetic preganglionic neurons start in the brainstem and the sacral cord, hence craniosacral. Cranial nerves III, VII and IX serve the eye, tear glands and salivary glands, while the vagus (X) carries outflow to the thoracic and abdominal organs, including the heart and upper gut. Sacral nerves supply the bladder, lower bowel and sexual organs.

Every preganglionic fiber, sympathetic or parasympathetic, releases acetylcholine onto nicotinic receptors on the ganglion neuron. After the ganglion the divisions split: parasympathetic fibers release acetylcholine onto muscarinic receptors, such as M2 at the heart's pacemaker, while most sympathetic fibers release norepinephrine onto alpha and beta receptors. Sympathetic fibers to sweat glands are the exception and release acetylcholine.

Sympathetic activity raises heart rate, blood pressure and breathing rate, widens the bronchial tubes and enlarges the pupils. Parasympathetic activity slows the heart and breathing, narrows the bronchial tubes, shrinks the pupils and increases digestive juices. Where an organ gets fibers from both divisions, one stimulates and the other inhibits, and the balance between them sets its activity.

Stretch receptors in the aortic arch and carotid sinus report beat-to-beat changes in arterial stretch to the solitary nucleus of the medulla, which answers by lowering or raising sympathetic activity. Chemoreceptors add a second input: the carotid and aortic bodies sense low oxygen and high carbon dioxide, the medulla's mainly carbon dioxide and pH. Higher centers, the hypothalamus among them, are thought to reset these reflexes.

Nerves act through impulses and neurotransmitters, so their effects last seconds and stay local. Hormones travel in the blood to every tissue but act only on cells with matching receptors, and their effects take minutes, hours or weeks and spread more widely. That receptor requirement is why a hormone carried through the whole bloodstream still has a defined target tissue.

Protein hormones bind receptors on the cell surface, and the chain of events they start is relatively fast. Steroid hormones usually bind receptors inside the cell, and because their effect involves making new proteins, it is relatively slow. So the speed of a hormone's action follows from where its receptor sits and whether new protein has to be built first.

Most hormone output is held steady by negative feedback: the gland senses the level of the substance it controls and reverses any drift. Some glands secrete in response to another gland's tropic hormone, as when pituitary TSH drives the thyroid, and some respond to direct nerve stimulation, as the adrenal medulla does. Because hormones are potent, small changes in output matter.

The two lobes are controlled differently. Releasing hormones from the hypothalamus govern the anterior lobe, which secretes growth hormone, TSH, ACTH, prolactin and the gonadotropins FSH and LH. Nerve stimulation governs the posterior lobe, which releases antidiuretic hormone to conserve water at the kidney tubules and oxytocin to contract the uterus and eject milk.

Thyroid follicles make thyroxine (T4, about 95 percent of the active hormone) and triiodothyronine (T3), and both need iodine. Without iodine the gland cannot make enough hormone, so the pituitary keeps sending TSH and the gland swells into a goiter. Calcium is handled by an opposed pair: calcitonin lowers blood calcium, while parathyroid hormone, released when calcium falls, raises it.

The cortex and medulla come from different embryonic tissues and are run differently: the cortex by ACTH feedback, the medulla by nerve impulses. The cortex, essential to life, makes steroids in three zones: aldosterone outermost, cortisol in the middle, sex hormones innermost. The medulla releases epinephrine and norepinephrine on sympathetic stimulation and can be removed without life-threatening effects.

The islets work as a pair: alpha cells release glucagon when blood glucose is low, and beta cells release insulin when it is high. Other organs carry hormone duty too: the pineal makes melatonin, the thymus thymosin, the stomach gastrin, the small intestine secretin and cholecystokinin, the heart's atria atrial natriuretic hormone, and the placenta hCG, which signals the ovaries to keep the uterine lining from being shed.

Hormones fall into chemical classes that differ sharply in size: amines and steroids are small, peptides larger, and glycoproteins larger still. The steroids include the sex hormones and every hormone of the adrenal cortex; nearly all others are proteins or protein derivatives. Some hormones travel bound to carrier proteins, so only a fraction circulates free, and that free fraction differs by class.

Plasma, the liquid part, is water, salts and protein and makes up over half of blood. The rest is formed elements: red cells carry oxygen, white cells fight infection and platelets help blood clot. A hematocrit is the percent of blood made up of red cells, so a high or low value can reflect dehydration as well as a change in the cells themselves.

Blood cells come from stem cells in the bone marrow. One progenitor line gives rise to neutrophils, eosinophils, basophils, monocytes and other innate immune cells; a lymphoid line gives rise to B cells, T cells and natural killer cells. Red cell output is tuned to demand, and the kidneys help set it by secreting the hormone erythropoietin.

A mature red cell lacks a nucleus and has a biconcave disc shape that lets it bend through narrow capillaries and spring back. It is packed with hemoglobin, an iron-rich protein that carries oxygen from the lungs to the tissues. Red cells circulate for about 120 days on average, and the spleen and liver then remove the old and damaged ones.

Old red cells are cleared by macrophages, largely in the spleen and liver. Heme oxygenase breaks heme into biliverdin, releasing iron that macrophages pass on for reuse, and biliverdin becomes bilirubin. The liver sends bilirubin out in bile, so a damaged liver, blocked bile ducts or red cells destroyed too fast can push bilirubin into the blood and turn skin yellow.

Neutrophils are the most numerous innate immune cells and ingest bacteria. Eosinophils and basophils defend against parasites and take part in allergic reactions. Monocytes mature into macrophages, the “big eaters” that also clear dead cells. Lymphocytes include B and T cells, which respond to specific microbes, and natural killer cells, which kill virus-infected and tumor cells.

Hemostasis runs in three phases: the vessel constricts, platelets gather into a temporary plug, and the coagulation cascade reinforces that weak plug. In the cascade, thrombin turns fibrinogen into fibrin, whose cross-linked strands make the clot stable. Once healing is underway the clot is broken down, mainly by plasmin, so a working system needs dissolving as much as clotting.

ABO type names the antigens on red cells, and plasma carries antibodies against the ones that are missing: A has anti-B, B has anti-A, AB has neither, and O has both. These antibodies are naturally occurring, so a transfusion given without compatibility testing can trigger a hemolytic reaction. Rh differs: an Rh-negative mother makes anti-Rh antibodies only after Rh-positive fetal blood reaches hers, so trouble usually comes in a later pregnancy.

Atria receive and ventricles pump, and wall thickness tracks the job: the ventricles carry more myocardium because they must push blood out forcefully. The right atrium takes oxygen-poor blood from the venae cavae and the left atrium oxygen-rich blood from the pulmonary veins; the right ventricle sends blood into the pulmonary trunk and the left into the aorta. The septum keeps the two sides apart.

Atrioventricular valves (tricuspid on the right, mitral on the left) close when the ventricles contract, so blood cannot flow back into the atria. Semilunar valves (pulmonary and aortic) close when the ventricles relax, so blood cannot fall back from the arteries. Both sides pump at once: right ventricle to the lungs, back to the left atrium, then left ventricle to the body.

The myocardium works without pause, so it needs a constant oxygen supply of its own. The right and left coronary arteries branch from the ascending aorta, and they are the only vessels that do. After the heart's own capillary beds, cardiac veins drain mostly into the coronary sinus, which empties into the right atrium.

The SA node in the right atrium fires 70 to 80 times a minute with no nerve input, which makes it the pacemaker. Its signal crosses the atria, then slows at the AV node so the ventricles can finish filling, before running down the AV bundle, bundle branches and conduction fibers. Nerves and hormones raise or lower that rate rather than start the beat.

One round of contraction (systole) and relaxation (diastole) lasts about 0.8 second at a normal rate. The heart sounds come from valve closure: “lub” as the AV valves shut after the atria fill the ventricles, “dub” as the aortic and pulmonary valves shut after the ventricles eject. Arterial pressure peaks during ventricular contraction and is lowest between beats.

The heart sits in the middle of the chest between the lungs, in the mediastinum, with about two-thirds of its mass left of the midline. It is enclosed in the pericardium, a sac whose serous layers line the sac and form the heart's outer surface, the epicardium. A small amount of fluid between the layers cuts friction as the heart beats.

Cardiac output is heart rate times stroke volume, the blood each beat ejects. Stroke volume rises with preload (more blood volume fills the ventricle further) and with stronger contraction, and falls with higher afterload, which slows ejection. Under the Frank-Starling mechanism a fuller ventricle ejects more, but only while it works on the steep part of its curve.

Arteries and veins share three layers: the tunica intima, a thin endothelium; the tunica media, mostly smooth muscle that changes diameter to steer flow and pressure; and the tunica externa, connective tissue that anchors the vessel. Veins keep all three but with less muscle and connective tissue, which fits the lower pressure inside them.

Large elastic arteries take blood from the ventricles and branch into ever smaller arteries and then arterioles. The arterioles hold the key position: smooth muscle where they branch into capillaries controls how much blood enters each capillary bed. Only about 10 percent of total blood volume is in the systemic arteries at any moment.

Little pressure is left once blood leaves the capillaries, so venous return depends on skeletal muscle action, breathing movements and smooth muscle in vein walls. Valves in medium and large veins, especially in the limbs, stop backflow against gravity. Thin, less rigid walls let veins stretch, so they hold almost 70 percent of blood volume.

Blood flows down a pressure gradient, from arteries to capillaries to veins. Most resistance comes from vessel diameter: as a vessel narrows, resistance rises and flow falls. Speed depends on the total cross-sectional area of a vessel class rather than one vessel's width, so blood moves slowest in the capillaries, which leaves time for exchange.

Systolic pressure comes from ventricular contraction, diastolic from relaxation, and the gap between them is pulse pressure. Mean arterial pressure equals cardiac output times peripheral resistance. Baroreceptors handle short-term changes, while the kidneys and hormones such as angiotensin II, aldosterone and vasopressin defend blood volume by conserving salt and water.

The right ventricle pumps the pulmonary circuit and the left ventricle the systemic circuit. Blood from the digestive tract takes a detour, the hepatic portal vein, through the liver first. Before birth the lungs, gut and kidneys are not working, so fetal blood takes shortcuts: the ductus venosus bypasses the liver, and the foramen ovale and ductus arteriosus bypass the lungs.

Capillary linings range from continuous, tightly sealed endothelium, as in the brain's barrier, to discontinuous, fenestrated lining with open pores, as in the liver's sinusoids. Across any wall, fluid movement is set by hydrostatic and osmotic pressure together, so substances tend to leave the blood at one end of a capillary and return at the other.

Lymph is the interstitial fluid that blood capillaries fail to take back. Blind-ended lymph capillaries collect it, their overlapping wall cells letting fluid in but not out. They merge into vessels, trunks and two ducts, the right lymphatic duct for the upper right quadrant and the thoracic duct for the rest, which return lymph to venous blood. With no pump, lymph moves by muscle action, breathing and vessel smooth muscle.

Lymphocytes arise in the red bone marrow, and T cells mature in the thymus, which shrinks after puberty. Lymph nodes filter lymph and serve as hubs where immune cells sample material from the body; the spleen filters blood in the same way. Tonsils and gut sites such as Peyer's patches guard mucosal surfaces where pathogens often enter.

Skin is usually the first line of defense: its cells secrete antimicrobial proteins, and immune cells sit in specific layers. Mucosal surfaces such as the airways and gut are prime entry points, so specialized immune hubs sit in those tissues. In the trachea, mucus from goblet cells traps particles and microorganisms, and cilia sweep it upward to be swallowed or expelled.

Innate cells respond quickly using receptors, such as Toll-like receptors, that recognize broad patterns rather than one strain. Neutrophils and macrophages ingest microbes, natural killer cells kill virus-infected and tumor cells, complement proteins punch holes in bacteria, and type I interferons drive antiviral responses. The trade-off is breadth without strain-level precision.

The classic signs (redness, swelling, heat and pain) come from mediators that relax blood vessels and make them leakier, so large molecules and immune cells reach the injured tissue. In fever, prostaglandin E2 acts on thermoregulatory neurons in the hypothalamus's preoptic area and the set point rises. The higher temperature makes survival harder for many microbes and activates the immune system.

CD8+ cytotoxic T cells kill infected and cancer cells, which show viral antigens on MHC class I, a protein nearly every cell carries. CD4+ helper T cells see antigen on MHC class II, which mainly antigen-presenting cells such as dendritic cells and macrophages express, and then coordinate other immune cells. Regulatory T cells restrain the rest to keep the body's own tissue safe.

A B cell's receptor is itself an antibody; when it recognizes its target and the cell activates, it divides and secretes that same antibody in soluble form. Antibodies neutralize pathogens, flag them for phagocytes (opsonization) and trigger complement. The five classes lean toward different jobs: IgM complement, IgG neutralization and opsonization, IgA the gut, IgE mast cells, IgD basophils.

After an infection clears, some activated B and T cells stay on as memory cells, so a second encounter is met before the pathogen takes hold. Active immunity, from infection or vaccination, takes weeks to build but is long-lasting, sometimes lifelong. Passive immunity, from maternal antibodies across the placenta or from immune globulin, protects at once but fades within weeks or months.

The nose warms and moistens incoming air, and nose hairs trap large particles. The pharynx has three regions: the nasopharynx carries air only, while the oropharynx and laryngopharynx pass air, food and fluids. In the larynx, nine cartilages frame the airway; the true vocal cords vibrate to make sound, and the epiglottis keeps food out of the larynx during swallowing.

The trachea divides at the fifth thoracic vertebra into the primary bronchi, which branch down to bronchioles and alveoli. Along the way cartilage fades and smooth muscle increases, and the lining thins from ciliated columnar to simple squamous epithelium. The right lung has three lobes; the left has two, plus a cardiac notch for the heart.

Air flows from higher to lower pressure. When the diaphragm contracts, the thoracic cavity grows and alveolar pressure drops below atmospheric, so air flows in; quiet expiration is passive, as elastic recoil shrinks the volume and raises alveolar pressure. Pleural pressure stays negative because the lungs recoil inward while the chest wall pulls outward, and that suction helps hold small airways open.

Gas exchange happens only across the alveolar ducts and alveoli, whose simple squamous walls allow rapid diffusion; the branching tubes above them only conduct air. Each alveolus is wrapped in capillaries. Surfactant coats the inside of the alveoli, easing inflation as you breathe in and keeping the lungs from collapsing as you breathe out.

Four standard volumes (tidal, inspiratory reserve, expiratory reserve and residual) combine into four capacities (inspiratory, functional residual, vital and total lung). Residual volume stays behind because small airways close before the lungs can empty, so any capacity that contains it, such as total lung capacity, needs plethysmography or gas dilution rather than a simple spirometer.

At the alveoli, oxygen passes through thin walls into capillary blood while carbon dioxide moves the other way, each by diffusion. How well that works depends on matching ventilation to blood flow. Mismatch runs from shunt to dead space; in a shunt, blood keeps flowing past alveoli that get no air, so it leaves without its share of oxygen and arterial oxygen falls.

Oxygen rides mainly on hemoglobin, and its grip can shift: acidity, more carbon dioxide, more 2,3-DPG and higher temperature shift the dissociation curve right, so working tissue unloads more oxygen. Carbon dioxide travels largely as bicarbonate, made by carbonic anhydrase in red cells as blood passes the tissues, and partly bound to hemoglobin, which holds it better after giving up oxygen.

Breathing centers in the medulla and pons set the rhythm. Central chemoreceptors in the medulla track the pH and carbon dioxide of the cerebrospinal fluid, while peripheral chemoreceptors in the carotid bodies also respond to oxygen. Carbon dioxide is the main driver of normal breathing; low oxygen mainly adds to the drive once it becomes severe.

Food passes through ingestion, mechanical digestion (chewing, stomach churning), chemical digestion by enzyme-driven hydrolysis, movement, absorption and elimination. Two movements are easy to confuse: peristalsis is a wave that pushes contents forward, muscle behind contracting and muscle ahead relaxing, while local segmenting contractions mix food with enzymes and fluids.

Saliva moistens food, and its amylase starts breaking down starch. Swallowing starts when the tongue pushes food into the pharynx, then becomes an involuntary reflex: the uvula rises to block the nasal passage, the epiglottis covers the larynx, and peristalsis carries food down the esophagus. Upper and lower esophageal sphincters gate food in and out; the lower one opens for food and otherwise stays closed.

The small intestine runs from the pyloric sphincter to the ileocecal valve in three parts: duodenum, jejunum and ileum. It finishes digestion and absorbs most nutrients, and its circular folds, villi and microvilli multiply the absorptive surface. Sugars and amino acids go to the blood and on to the liver, while fatty acids travel by lymph.

The large intestine runs from the cecum through the colon and rectum to the anal canal. It makes no digestive enzymes: it absorbs water and electrolytes, its bacteria break down leftover nutrients and make vitamin K, and the rectum stores stool. At the outlet, an involuntary internal sphincter of smooth muscle sits above a voluntary external sphincter of skeletal muscle.

The liver's units are lobules, whose sinusoids carry blood to a central vein; it takes oxygen from the hepatic artery and gut nutrients from the hepatic portal vein, and it handles carbohydrate, fat and protein metabolism. It makes bile, whose salts emulsify fat, while the gallbladder only stores it. Pancreatic enzymes such as amylase, trypsin and lipase answer to secretin and cholecystokinin.

From the mouth to the anus the gut wall keeps four layers. The mucosa lines the lumen and secretes mucus, enzymes and hormones; the submucosa carries blood vessels, lymphatics and nerves; the muscular layer has inner circular and outer longitudinal smooth muscle with the myenteric plexus between them. The outer layer is called adventitia above the diaphragm and serosa below it.

The stomach has fundic, cardiac, body and pyloric regions, and its glands hold mucous, parietal, chief and endocrine cells. The first three make gastric juice; the endocrine cells release gastrin into the blood, which drives acid and pepsin output. The stomach also makes intrinsic factor, which the body needs to absorb vitamin B12. Secretion runs in cephalic, gastric and intestinal phases.

Each juice has its targets: saliva starts on starch, stomach acid and enzymes work on protein, bile helps break down fat, and pancreatic juice acts on carbohydrates, fats and proteins, with the small intestine's own enzymes finishing the job. Absorbed simple sugars, amino acids and glycerol go to the blood and the liver, while fatty acids and fat-soluble vitamins enter the lymph.

Catabolism breaks food molecules down for fuel, while anabolism builds new molecules, such as nonessential amino acids, from the carbon intermediates that breakdown supplies. Fully oxidizing one glucose through glycolysis, the citric acid cycle and oxidative phosphorylation yields about 30 to 32 ATP, most of it from the last, oxygen-dependent step.

Glycolysis splits glucose into pyruvate in the cytosol. With enough oxygen, pyruvate becomes acetyl coenzyme A and is fully oxidized in the mitochondria; without it, pyruvate is diverted to lactate and the yield falls to two ATP per glucose. The liver can turn that lactate back into glucose by gluconeogenesis, a loop known as the Cori cycle.

During fasting or a low-carbohydrate diet, fatty acids are broken down by beta-oxidation into acetyl-CoA. When acetyl-CoA builds up, the liver turns it into ketone bodies (acetoacetate, beta-hydroxybutyrate and a little acetone), which travel in the blood to other organs, the brain included, and are converted back to acetyl-CoA for energy.

Amino acids lose their amino groups by transamination and deamination, and the remaining carbon skeletons can fuel energy production. The nitrogen freed as ammonia is toxic, so liver cells run the urea cycle to turn it into urea for the kidneys to excrete. Nine essential amino acids cannot be made in the body and must come from food; the nonessential ones can.

After a meal, rising blood glucose triggers insulin, which helps glucose enter muscle, fat and liver cells, and spare fuel is stored in the liver, muscles and body fat. Between meals, falling glucose triggers glucagon instead. The liver keeps supplying glucose, partly by gluconeogenesis, and during fasting ketone bodies can become the brain's main alternative fuel.

Weight holds steady over time when the calories taken in match the calories burned. Calories beyond what activity and daily living use may be stored as fat. Energy out is shaped by more than exercise: everyday movement counts, and some medicines can shift energy balance by increasing hunger or making the body burn fewer calories.

Heat-sensitive neurons in the hypothalamus's preoptic area integrate temperature signals and steer the effectors. To shed heat, sympathetic vasoconstriction eases so skin vessels widen, and sweating rises; to keep heat, skin vessels narrow. To make heat, brown fat releases energy as heat instead of ATP, and in harder cold, shivering skeletal muscle becomes the main source.

The body needs 13 vitamins and can make two of them, D and K. Vitamin D is fat-soluble, and fat-soluble vitamins are absorbed with fats into lymph lacteals rather than blood capillaries. Minerals split by amount needed: macrominerals such as calcium, sodium and potassium in larger amounts, trace minerals such as iron, zinc and iodine in small ones.

Each kidney lies behind the parietal peritoneum, so it is retroperitoneal, held by renal fascia, cushioned by perirenal fat and wrapped in a fibrous capsule. Inside, the outer cortex surrounds the medulla's pyramids, whose papillae drain into minor calyces; these join into major calyces and then the renal pelvis, which continues as the ureter.

Blood enters at the hilum through the renal artery, which branches down to afferent arterioles that feed each glomerulus. An efferent arteriole drains the glomerulus and gives rise to peritubular capillaries around the tubules, which in the medulla form the vasa recta. Two capillary beds in series let the kidney filter blood first and then take back what the tubules return.

Ureters push urine to the bladder by peristalsis, and mucosal flaps where they enter act as valves against backflow. The bladder's folds and transitional epithelium let it stretch as it fills. The detrusor muscle contracts to expel urine, and two sphincters control the flow: the internal one is involuntary smooth muscle, the external one voluntary skeletal muscle.

Each kidney has over a million nephrons. A nephron is a renal corpuscle (the glomerulus inside a double-layered glomerular capsule) plus a renal tubule: proximal tubule, loop of Henle and distal convoluted tubule, draining into collecting ducts. Where the afferent arteriole meets the loop's ascending limb, the juxtaglomerular apparatus monitors blood pressure and secretes renin.

The thin glomerular walls let water, small molecules and wastes pass into the tubule while proteins and blood cells stay in the blood. The kidneys filter about 150 quarts of blood a day, yet only 1 to 2 quarts become urine, because the tubules return most of the filtrate. An estimated GFR tracks this filtering, often starting from blood creatinine.

Across a wide range of arterial pressure, the kidney holds blood flow and GFR steady by autoregulation: about half comes from the myogenic response of renal arterioles and about a third from tubuloglomerular feedback. Below that range, flow follows pressure. Low blood pressure or sympathetic activity also releases renin, which starts the angiotensin and aldosterone cascade.

The proximal tubule does the bulk work, reabsorbing about two-thirds of filtered water and sodium chloride and all the filtered glucose and amino acids. Glucose return has a ceiling: above a blood level commonly put near 180 mg/dL, glucose spills into the urine, and the threshold varies between people. Tubules also secrete hydrogen ions and other wastes into the filtrate.

Loops of Henle and the vasa recta run side by side in opposite directions. Countercurrent multiplication in the loops builds an osmotic gradient in the medulla, and countercurrent exchange in the freely permeable vasa recta keeps blood flow from washing it out. Antidiuretic hormone sets how much water is drawn back down that gradient; without it, distal segments stay impermeable to water.

About two-thirds of total body water is inside cells, the intracellular fluid, and one-third is outside, the extracellular fluid. The extracellular share divides again into plasma and the interstitial fluid around cells. Water crosses between compartments when their solute concentrations differ, so a change on one side pulls water from the other.

Water taken in should equal water lost; when it does not, the result is dehydration or overhydration. When blood volume falls, brain sensors trigger thirst, release of vasopressin (antidiuretic hormone) and more sympathetic activity, and vasopressin makes the kidneys reabsorb more water. Intake rises and urine loss falls together.

Electrolytes are minerals that carry a charge in body fluids. Sodium and chloride govern fluid volume, potassium supports heart and muscle function, calcium and phosphate build bone, and bicarbonate carries carbon dioxide and helps set pH. An imbalance can follow a change in body water, and the kidneys correct it by adjusting what they excrete.

Water follows differences in solute concentration and pressure. Losing dilute (hypotonic) fluid leaves the blood more concentrated, so water shifts from cells into the circulation. In tissues, protein leaking out of capillaries raises interstitial osmotic pressure and holds fluid there, and edema builds unless lymph capillaries carry the excess back.

Acid produced by metabolism is first titrated by bicarbonate, the body's most extensive buffer system. The kidney then replaces the bicarbonate used up, making new bicarbonate as it excretes ammonium in the urine. Inside the tubule, phosphate and ammonia take up the hydrogen ions secreted into the filtrate, which limits how far urine pH falls.

Two organ systems correct pH. The lungs act through gas exchange and work within minutes to hours, changing how much carbon dioxide stays in the blood. The kidneys act through hydrogen-ion secretion and bicarbonate reabsorption and take hours to days, but they can also generate new bicarbonate as they excrete ammonium.

Carbon dioxide tracks ventilation and bicarbonate the metabolic side: retained CO2 lowers pH (respiratory acidosis), blown-off CO2 raises it (respiratory alkalosis). A first read: if pH and CO2 move the same way, the primary problem is metabolic; if opposite, respiratory. Compensation narrows the pH change, but the prevailing view is that it never restores it, so a normal pH with abnormal values suggests a mixed disorder.

The testes hang in the scrotum, about 3 degrees Celsius below body temperature, because viable sperm need that cooler setting; the cremaster muscle pulls them closer in the cold. Sperm pass through the epididymis, where they mature, then the ductus deferens, ejaculatory duct and urethra. Seminal vesicles supply about 60 percent of semen volume, most of the rest coming from the prostate.

In the seminiferous tubules, spermatogonia divide by mitosis from puberty on, and each primary spermatocyte goes through two meiotic divisions with one DNA replication, giving four haploid spermatids. Sertoli cells support the developing cells, and Leydig cells between the tubules make testosterone. FSH drives sperm production and LH drives testosterone, held in check by negative feedback.

The ovaries are not joined to the uterine tubes, so an ovulated oocyte enters the peritoneal cavity first and is swept in by the fimbriae. Fertilization usually happens in the tube, because the oocyte is fertile for only 24 to 48 hours of a trip that lasts about 7 days. The endometrium's functional layer sheds at menstruation, and its basal layer rebuilds it.

Primary oocytes start meiosis I before birth and stop until puberty; each month one resumes and divides unequally into a secondary oocyte and a small polar body. Meiosis II then stops at metaphase and finishes only if a sperm enters. After ovulation, prompted by LH, the remains of the follicle form the corpus luteum, which secretes progesterone and some estrogen.

GnRH from the hypothalamus drives FSH and LH release from the anterior pituitary. After menstruation, rising estrogen thickens the endometrium as follicles grow, and around day 12 to 14 of a 28-day cycle LH prompts ovulation. Progesterone from the corpus luteum readies the lining further, and if no pregnancy occurs, falling hormone levels signal the lining to shed.

Sperm are not ready on arrival: capacitation, which takes several hours in humans, prepares them for the acrosome reaction that lets one cross the zona pellucida. Once one sperm enters, the zona hardens and blocks the rest. The embryo reaches the uterus, attaches about 5 or 6 days after ovulation, and implantation releases hCG, the hormone pregnancy tests detect.

In the blastocyst, the outer trophectoderm gives rise to the placenta and the inner cell mass to the fetus. After implantation, gastrulation forms three germ layers, ectoderm, mesoderm and endoderm, from which every tissue develops. Two umbilical arteries carry fetal blood to the placenta and one vein brings it back, and pregnancy runs about 40 weeks across three trimesters.

Labor has three stages: the cervix opens to full dilation, about 10 centimeters; the baby is delivered; and the placenta separates and is delivered. Oxytocin contracts the uterine muscle and also ejects milk. Milk production itself is driven by prolactin from the anterior pituitary, after estrogen and progesterone build up the glands during pregnancy.

A karyotype is a person's full set of chromosomes, laid out in order to check number and structure. For one gene, a single altered copy can cause an autosomal dominant condition, while a recessive one needs both copies. X-linked recessive conditions affect males far more often, and a father cannot pass an X-linked trait to his son. Phenotype reflects genotype plus environment.