HESI A2 Anatomy and Physiology study guide

What this topic actually asks of you

Anatomy is the study of structure; physiology is the study of function. This topic asks you to hold the two together: name the part, say what it does, and say what changes when it stops doing it. The vocabulary is large, but it is not random — every term on the page is a position, a layer, a pathway or a product.

Elsevier lists anatomy and physiology among the areas the HESI A2 covers. The publisher releases no national content specification — no item count, no clock, no cut score — so nothing here mirrors an official weighting, and nothing here claims to. The scope is a teaching scope: the body's systems, in the order that makes them readable, with the general ideas first and the organs after.

The anatomy on this page follows the US National Cancer Institute's SEER training modules on anatomy and physiology, and MedlinePlus, the US National Library of Medicine's consumer health service, for organ-level descriptions. Both are US federal government publications, which are not subject to copyright in the United States. Where a number is stated — the count of bones, the volume of a breath — the number is the standard teaching value and is named as such.

The eighteen ideas this page teaches

These are the ideas a question is built from, and they are the headings this page uses.

1. Levels of organization — chemical, cellular, tissue, organ, system, organism.
2. Anatomical position and directional terms — the reference posture, and the words that assume it.
3. Body planes — sagittal, frontal and transverse cuts, and what each one separates.
4. Body cavities and membranes — the dorsal and ventral cavities, the serous membranes and the four quadrants.
5. The four tissue types — epithelial, connective, muscle and nervous tissue.
6. Homeostasis and feedback — the receptor–control center–effector loop, and negative versus positive feedback.
7. The integumentary system — the layers of the skin, its glands and its roles.
8. The skeletal system — axial and appendicular bone, bone tissue, and the joints.
9. The muscular system — three muscle types, the sarcomere, and how a contraction happens.
10. The nervous system — the neuron, the impulse, the central and peripheral divisions, the brain regions, the spinal cord and the reflex arc.
11. The special senses — the eye and the ear as structures that turn a stimulus into a signal.
12. The endocrine system — the glands, the hormones, and the loops that set their levels.
13. The cardiovascular system — the heart as a pump, the paths of blood flow, the vessels and the blood itself.
14. The lymphatic and immune systems — lymph, nodes, the spleen and the thymus.
15. The respiratory system — the airway, the alveoli, the mechanics of breathing and gas transport.
16. The digestive system — the tract in order, the accessory organs, and where each nutrient is handled.
17. The urinary system — the kidney, the nephron, and the three processes that make urine.
18. The reproductive systems — the male and female organs, the gametes and the cycle.

Levels of organization, and the words for position

The body is built in steps, and each step has a name. Placing a structure at the right step is a matter of asking what sits above it and what sits below it.

Anatomical position is the reference point for every direction. A body in the anatomical position is standing upright, face forward, feet flat and slightly apart, with the arms at the sides and the palms facing forward. Every directional word below assumes that posture, so the palm is an anterior surface and the little finger is medial to the thumb. A body lying face up is supine; face down is prone.

Directional terms come in pairs. Learn each term against its opposite, and remember that the words describe the body's own geometry, not the page: a body drawn lying on its side still has a medial side and a lateral side.

The two pairs do different work. Superior and inferior are the pair used along the trunk, measured toward the head or the feet. Proximal and distal are the pair used along a limb, measured from where the limb meets the trunk. The elbow is proximal to the wrist because it is nearer the shoulder joint, and the wrist is distal to the elbow for the same reason.

A plane is a flat surface the body is cut or viewed through. The name of the plane says what the cut separates, not which surface you end up looking at.

The figure is a teaching diagram, not an anatomy: the body is drawn face-on so that the midline and the waistline can be seen, and the frontal plane is drawn as a tilted sheet because a plane that separates front from back cannot be seen edge-on from the front. The small side view beside the body shows that same plane edge-on, as the vertical dashed line running front to back through the trunk.

Cavities, membranes and the four quadrants

The trunk holds two great cavities. The dorsal cavity is at the back and is continuous from the skull to the sacrum: the cranial cavity holds the brain and the vertebral (spinal) cavity holds the spinal cord. The ventral cavity is at the front and is divided by the diaphragm into the thoracic cavity above and the abdominopelvic cavity below.

The thoracic cavity holds two pleural cavities (one per lung) around a central mediastinum, which contains the heart in its own pericardial cavity. The abdominopelvic cavity holds the abdominal part, with the digestive organs, and the pelvic part, with the bladder and the internal reproductive organs.

Enlarge

Image: Scheme body cavities-en.svg by Mysid, public domain, via Wikimedia Commons.

Each cavity is lined by a serous membrane that folds back on itself: the parietal layer lines the wall and the visceral layer covers the organ, with serous fluid between them to let the organ slide. The membrane is named for its cavity — pleura around the lungs, pericardium around the heart, peritoneum around the abdominal organs.

The four abdominopelvic quadrants are named from the body's own right and left, not the viewer's, with the lines crossing at the umbilicus: right upper, left upper, right lower, left lower. Clinicians also use nine regions — right hypochondriac, epigastric and left hypochondriac across the top, then the right and left lumbar regions either side of the umbilical region, then the right and left iliac regions either side of the hypogastric region.

The four tissue types

Every organ is a mixture of four tissues, and the mixture is the function. A gland is mostly epithelium sitting on connective tissue; a muscle is muscle cells wrapped in connective tissue with nerves and vessels running through it.

Epithelium is named twice: once for the shape of its cells and once for how many layers it has. Squamous cells are flat, cuboidal cells are as tall as they are wide, and columnar cells are taller than they are wide. Simple epithelium is one layer thick, which suits absorption and filtration; stratified epithelium is several layers thick, which suits protection. Skin is stratified squamous epithelium; the air sacs of the lung are simple squamous epithelium, thin enough for gas to cross.

Connective tissue is named for its matrix rather than its cells. Loose connective tissue (areolar, adipose, reticular) packs organs and stores fat; dense connective tissue forms tendons and ligaments; cartilage gives flexible support; bone is connective tissue with a mineralized matrix; and blood is a connective tissue whose matrix is plasma.

Homeostasis: the loop every system serves

Homeostasis is the maintenance of a stable internal environment while the outside world changes. It is not stillness: blood glucose, body temperature and blood pressure all move, and the point of the system is to bring them back. Most loops are negative feedback, in which the response cancels the change. A few are positive feedback, in which the response amplifies the change until an event finishes it.

Two negative loops are worth being able to trace. Body temperature: a rise is detected by receptors, the control center in the hypothalamus orders sweating and vasodilation, and the temperature falls. Blood glucose: a rise is detected by the pancreas itself, which releases insulin so that cells take up glucose and the liver stores it; a fall releases glucagon, and the liver releases glucose.

Positive feedback is rarer, and its purpose is a finish line rather than a balance. During childbirth, pressure on the cervix triggers oxytocin release, which strengthens contractions, which increases the pressure — the loop runs until the baby is delivered. In blood clotting, each activated factor activates the next, and the clot grows until the vessel is sealed.

The integumentary system

The skin is the body's largest organ. It has two named layers and a layer beneath them: the epidermis, a stratified squamous epithelium with no blood vessels of its own; the dermis, a connective-tissue layer carrying the vessels, glands and nerve endings; and the hypodermis, the subcutaneous layer of fat and loose connective tissue that anchors the skin to what lies under it.

The epidermis renews from its deepest layer, the stratum basale, and the cells flatten and fill with keratin as they move outward, so the surface layer, the stratum corneum, is dead keratin and is what keeps water in and microbes out. The dermis carries the arteriovenous anastomoses — direct artery-to-vein connections that let skin blood flow change quickly, which is part of how the skin controls heat loss — and it is the layer that holds the glands and the receptors for touch, pressure, pain and temperature.

Enlarge

Image: Skin layers.svg by Madhero88 and M.Komorniczak, CC BY-SA 3.0, via Wikimedia Commons.

The skin does six jobs: protection (a physical, chemical and immune barrier), thermoregulation (sweating and changing the width of skin blood vessels), sensation (receptors for touch, pressure, temperature and pain), vitamin D synthesis, excretion (small amounts of water, salt and nitrogenous waste) and blood storage (the dermis can hold a share of the body's blood volume). Hair, nails and glands are its accessory structures, and a burn is described by how deep it reaches: epidermis only, into the dermis, or through it.

The skeletal system

An adult skeleton has 206 bones, counted by standard teaching convention, and it is divided into two parts. The axial skeleton (80 bones) is the body's axis — skull, vertebral column, ribs and sternum — and its job is protection and support. The appendicular skeleton (126 bones) is the limbs plus the girdles that attach them to the axis, and its job is movement.

Enlarge

Image: Human skeleton front en.svg by Mariana Ruiz Villarreal (LadyofHats), public domain, via Wikimedia Commons.

Bone does five things besides hold the body up: it protects organs (the skull the brain, the rib cage the heart and lungs), it stores minerals (calcium and phosphorus, released or taken up under hormonal control), it produces blood cells (hematopoiesis in the red marrow), it stores fat (yellow marrow) and it acts as a lever for muscles. A bone is not dead material: it is remodeled throughout life by osteoblasts, which build bone, and osteoclasts, which break it down, with osteocytes maintaining the finished tissue.

Bones are classified by shape — long, short, flat, irregular and sesamoid — and the shapes follow the job: long bones are levers, flat bones shield organs and give muscle a broad attachment, and sesamoid bones such as the patella sit inside a tendon to change its line of pull. Most bones form by endochondral ossification, in which a cartilage model is replaced by bone; the flat bones of the skull form by intramembranous ossification, directly in membrane.

The vertebral column is read from the top: seven cervical vertebrae, twelve thoracic vertebrae carrying the ribs, five lumbar vertebrae, then the sacrum (five fused vertebrae) and the coccyx (three to five fused vertebrae). The curves are not a defect; they let the column act as a spring.

A joint is named for the material that holds it together and for how much it moves. Fibrous joints hold bones with dense connective tissue and barely move (the sutures of the skull). Cartilaginous joints hold bones with cartilage and allow a little movement (the joints between vertebrae, and where the ribs meet the sternum). Synovial joints have a cavity between the bones, filled with synovial fluid and lined by a capsule, and they are the freely movable ones.

The muscular system

Muscle does four things: it produces movement, maintains posture, stabilizes joints and generates heat. There are three muscle types, and a question that names one of their properties is really asking which type it is.

Enlarge

Image: Muscles anterior labeled.png by Mikael Häggström, public domain, via Wikimedia Commons.

A skeletal muscle is built in nested bundles: the whole muscle is made of fascicles, each fascicle of muscle fibers (the muscle cells), each fiber of myofibrils, and each myofibril of repeating units called sarcomeres. The sarcomere is where contraction happens. It contains thin actin filaments and thick myosin filaments, and in a contraction the myosin heads bind the actin and pull the thin filaments toward the center of the sarcomere, shortening it. The filaments themselves do not shorten — they slide past one another, which is why it is called the sliding-filament model.

Two inputs are needed for that to happen. A motor neuron releases acetylcholine at the neuromuscular junction, which starts an electrical impulse along the muscle fiber; the impulse releases calcium from the sarcoplasmic reticulum; and the calcium binds troponin, which shifts tropomyosin off the binding sites on actin so that myosin can reach it. Contraction also needs ATP, both for the power stroke and to detach the myosin head afterward.

Muscles work in groups, and the names describe the roles: the prime mover (agonist) produces the main action, the antagonist opposes it and controls the return, and the synergist helps the prime mover or steadies a joint. A muscle's name often tells you its job — size (gluteus maximus), direction of fibers (rectus, meaning straight), location (frontalis), attachment (sternocleidomastoid) or action (flexor).

Movement terms: which way the body part goes

Muscle actions are named for the direction of the movement, not for the muscle, so one muscle can produce more than one of these depending on which of its parts is fixed.

Two pairs are the ones that get confused. Abduction and adduction differ only in direction — away from the midline against toward it — and the mnemonic is that abduction is the one that takes the limb away. Pronation and supination are the forearm only: the palm turns down in pronation, which is the position a hand is in when it types. Circumduction is the circular combination of flexion, extension, abduction and adduction at a ball-and-socket joint, as the arm makes when it draws a circle in the air.

The sliding filament sequence, step by step

A muscle fiber holds myofibrils, and each myofibril is a chain of sarcomeres — the unit that shortens. One sarcomere runs from one Z line to the next and holds two kinds of filament: thin actin filaments anchored at the Z lines, and thick myosin filaments in the middle, with heads that reach toward the actin. Contraction does not shorten the filaments; it slides them past each other, which is why the model is called sliding filament.

1. A motor neuron releases acetylcholine at the neuromuscular junction, and the muscle fiber's membrane depolarizes.
2. The signal travels into the fiber and the sarcoplasmic reticulum releases stored calcium ions.
3. Calcium binds troponin on the thin filament, and tropomyosin shifts off the myosin-binding sites.
4. A myosin head, already energized by ATP, binds actin and pulls — the power stroke — dragging the thin filament toward the sarcomere's center.
5. ATP binds the head and detaches it, and the head re-cocks for the next stroke.
6. When the signal stops, calcium is pumped back into the reticulum, tropomyosin returns, and the fiber relaxes.

The visible result is that the sarcomere's light bands and central gap narrow while the dark band stays the same width. ATP is needed twice: to detach each head and to pump the calcium back. Without ATP the heads stay bound, which is the chemistry behind rigor mortis.

The nervous system

The nervous system is divided by position and by job. By position, the central nervous system (CNS) is the brain and the spinal cord, and the peripheral nervous system (PNS) is everything else — twelve pairs of cranial nerves and thirty-one pairs of spinal nerves. By job, sensory (afferent) neurons carry information toward the CNS and motor (efferent) neurons carry commands away from it. The motor half divides again: the somatic division controls skeletal muscle, and the autonomic division controls smooth muscle, cardiac muscle and glands. The autonomic division has two opposed branches — sympathetic (arousal and "fight or flight") and parasympathetic (rest, digestion and repair).

Enlarge

Image: Complete neuron cell diagram en.svg by Mariana Ruiz Villarreal (LadyofHats), public domain, via Wikimedia Commons.

A neuron has three functional parts. Dendrites receive signals and carry them toward the cell body. The cell body (soma) holds the nucleus and does the cell's metabolic work. The axon carries the impulse away, sometimes wrapped in myelin, a fatty insulation laid down by Schwann cells in the PNS and oligodendrocytes in the CNS, with gaps called nodes of Ranvier where the impulse is regenerated. At the far end the axon branches into terminals that release neurotransmitters into a synapse, the gap between one neuron and the next.

An impulse begins with the resting membrane potential, about −70 mV inside relative to outside, held by the distribution of sodium and potassium ions. A stimulus that reaches the threshold opens sodium channels, sodium enters, and the membrane depolarizes; potassium channels then open and the membrane repolarizes. The event is all-or-none — a stronger stimulus produces more impulses, not a bigger one — and it travels without decreasing in size because it is regenerated at each node.

Enlarge

Image: Blausen 0111 BrainLobes.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

The cerebrum is the two hemispheres, joined by the corpus callosum, and its surface is folded into gyri (ridges) and sulci (grooves) to fit a large area of cortex into the skull. The diencephalon sits beneath it: the thalamus relays sensory information to the cortex, and the hypothalamus is the body's homeostatic control center, linking the nervous system to the endocrine system through the pituitary.

Enlarge

Image: Blausen 0115 BrainStructures.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

The figure above is a side view drawn with the cerebrum see-through so that the structures beneath it can be labeled; it is not a cut through the midline. Beneath the cerebrum, the brainstem runs from the midbrain through the pons to the medulla oblongata, which continues into the spinal cord. The medulla holds the vital centers for heart rate, breathing and blood pressure, so those functions are controlled there, not in the cerebrum or the cerebellum.

The brain and spinal cord are wrapped in three meninges — dura mater, arachnoid and pia mater — and bathed in cerebrospinal fluid, which cushions them and is produced by the choroid plexus in the ventricles. A reflex arc is the shortest nervous pathway: a receptor detects the stimulus, a sensory neuron carries it to the spinal cord, an interneuron passes it to a motor neuron, and the motor neuron drives the effector — all before the brain has registered the event.

The action potential, and saltatory conduction

A neuron at rest sits at a resting membrane potential of about −70 mV inside, held there by the sodium-potassium pump pushing three sodium ions out for every two potassium ions it brings in, and by leaks that let a little of each back. The inside is negative relative to the outside, and the fiber is polarized.

A stimulus that reaches the threshold — about −55 mV — opens voltage-gated sodium channels, and sodium rushes in. That rush is depolarization: the inside becomes positive, briefly to about +30 mV. The sodium channels then close and voltage-gated potassium channels open, potassium leaves, and the membrane returns toward rest — repolarization, overshooting a little. The brief refractory period that follows is why the impulse travels one way along the fiber and not backward. The action potential is all-or-nothing: a stronger stimulus produces more impulses per second, not a bigger one.

Myelin is the lipid wrapping that Schwann cells lay down in the peripheral nervous system and oligodendrocytes in the central one. Myelin insulates, so the current cannot cross the membrane where it is wrapped and instead jumps between the unmyelinated gaps called nodes of Ranvier, where the channels sit. That jumping is saltatory conduction, and it makes a myelinated fiber conduct many times faster than a bare one of the same width. Damage to myelin — as in multiple sclerosis — slows or blocks the impulse rather than weakening the muscle, because the signal may fail at a node before it reaches the end.

The special senses: the eye and the ear

The eye turns light into a signal. Light passes the cornea, which does most of the bending, then the pupil, whose size is set by the iris, then the lens, which fine-tunes the focus by changing shape — a process called accommodation. It crosses the vitreous humor, the jelly that fills the back of the eye, and lands on the retina, where rods respond in dim light and at the edges of the field and cones respond to color and fine detail, most densely at the fovea.

Enlarge

Image: Schematic diagram of the human eye en.svg by Rhcastilhos and Jmarchn, CC BY-SA 3.0, via Wikimedia Commons.

The ear turns pressure waves into a signal, in three stages. The outer ear collects sound: the auricle funnels it into the external auditory canal, which ends at the tympanic membrane. The middle ear is an air-filled space where three tiny bones — the malleus, incus and stapes — pass the vibration on and amplify it, and the eustachian tube keeps the pressure on both sides of the membrane equal. The inner ear contains the cochlea, where hair cells in the organ of Corti convert the vibration into nerve impulses, and the semicircular canals with the vestibule, which report the position and movement of the head.

Enlarge

Image: Blausen 0328 EarAnatomy.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

The endocrine system

The endocrine system sends chemical messages through the blood, and its hormones act on cells that carry a receptor for them. That is the whole logic: no receptor, no response. A steroid hormone crosses the cell membrane and binds a receptor inside the cell, changing gene activity; a nonsteroid (protein or peptide) hormone binds a receptor on the surface and sets off a second messenger inside the cell.

Enlarge

Image: Blausen 0345 EndocrineSystem Female2.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

The figure is drawn on a female body, so the gonads it labels are the ovaries; the testes sit outside the body and produce the same class of hormones, driven by the same pituitary signals. The pituitary is labeled in the figure; the hypothalamus that controls it sits in the brain just above, and is not labeled there.

Endocrine control runs on axes, and the thyroid axis is the model to know. The hypothalamus releases thyrotropin-releasing hormone, the anterior pituitary releases thyroid-stimulating hormone (TSH), and the thyroid releases T3 and T4 — which then suppress the first two steps, so the level settles at a set point. That is negative feedback applied to a gland, and it explains why a failing thyroid produces a high TSH: the brake is gone, so the pituitary keeps pushing.

The cardiovascular system

The heart is a double pump, and the two circuits are named for where they go. The pulmonary circuit carries blood to the lungs to pick up oxygen; the systemic circuit carries it to the rest of the body. The right side of the heart serves the first and the left side the second, which is why the left ventricle's wall is the thickest — it pumps against the whole body's resistance.

Enlarge

Image: Blausen 0457 Heart SectionalAnatomy.png by Blausen Medical Communications, Inc., CC BY 3.0, via Wikimedia Commons.

Blood follows one path, and the valves sit along it in order. From the body it enters the right atrium, passes the tricuspid valve into the right ventricle, leaves through the pulmonary semilunar valve into the pulmonary trunk, picks up oxygen in the lungs, returns through the pulmonary veins to the left atrium, passes the bicuspid (mitral) valve into the left ventricle, and leaves through the aortic semilunar valve into the aorta.

The heart's conduction system sets its rhythm: the sinoatrial (SA) node in the right atrium fires first and is the pacemaker, the impulse spreads to the atrioventricular (AV) node, then down the bundle of His, along the bundle branches and through the Purkinje fibers, which trigger the ventricles to contract. Keep that order: it places the pacemaker, and it explains a dropped beat the AV node can cause.

Cardiac output is the volume the heart moves in a minute, and it is the product of two numbers: stroke volume (the volume of one beat) multiplied by heart rate (beats per minute). At rest that is about 5 liters a minute. Blood pressure is cardiac output multiplied by the resistance of the vessels, which is why a narrowed arteriole raises pressure even when the heart has not changed.

Enlarge

Image: Circulatory System en.svg by Mariana Ruiz Villarreal (LadyofHats), public domain, via Wikimedia Commons.

Blood is about 55 percent plasma and 45 percent formed elements by volume. Erythrocytes (red cells) carry oxygen on hemoglobin; they have no nucleus in humans, live about 120 days, and are made in red marrow under the control of erythropoietin, a hormone released by the kidneys when oxygen is low. Leukocytes (white cells) defend the body — neutrophils are the most numerous and are the first to a bacterial infection, lymphocytes carry specific immunity, monocytes become macrophages, and eosinophils and basophils handle parasites and allergic responses. Platelets are cell fragments that form the first plug at a broken vessel.

The lymphatic and immune systems

The lymphatic system is the drainage half of the circulation. Not all of the fluid that leaves a capillary returns to it, and the remainder is collected as lymph by lymphatic capillaries, filtered through lymph nodes and returned to the blood at the subclavian veins. Along the way it does two other jobs: lymphatics in the small intestine absorb fats, and the nodes are where immune cells meet what the body has collected.

Enlarge

Image: Blausen 0623 LymphaticSystem Female.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

The respiratory system

Air follows one route: nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. Learn that order first. The epiglottis closes the larynx during swallowing so food is diverted into the esophagus; the trachea is held open by C-shaped cartilage rings; and the bronchi branch repeatedly into narrower bronchioles that end in clusters of alveoli.

Enlarge

Image: Blausen 0770 RespiratorySystem 02.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

The alveoli are the point of the system. Each is a thin-walled sac surrounded by capillaries, so the distance between air and blood is two cells — the alveolar wall and the capillary wall. Surfactant lines the alveoli and lowers surface tension, which keeps them from collapsing on themselves when the air leaves.

Inspiration is active; quiet expiration is passive. The diaphragm contracts and flattens while the external intercostals lift the ribs, which enlarges the thoracic cavity; the pressure inside falls and air moves in. Quiet expiration is mostly elastic recoil — the lungs and chest wall spring back — with the internal intercostals and abdominal muscles called on for a forced breath out. A tidal volume is the air moved in one quiet breath, about 500 mL in an adult; vital capacity is the largest breath you can move, and residual volume is what stays in the lungs and cannot be exhaled.

Oxygen travels mostly bound to hemoglobin, and carbon dioxide travels mostly as bicarbonate in the plasma, with smaller amounts on hemoglobin and dissolved in the plasma. Those smaller routes are real, but they are not the main one.

The digestive system

Digestion is both mechanical and chemical, and the tract is arranged so that each region does the job the next one needs done. Mechanical digestion is chewing, churning and segmentation; chemical digestion is enzymes breaking bonds, and it begins in the mouth.

Enlarge

Image: Blausen 0316 DigestiveSystem.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

The alimentary canal runs mouth → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus. The accessory organs — teeth, tongue, salivary glands, liver, gallbladder and pancreas — sit outside that tube and add what it cannot make: saliva, bile and pancreatic enzymes.

The small intestine has three parts with different emphasis: the duodenum receives bile and pancreatic juice and does most of the chemical work, the jejunum absorbs most nutrients, and the ileum absorbs what is left, including vitamin B12 and bile salts. The pancreas supplies amylase for starch, lipase for fat and trypsin for protein, and the brush border of the intestine finishes the job with enzymes such as lactase and maltase.

The urinary system

The urinary system filters the blood, and everything else it does follows from that: it regulates the volume and composition of body fluids, holds acid–base balance, contributes to blood pressure through the hormone renin, stimulates red cell production through erythropoietin, and activates vitamin D. The organs are the two kidneys, the two ureters, the urinary bladder and the urethra.

Enlarge

Image: Urinary System (Male).png by BruceBlaus, CC BY-SA 4.0, via Wikimedia Commons.

The figure shows a male tract, so it labels the prostate, which sits below the bladder and surrounds the urethra. The urinary organs themselves are the same in both sexes; the differences are the length of the urethra and the presence of the prostate.

The functional unit is the nephron, and there are about a million in each kidney. Blood arrives at the glomerulus, a knot of capillaries inside Bowman's capsule, where pressure forces water and small solutes out and leaves cells and large proteins behind. The filtrate then passes the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct, and what leaves the collecting duct is urine.

Enlarge

Image: Kidney Nephron.svg by Holly Fischer, CC BY 3.0, via Wikimedia Commons.

Filtration is passive and pressure-driven. Reabsorption moves useful material back into the blood — glucose and amino acids in the proximal tubule, water throughout, and water again in the collecting duct when ADH is present. Secretion moves material the body wants rid of from the blood into the tubule, which is how hydrogen ions and some drugs are cleared. That is why a question about a substance "appearing in the urine" can be asking about filtration, about failed reabsorption, or about secretion — the three are different routes to the same fluid.

The reproductive systems

The reproductive systems produce gametes, produce the hormones that maintain the secondary sex characteristics, and in the female sustain a pregnancy.

Enlarge

Image: Male genital system - Sagittal view.svg by R. Dewaele (Bioscope, Unige), J. Abdulcadir (HUG), C. Brockmann (Bioscope, Unige), O. Fillod and S. Valera-Kummer (DIP), Université de Genève, CC BY-SA 4.0, via Wikimedia Commons.

In the male, the testes sit in the scrotum, where the temperature is a little below body temperature — the condition sperm production needs. Inside each testis, seminiferous tubules produce sperm, and the interstitial (Leydig) cells between them produce testosterone. Sperm mature and are stored in the epididymis, travel through the vas deferens, and are joined by fluid from the seminal vesicles, the prostate and the bulbourethral glands before leaving through the urethra.

Enlarge

Image: Blausen 0399 FemaleReproSystem 01.png by BruceBlaus, CC BY 3.0, via Wikimedia Commons.

In the female, the ovaries produce the oocytes and the hormones estrogen and progesterone. Each month an oocyte is released at ovulation and is drawn into a uterine (fallopian) tube, where fertilization normally happens in its wider section, the ampulla. The uterus has a muscular wall, the myometrium, lined by the endometrium, which is built up and shed in the menstrual cycle and is where a fertilized egg implants. The cervix is the narrow neck of the uterus, and the vagina is the passage to the outside.

The traps, collected

The traps on this topic are terms used in the wrong frame and structures placed one step off in a sequence — not missing facts.

How to practice this

• Draw the path before you name the parts. Blood, air, food and urine each follow one route. If you can draw the route, a structure becomes "where on the route?", which is answerable.
• Say the function in the same breath as the name. "Nephron — filters, reabsorbs, secretes" is a fact you can use; "nephron" on its own is a word.
• Learn terms in pairs, never singly. Superior without inferior is half a fact — learn the pair together, in both directions.
• Sort by system before you answer. A hormone and a gland belong to the endocrine system; a vessel and a chamber belong to the cardiovascular one. The system narrows the possibilities before the fact does.
• Read the direction word. "Carries blood away from the heart" points to an artery. "Carries oxygen-poor blood" points to the pulmonary artery. They are different facts.

Where to go next

• The subject's own page — Anatomy and Physiology lists what is published for this subject, including its guide and its practice set as they land.
• The cell and the gene behind the systems — Biology: organelles, DNA, cell division and inheritance, which is the level below this one.
• The measurements a body system is described with — Measurement and Conversion: temperature, mass and volume, the units a physiology description is stated in.

Sources. The structural and functional descriptions on this page follow the US National Cancer Institute's SEER Training Modules on anatomy and physiology, training.seer.cancer.gov, which are US federal government material and not subject to copyright in the United States, together with MedlinePlus (medlineplus.gov), the US National Library of Medicine's consumer health service, for organ-level descriptions. The hormone actions, the conduction pathway of the heart, the segments of the nephron and the phases of the menstrual cycle are the standard teaching descriptions carried by those sources. Counts and volumes are standard teaching values stated as such: 206 bones in the adult skeleton (80 axial, 126 appendicular), twelve pairs of cranial nerves and thirty-one pairs of spinal nerves, a resting membrane potential of about −70 mV, a tidal volume of about 500 mL and a resting cardiac output of about 5 L per minute. The publisher's own account of the areas the exam covers is quoted from its HESI A2 page. Each picture on this page carries its source, author and license in the credit beneath it. Accessed 2026-10-25.

HESI A2 is a trademark of Elsevier Inc. Elk Ridge Academy is an independent study resource and is not affiliated with, endorsed by, or sponsored by Elsevier Inc. Exam names are used descriptively only.