Anatomy and Physiology: How Structure and Function Fit Together

Human anatomy is the scientific study of the body's structures. Human physiology is the scientific study of the chemistry and physics of those structures and of how they work together to support life. Anatomy asks what a part is and where it lies; physiology asks how it works. Physiologists specialise too: neurophysiology, for example, studies the brain, spinal cord and nerves, from the organ level down to molecules. (OpenStax Anatomy & Physiology 2e, 1.1 Overview of Anatomy and Physiology.)

In the wider life sciences, physiology is the biological science concerned with the life-supporting properties, functions and processes of living organisms or their parts. Its recognised specialties include electrophysiology, endocrinology, neurophysiology, comparative physiology and psychophysiology. (NLM Medical Subject Headings (MeSH), Physiology.)

The word physiology was first recorded in English between 1555 and 1565, from Latin physiologia and Greek physiología, 'the science of natural causes and phenomena'. Anatomy, by contrast, means 'cutting up'. (Dictionary.com, physiology.)

The idea that links the two subjects is that form is closely related to function in all living things. OpenStax gives two everyday examples: your eyelid can close and reopen in a blink to clear dust because of how its nerves and muscles are arranged, and your thumb can be opposed to the four fingers, which is what lets the hand pinch, grasp and use a pen or phone. Its advice to students is to keep relating the form of each structure to its function. (OpenStax Anatomy & Physiology 2e, 1.1 Overview of Anatomy and Physiology.)

The four examples below are the ones that come up again and again in A&P courses. Each shows a structure whose shape only makes sense once you know its job.

Gas exchange happens in the alveoli, air sacs about 200 µm across. Most of their surface (up to 97%) is covered by type I alveolar cells, flat cells that are highly permeable to gases. With the capillary wall they form the respiratory membrane, only about 0.5 µm thick, across which oxygen and carbon dioxide move by simple diffusion. Type II cells secrete surfactant, which lowers surface tension and helps keep the alveoli open. (OpenStax Anatomy & Physiology 2e, 22.1 Organs and Structures of the Respiratory System.)

Add up all the alveoli and the lungs exchange gases across a very large, highly permeable surface of about 70 square metres, wrapped in capillaries with very thin walls and no smooth muscle. (OpenStax Anatomy & Physiology 2e, 22.2 The Lungs.)

Kenhub's figures agree closely: alveoli 0.2–0.5 mm across, a respiratory membrane of only 0.5 µm (a red blood cell is about 7.5 µm wide), type I cells over about 95% of the surface and a total area of around 75 square metres. Structure → function: a thin barrier keeps the diffusion distance short, and a vast area moves enough gas for the whole body. (Kenhub, Alveoli (Ferng, reviewed by Mytilinaios).)

The small intestine is about 3.05 m long in a living person, and its lining is folded at three scales: circular folds of the mucosa and submucosa; villi 0.5–1 mm long, about 20 to 40 per square millimetre; and microvilli about 1 µm long, an estimated 200 million per square millimetre, forming the brush border. Each villus holds a capillary bed and a lymphatic capillary, the lacteal, which carry absorbed nutrients away. OpenStax says these folds increase the absorptive surface more than 600-fold, to about 200 m². (OpenStax Anatomy & Physiology 2e, 23.5 The Small and Large Intestines.)

That textbook total is disputed. Helander and Fändriks (2014) recalculated the area from an average small-intestine diameter of 2.5 cm, with villi enlarging the surface about 6.5 times and microvilli about 13 times, and arrived at about 30 m² for the small intestine and about 32 m² for the whole digestive tract. Whichever figure you use, the principle is the same: folding packs a large absorptive surface into a narrow tube. (BioNumbers (Harvard), Average surface area of parts of the gastrointestinal tract, from Helander & Fändriks, Scand J Gastroenterol 49:681 (2014).)

The heart has four valves. The atrioventricular valves (the tricuspid on the right and the mitral, or bicuspid, on the left) are anchored by chordae tendineae to papillary muscles, which stop the flaps being forced back into the atria when the ventricles contract. The semilunar valves (pulmonary and aortic) have no cords; their pocket-like cusps fill with blood and seal the opening when the ventricles relax. The valves open and close passively: blood flows towards lower pressure, and the pressure difference across each valve opens or seals it. (OpenStax Anatomy & Physiology 2e, 19.1 Heart Anatomy.)

The chambers show the same principle. The muscle of the left ventricle is much thicker and better developed than that of the right, because the left ventricle must generate enough pressure to drive blood through the long, high-resistance systemic circuit, while the right ventricle pumps only through the pulmonary circuit to the lungs. (OpenStax Anatomy & Physiology 2e, 19.1 Heart Anatomy.)

A neuron's shape follows its job. Branched dendrites receive information from other neurons at synapses; the cell body holds the nucleus and most organelles; and one axon carries the nerve impulse to target cells, starting at the trigger zone by the axon hillock and ending in synaptic end bulbs. Many axons are wrapped in myelin, made by glial cells, which insulates them much like the plastic around an electrical wire, with bare gaps called nodes of Ranvier. (OpenStax Anatomy & Physiology 2e, 12.2 Nervous Tissue.)

That insulation changes the physiology. In an unmyelinated axon the signal spreads continuously, which is slow; in a myelinated axon it effectively jumps from node to node (saltatory conduction), which is faster. A wide axon also conducts faster than a narrow one. (OpenStax Anatomy & Physiology 2e, 12.4 The Action Potential.)

The body is built in levels of increasing complexity, each made from the one below. OpenStax's figure uses six: chemical, cellular, tissue, organ, organ system and organism. At the chemical level, atoms of elements such as hydrogen, oxygen, carbon, nitrogen, calcium and iron bond into molecules such as water, proteins and sugars, the building blocks of every structure. (OpenStax Anatomy & Physiology 2e, 1.2 Structural Organization of the Human Body.)

A cell is the smallest independently functioning unit of a living organism; inside it, organelles are tiny functioning units suspended in water-based cytoplasm. A tissue is a group of many similar cells that work together for a specific function. An organ is an anatomically distinct structure made of two or more tissue types. An organ system is a group of organs that work together to perform major functions, and an organism is a living being that can carry out all the functions of life on its own. (OpenStax Anatomy & Physiology 2e, 1.2 Structural Organization of the Human Body.)

There are four primary tissue types: epithelial tissue covers surfaces and forms glands, connective tissue binds and supports, muscle tissue contracts to produce movement, and nervous tissue carries electrochemical signals. Every organ is a combination of them. (OpenStax Anatomy & Physiology 2e, 4.1 Types of Tissues.)

OpenStax groups the organs into 11 organ systems. It warns that assigning organs to systems can be imprecise, because most organs contribute to more than one: the testes and ovaries, for example, are listed both as endocrine glands, which secrete hormones, and as reproductive organs, which make gametes. (OpenStax Anatomy & Physiology 2e, 1.2 Structural Organization of the Human Body.)

The count depends on the source. The National Cancer Institute's SEER training module lists ten major systems (skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary and reproductive) with no separate integumentary system; OpenStax adds the skin, hair and nails as the integumentary system to make 11. (National Cancer Institute SEER Training Modules, Introduction to the Human Body.)

Homeostasis is the state of steady internal conditions that living things maintain, and much of physiology is about how the body achieves it. Every regulated value has a set point, the value around which its normal range fluctuates, and a normal range, the restricted set of values that is optimally healthful and stable. Keeping inside that range requires the body to monitor its internal conditions continuously. (OpenStax Anatomy & Physiology 2e, 1.5 Homeostasis.)

Body temperature is the classic example. The average normal body temperature is generally accepted as 37 °C (98.6 °F), but studies have found a normal range from about 36.1 °C (97 °F) to 37.2 °C (99 °F). (MedlinePlus, Body temperature norms.)

Negative feedback reverses a deviation from the set point. It needs three parts: a sensor (receptor) that monitors the value, a control centre that compares it with the normal range, and an effector that changes it back. When body temperature rises above 37 °C, nerve cells in the skin and brain (sensors) signal the brain's temperature-regulatory centre (control centre); it drives the sweat glands (effectors) and dilates the blood vessels of the skin, so heat is lost. In the cold, the heat-gain centre cuts blood flow to the skin, and if heat loss is severe the brain triggers shivering, whose muscle contractions release heat. (OpenStax Anatomy & Physiology 2e, 1.5 Homeostasis.)

Blood glucose is regulated the same way. When endocrine cells in the pancreas detect excess glucose in the blood (the stimulus), the beta cells release the hormone insulin; as glucose falls back to normal, insulin release stops, which keeps blood sugar from dropping below the normal range. (OpenStax Anatomy & Physiology 2e, 1.5 Homeostasis.)

Positive feedback intensifies a change instead of reversing it. In childbirth, the baby's head stretching the cervix triggers the release of oxytocin, which makes the uterine muscle contract harder, which pushes the head harder against the cervix; the loop stops only when the baby is born. Blood clotting is another positive feedback cascade: each step stimulates the release of more clotting substances. (OpenStax Anatomy & Physiology 2e, 1.5 Homeostasis.)

Anatomy and physiology is usually taught as a two-semester sequence, A&P 1 and A&P 2. The Human Anatomy & Physiology Society (HAPS) publishes more than 800 learning outcomes for it, organised into lettered modules; it notes that most first-semester courses cover modules A to J and second-semester courses the rest, and it takes no position on the order. (Human Anatomy & Physiology Society, HAPS A&P Learning Outcomes.)

HAPS's end-of-course exams show the usual split. The A&P 1 exam covers the body plan, homeostasis, chemistry and cell biology, histology, and the integumentary, skeletal, muscular and nervous systems with the special senses. The A&P 2 exam covers the endocrine, cardiovascular, lymphatic and immune, respiratory, digestive, urinary and reproductive systems, plus metabolism and fluid, electrolyte and acid–base balance. The nervous and cardiovascular systems carry the most questions, 14 each. (Human Anatomy & Physiology Society, HAPS A&P I and A&P II Exams.)

The endocrine system is the swing topic: the exam places it in A&P 2, while the learning-outcomes guidance counts module J in the first semester. Check your own syllabus. (Human Anatomy & Physiology Society, HAPS A&P Learning Outcomes.)

Labs bring the anatomy to life with anatomical models, skeletons, diagrams and photographs, and with dissection and the inspection of cadavers. (LibreTexts Medicine, 1.1A Defining Anatomy (A and P for STEM Educators, James Madison University).)

A 2013 review in Psychological Science in the Public Interest by John Dunlosky and colleagues rated ten common study techniques. Only two received the highest utility rating: practice testing (quizzing yourself with flash cards or end-of-chapter questions) and distributed practice (spreading study out over time). Summarising, highlighting and underlining, and rereading were among the five rated low. (Association for Psychological Science, Which Study Strategies Make the Grade? (Dunlosky et al. 2013).)

The effect holds for anatomy. Dobson, Perez and Linderholm (2017) had students learn skeletal muscle anatomy with the same 24 minutes of study arranged in five ways. Retrieval beat restudying, spacing beat cramming, and the combination was best: a week later, students who studied and retrieved across seven days recalled about 33.9 items on average, against about 12.9 for one massed session with retrieval. (Dobson, Perez & Linderholm, Distributed Retrieval Practice Promotes Superior Recall of Anatomy Information, Anatomical Sciences Education 10:339 (2017).)

1. Learn the language first. Anatomical terms come from Greek and Latin and their meanings are fixed, so learning common roots makes each new name easier to decode. Start with the anatomical terms, directional terms, body planes.
2. Pair every structure with its function. Write the function next to each name, and ask what the shape lets it do.
3. Test, don't reread. Cover the labels on a diagram and name each part from memory, for example the label-it-yourself diagram on the heart blood flow page or the anatomy quiz.
4. Space it out. Revisit each system on several days spread over the week instead of in one long session before the exam.
5. Draw the loop. For every hormone or reflex, sketch stimulus, sensor, control centre, effector and response, and mark whether the feedback is negative or positive.
6. Study in 3D. Rotate the 3D human body, fade the skin and muscles, and find each organ in place before you learn its details.