Anatomy is the study of structure; physiology is the study of what that structure does. Together they describe a living architecture composed of roughly thirty-seven trillion cells organized into tissues, organs, and eleven major systems.
The Western tradition begins with Galen of Pergamon (129 — c. 216 CE), whose dissections of monkeys and pigs guided medicine for fourteen centuries. Andreas Vesalius's De humani corporis fabrica (1543) replaced inference with direct human dissection, and modern anatomy was born. Today, MRI, CT, and cryosection atlases let us see the living body in three dimensions and four when motion is added.
This deck walks through the body system by system. Each plate pairs a labelled figure with the essential vocabulary you would find in a first-year medical text — but written for the curious reader, not the examinee.
Biological structure nests cleanly. Atoms compose molecules; molecules build organelles; organelles populate cells. Cells of similar specialization form tissues — epithelial, connective, muscular, nervous. Tissues combine into organs, organs cooperate as systems, and systems together constitute the organism.
Four primary tissue types do nearly all the work. Epithelium covers and lines. Connective tissue (including bone, blood, fat) supports and transports. Muscle contracts. Nervous tissue conducts and integrates.
Bone is living connective tissue — a protein matrix (mostly type I collagen) hardened with hydroxyapatite. The skeleton stores 99% of the body's calcium, houses red marrow that produces ~2 million red blood cells per second, and provides the levers that muscles pull on.
The body has three muscle tissues: skeletal (voluntary, striated), cardiac (involuntary, striated, branched), and smooth (involuntary, in vessel walls and gut). Roughly 600 named skeletal muscles account for 30–40% of body mass.
Contraction is the sliding-filament action: myosin heads, fueled by ATP, walk along actin filaments. Calcium release from the sarcoplasmic reticulum unlocks the binding sites; relaxation requires its reuptake.
The nervous system divides into the central nervous system (CNS — brain and spinal cord) and the peripheral nervous system (PNS — nerves to and from the rest of the body). The PNS subdivides into somatic (voluntary) and autonomic (involuntary), and the autonomic into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest).
The basic signaling unit is the neuron: dendrites collect input; the soma integrates; the axon — sometimes a meter long — transmits an action potential to a synapse, where neurotransmitters (acetylcholine, glutamate, GABA, dopamine, serotonin) hand off the signal.
The heart is a fist-sized muscular pump that beats ~100,000 times a day, moving five liters of blood per minute through ~100,000 km of vessels. Deoxygenated blood enters the right atrium, passes to the right ventricle, and is sent to the lungs via the pulmonary artery — the only artery carrying deoxygenated blood. Oxygenated blood returns to the left atrium and ventricle, then exits via the aorta.
| Vessel | Pressure | Function |
|---|---|---|
| Aorta | ~120/80 mmHg | Distribution |
| Arterioles | ~60 mmHg | Resistance, BP control |
| Capillaries | ~30 mmHg | Exchange (O₂, CO₂, nutrients) |
| Venules / veins | ~5 — 15 mmHg | Return (with one-way valves) |
"The heart's right side serves the lungs; the left side serves the world." — clinical aphorism.
The respiratory tract begins at the nose and ends at the alveoli — ~480 million tiny sacs whose combined surface area (~70 m²) approximates a tennis court. Each breath moves air across a membrane only 0.5 μm thick, where oxygen diffuses into pulmonary capillaries and CO₂ diffuses out.
The diaphragm — a dome-shaped skeletal muscle innervated by the phrenic nerve (C3–C5) — drives the bellows. Contraction lowers it, expanding the thorax and pulling air in. Resting tidal volume is ~500 mL; vital capacity is 4–5 L. Adults breathe 12–20 times per minute.
From mouth to anus the alimentary canal runs roughly nine metres, lined throughout by mucosa adapted to its task. Mechanical breakdown begins with chewing; chemical digestion begins with salivary amylase. The stomach uses HCl (pH ~1.5–3.5) and pepsin to denature proteins. The small intestine — duodenum, jejunum, ileum — is the site of most absorption, aided by bile from the liver/gallbladder and enzymes from the pancreas.
The large intestine reclaims water and hosts the gut microbiome — tens of trillions of bacteria, with estimates around 38 trillion bacterial cells across hundreds to thousands of taxa, producing short-chain fatty acids, vitamins K and B12, and signals that influence immunity and even mood.
Where the nervous system signals in milliseconds with electrical impulses, the endocrine system signals in minutes to hours with hormones — chemical messengers carried in blood. Major glands include the hypothalamus, pituitary ("master gland"), thyroid, parathyroids, adrenals, pancreas, ovaries, and testes.
| Gland | Hormone | Effect |
|---|---|---|
| Thyroid | T3, T4 | Sets metabolic rate |
| Adrenal medulla | Epinephrine | Fight-or-flight |
| Adrenal cortex | Cortisol | Stress, glucose |
| Pancreas (β-cells) | Insulin | Lowers blood glucose |
| Pancreas (α-cells) | Glucagon | Raises blood glucose |
| Posterior pituitary | ADH, oxytocin | Water balance, bonding |
Two kidneys filter ~180 L of plasma daily, returning ~178 L and excreting ~1.5 L of urine. The functional unit is the nephron (~1 million per kidney), which concentrates wastes and balances water, sodium, potassium, and acid–base.
Skin is the largest organ — ~2 m², 16% of body weight. Three layers (epidermis, dermis, hypodermis) protect, regulate temperature, sense, and synthesize vitamin D from UVB.
Gonads (ovaries, testes) produce gametes and sex hormones (estrogen, progesterone, testosterone). Fertilization in the fallopian tube initiates 38 weeks of development.
X-rays (Röntgen, 1895) reveal dense tissues. CT stacks x-rays into 3D. MRI uses magnetic fields and radiofrequency to differentiate soft tissues. Ultrasound bounces sound off interfaces. PET tracks metabolism with radiotracers. Each modality answers a different clinical question.
Crash Course Anatomy & Physiology — a 47-episode series by Hank Green covering every system.
Watch on YouTube →
Anatomy is among the best-evidenced areas of biology — the body has been dissected for centuries and imaged for one. Functional claims (which organ does what) rest on millions of patient–years of clinical observation plus controlled physiology research. Where uncertainty persists, it is mostly at the molecular and microbiome scales.
Educational content only. Not a substitute for medical advice, diagnosis, or treatment.