Anatomia humana

The skeleton is conventionally divided into the axial — the skull, vertebral column, and ribcage that defend the brain, cord, and viscera — and the appendicular, the limbs and their girdles, by which the body acts upon the world.

Bone is not the dry chalk of memento mori. It is a living tissue, woven of collagen and mineral, threaded with vessels, populated by osteoblasts that build and osteoclasts that pare back. The adult skeleton renews itself entirely every decade.

• Axial: 80 bones — cranium, vertebrae, ribs, sternum.
• Appendicular: 126 bones — pectoral and pelvic girdles, arms, legs.
• Marrow: site of haematopoiesis — the daily birth of blood cells.

Three kinds of muscle compose the body's musculature, each with its own architecture and obedience — some willing, some unbidden.

• Skeletal — striated, voluntary, attached to bone by tendons. The muscles of expression and labour.
• Smooth — unstriated, involuntary, lining the gut, the vessels, the airways. The quiet workforce.
• Cardiac — striated yet involuntary, branched and intercalated, contracting in concert at its own metronome.

Muscles act only by pulling. Every push is a pull elsewhere. Every step is a chorus — agonist, antagonist, synergist, and stabilizer in their counterpoised duet.

The heart is a four-chambered pump no larger than its owner's fist, contracting some 100,000 times a day, moving roughly 7,000 litres of blood through the long circuit of arteries, capillaries, and veins.

Two loops, intertwined:

• Pulmonary — right ventricle to lungs and back. Spent blood reloads with oxygen.
• Systemic — left ventricle to the body's furthest reach. Oxygen delivered, carbon dioxide collected.

Capillaries, where the actual commerce happens, are so fine that red cells pass through in single file.

Air enters by the nose, where it is warmed, moistened, and filtered. It descends through the trachea and divides at the carina into two bronchi; these branch and re-branch some twenty-three times into a microscopic foliage.

At the tips: alveoli. Some 300 to 500 million of them, gossamer sacs whose unfurled surface measures ~70 square metres — the area of a respectable parlour. There, oxygen crosses one cell's thickness into the blood and carbon dioxide departs.

Twelve to twenty breaths a minute. About half a billion in a long life.

From mouth to anus, the digestive tract is a single tube about nine metres long, folded with great economy into the abdomen. It is, topologically, the body's outside running through its inside.

• Mouth & oesophagus — mastication, deglutition.
• Stomach — acid bath, churning vat (pH ~ 1.5).
• Small intestine — ~6 m of villi and microvilli; the seat of absorption.
• Large intestine — water reclamation, fermentation.
• Liver, pancreas, gallbladder — the chemical works alongside.

And the microbiome: some 38 trillion bacterial cells — rivalling our own count — that fashion a virtual organ within us, helping digest, train the immune system, and synthesize what we cannot.

About 86 billion neurons compose the brain — each averaging some 7,000 connections — plus an even larger company of glial cells that nurture and insulate them.

• Central (CNS) — brain and spinal cord. The seat of cognition and reflex.
• Peripheral (PNS) — cranial and spinal nerves, the wiring to every organ and limb.

The peripheral system itself divides:

• Somatic — voluntary, the nerves of the will.
• Autonomic — involuntary, splitting again into sympathetic (fight or flight) and parasympathetic (rest and digest).

Signals travel by electrochemical wave at up to 120 m/s — faster than the swiftest train.

Where the nervous system speaks in milliseconds, the endocrine system speaks in minutes and hours, by the slower medium of hormones — molecules released into the bloodstream that find, by exquisite key-and-lock, only their intended cells.

The choir is conducted from the head:

• Hypothalamus — the bridge between brain and gland; sets the tempo.
• Pituitary — the master gland, releasing trophic hormones to direct others.
• Thyroid & parathyroids — metabolism, calcium.
• Adrenals — cortisol, adrenaline; stress and salt.
• Pancreas (islets) — insulin and glucagon; sugar's regulator.
• Gonads — the sex steroids that mark each life stage.

A picogram of the right hormone, in the right place, can reshape a whole organism.

The body's defence operates in two registers, both indispensable.

• Innate — ancient, fast, unspecific. Skin and mucosae, complement proteins, neutrophils, macrophages, natural killer cells. The first responders.
• Adaptive — slow to mount, but exquisitely specific and memorial. B cells manufacture antibodies; T cells kill infected cells and orchestrate the response.

The two communicate by cytokine, by direct contact, by displayed antigen. The whole network is plumbed by the lymphatic system — vessels that drain interstitial fluid, lymph nodes that filter and sample, the spleen, thymus, and tonsils that train and station the lymphocytes.

Its defining trick is memory. A first encounter teaches; a second is dispatched in days.

The skin is, by area and by mass, the body's largest organ — some 1.5 to 2 square metres, weighing several kilograms in an adult.

Three strata, ordered outward to in:

• Epidermis — layered keratinocytes, melanocytes; the renewable barrier.
• Dermis — collagen and elastin, with vessels, nerves, glands, and hair follicles.
• Hypodermis — subcutaneous fat for insulation and ballast.

The skin defends, regulates temperature, synthesises vitamin D under sunlight, signals emotion (blush, pallor, gooseflesh), and reads the world by touch — some four million sensory receptors of pressure, vibration, temperature, and pain.

Every twenty-eight days or so, the epidermis renews itself: the body literally moults.

To divide the body into systems is a teacher's convenience, not nature's design. Every system relies on every other.

Lift a cup of tea and observe it: bones and joints provide the lever; muscles do the work; the nervous system commands and corrects; the heart and lungs replenish what is burned; the endocrine system tunes the pace; the skin reports temperature; the immune system guards the cut on your finger; the kidneys keep the chemistry inside its narrow ledger.

This dynamic equilibrium is homeostasis — the body's quiet, ceaseless project of remaining itself. Body temperature within half a degree. Blood pH within a tenth. Glucose within a band. Hour by hour, year by year, the body negotiates with the world to stay the body.

“The constancy of the internal environment is the condition of free life.” — Claude Bernard