Classification: Homo sapiens Seven systems · 78 organs · 37 trillion cells
The Human Body
An annotated reference to the machine you live inside. Every plate is labelled, every figure sourced, and every explanation available in two registers — plain language, or clinical terminology.
System 01 · Skeletal systema skeletale
A frame that carries you — and rebuilds itself while you sleep.
Bone looks finished, but it isn’t. Cells tear it down and lay it back again continuously, so the skeleton you carry today is not the one you had a decade ago. It also stores minerals, manufactures blood, and absorbs the load of every step you take.
Osseous tissue undergoes continuous remodelling: osteoclasts resorb mineralised matrix while osteoblasts deposit osteoid, replacing roughly 10% of adult skeletal mass annually. The skeleton is the principal calcium–phosphate reservoir and houses haematopoietic marrow within trabecular compartments.
- 206
- Bones, adult
from ~270 at birth - 10%
- Remodelled
per year - 15%
- Of total
body mass
Source: Standring (ed.), Gray’s Anatomy, 42e, 2020
Sense of scale — where the bones are
Twenty-seven bones in each hand, twenty-six in each foot. Just over half the skeleton is given to grasping and walking.
System 02 · Muscular systema musculare
Six hundred engines, and every one of them can only pull.
Muscle never pushes. Each one shortens by up to a third of its length and then has to be pulled back by an opposing partner, which is why the body is built in pairs. Precision comes from how few fibres you recruit, not how many.
Skeletal muscle generates force through actin–myosin cross-bridge cycling; sarcomeres shorten up to ~30% of resting length. Movement arises from agonist–antagonist pairs acting across joints, with force graded by orderly motor-unit recruitment from small to large.
- 600
- Skeletal
muscles (approx.) - 40%
- Of body mass
upper range - 30%
- Max fibre
shortening
Source: Gray’s Anatomy, 42e · Guyton & Hall, 14e, 2021
Sense of scale — muscle against frame
The engine outweighs the chassis it moves by more than two to one.
System 03 · Nervous systema nervosum Deep dive
Eighty-six billion cells, arguing at 120 metres per second.
Your brain runs on about twenty watts — less than a household bulb — while handling balance, language, memory and grief at once. Signals race down insulated fibres as electrical pulses, then leap chemical gaps thinner than a wavelength of light.
Action potentials propagate saltatorily along myelinated Aα fibres at up to 120 m/s, then transduce chemically across synaptic clefts of 20–40 nm. The CNS accounts for ~2% of body mass yet ~20% of resting oxygen and glucose consumption.
- 86B
- Neurons
Azevedo, 2009 - 120
- m/s peak
conduction - 20W
- Resting power
draw, brain
Source: Azevedo et al., J. Comp. Neurol., 2009 · Guyton & Hall, 14e
Sense of scale — signal speed
A reflex reaches your spinal cord faster than any train you can buy a ticket for.
System 04 · Cardiovascular systema cardiovasculare
A pump the size of your fist, on a hundred-thousand-kilometre circuit.
The heart is two pumps in one housing: the right side sends blood a short way to the lungs, the left drives it everywhere else. It repeats the cycle roughly once a second, without a single day off, for a lifetime.
The heart functions as a dual pump in series: the right ventricle perfuses the pulmonary circuit at ~25/10 mmHg, the left the systemic circuit at ~120/80 mmHg. Resting cardiac output is ~5 L/min (stroke volume ~70 mL × heart rate ~70 bpm).
- 5L
- Per minute
at rest - 100k
- km of vessels
estimated - 2.5B
- Beats in an
average life
Source: Guyton & Hall, 14e, 2021 · Gray’s Anatomy, 42e
Sense of scale — total vessel length
Laid end to end, the circuit would wrap the planet two and a half times.
System 05 · Respiratory systema respiratorium
Seventy square metres of surface, folded into your chest.
Lungs don’t breathe — they’re passive bags. The diaphragm drops, pressure inside the chest falls below the air outside, and the atmosphere is pulled in. Oxygen then crosses a barrier thinner than a soap film.
Ventilation is driven by diaphragmatic contraction generating sub-atmospheric intrapleural pressure. Gas exchange proceeds by passive diffusion across the alveolar–capillary membrane (~0.2–0.6 µm) along partial-pressure gradients.
- 480M
- Alveoli
Ochs, 2004 - 70m²
- Gas exchange
surface - 11k
- Litres of air
per day
Source: Ochs et al., Am. J. Respir. Crit. Care Med., 2004
Sense of scale — the tennis-court myth
Lungs are often said to equal a tennis court. They are closer to a third of one — which is still seventy square metres folded inside your ribs.
System 06 · Digestive systema digestorium
Nine metres of chemistry, turning matter into you.
The tract is one continuous tube from mouth to end — technically the outside world, threaded through you. Food only becomes part of your body once it has been broken small enough to cross that wall, a journey of one to three days.
The alimentary canal is a continuous mucosa-lined tube; absorption occurs chiefly in the jejunum, where plicae, villi and microvilli amplify surface area 60–120×. Roughly 5×10⁸ enteric neurons coordinate motility semi-autonomously of the CNS.
- 9m
- Tract length
mouth to anus - 30m²
- Absorptive area
Helander, 2014 - 24–72
- Hours total
transit time
Source: Helander & Fändriks, Scand. J. Gastroenterol., 2014 · Sender et al., 2016
Sense of scale — the tennis-court myth, again
Helander and Fändriks measured the real figure at about thirty square metres — nearer half a badminton court than a tennis court.
Section 07 · Connections Part two
Seven systems is a filing convenience. The body has never observed it.
Dividing the body into systems is how it gets taught, not how it works. The most interesting structures belong to two or three at once, and the divisions fall apart exactly where they are load-bearing. Six places the boundaries fail.
-
02Muscular 05Respiratory 03Nervous
The diaphragm answers to two masters
It is skeletal muscle — the voluntary kind — which is why you can hold your breath. But it is also the only muscle you cannot abandon: the brainstem drives it whether or not you attend to it, about 20,000 times a day. It fails upward through the neck, not the chest, because it forms early and migrates down, dragging its nerve behind it.
C3–C5Phrenic nerve roots. A neck injury above them stops breathing; below them does not.
Source: Gray’s Anatomy, 42e · Guyton & Hall, 14e
-
Your bones make your blood
The skeleton is not scaffolding with marrow stored inside it — the marrow is the point. Red marrow in the pelvis, sternum, ribs and vertebrae runs the body’s largest manufacturing operation, and it never stops, because every cell it makes is on a countdown from the day it is released.
2M/secRed cells produced, each lasting 100–120 days before the spleen retires it.
Source: Chen et al., Front. Endocrinol., 2018 · Gray’s Anatomy, 42e
-
01Skeletal 02Muscular 04Cardiovascular
The skeleton is a bank, and muscle spends the deposit
Calcium is what lets a muscle fibre contract at all, and what lets the heart keep its rhythm. Almost all of it is locked in bone as hydroxyapatite — which means your skeleton is not only holding you up, it is underwriting every heartbeat. When blood calcium falls, the body will dismantle bone to defend it, because the rhythm matters more than the frame.
99%Of body calcium held in bone, drawn on by PTH, calcitonin and calcitriol.
Source: StatPearls, Physiology — Calcium, 2023
-
Your gut has its own brain, and does not consult you
Embedded in the wall of the intestine is a nervous system large enough to run the tract by itself. Sever every connection to the brain and the gut carries on digesting — coordinating peristalsis, secretion and blood flow without instruction. It is the only organ that would keep working if you cut it off from the central nervous system entirely.
5×108Enteric neurons, coordinating motility semi-autonomously of the CNS.
Source: Gray’s Anatomy, 42e · Guyton & Hall, 14e
-
05Respiratory 04Cardiovascular
Two systems meet across a film you could see through
The whole purpose of breathing and the whole purpose of circulation converge at one boundary: the sheet where air and blood are separated by almost nothing. No pump pushes oxygen across it. It moves by diffusion alone, down a pressure gradient, over a distance shorter than a wavelength of visible light.
0.2–0.6µmAlveolar–capillary membrane. Seventy square metres of it, folded into the chest.
Source: Ochs et al., Am. J. Respir. Crit. Care Med., 2004
-
The heart cannot get blood back from your feet
Arteries have the pump behind them. Veins do not — by the time blood reaches your ankles the pressure from the heart is largely spent, and it still has to climb back up. What lifts it is your calf muscles squeezing the veins between them, with one-way valves keeping each squeeze. Standing perfectly still is the one posture that defeats it, which is why soldiers on parade faint.
64%Of total blood volume sitting in the systemic veins at rest, waiting to be moved.
Source: Guyton & Hall, 14e, 2021
Part Three
When something goes wrong
Three ordinary complaints, traced back to the mechanism that produces them. These pages explain how a symptom is generated — they do not diagnose one. Nothing here is a substitute for a clinician.
Condition 08 · Headache cephalalgia
The brain cannot feel pain. A headache is everything around it complaining.
Cut into brain tissue and it registers nothing — there are no pain receptors in it at all. Surgeons operate on a conscious brain for exactly this reason. What hurts is the packaging: the tough membrane wrapping the brain, the big arteries at its base, and the muscles and vessels of the scalp outside the skull.
The cranium, brain parenchyma, ependymal lining and choroid plexus are pain-insensitive. Nociception arises from the dura mater and dural arteries, the proximal arteries of the circle of Willis, the great venous sinuses, cranial nerves, and extracranial muscle and vasculature — transmitted largely via trigeminal afferents with peripheral and central sensitisation.
- 40%
- Of people affected
3.1 billion, 2021 - 70%
- Report episodic
tension-type - 3rd
- Migraine rank,
global DALYs
Source: WHO headache disorders, 2024 · Clinical Methods, 3e, ch. 59
- 01The insensitive core. Skull, brain tissue, the ventricle lining and choroid plexus carry no nociceptors. Damage there produces no sensation of its own.
- 02The sensitive wrapper. The dura and its arteries, the large vessels at the base of the brain and the great venous sinuses are richly supplied with pain fibres.
- 03Trigeminal alarm. Dural nerve endings release CGRP and substance P, widening vessels and inflaming the membrane. The fibres then become easier to trigger than before.
- 04The outside layer. In tension-type headache, sustained contraction of scalp and neck muscles adds a second source — which is why the ache feels like a band rather than a point.
Condition 09 · Abdominal pain dolor abdominis
Your gut cannot feel a cut. It can only feel a stretch.
The nerves in your intestines ignore things that would be agony on skin. You can cut or burn the bowel wall and feel nothing. What they do report is stretching — a tube being pulled wider than it should be. That is why a stomachache is a vague, deep, hard-to-point-at ache rather than a sharp spot.
Visceral nociceptors respond to stretch as the primary stimulus, along with distension, contraction, traction, torsion and ischaemia, but not to cutting or thermal injury. Afferents travel in unmyelinated C-fibres entering the cord bilaterally at multiple levels, producing dull, poorly localised midline pain.
- C
- Unmyelinated fibres
slow, diffuse - 3
- Gut divisions
fore, mid, hind - Aδ
- Fibres that localise
once lining inflames
Source: StatPearls, Acute Abdomen, 2024 · Gray’s Anatomy, 42e
- 01A tube is stretched. Gas, obstruction or inflammation distends a hollow organ. Stretch — not cutting — is what the receptors in the wall are built to detect.
- 02The signal arrives everywhere at once. Slow C-fibres enter the spinal cord at several levels on both sides, so the brain gets no clear return address.
- 03The brain guesses the body wall. Gut and skin nerves converge on the same cord segments, so the ache is felt on the surface — foregut at the stomach, midgut at the navel, hindgut low down.
- 04Then it sharpens and moves. Once inflammation reaches the abdominal lining, fast Aδ fibres take over and pin it precisely — the reason appendicitis starts at the navel and ends in the right lower corner.
Condition 10 · Common cold rhinitis acuta
The symptoms are your defence, not the damage.
A cold virus barely harms you. Unlike flu, it does not wreck the lining of your nose — it slips into a few cells and copies itself. Almost everything you feel is your own immune system responding: the streaming, the blocked nose, the sore throat. You are not feeling the virus. You are feeling the reaction to it.
Rhinovirus binds ICAM-1 on nasal epithelium but, in contrast to influenza, produces no significant direct cytotoxicity. Symptoms derive from the innate immune response — bradykinin generation drives sore throat, rhinorrhoea and congestion, with interleukins and prostaglandins contributing systemic features.
- 2–5
- Colds per year
adult average - 160
- Rhinovirus subtypes
and counting - 1–4
- Days incubation
before symptoms
Source: StatPearls, Upper Respiratory Tract Infections, 2023 · NHS, Common cold
- 01The virus docks. Rhinovirus latches onto ICAM-1, a protein already sitting on your nasal cells, and gets inside. One to four days pass before you notice anything.
- 02Almost nothing is destroyed. Unlike influenza, rhinovirus causes no meaningful cell death. If damage caused symptoms, you would have none.
- 03The response arrives. Infected cells summon the innate immune system. Bradykinin produces the sore throat and the running nose; interleukins and prostaglandins produce the ache and fever.
- 04The block is blood, not snot. Veins in the turbinates dilate and engorge, narrowing the airway. That is why blowing your nose does not clear it — and why decongestants target vessels.
Section 11 · References
Every figure on this site traces back to a printed source.
Anatomical counts vary between authorities and across individuals. Where estimates differ, the figure quoted is the one most commonly reported in current reference texts, with the primary study named.
- 01Standring S. (ed.). Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 42nd ed. Elsevier, 2020.
- 02Hall J.E. Guyton & Hall Textbook of Medical Physiology, 14th ed. Elsevier, 2021.
- 03Azevedo F.A.C. et al. Equal numbers of neuronal and non-neuronal cells make the human brain an isometrically scaled-up primate brain. J. Comp. Neurol., 2009.
- 04Ochs M. et al. The number of alveoli in the human lung. Am. J. Respir. Crit. Care Med., 2004.
- 05Helander H.F. & Fändriks L. Surface area of the digestive tract — revisited. Scand. J. Gastroenterol., 2014.
- 06Sender R., Fuchs S., Milo R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol., 2016.
- 07Blanda M. Headache. In: Walker H.K. et al. (eds). Clinical Methods: The History, Physical, and Laboratory Examinations, 3rd ed. Butterworths, 1990. NCBI Bookshelf.
- 08World Health Organization. Headache disorders — fact sheet. WHO, 2024. who.int.
- 09Patterson J.W. et al. Acute Abdomen. StatPearls. StatPearls Publishing, 2024. NCBI Bookshelf.
- 10Thomas M., Bomar P.A. Upper Respiratory Tract Infection. StatPearls. StatPearls Publishing, 2023. NCBI Bookshelf.
- 11National Health Service. Headaches and Common cold — condition guidance. NHS, 2024. nhs.uk.