Why every cell needs oxygen
Every cell in your body is at work all the time, building proteins, moving salts in and out, contracting or sending signals. That work runs on energy, and a cell gets it by breaking down food, above all the sugar glucose. The richest way to do it uses oxygen. In a chain of small steps inside the cell, glucose and oxygen become carbon dioxide and water, and the energy set free is caught in tiny packets the cell can spend. Biologists call this respiration.
In everyday speech respiration means breathing, but in biology it is the chemistry inside each cell. Breathing is how that chemistry is supplied: the lungs take in the oxygen respiration uses up and get rid of the carbon dioxide it leaves behind. The heart, the blood and the lungs all exist to keep that trade going in trillions of cells at once.
Why a body needs a pump
A single cell can get its oxygen without help. Molecules drift from where they are crowded to where they are sparse, a spreading called diffusion, and across the width of a cell oxygen arrives in a few hundredths of a second. But diffusion slows badly as the distance grows: twice as far takes four times as long. Over one centimetre, oxygen would need about seven hours. A body as thick as yours would suffocate at its core long before any arrived, because diffusion is far too slow over such distances.
So large animals move their supplies in a stream. Blood, driven by a pump through a closed loop of tubes, brings oxygen and food to within a hair's breadth of every cell, and diffusion only has to cover that last tiny gap. This endless pumped loop is the circulation.
A pump made of muscle
The pump is the heart, a hollow organ about the size of a fist, just left of the middle of the chest. Its walls are built of cardiac muscle, a kind found only in the heart. Like the muscle of an arm, it works by shortening: its fibres pull in, the chamber inside shrinks, and blood is squeezed out. Unlike arm muscle, it is built to keep going without tiring. It relaxes for only a fraction of a second between beats, and in that pause the chamber fills again.
At rest an adult heart beats about sixty to a hundred times a minute, around a hundred thousand times a day. During hard exercise the rate can more than double. Working muscles burn through oxygen faster, so the heart must speed up to bring them more oxygen every minute.
Two pumps side by side
The heart is really two pumps joined together. Its right side collects blood returning from the body, dark and low in oxygen, and sends it on a short trip to the lungs. There the blood takes up oxygen, turns bright red, and flows back to the heart's left side. The left side then pumps it out on the long trip around the rest of the body. The short loop through the lungs is the pulmonary circulation, from pulmo, Latin for lung. The long loop is the systemic circulation.
Why not one loop? Blood that has squeezed through a bed of fine vessels comes out with little push left, and the lungs' delicate walls could not stand a hard shove anyway. So the blood returns to the heart, and a second squeeze restores its pressure: the lungs get a gentle flow, and the body gets a forceful one.
That answers the question you started with: Why does your blood pass through your heart twice on every lap of your body?
The route Galen missed
For more than a thousand years, physicians followed the Greek doctor Galen, who taught that blood crossed from the heart's right side to its left through tiny, invisible pores in the thick wall between them. Around 1242, Ibn al-Nafīs, a physician trained in Damascus and working in Cairo, rejected this in his commentary on the anatomy in Ibn Sīnā's Canon of Medicine. The wall, he wrote, is thick and has no pores, visible or invisible. Blood from the right side must travel to the lungs, spread through them and mingle with air, then return to the left side.
It is the earliest known correct description of the pulmonary circulation. Europe reached the same route only in the sixteenth century, with Michael Servetus in 1553 and Realdo Colombo in 1559. Ibn al-Nafīs's commentary was rediscovered in a Berlin library in 1924.
Out at high pressure, back at low
Blood leaves the heart in arteries, thick-walled tubes with layers of muscle and elastic tissue. Each beat slams a surge of blood into them, and their walls stretch and spring back to take it; that surge is the pulse you feel at your wrist. The push of blood against the artery walls is your blood pressure, measured with an inflatable cuff around the upper arm. A reading of around 120 over 80 is typical for a healthy adult.
By the time blood has passed through the body's finest vessels, almost all that pressure is spent. It returns to the heart in veins, which have thinner walls and wider channels. With so little push behind the blood, many veins, especially in the legs, have one-way valves that stop it sliding backwards, while the squeeze of the muscles around them helps it along.
Where the real exchange happens
Between the arteries and the veins lies a mesh of the smallest vessels of all, the capillaries. A capillary is so narrow that red blood cells pass through it in single file, and its wall is just one cell thick. No cell in your body is far from one.
This is where the circulation does its real job; everything before it is delivery. Here the wall is thin enough for substances to cross by diffusion, each drifting from where it is plentiful to where it is scarce. Oxygen and glucose, abundant in the arriving blood, pass out to the cells around. Carbon dioxide and other wastes, abundant in the cells, pass in, to be carried away through the veins. The arteries and veins are only the roads. The capillaries are the doorsteps where the goods change hands.
What blood is made of
Spin a tube of blood fast in a machine and it separates. The cells pack down at the bottom as a dark red layer, and above them sits a clear, straw-coloured liquid called plasma. Plasma is a little over half of blood by volume, about fifty-five parts in a hundred, and it is mostly water. Red blood cells make up nearly all the rest, and the white blood cells and platelets form a thin pale band between the two.
Plasma is the carrier. Dissolved in it travel salts, proteins, hormones and the fuel cells burn, above all the sugar glucose, taken up from the gut and delivered to every cell. It carries waste away too: most of the carbon dioxide the cells give off travels back to the lungs in the plasma, changed into a salt called bicarbonate.
Red cells and their cargo
Oxygen dissolves poorly in water, so plasma alone could carry only a trickle of it. The bulk rides on red blood cells, or red cells for short, the most numerous cells in the body. Each is a flexible disc about eight thousandths of a millimetre across, with no nucleus, which leaves room for its cargo: some 270 million molecules of haemoglobin, an iron-containing protein that gives blood its colour. Each haemoglobin molecule can hold four molecules of oxygen.
Haemoglobin grips oxygen where it is plentiful, in the lungs, and lets go where it is scarce, in working tissue. Carbon monoxide, a gas made when fuel burns without enough air, as in a faulty heater, is deadly because haemoglobin grips it about two hundred times more tightly than oxygen. Once it is attached, those molecules stop carrying oxygen, and the body suffocates with air in its lungs.
Made in the bone
Blood cells do not last. A red blood cell wears out after about four months, so the body must replace them at a startling rate, more than two million every second. The factory is bone marrow, the soft tissue filling the hollow centres of bones such as the hips, ribs and breastbone, and it makes every kind of blood cell.
Besides red cells, it turns out white blood cells, far fewer in number, which are the body's defenders: they hunt down bacteria and viruses, swallow invaders and make antibodies against them. And it makes platelets, tiny fragments pinched off from giant cells in the marrow. When a vessel is cut, platelets stick to the torn edge and to each other, forming a plug, and they set off the clotting that seals the wound.
How air gets in
Air enters through the nose or mouth and travels down the windpipe, the trachea, a tube held open by C-shaped rings of cartilage that you can feel at the front of your neck. In the chest it splits into two bronchi, one for each lung, and these divide again and again into ever finer tubes, like the branches of an upside-down tree.
The lungs cannot pull air in by themselves. The work is done mostly by the diaphragm, a dome-shaped sheet of muscle beneath them. When it contracts it flattens and moves down, while muscles between the ribs lift the ribcage. The chest grows larger, the pressure inside the lungs drops below the pressure of the air outside, and air rushes in. At rest, breathing out is mostly the diaphragm relaxing and the stretched lungs springing back.
Where air meets blood
The finest airways end in clusters of tiny air sacs, the alveoli, around 480 million of them in a pair of adult lungs. Each is wrapped in a net of capillaries, and the barrier between the air in a sac and the blood beside it is less than a thousandth of a millimetre thick. Spread flat, all those sac walls together would cover seventy square metres or more, a surface folded into the space of your chest.
Oxygen crosses that barrier by diffusion, from the air, where it is plentiful, into the blood, where it has been used up. Carbon dioxide diffuses the other way, out of the blood and into the air, to be breathed out. So each breath out carries about a hundred times more carbon dioxide than the air you took in, while still keeping most of its oxygen.
When oxygen runs short
In a flat-out sprint, your leg muscles use energy faster than the heart and lungs can bring them oxygen. They do not simply stop. They switch to anaerobic respiration, which means respiration without air: glucose is broken down only part of the way, into lactic acid, with no oxygen needed.
The catch is the yield. Respiration with oxygen gets about thirty packets of usable energy from each glucose molecule; the anaerobic route gets two. With far less energy per glucose, the muscle must burn through its sugar many times faster for the same work, and the acid it makes builds up. That is why an all-out effort lasts seconds or a minute, not an hour. Afterwards you keep breathing hard while the body restores its energy stores and deals with the lactic acid, using the oxygen it went without.