How does the circulatory system work?

Your circulatory system is a delivery network. A pump, the heart, pushes about 5 litres of blood round your body every minute, bringing oxygen and food to every cell and carrying waste away. We are facing the person, so their right side is on your left.

Arteries carry blood away from the heart. The biggest, the aorta, is about as wide as your thumb. It arches over the heart, sends branches up to the head and arms, and runs down in front of the spine to the gut, kidneys and legs.

Veins bring blood back. They join into two huge veins: the superior vena cava from the head and arms and the inferior vena cava from everything below. One odd vein, the portal vein, takes blood from your gut to the liver first, so food is checked before it reaches the rest of you.

Between them, in every organ, is a mesh of capillaries, tubes so thin that red blood cells go through in single file. You have about 10 billion of them. Laid end to end, your capillaries alone would stretch thousands of kilometres. (The famous figure of 100,000 km comes from a rough 1922 guess and is too big: newer estimates are about 9,000 to 19,000 km.)

The thin green threads are the lymph vessels. They collect fluid that leaks out of capillaries and return it to the veins near your neck, passing through bean-shaped lymph nodes on the way. The heart itself has its own box: see HeartClear. Click it on the model.

Your blood goes round two loops, and passes through the heart once for each. That is why it is called a double circulation.

In the pulmonary loop, the right side of the heart pumps oxygen-poor blood through the pulmonary artery to the lungs. In the lung capillaries, next to the air sacs, it gives up carbon dioxide and fills up with oxygen in less than a second. It returns to the left side of the heart about 98% full. (How the lungs do it is in LungsClear.)

In the systemic loop, the left side pumps it through the aorta to an organ. In the organ's capillaries it hands over oxygen and picks up waste, then the veins carry it back. At rest, blood returning to the heart is still about 75% full: a reserve for when you need it. Blood from the brain comes back about 67% full, blood from the kidneys about 91%, because they take very different amounts.

You have about 5 litres of blood and at rest the heart pumps about 5 litres a minute, so an average lap takes about a minute. Most of that time is spent slowly drifting back through the veins, which hold about two thirds of your blood.

When you run, the heart can pump four or five times as much. Small arteries in the working muscles open wide while those to the gut and kidneys narrow, so muscles go from about a sixth of the flow to over three quarters.

Blood flows because of a difference in pressure, from high to low. In the aorta it swings between about 120 and 80 mmHg with each beat, a mean of about 93. By the far end of the capillaries it is about 15, and it arrives back at the heart with almost none left.

The biggest drop is in the arterioles, the smallest arteries. They have thick rings of smooth muscle, so they act as taps. Poiseuille's law says the resistance of a tube goes up with the fourth power of how much narrower it gets: squeeze an arteriole to half its width and it lets through only one sixteenth as much. By opening some taps and closing others, your body sends blood where it is needed.

Blood also changes speed. The aorta is one tube of about 2.5 cm², but the capillaries together are about 1,000 times wider, so blood slows from about 33 cm a second to about 0.3 mm a second, giving time to swap oxygen and food.

The walls match the job. Arteries have thick, stretchy walls to take the pounding. Capillaries are one cell thick so things can pass through. Veins have thin, floppy walls and hold most of your blood at low pressure.

Your body watches the pressure too. Stretch sensors called baroreceptors in the neck and aorta report to the brainstem (see BrainClear). If you stand up fast and pressure dips, within seconds your heart speeds up and arterioles tighten. Over hours and days the kidneys adjust it with a hormone called renin (see KidneyClear).

A doctor's cuff reads the top and bottom of the swing in your arm's artery: 120/80 is the classic normal. HeartClear lets you work a cuff yourself.

Everything the heart does is for this moment. In a capillary, only one cell thick and just wide enough for one red blood cell at a time, blood slows to about 0.3 mm a second and swaps things with the cells around it.

Oxygen and glucose are more crowded in the blood than in the cells, so they diffuse out. Carbon dioxide is more crowded in the cells, so it diffuses in. Nothing pumps them: they just spread from crowded to less crowded.

Water moves too, pushed and pulled by Starling forces. Blood pressure pushes fluid out, about 35 mmHg at the start of the capillary. Proteins in the plasma, mainly albumin, pull water back in by osmosis, about 28 mmHg (see OsmosisClear). At the start the push wins and fluid leaks out; by the end the pressure has dropped to about 15 and some is drawn back. What is left over, about 2–3 litres a day, is collected by lymph vessels.

If too much leaks, tissues swell: oedema. It happens when capillary pressure rises (as in heart failure) or when blood has too little protein (as in severe malnutrition or liver disease).

Then blood has to get home. In your legs that means climbing more than a metre with very little pressure left. Veins have one-way valves, and every time your calf muscles squeeze, they milk blood upwards: the muscle pump. Stand still for long and blood pools in your feet; walk, and ankle vein pressure falls from about 90 to about 20 mmHg.

Spin a tube of blood fast and it separates. About 55% is plasma, a straw-coloured liquid that is mostly water, carrying salts, sugar, hormones and proteins such as albumin. Just under half is red blood cells. The share of red cells is the haematocrit, about 40–45%: a bit higher in men, lower in women, higher again in people living high in the mountains.

A drop the size of a pinhead (one microlitre) holds about 5 million red cells, which carry oxygen on haemoglobin; about 5,000–10,000 white cells, which fight germs; and about 150,000–450,000 platelets, tiny cell pieces that help plug leaks. Red cells live about 120 days, so your bone marrow makes about 2–3 million every second. When oxygen is low, the kidneys send a hormone, EPO, to speed it up (see KidneyClear).

Cut a small vessel and three things happen. The vessel narrows. Platelets stick to the torn edge and to each other, making a soft plug. Then the clotting cascade runs: a chain of proteins in the plasma, each switching on many copies of the next, until thrombin turns soluble fibrinogen into sticky fibrin threads that knit the plug into a firm clot. Because each step multiplies the last, a tiny trigger quickly makes a big clot.

People with haemophilia lack one of the proteins, usually factor VIII, so their blood clots slowly.

Most problems with the circulation build up quietly over years, so regular check-ups matter. This chapter explains them; it cannot tell you what is going on in your own body. For that, see a doctor.

High blood pressure (hypertension) means a reading that stays at 140/90 mmHg or more. It usually causes no symptoms, yet it strains the heart, kidneys, brain and eyes. About 1 in 3 adults worldwide has it, and nearly half don't know. In India, the NFHS-5 survey found raised blood pressure in about 1 in 4 adults. A simple cuff check at a clinic is the only way to find out.

Atherosclerosis is fatty plaque building up inside artery walls, helped by high blood pressure, smoking, diabetes and high cholesterol. Because of Poiseuille's r⁴ rule a narrowing matters a lot, though small arteries downstream can open up and hide it until it is severe.

If a plaque cracks, a clot can form on it and block the artery. In a coronary artery that is a heart attack: chest pain or pressure that may spread to the arm, jaw or back, with sweating or breathlessness. In the brain it is a stroke: think FAST, face drooping, arm weakness, speech trouble, time to call. Either way, call an ambulance at once (in India 112 or 108): every minute counts.

Varicose veins are swollen, twisted veins, usually in the legs, where valves have stopped working. Anaemia means too little haemoglobin, so you feel tired and breathless; NFHS-5 found it in about 57% of Indian women and 67% of young children. Iron-rich food helps, and a doctor can test for it and find the cause.

You can help others too. A healthy adult can give blood: in India, aged 18–65 and at least 45 kg. One donation of 350–450 mL is less than a tenth of your blood; the plasma is replaced in about a day and the red cells in a few weeks.