Where does your blood go? Two loops, three vessels and the heart’s per-minute maths

How to use this simulation

1. Press play and follow the red and blue cells: right ventricle to lungs, lungs to left atrium and left ventricle, then round the body and back to the right atrium. Blue cells turn red in the lung capillaries, and red cells turn blue in the body capillaries.
2. Use “Highlight a loop” to pick pulmonary or systemic. The other loop fades, so you can follow one circuit on its own.
3. Raise the heart rate. The cells travel faster and cardiac output climbs in the readings. Raise the stroke volume and more cells go out with each beat, so the stream gets denser.
4. Try At rest, Exercise and Trained athlete in turn and compare the three cardiac outputs. The athlete’s rate is much lower, yet the output is about the same.
5. Keep the artery, vein and capillary switch on: arteries get thick walls, veins show valves, and the cells slow down in the capillaries, where exchange happens.

Where does blood actually go?

Start with a simple question. Is blood made, used up and replaced, or is it the same blood going round and round? For more than a thousand years doctors believed the liver kept making blood and the tissues soaked it up, with the blood sloshing in and out like a tide.

In 1628 the English physician William Harvey made a clever argument. Multiply what the heart pushes out in one beat by the beats in an hour and you get far more blood than the body holds. With our simulation’s numbers the heart pumps about 302.4 litres an hour, yet the body contains only about 5 litres. So the same blood must pass through the heart roughly 60 times every hour.

No body could manufacture that much new blood every hour, so the blood has to be going round. Harvey concluded that it travels in a closed circuit with the heart as the pump. He had no microscope, so he never saw the tiny vessels joining arteries to veins, the capillaries. Marcello Malpighi found them a few decades later.

The simulation on this page lets you watch Harvey’s circuit run in front of you.

What circulation is, from scratch

Every cell in your body needs oxygen and food, and every cell produces waste that has to be taken away. Think of a big city: groceries must reach every home, and rubbish must be carried out. The body’s delivery network is the circulatory system. It has three parts: the blood (what is carried), the blood vessels (the roads) and the heart (the pump).

There are three kinds of vessel. A vessel that carries blood away from the heart is an artery. A vessel that brings blood back to the heart is a vein. Between them lies a mesh of very fine tubes called capillaries, and this is where the real exchange with the cells takes place.

Here is a rule that saves marks: a vessel is named by the direction of the blood, not by how much oxygen it holds. Leaves the heart, it is an artery. Enters the heart, it is a vein. Once you hold on to that single rule, many exam traps disappear.

A human has one heart, but blood passes through it twice in every full lap: once on the way round the lungs and once on the way round the rest of the body. That is exactly why we call it double circulation.

Glossary: refresh these before the exam

These terms appear again and again in the diagram and in the text. A quick read now makes the rest easier.

• Circulation: blood moving round the body in closed vessels, pushed by the heart.
• Double circulation: blood passing through the heart twice in one full lap, once to the lungs and once to the body.
• Pulmonary circulation: the loop from the right ventricle through the lungs to the left atrium.
• Systemic circulation: the loop from the left ventricle through the body tissues to the right atrium.
• Artery: a vessel carrying blood away from the heart; thick and elastic walls.
• Vein: a vessel carrying blood back to the heart; thin walls, with valves inside.
• Capillary: a microscopic vessel with a wall one cell thick, where gases and nutrients are exchanged.
• Aorta: the largest artery, leaving the left ventricle.
• Vena cava: the large veins (superior and inferior) bringing body blood into the right atrium.
• Stroke volume: the volume pumped out by one ventricle in one beat.
• Cardiac output: the volume pumped by one ventricle each minute, which is heart rate × stroke volume.

The heart’s four chambers, briefly

The heart has four chambers: two atria on top (the receiving rooms) and two ventricles below (the pumping rooms). A wall called the septum keeps the right and left sides apart. In the simulation the right-hand chambers hold blue blood and the left-hand ones red, because the septum stops the two kinds from mixing.

The right atrium takes in oxygen-poor blood returning from the body, and the right ventricle sends it to the lungs. The left atrium takes in oxygen-rich blood from the lungs, and the left ventricle drives it round the whole body through the aorta. The left ventricle has the thickest wall because it has to push blood the farthest.

How the valves open and shut, and the stages of one heartbeat, are shown in detail in the separate human heart animation. This page has a different focus: the whole route of the blood outside the heart, the types of vessel, and how much blood moves each minute.

The two loops, step by step

The easiest way to understand circulation is to ride along with one blood cell. Do exactly that in the simulation.

Artery, vein and capillary compared

The three vessel types are built to suit their jobs. An artery takes the full thump of the heart, so it is thick and elastic. A vein is under low pressure, so it is thin, but it has valves to stop the blood slipping backwards. A capillary exists for exchange, so its wall is as thin as a wall can be.

Single versus double circulation

Not every animal circulates like us. A fish’s heart has two chambers, one atrium and one ventricle. The ventricle pushes blood to the gills, from the gills the blood goes straight on to the body, and then returns to the heart. Blood visits the heart once in a lap, so this is single circulation.

The weakness of single circulation is that the blood loses most of its pressure squeezing through the fine gill vessels, so it reaches the body slowly. Birds and mammals are warm-blooded and burn a lot of energy, so they need oxygen delivered fast. After the lungs, their blood goes back to the heart for a fresh push. That is the big advantage of double circulation.

Cardiac output: the heart’s per-minute maths

Stroke volume is the blood one ventricle pushes out in a single beat. Heart rate is the number of beats a minute. Multiply them and you get how much blood leaves the heart each minute, called cardiac output.

Try it yourself in the simulation

Five quick experiments, a minute each. Predict first, then move the sliders.

• Experiment 1 (raise the rate): keep the stroke volume at 70 mL and move the rate from 60 up to 150. How many times larger is the output? Double the rate gives double the output, because the formula is a plain product.
• Experiment 2 (raise the stroke volume): hold the rate at 72 and raise the stroke volume from 70 to 110 mL. The number of cells goes up while their speed does not. Output rises, but each cell still travels at the same pace.
• Experiment 3 (three states): choose At rest, Exercise and Trained athlete in turn. Exercise multiplies the output by about 3.3. The athlete’s rate is only 50, yet the output is about 5.0 L/min.
• Experiment 4 (follow one loop): choose pulmonary and follow a single cell. Where does it change from blue to red? Then choose systemic and watch the same cell. Where does it change from red to blue?
• Experiment 5 (speed versus vessel): with the artery, vein and capillary switch on, notice where the cells slow down. Now switch it off: the speed becomes the same everywhere, which does not happen in a real body.

Solved problems

In every problem the numbers come from the simulation’s own values. Try each one yourself before you read the working.

Mistakes everyone makes

These slips cost the most marks. Each one comes with the correct version.

• Wrong: an artery always carries oxygen-rich blood. Right: an artery carries blood away from the heart. The pulmonary artery carries oxygen-poor blood!
• Wrong: a vein always carries oxygen-poor blood. Right: the pulmonary veins bring oxygen-rich blood to the left atrium.
• Wrong: double circulation means two hearts. Right: there is one heart, but blood passes through it twice in each lap.
• Wrong: gas exchange for the body happens in the lungs only. Right: exchange at the lungs happens in the alveolar capillaries, and tissue exchange happens separately in the systemic capillaries.
• Wrong: blood in veins is blue. Right: oxygen-poor blood is a dark red. Diagrams use blue only to tell the two kinds apart.
• Wrong: capillary walls contain muscle. Right: the wall is a single layer of flat cells, with no muscle and no valves.
• Wrong: a faster heart rate always raises cardiac output. Right: at very high rates the ventricles do not have time to fill, the stroke volume falls, and the output stops climbing.

Links to real life

Taking blood pressure means measuring the pressure in an artery. Two numbers are written, for example 120/80 mm Hg (a textbook approximation): the first is the pressure while the ventricles contract, the second while they relax. Pressure in the pulmonary artery is much lower, about 25/8 mm Hg, because the lungs are close to the heart and their vessels are soft. That is why the right ventricle’s wall is thinner than the left’s.

The pulse at your wrist is an artery swelling and relaxing. Arteries are elastic, so each beat stretches them a little. You cannot feel a pulse in a vein, because the jolt of each beat has faded by then. Blood climbs the leg veins thanks to squeezing by the muscles and the help of the valves; standing still for hours can make the feet swell.

During exercise the muscles need more oxygen, so both heart rate and stroke volume rise. With regular exercise the heart muscle gets stronger, the stroke volume grows and the resting rate falls. That is why doctors encourage walking and cycling.

When fatty deposits narrow an artery, the flow drops. If that happens in a coronary artery, the heart muscle gets too little oxygen, and the extreme result is a heart attack. High blood pressure, smoking and fatty diets raise the risk. This is general knowledge, not medical advice; see a doctor if you feel unwell.

A red blood cell lives about 120 days, so a single cell makes several hundred thousand trips in its life. On every trip it picks up oxygen in the lungs and drops it off in the tissues.

Exam corner (CBSE and general boards)

These questions come up again and again. Draw a labelled diagram wherever you can, because it earns marks.

• What is double circulation? Write: the circulation in which blood passes through the heart twice in one complete circuit.
• Write five differences between an artery and a vein: wall, lumen, valves, direction of flow and blood pressure.
• Differentiate between pulmonary and systemic circulation: start and end points, vessels, type of blood, function.
• Describe the path of blood with a diagram, starting at the right atrium and returning to it.
• State the function of capillaries and say why their walls are so thin.
• Calculate the cardiac output for a rate of 80 beats/min and a stroke volume of 65 mL. (Answer: 5.2 L/min)
• Why is the wall of the left ventricle thicker than that of the right ventricle?

Revision summary in a minute

Read this list once the night before the exam and you are done.

• Blood moves round in closed vessels, pushed by the heart; this is circulation.
• Human circulation is double: one heart, two loops, four chambers.
• Pulmonary loop: right ventricle, pulmonary artery, lungs, pulmonary veins, left atrium.
• Systemic loop: left ventricle, aorta, body tissues, vena cava, right atrium.
• Arteries take blood away, veins bring it back, and exchange happens in capillaries.
• The pulmonary artery carries oxygen-poor blood, and the pulmonary veins carry oxygen-rich blood.
• Cardiac output = heart rate × stroke volume, about 5 L/min at rest.

Frequently asked questions