Why Is the Heart Called a Double Pump?

The heart is called a double pump because it is really two pumps built side by side into one organ. The right side sends oxygen-poor blood a short way to the lungs, and the left side sends the oxygen-rich blood that comes back from the lungs out to the whole body. Each pump pushes blood around its own loop.¹ That means every drop of blood passes through the heart twice on one full trip, once to pick up oxygen and once to deliver it. Humans share this fully split heart with birds and other mammals, while a fish gets by with a single pump.²

Two Pumps Sharing One Heart

Each side of the heart is a complete two-chamber pump. A thin-walled upper chamber, called an atrium (plural atria), sits on top of a lower pumping chamber, called a ventricle.³ The atrium is the weaker pump that fills the ventricle, and the ventricle supplies the main force.⁴ A solid wall of muscle runs down the middle and keeps the right side’s blood apart from the left side’s.⁵ That wall is called the septum, and it is what makes the heart two pumps instead of one big chamber. Because it is solid, blood returning from the body never mixes with blood returning from the lungs inside a healthy heart.

The two sides have different jobs. The right ventricle drives the lung loop.⁶ The left ventricle drives the body loop, which feeds every tissue cell.⁷ So the right pump handles blood low in oxygen and the left pump handles blood high in oxygen.⁸ Blood travels in a figure eight, because each pump feeds the other’s loop. One quirk: “right” and “left” mean the person’s own sides, so on a textbook diagram the right side appears on the left of the page. Both sides push out about the same volume of blood with each beat; if one moved more, blood would build up in one loop. What differs is how hard they push.

The Path Blood Takes Through Both Pumps

Oxygen-poor blood from the body returns through two large veins, the venae cavae, into the right upper chamber.⁹
It passes through the tricuspid valve into the right lower pumping chamber.¹⁰
It is pushed through the pulmonary valve into the pulmonary arteries, which lead to the lungs.¹¹
Blood that has picked up oxygen returns through the pulmonary veins to the left upper chamber.¹²
It passes through the mitral valve into the left lower pumping chamber.¹³
It is pushed through the aortic valve into the aorta, the body’s main artery, which carries it out to the body.¹⁴

After the blood has delivered oxygen and nutrients to the cells, it returns to the right upper chamber and the cycle begins again.¹⁵

The valves are what keep this orderly. Each is a one-way door that opens forward and snaps shut behind the blood. There are four, one at the exit of each chamber, and they stop blood from sliding backward. Their closing makes the “lub-dub” a doctor hears through a stethoscope.

Naming can trip people up. Arteries carry blood away from the heart and veins carry it toward the heart, whatever the oxygen level. So the pulmonary artery carries oxygen-poor blood, unlike the body’s other arteries, and the pulmonary veins carry oxygen-rich blood. Textbooks colour these vessels blue and red by oxygen level, not by whether they are arteries or veins.

Why the Left Pump Is Stronger

The two loops run at very different pressures. Average pressure in the body’s arteries is about 93 mmHg (millimetres of mercury), the unit on your blood pressure reading, versus about 5 to 15 in the lung arteries.¹⁶ The body loop has to push blood much farther, through far more vessels, than the short lung loop.¹⁷ Lung vessels are also thin and stretchy, so they take in blood easily at low pressure.¹⁸ One shared pressure wouldn’t work: body-level pressure would push fluid out of the fragile lung vessels, while lung-level pressure could not reach the toes or the brain.

The walls are built to match. The left ventricle wall is about 8 to 12 mm thick, while the right is about 3 to 5 mm, so the left wall is roughly three times thicker.¹⁹ The thicker wall spreads the higher pressure, so each muscle fibre carries a similar workload.²⁰ The thin right wall, though, copes poorly with a sudden rise in pressure, which makes it the more easily strained side.²¹ Both ventricles contract on the same beat; the thicker left wall just gives each beat a stronger squeeze.

Your blood pressure reading reflects the left pump. It is taken from an artery in your arm, so it shows the body loop’s high pressure, not the lung loop’s. Its two numbers capture the pressure when the left ventricle squeezes and when it rests. Lung pressure cannot be measured with an arm cuff, which is why doctors use other tests, such as ultrasound or a catheter, when they need it. An artificial heart must keep the flow in the two loops balanced, because a mismatch can cause life-threatening lung damage.²²

How the Two Sides Beat in Time

Each beat begins at the sinus node, a small patch of tissue in the right upper chamber.²³ It fires about 60 to 100 times a minute at rest.²⁴ The signal spreads through both upper chambers and makes them squeeze blood down into the lower ones.²⁵

Then the signal is held back briefly at a relay point between the upper and lower chambers.²⁶ It slows the signal for a fraction of a second. That gives the lower chambers time to finish filling.²⁷

After the pause, the signal splits down right and left branches to reach each ventricle.²⁸ The two ventricles contract together, sending blood to the lungs and the body at the same moment.²⁹

Sometimes the timing slips. If one of the branches is blocked, one ventricle can lag behind the other, and an ECG can show this as a widened wave. Some people with heart failure get a special pacemaker that brings both sides back into time.³⁰

Why Not Just One Pump

A fish heart has one atrium and one ventricle, which push blood through the gills and then on around the body.³¹ Blood loses much of its pressure in the gills before it reaches the body.³² Cold-blooded fish burn fuel slowly, so a lower-pressure supply meets their needs.

Reptiles sit in between. Most lizards and turtles have a partly divided ventricle that serves both loops, so some oxygen-rich and oxygen-poor blood can mix inside it.³³ They can send blood away from the lungs when needed, for example during a dive.³⁴ Crocodiles are an exception and have a fully divided ventricle, though they keep a way to shunt blood when diving. Only a complete split allows the two-pressure system that warm-blooded animals need.³⁵ That suits a warm-blooded body, which burns a lot of fuel every minute and needs a constant oxygen supply.

In birds and mammals, the fully divided heart keeps oxygen-poor and oxygen-rich blood completely apart.³⁶

The split forms before birth. An embryo’s heart starts as a single tube, and walls grow inside it, dividing it into two channels with one-way valves.³⁷ That is why some heart defects are holes in those walls.

The Double Pump Before Birth

A fetus gets its oxygen from the placenta rather than from its lungs.³⁸ So about 80 to 90% of the blood the right ventricle pumps skips the lungs through a short connecting vessel.³⁹ A flap-like opening between the upper chambers also lets blood cross from right to left.⁴⁰

The lungs can be bypassed safely because before birth they are filled with fluid, not air, and the vessels inside are tightly constricted, so blood meets high resistance there. Blood takes the easier route through the connecting vessel into the body’s main artery.

At the first breath, the lungs inflate and their resistance drops, and blood begins flowing through them. The connecting vessel and the opening between the upper chambers close within days to weeks. Then the two pumps work fully in series.

Sometimes a shortcut stays open and doctors hear a murmur. Many small openings close on their own, and larger ones may need a procedure.

When One Side Struggles or the Sides Mix

Failure tends to spread from left to right. When the left side weakens, pressure backs up into the lungs.⁴¹ Over time the right ventricle becomes overloaded and can fail as well.⁴²

When the left side struggles, the signs come from fluid backing up into the lungs: breathlessness, especially when lying flat, and fatigue during activity. When the right side struggles, blood backs up before it reaches that pump, causing swollen ankles and legs, fluid build-up in the abdomen, and bulging neck veins. If these symptoms are new or getting worse, see a doctor.

Holes between the sides cause a different problem. A hole in the wall between the ventricles lets blood leak from the high-pressure left side into the right, so the lungs receive too much blood.⁴³ A small hole may cause little more than a murmur, while a large one floods the lungs. With a large hole, the extra flow can make the lung vessels stiffen over the years, raising lung pressure. Doctors decide how to treat it by looking at its size and at how the heart and lungs are coping.

In some birth defects the main arteries are swapped.⁴⁴ The body loop and lung loop become two separate circles instead of one figure eight. Oxygen-poor blood just circulates around the body and oxygen-rich blood just circulates around the lungs, so neither gets what it needs. Survival depends on openings between the sides that let a little blood cross, which keeps the baby alive until surgery.

Children born with only one working ventricle can have surgery that routes blood from the body’s veins straight to the lungs without a pump.⁴⁵ But without a pump on the lung side, the heart pumps less blood and pressure builds in the veins, which causes complications over time.⁴⁶

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