If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

The heart is a double pump

The heart is a double pump
Magnified view of blood cells in the human body (Photo courtesy of National Cancer Institute)

What cells need

To understand the critical importance of the heart requires taking a step back so we understand the needs of each cell in our body.  Remember that our body is composed of over 10 trillion cells that work together in remarkable unity (a lesson in good governance!).  Cells have basic needs, and at the top of the list would be these four things:
1) access to oxygen
2) a source of glucose
3) a balanced fluid environment with the right amount of water/electrolytes
4) removal of waste (such as carbon dioxide)
Consider how this compares to basic human needs: breathing air in and breathing out, eating food, drinking water, and getting rid of urine/stool.  When you really stop and think about it, many of the things that we do can be traced back to our cellular needs.

A breath of air

The lungs are composed of a few hundred million tiny air sacs called alveoli, each of which are surrounded by a network of blood vessels (capillary bed) which carry deoxygenated blood, shown in blue, and carry out oxygenated blood, shown in red. (Adapted from Wikipedia image)
Now let’s follow a single breath of air.  21% of the molecules in this breath are oxygen molecules, and as they race down into the lungs, they end up in the alveoli which are tiny air-filled sacs.  The story could end there, if not for the remarkable nature of lungs.  The lungs allow the oxygen molecules to continue their journey from the gas phase into a new liquid phase. Meanwhile carbon dioxide molecules make the opposite trip from liquid to gas similar to what happens at the surface of a carbonated beverage. The oxygen diffuses (think of the drop of ink in a pool of water) into the fluid interstitial space of the lung, and is then absorbed into the blood stream, and then enters into the red blood cells themselves. This diffusion occurs in a fraction of a second because the distance between the alveoli and the red blood cell is so tiny.
The white balls start out on the top and then move all over the matrix over time through random movements. If the white balls represent O2 molecules, then this illustrates how random movements allow O2 to move from an area of high concentration (alveoli) to an area of low concentration (blood)—a process called diffusion. (Adapted from Wikipedia from Runningamok19)

Why you need your heart

Now let’s pause and ponder the following:
What would happen if there was no heart? Well, diffusion of oxygen works wonders when the distances are very small, but what about large distances like the distance from your lungs to your feet?  Could a single molecule of oxygen simply diffuse all the way there?  In theory, it could—but it would take a really long time!  By the time the oxygen arrived in your toes by simple diffusion, they would have died and fallen off.
Once the oxygen has gotten into the blood stream, there has to be a way to rapidly “move” the oxygen molecules from one place to another. This is where hemoglobin, a protein that uses iron to help bind to O2 molecules, comes to the rescue.  Each red blood cell is filled with ~250 million hemoglobin proteins, and each hemoglobin protein can bind to 4 O2 molecules (the bound form is called “oxyhemoglobin”).  That means that each red blood cell can bind ~1 billion oxygen molecules!  As a result, the vast majority (>97%) of the O2 molecules are actually bound to oxyhemoglobin; with only a minority of O2 molecules floating freely in the blood.
Cross-section of the human heart, with the right ventricle labeled “pump 1” and the left ventricle labeled “pump 2”. (Adapted from Wikipedia image from Zoofari)
While air is going in and out of the lungs, the heart is busy working as well.  Blood enters the heart through the superior and inferior vena cava, which are the large veins that bring blood back from the top and bottom of the body respectively. Then, the blood remains in the right atrium, which can be thought of as a waiting room for the right ventricle. The right ventricle (pump #1) has muscular walls that squeeze down and softly push the blood into the arteries, arterioles, and capillaries of the lungs. Next, the oxygen diffuses from an area of high concentration (alveoli) to an area of low concentration (blood), before the blood returns (through pulmonary veins) to the left of the heart. Just like the right atrium, the left atrium can be thought of as a waiting room for the left ventricle.  The left ventricle is a room with even stronger, thicker, and more muscular walls than the right ventricle. As a result, the left ventricle (pump #2) forcefully pushes the blood through the arteries and capillaries of the body to get to the trillions of cells in need of oxygen. For the return trip, blood travels through the veins of the body to get back to the right side of the heart and repeat the process. So there you have it – one heart – two pumps: the right ventricle and the left ventricle.

Why are there two ventricles?

Now here’s a thought experiment: Why not just have just one ventricle (single pump) that moves blood to the lungs and then onwards to the rest of the body?
The heart functions as a double ventricle. Blood moves from the body into the right atrium, and then into the right ventricle where it gets pumped into the lungs. Blood gets oxygenated in the lungs, moves into the left atrium, and into the left ventricle where it gets pumped into the body again.
This diagram shows the heart as having a single atrium and single ventricle. Blood moves from the body into the atrium, and then into the ventricle where it gets pumped into the lungs, gets oxygenated, and then goes back to the body again.
It’s actually a great question, since at first glance it seems like it would be more efficient to just allow the blood to go out to the body instead of taking a return trip to the heart.  Think of it this way using numbers. Pressure is needed to move blood through the resistance of a large network of blood vessels like arteries, capillaries, and veins. Even if the right ventricle squeezes down and raises the pressure of the blood to about 25mmHg, after passing through the lungs, the blood pressure is back down to about 5mmHg (a reduction of 20mmHg).  It goes into the left ventricle where it gets a second squeeze causing the pressure to rise back up to about 120mmHg (almost 5 times the pulmonary pressure!).  That’s enough pressure to make it through all of the organs in the body.

Getting the pressure right

Now, let’s say that the right ventricle raised the pressure up to 140mmHg, then you may be able to have the blood pressure drop 20mmHg and still be at 120mmHg.  That sounds like a great solution, except for the fact: 1. If exposed to those high pressures, fluid would get pushed right out of the capillaries and into the lungs (some capillaries would actually break!), and 2.At high pressures, blood would move past the alveoli so quickly that O2 molecules wouldn't have time to diffuse into the blood and bind to hemoglobin. This makes sense when you remember that none of the capillaries in the body are exposed to extremely high pressures (120-140mmHg), because by the time blood gets down to the capillaries it has already passed through arteries (and arterioles), and the pressure has dramatically fallen.  Having lower pressures in the pulmonary circulation is particularly important given the large amount of O2 that needs to diffuse across from the alveoli to the capillaries—every extra millisecond helps!
That’s why the human body needs two pumps working at different pressures, high pressure to allow the blood to circulate around the body, and low pressure to allow for optimal gas exchange in the lungs without broken capillaries!

Want to join the conversation?

  • leafers seed style avatar for user sujitha radhakrishnan
    What is diastolic and systolic blood pressure??And please tell me how and why do we get high blood pressure and low blood pressure??
    (152 votes)
    Default Khan Academy avatar avatar for user
    • piceratops seed style avatar for user Ale Mellina
      Diastolic blood pressure measures the pressure in your blood vessels between heartbeats (when your heart is resting). Systolic pressure is the force of blood in the arteries as the heart beats. It is shown as the top number in a blood pressure reading(it should be the higher number between the two). High blood pressure is 140 and higher for systolic pressure. To answer your other questions, there are many reasons blood pressure fluctuates. High blood pressure, known as hypertension, can be caused by narrowing of the arteries, a greater than normal volume of blood, or the heart beating faster or more forcefully than it should-which all force the heart to pump harder then it should have to/ increased pressure against the artery walls. Low blood pressure, known as hypotension, disrupt the body's ability to control blood pressure. Common causes of this commonly include medications and can be a sign to a much more serious problem if detected with more underlying symptoms of illness. There are many types of hypotension and each come with their own causes and implications.
      (284 votes)
  • leaf green style avatar for user Reid Wilson
    How much blood is in the lungs getting oxygenated? Does blood go from the right ventricle PUMP, then into the lungs PUMP, then into the left atrium PUMP, then into the aorta? What I'm trying to ask is how long the blood is in the lungs before it gets pumped back into the heart for distribution?
    (16 votes)
    Default Khan Academy avatar avatar for user
  • hopper cool style avatar for user kyrapadam
    How long does it take for the heart to pump all the blood in your body?
    (17 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user johnnybres
      Firstly, it is important to try and think about the blood flow being constantly ongoing rather than it being pumped round at intervals. For example, if you take a 70kg man, he may have a total volume of blood of 5L. Some of this will be in the lungs being oxygenated; some will be in the aorta being pumped around the body; some in the brain or other tissues giving up its oxygen to the cells etc. This means that once one group of blood cells have released their oxygen to the tissues, the ones behind in the flow immediately take their place and so on. This is happening constantly, not at intervals, so thinking about certain tissues being oxygenated “more frequently” is not correct. Some organs, like the brain kidneys and heart need more oxygen than others, like the skin, but this is catered for by having more blood flowing to these organs. In some organs it will be faster and some slower, depending on how much that organ needs.

      Now that said, let’s work out how long it would take for a heart to pump that much blood through itself. Let’s take our 70kg man with an estimated 5L of blood. Now the amount the heart pumps depends on how much blood it pumps in one beat and also how often it beats. Both of these vary depending on whether the person is at rest or doing exercise, and also how well his heart works. If we take our man to have a resting heart rate of 70 beats per minute and take a stroke volume (amount the heart pumps on each beat) of 70ml (from Wikipedia), he would pump 4900ml (70 x 70=4900) in 1 minute. So he would pump almost the entire volume of his blood through his heart in 1 minute.

      However, if we now send our man to run up and down several flights of stairs, his heart rate may then increase to 120 beats per minute. The stroke volume is also likely to increase after the exercise, let’s say to 90ml. Then he would be pumping 10800ml (120 x 90) in one minute. That means that his heart pumps over 5 litres in 30 seconds.

      The values here are just estimates to illustrate that the amount the heart pumps can alter depending on what is going on to the body.
      (35 votes)
  • hopper cool style avatar for user Madeliv
    What can I compare a 5 mmHg and 120 mmHg pressure to? Is the pressure comparable to e.g. the pressure of a car on your foot? More/less?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • aqualine ultimate style avatar for user Brandon Wise
      The pressure of the atmosphere on a surface near sea level is about 15 PSI (pounds per square inch). There are about 52 mmHg in a single unit of PSI. So if you think about it this way 120 mmHg is equal to somewhere around 2.32 PSI. In my experience the average inflation pressure for a car tire is 22 PSI so that should give you a reference point. We are not tires.
      (32 votes)
  • blobby green style avatar for user Vrunda Monani
    for the blood test blood is taken out from arteries or vein?
    (7 votes)
    Default Khan Academy avatar avatar for user
    • mr pants teal style avatar for user Spoorthi😀
      There are a number of reasons why veins receive such preferential treatment by medical professionals around the globe. Firstly, veins are comparatively easier to draw blood from, physically speaking, as the placement of veins is such that they are close to the surface of skin. This makes the process easier by avoiding a deep needle plunge just to draw a bit of blood. On the contrary, arteries are located a bit deeper in the skin, so it doesn’t make much sense to make the process unnecessarily difficult – and potentially dangerous.

      The walls of veins are also thinner than arteries, which enables them to hold more blood (more volume). This quickens the process of blood collection and simultaneously results in more blood released into the sample tube. It’s also easier to pierce a vein than it is to pierce an artery, so drawing blood from a vein is less painful for the subject.

      The pressure in the veins is less than that of the arteries, so there’s a smaller chance of blood coming back through the spot where you were punctured by the needle before the tiny wound is healed.
      (13 votes)
  • piceratops ultimate style avatar for user v.tricia
    I'm going through these as a nursing student and I guess it only got interesting asking why have two ventricles. (don't reptiles have 3 chambers?) Not necessarily a problem with your program, more my instructor, but you might assume the student knows the proceeding material at least.
    (6 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user timo.honkonen89
      Fishes have simple blood circulation: one atrium, one ventricle that pumps blood to gills where the blood flows to tissues and back to the heart.

      Reptiles have imperfect double circulation: two atrium and one chamber. Blood flows from tissues to one atrium and to the only ventricle. ventricle pumps blood to the lungs and also to the systemic flow. From the lungs blood comes back to the heart (another atrium) and to the only ventricle where oxygenated blood mixes with unoxygenated.

      Birds and mammals have perfect double blood circulation. It's the most efficient because it generates the highest oxygen levels in the systemic flow. So tissues have more oxygen and therefor cells can keep up higher metabolic rate than for example reptile-cells.
      Birds and mammals are warm-blooded and our metabolic rate and capability to action is not so determined by the temperature of environment.
      (3 votes)
  • hopper happy style avatar for user Calin Tif
    On the last part where they debunk the suggestion that a system without the blood going back to the heart, instead going straight to the lungs, they say it won't work because the amount of pressure required to make the blood move that much would break the capillaries. What if we just had stronger capillaries? Isn't that a valid solution?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Satwik Pasani
      It is a valid solution, but not an easily attainable one. To strengthen the capillaries, the wall needs to be thickened at the cost of easy diffusion of materials across it. Or, the collagenous material impregnated with hard salts, loosing the highly flexible nature of capillaries.
      Nature, after millions of years of evolution, came up with the perfect amount of tradeoff for a good capillary, which needs to be accompanied by a system which saves it from rupture.
      (9 votes)
  • aqualine ultimate style avatar for user Isaiah Pannell
    Why does the heart need to be closer to the left side of our body??
    (4 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user hrconsultingmexico
    When you described a hemoglobin being a protein that uses iron to help bind to O2 molecules, what happens when the body lacks iron? Does something happen with the blood flow or does it interfere with binding the oxygen molecules to the hemoglobin?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • duskpin tree style avatar for user Brooke
      A lack of iron is called anaemia and means that the haemoglobin protein can't be formed so less oxygen can bind. General symptoms are weakness, fatigue, pale skin and dizziness. On a blood film, red blood cells are small (microcytic) and pale (hypochromic).
      (4 votes)
  • hopper happy style avatar for user Tanzim Hossain
    In the last Paragraph, it's said that blood pressure is low in capillary tubes. But as blood flows from the capillaries to the veins, what causes the blood pressure in veins?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Noorani Tejani
      blood flows in the venules and veins because of more blood coming from the arteries and arterioles and capillaries.In veins within muscles muscle contractions aid in the flow of blood to the heart.Veins have valves which prevent the backflow of blood.
      (3 votes)