How the Heart Moves Blood — Four Chambers, Four Doors, Two Loops

The Door That Doesn't Stop

Even while you read this sentence, thin doors are opening and closing in the middle of your chest. It happens more than 100,000 times a day, reaching 3 billion times over a person's lifetime (PMID 33339213, PMID 17588873). Opening and closing the same door 3 billion times without a leak is a condition hard to meet for any machine built by humans.

What these doors do is send blood in only one direction. The reason blood circulates is oxygen. Oxygen that enters through the breath cannot move on its own, and is carried by blood all the way to the muscles. Therefore, to keep using force, the blood must ferry that oxygen as much as is needed (PMID 40121540).

This article follows three questions in turn. What does the heart look like? Why does blood circulate in only one direction? What decides how much to send? Unfamiliar terms will be explained right where they appear, so there are no terms you need to know in advance.

There Are Four Chambers and Two Paths

The heart is shaped like four muscular chambers stacked top and bottom. The atria are the two upper chambers, which first catch and hold the blood that returns. The ventricles are the two lower chambers, which squeeze the held blood out of the heart. The top receives, and the bottom squeezes.

The tubes the blood travels through also split into two paths. Arteries are the tubes through which blood leaves the heart. Veins are the tubes through which blood returns to the heart. These names divide paths only by the direction the blood goes, and have nothing to do with whether the blood is rich in oxygen. Thus, the artery going out to the lungs carries oxygen-depleted blood (PMID 37816344), and the vein returning from the heart to the lungs carries oxygen-loaded blood (PMID 33809829).

The reason there are four chambers is that a single heart runs two tracks at the same time. The two right chambers receive blood returned from the body and send it to the lungs. The two left chambers receive blood from the lungs and send it to the body. Blood moves along this path in only one direction. It leaves the heart, passes through the lungs, goes to the rest of the body, and returns (PMID 32620577).

The left and right differ even in wall thickness. The right ventricle has a thin wall. Even if the blood returning from the usual range increases, the right ventricle accepts that blood without driving the pressure in the right atrium up to a harmful level. This is because the path toward the lungs where the right side sends blood has low pressure, offers small resistance to pushing the blood out, and stretches well (PMID 29945812). Therefore, it accomplishes its work with a thin wall.

The chambers are four and the paths are two. The upper chambers receive, and the lower chambers squeeze.

Why Blood Only Flows in One Direction

To write the answer right away, it is because there are doors that open in only one direction inside the heart and veins. It took a long time for people to figure this out.

For nearly 1,500 years before Harvey revealed that blood circulates, medicine in Europe and the Middle East largely followed the teachings of Galen. People viewed that blood was made in the liver, flowed to various parts of the body, and was used up there (PMID 27791994). In that view too, blood flowed in one direction. It was just a one-way trip with no return. That meant the liver kept producing fresh blood as much as the body used.

The clue came from a single anatomical drawing. Fabricius, an anatomist in Padua, was the first to draw and leave behind the small doors inside the veins. That drawing served as a foundation for his student Harvey (PMID 32499171). If the doors open in only one direction, blood can only go that way. If blood keeps going in only one direction yet does not run out, only one answer remains. The blood returns to where it started.

That is what the book Harvey published in 1628 says. Blood makes a full circuit of the body and returns to the heart (PMID 27791994). Ibn al-Nafis of Damascus had written earlier about the short path passing through the lungs. Between Galen and Harvey, figures such as Razi, Ahwazi, Vesalius, and Servetus also left behind their own views (PMID 18426332, PMID 27791994).

Today we know the names of those small doors. Valves are thin membranes inside the heart and blood vessels that open in only one direction to prevent blood from flowing backward. There are four of these doors inside the heart. Two sit at the junction where blood passes from the atria to the ventricles. Slender tendons are attached to these two valves, holding them so they do not flip backward (PMID 32178262). The other two sit at the exits from the ventricles into the arteries. One is the door going out to the lungs, and one is the door going out to the body, and the two look similar to each other (PMID 21459245).

The doors determine the direction blood goes. And the arteries and veins are connected in a single line to form one loop. Therefore, in a steady state where what the body does continues without change, the amount that leaves the heart equals the amount that returns (PMID 35610620). This is because the blood that went out eventually returns along that loop.

The reason blood circulates in only one direction is that the doors open in only one direction, and for Harvey, the drawing of the vein doors made by his teacher was one of the clues.

The Two Paths

The path where blood circulates consists of two loops. The pulmonary circulation is the short path that goes from the heart to the lungs and returns to the heart. This path takes oxygen-depleted blood to the finest vessels of the lungs, picks up oxygen, and returns. The places where gases like oxygen move in and out of the blood are the walls of those thin vessels (PMID 37816344). The systemic circulation is the long path that goes from the heart, through the entire body, and returns to the heart. This path drops off the oxygen it carried to every corner of the body.

Both paths meet the same kind of vessel at their ends. Capillaries are the finest vessels connecting arteries and veins, and they are the sites where oxygen and substances actually exchange between the blood and tissues. If an artery is a highway, capillaries are the alleys in front of houses, and cargo is unloaded only in the alleys.

Now, let us follow a single drop of blood for one complete circuit. Blood that has dropped off its oxygen in the leg muscles travels up the veins and reaches the right atrium. It passes through the door between the right atrium and the ventricle and descends into the right ventricle. When the right ventricle squeezes, the blood pushes past the outlet door and into the artery heading to the lungs. The blood inside this artery is still depleted of oxygen.

Upon reaching the capillaries of the lungs, the blood loads oxygen across the wall between them. Blood loaded with oxygen enters the left atrium via the veins coming out of the lungs (PMID 33809829). Up to this point, the first section going from the right heart to the lungs is the pulmonary circulation.

The blood passes through the door once more and descends into the left ventricle. When the left ventricle squeezes, it heads out toward the entire body through the exit door leading to the body. It branches from thick arteries into progressively finer vessels, reaches the alleys of the leg muscles, and drops off oxygen. Then it travels back up the veins to the right atrium. This back section, which has returned from the entire body through the left heart, is the systemic circulation.

During this single circuit, the blood passes through all four chambers and all four doors, and rides the loops twice. The heart runs these two paths simultaneously. The exact moment the right side sends blood to the lungs, the left side sends blood to the body.

The short path and the long path have different properties. The path toward the lungs accepts all the blood the heart pumps out, even though the pressure is low and resistance is small (PMID 37816344, PMID 27065170). The artery going to the lungs has thinner walls than the body-side arteries, and far less muscle in its wall (PMID 27065170). This matches the thin wall of the right ventricle we saw earlier.

On the body-side path, thick arteries smooth out the flow. Thick arteries bulge while the heart squeezes and snap back while it rests. Thanks to this recoil, a choppy, interrupted flow becomes steady by the time it reaches the alleys (PMID 28954852). Arteries becoming stiff and losing this property is intertwined with various cardiovascular diseases and is a major cause of illness and death worldwide (PMID 28954852).

The paths are two. One makes a short trip to the lungs, and one makes a long trip to the body, and cargo is unloaded only at the alleys at the end of both paths.

Within a Single Beat

A single beat is divided into squeezing time and resting time. Systole is when the ventricles squeeze and blood is pushed out of the heart. Diastole is when the ventricles relax and refill with the blood they will send out next. The ventricles can only fill during diastole. That is why neither phase can be skipped.

The squeezing force comes from inside the muscle cells. Heart muscle cells are brick-shaped cells with cross-striations. Inside them, proteins responsible for contraction are lined up regularly (PMID 38884723). What turns these proteins on and off is calcium. Calcium inside the cell must be high enough when squeezing and low enough when resting for the heart to work properly (PMID 28684623).

The four chambers beat in rhythm thanks to electricity. Heart muscle cells are connected to one another and move like a single mass. An electrical signal that starts in one cell continues and spreads to neighboring cells (PMID 30604919).

There is also a value that measures the squeezing side. Ejection fraction is the percentage of blood in the filled ventricle that is pushed out in a single contraction. If half of the blood in a full room went out, it is 50%. This value only tells you about the squeezing side. However, in people whose hearts can no longer send out their share of blood, how severe the disease is and what the future holds are more closely tied to how misaligned the filling side is than to the ejection fraction (PMID 18313620). This means the resting side is just as important as the squeezing side.

A single beat is squeezing and filling, filling only happens when resting, and the resting side is just as important as the squeezing side.

How Much Gets Sent Is Determined

Sitting quietly and climbing stairs require very different amounts of oxygen from the body. Yet the blood still arrives as needed. There is a value that measures that amount. Cardiac output is the amount of blood the heart pumps out into the body in one minute. It is the value obtained by multiplying the amount that goes out in one beat by the number of times the heart beats in one minute. The amount that goes out in a single beat can be measured with an echocardiogram (PMID 35085589).

In determining this value, the path back is just as important as the path out. Venous return is the flow of blood returning from the body's veins to the right side of the heart. Arteries and veins are connected in a single line. Therefore, in a steady state, the amount returning equals the cardiac output (PMID 35610620). The heart cannot pump out more than what has returned (PMID 27613307).

Unlike arteries, veins are a stretchy reservoir. The blood in the body is divided into two shares. One share pushes tightly against the vessel walls and actually creates flow, while the other share is simply contained within them (PMID 27613307). The blood that is merely contained in the veins serves as a reserve to be drawn upon when needed (PMID 35610620).

The properties of the heart itself also play a part. The Frank-Starling mechanism is the property where the heart squeezes harder the more it is filled and stretched. This property stops at pumping out as much as came in. It does not determine how much will come in. A 2026 review calls this a passive match that fits the output to the input, and does not view it as a device that actively regulates cardiac output (PMID 42157570). A 2004 comparative physiology review points in the same direction (PMID 14748547).

Now let us imagine that a person who was sitting starts to climb stairs. Their leg muscles begin to use more oxygen. In a healthy person, for every additional liter of oxygen used per minute, cardiac output increases by about 6 liters along with it (PMID 40121540). In a healthy person, this means that as the oxygen used increases, the blood follows suit.

Several regulatory mechanisms are involved together in this matching. These are commands descending from the brain, reflexes ascending from the exercising muscles, and sensors that detect blood pressure. The right ventricle squeezes harder while also relaxing better, thereby accepting the increased venous return (PMID 40121540).

The increased flow is created by redistributing the blood that is already present. In a healthy young man, cardiac output during exercise can increase up to fivefold. During this time, the share of blood that creates flow changes only slightly (PMID 27613307). This is because as the resistance of the arteries and veins changes, the blood already present is redistributed within the blood vessels (PMID 27613307). Separately from that, the reserve simply contained in the veins remains as a share that can be drawn out by the stimulation of the sympathetic nervous system, which puts the body into a state of tension (PMID 35610620).

What the Frank-Starling mechanism does in this scene is a matching that pumps out just as much as came in (PMID 42157570). Ultimately, how much gets sent is determined jointly by the properties of the atria and ventricles and the properties of the blood vessels (PMID 27613307).

However, textbooks are still refining how to draw and divide those shares today. A 2026 narrative review views it this way. The heart supplies the energy, the volume, elasticity, and resistance of the blood vessels determine the range of possible flow, and pressure is a value that reports that state (PMID 42157570). The opposing side argues that Guyton's curves centered on venous return should still be taught as they are. If the share of blood creating flow is constant, venous return and venous pressure move interdependently, making it meaningless to debate which side determines it first (PMID 28349325). A review stating that the physiology of this curve helps understand the effects of critical care is on the same side (PMID 35610620). Because this debate is large enough to warrant a separate article, we take here only a single line that both sides acknowledge together.

How much gets sent is determined not by the heart alone, but by the heart and blood vessels together, and the details are still under debate.

The Places Where This Heart Breaks Down

Heart failure is a state in which the heart either fails to pump as much blood as the body needs, or can only pump that much by raising the filling pressure (PMID 18313620). It is a state of continually overworking to push out the same amount. Based on a 2012 review, more than 5 million people suffer from this disease in the United States alone (PMID 22227365).

The places that break down are three: the valves, the muscle, and the electricity.

The ways valves break down are broadly two. They either narrow and fail to open properly, or they fail to close properly and leak. When the net-like structure forming the skeleton of a valve is disrupted, its function collapses into one of these two (PMID 33339213). Even if the cords holding the valve break, the thin leaflets of the valve flip backward and blood leaks in reverse (PMID 32178262).

Comparing this breakdown to the path of a single drop of blood we followed earlier makes clear what is happening. When a valve narrows, a single drop has a hard time passing through that valve. When a valve leaks, a single drop moves forward, and then part of it is pushed backward. Either way, the heart must try harder to pump the same amount, and leaving this state alone can lead to heart failure (PMID 33339213). However, when the valve leading out to the lungs narrows, mild or moderate cases generally tolerate it well (PMID 21459245).

Muscle-side breakdowns commonly happen because another disease injures the heart muscle. Ischemic heart disease caused by a lack of blood supply to the heart, high blood pressure, and diabetes are prime examples (PMID 22227365). Heredity is another path. For a healthy heart to work efficiently, it must tightly regulate how many of the contractile proteins within the muscle to turn on. Gene mutations linked to inherited heart diseases often disrupt this regulation (PMID 39030271). The electrical side—that is, diseases where the rhythm is disrupted—is covered in the next part.

This failure first reveals itself when exerting effort. As the disease progresses, abnormalities in blood flow and pressure become apparent even when resting (PMID 34389460). Commonly, this appears as a person who once felt short of breath when climbing stairs later feeling short of breath even while sitting still.

Symptoms are visible outside the heart. Blood pools in the lungs, causing shortness of breath. The return flow is blocked, causing the legs to swell and fluid to build up in the abdomen. Whole-body symptoms such as nausea, loss of appetite, and fatigue are also common (PMID 22227365).

Things the body does to cope can ultimately catch up with it. The body uses increased filling to boost the pumped amount, alters the ventricle's volume and wall thickness, and turns on systems regulated by nerves and hormones throughout the entire body to maintain pressure. All three help initially, but eventually lead to a vicious cycle where the disease worsens progressively (PMID 22227365). The Frank-Starling mechanism seen in the stairs scene is the first of these.

The numbers are also heavy. Based on the same review, the 5-year mortality rate for people with heart failure is 50% (PMID 22227365). That means if you watch over that period, about one in two people dies. Treatments have been built upon an understanding of these very mechanisms. Options include draining pooled water, suppressing the overactive neurohormonal system, and reinforcing contractility, along with surgical choices such as cardiac resynchronization therapy, ventricular assist devices, and transplantation (PMID 22227365).

This heart can be looked into. The amount pumped out in a single heartbeat is measured with an echocardiogram (PMID 35085589). Magnetic resonance imaging allows us to see in three dimensions which direction and how fast blood is flowing inside the heart and blood vessels (PMID 32155346).

Breakdowns occur in the valves, muscle, and electricity, symptoms are seen outside the heart first, and treatments stand upon those mechanisms.

Where to Look Next

This is as far as the foyer goes. There are a few topics this article only mentioned in passing.

The story of a lump forming inside the heart, breaking off, and blocking an artery in another organ continues in What a Stroke Actually Is — Blocked, Burst, and the Clock. It also explains the path of the one-way road we followed in this article, and how that lump travels along its route to get there.

What tests that look inside the heart do and do not tell you is covered in What a Test Actually Tells You — An Introduction to Sensitivity, Specificity, and Pretest Probability.

Why aiming for values like pressure is a cautious business, and how improving numbers and improving people can diverge, is tackled head-on in Better Numbers, Better Patients? — An Introduction to Surrogate Endpoints.

How to read numbers like "50% 5-year mortality" is answered in What Does "50% Lower Risk" Mean in People? — Reading Relative Risk, Absolute Risk, and NNT.

What happens when the blood vessels supplying the heart itself narrow and block is continued in What Coronary Artery Disease Is — The Road That Feeds the Heart, Narrowed and Blocked.

How to read blood pressure is covered in What Blood Pressure Measures — Two Numbers, and an Answer That Changes With Where You Measure, and what changes when the heart's rhythm gets scrambled is covered in What Atrial Fibrillation Is — The Upper Chambers That Lost the Beat, and the Blood That Pools There. Why textbooks are being rewritten regarding how much blood to pump out was a topic this article only named and passed by. That topic has not been written yet.

Source Papers

• "Biology and Biomechanics of the Heart Valve Extracellular Matrix." (2020) — PMID 33339213 — Heart valves open and close more than 100,000 times a day and 3 billion times over a lifetime. When their scaffold gets disordered, they narrow or leak, and if left unattended, they progress to heart failure.
• "Heart valve function: a biomechanical perspective." (2007) — PMID 17588873 — Valves must repeat at least 3 billion cycles over the course of a lifetime.
• "Determinants of cardiac output in health and heart failure." (2025) — PMID 40121540 — In healthy people, every 1-liter increase in oxygen uptake per minute increases cardiac output by about 6 liters.
• "The Pulmonary Vasculature." (2023) — PMID 37816344 — The pulmonary circulation operates as a low-pressure, low-resistance circuit responsible for gas exchange. In local hypoxia, arterioles upstream of capillaries constrict to raise local resistance by up to 250% and redistribute blood flow, whereas systemic hypoxia raises pulmonary artery pressure and resistance.
• "The Pulmonary Venous Return from Normal to Pathological." (2021) — PMID 33809829 — Pulmonary veins carry oxygenated blood out of the lungs and into the left atrium.
• "Mechanisms of heart valve development and disease." (2020) — PMID 32620577 — Valves ensure the one-way flow of blood as it exits the heart, passes through the lungs, and returns to the rest of the body.
• "[Pathophysiology of right ventricular hemodynamics]." (2018) — PMID 29945812 — The right ventricle has thin walls and buffers physiological increases in venous return without a harmful rise in right atrial pressure. The pulmonary circulation is a low-pressure, low-resistance, high-compliance system.
• "The history of the theory of the circulation of the blood." (2016) — PMID 27791994 — Galen believed blood was produced in the liver and consumed, while Harvey established the concept of circulation in his 1628 book De Motu Cordis.
• "Fabrici and the functional power of the image." (2020) — PMID 32499171 — Fabrici left behind the first anatomical and physiological illustrations of venous valves, and that work served as a decisive inspiration for his student Harvey.
• "A brief history of the discovery of the circulation of the blood in the human body." (2008) — PMID 18426332 — From Galen to Harvey: contributions by Razi, Al-Qazwini, Ibn al-Nafis (pulmonary circulation), and Servetus.
• "Mechanics and Microstructure of the Atrioventricular Heart Valve Chordae Tendineae." (2020) — PMID 32178262 — Heart valve tendons distribute the force exerted on the leaflets during systolic closure to the papillary muscles, preventing prolapse and regurgitation.
• "The pulmonary valve." (2011) — PMID 21459245 — Review interspersed with Case Reports. The pulmonary valve is the least affected by acquired diseases, so most of its disorders are congenital. The most common is congenital pulmonary valve stenosis, which is generally well tolerated when mild or moderate.
• "Venous return and mean systemic filling pressure: physiology and clinical applications." (2022) — PMID 35610620 — The opposing side (Guyton advocates). Under normal conditions, venous return = cardiac output (arterial and venous systems are in series). The venous system is a high-compliance reservoir, and unstressed volume is mobilized as a reserve. The authors emphasize that Guytonian physiology helps understand the therapeutic effects in critically ill patients.
• "Lung Circulation." (2016) — PMID 27065170 — The pulmonary circulation accommodates the entire cardiac output at low arterial pressure, and pulmonary arteries have thinner walls and far fewer vascular smooth muscle cells than systemic arteries.
• "Vascular Smooth Muscle Cells and Arterial Stiffening." (2017) — PMID 28954852 — Large arteries stretch during systole and recoil during diastole, converting pulsatile flow into steady flow in the microcirculation. Arterial stiffening, the opposite condition, is a major cause of morbidity and mortality worldwide.
• "The Contractile Machines of the Heart." (2024) — PMID 38884723 — Cardiomyocytes are striated, brick-shaped cells with contractile proteins aligned in a sarcomere structure.
• "Calcium and Excitation-Contraction Coupling in the Heart." (2017) — PMID 28684623 — Normal function requires calcium to be sufficiently high during systole and sufficiently low during diastole, and under steady-state conditions, calcium influx equals efflux on every beat.
• "Conduction in cardiac tissue. Historical reflections." (2019) — PMID 30604919 — Myocardium is a functional syncytium, and the gap junction hypothesis is strongly supported by measurement evidence. Propagation is non-uniform, anisotropic, and discontinuous. However, the possibility that ephaptic conduction substitutes under conditions of abnormally reduced gap junction conductance has not been ruled out, and definitive confirmation is still needed.
• "The cardiac cycle and the physiologic basis of left ventricular contraction, ejection, relaxation, and filling." (2008) — PMID 18313620 — Definition of heart failure (failure to pump the required output or doing so only by elevating filling pressures). Severity and prognosis correlate more closely with the degree of diastolic filling abnormality than with ejection fraction.
• "Stroke Volume Determination by Echocardiography." (2022) — PMID 35085589 — Stroke volume is the net result of all events in the cardiac cycle and is measured by echocardiography.
• "Energy, flow and pressure in the cardiovascular system: a narrative review." (2026) — PMID 42157570 — A narrative review presenting a debated viewpoint. The heart supplies energy and vascular properties determine the feasible flow, while pressure reports the state. Mean systemic pressure does not drive flow; right atrial pressure and preload are dependent variables. The Frank-Starling mechanism is a passive matching mechanism, not active regulation. The authors write that interpreting pressure as a driving force has contributed to potentially harmful treatments.
• "Passive regulation of cardiac output during exercise by the elastic characteristics of the peripheral circulation." (2004) — PMID 14748547 — The inverse relationship between cardiac output and filling pressure itself acts as a passive (hydraulic) regulatory mechanism.
• "Volume and its relationship to cardiac output and venous return." (2016) — PMID 27613307 — Stressed volume, which creates flow, typically accounts for about 30% of total blood volume. In healthy young men, even when cardiac output increases up to fivefold during exercise, the change in stressed volume is small.
• "Guyton's venous return curves should be taught at medical schools." (2017) — PMID 28349325 — The opposing side (Guyton advocates). Guyton's venous return and circulatory equilibrium should be taught in medical schools, and arguing over which variable is independent is meaningless.
• "The pathophysiology of heart failure." (2012) — PMID 22227365 — Affecting over 5 million people in the U.S. with a 5-year mortality rate of 50%. Symptoms include pulmonary congested dyspnea, peripheral edema, ascites, and systemic symptoms. Compensatory mechanisms (Starling mechanism, remodeling, neurohormones) eventually turn into a vicious cycle, and treatments such as diuresis, neurohormonal inhibition, inotropic support, resynchronization, assist devices, and transplantation were built upon this understanding.
• "Functional control of myosin motors in the cardiac cycle." (2025) — PMID 39030271 — Only about 5% of myosin motors are needed to generate peak systolic pressure, but far more are required for ejection. Changes in motor pressure open and close valves and alter contraction patterns. Inherited heart disease mutations frequently disrupt this regulation.
• "Hemodynamics for the Heart Failure Clinician." (2022) — PMID 34389460 — Heart failure is a condition that cannot produce adequate blood flow without the cost of elevated filling pressures, manifesting first under stress and eventually appearing even at rest.
• "4D Flow with MRI." (2020) — PMID 32155346 — Three-directional velocity-encoded 4D flow MRI visualizes and quantifies circulatory blood flow in three dimensions.