The heart is a four-chambered muscular pump that moves roughly 5 quarts of blood every minute through two separate circuits: the pulmonary circuit to the lungs and the systemic circuit to the body. Deoxygenated blood enters the right atrium, passes through the tricuspid valve into the right ventricle, and is pushed through the pulmonary valve toward the lungs. Oxygenated blood returns to the left atrium, crosses the mitral valve into the left ventricle, and exits through the aortic valve into the aorta. The left ventricle does most of the work, generating pressures around 120 mmHg to drive blood through the entire body.
The Four Chambers and What Each One Does
The heart's map begins with its four chambers, and each has a distinct job defined by the pressure it must generate and the circuit it serves. The right atrium receives deoxygenated blood from the superior and inferior vena cava at a pressure of only 2 to 6 mmHg. The right ventricle then pumps that blood into the pulmonary artery at a peak pressure of about 25 mmHg, which is enough to reach the lungs but not enough to damage their delicate capillaries. This low-pressure design is why the right ventricle's wall is only 3 to 5 mm thick.
The left atrium receives oxygenated blood from four pulmonary veins and sits at a pressure of 8 to 12 mmHg, slightly higher than the right side because it feeds a more demanding pump. The left ventricle is the powerhouse, with a wall 8 to 15 mm thick, roughly three times that of the right ventricle. It must generate a peak systolic pressure of 110 to 130 mmHg to push blood through the aorta and into every organ system.
In plain terms: the right side is a low-pressure delivery service to the lungs, and the left side is a high-pressure engine for the whole body.
The interventricular septum separates the two ventricles and is the thickest part of the heart wall. A common misconception is that the heart is one pump. It is really two pumps wired to beat in sequence, and that separation is what keeps oxygenated and deoxygenated blood from mixing. When the septum develops a hole, called a ventricular septal defect, the pressure difference between the two sides determines how much blood shunts across, which is why small defects often close on their own while large ones need surgical repair.
The Valves: Four One-Way Gates
Four valves enforce one-way flow through the heart, and each opens or closes in response to a pressure difference rather than any active muscle contraction. The tricuspid valve sits between the right atrium and right ventricle and has three leaflets. The pulmonary valve guards the exit from the right ventricle into the pulmonary artery and has three cusps. The mitral valve, between the left atrium and left ventricle, has just two leaflets and is the valve most often damaged by rheumatic fever. The aortic valve, at the exit from the left ventricle, has three cusps and is the most commonly replaced valve in adults.
Valve openings are surprisingly small. A healthy aortic valve opens to about 3 to 4 square centimeters in an adult. Once stenosis narrows that opening below roughly 1 square centimeter, the heart must generate much higher pressures to maintain flow, and symptoms such as chest pain and shortness of breath typically appear. This threshold is why cardiologists measure valve area directly rather than relying only on pressure gradients. A gradient that looks modest at rest can hide severe disease once a patient exercises and demand rises.
The valves are anchored by the chordae tendineae, tough strings that connect the mitral and tricuspid leaflets to the papillary muscles. These muscles do not open the valves; they contract during systole to prevent the leaflets from prolapsing backward. This is a non-obvious detail: the papillary muscles are brakes, not drivers. If a papillary muscle ruptures after a heart attack, the valve flails and blood rushes backward, a mechanical emergency that can cause rapid heart failure.
The Cardiac Cycle: A Timeline of One Heartbeat
A single heartbeat lasts about 0.8 seconds at a resting rate of 75 beats per minute, and that interval splits into a filling phase and an ejection phase. The sequence below traces the cycle from the start of atrial contraction to the moment the ventricles refill. The numbers are resting adult values and shift with heart rate, age, and fitness.
| Phase | Duration | What Happens | Key Pressure or Volume |
|---|---|---|---|
| Atrial contraction (atrial systole) | 0.1 s | Atria squeeze the last 20% of blood into ventricles | Left atrial pressure rises to about 12 mmHg |
| Isovolumetric contraction | 0.05 s | Ventricles contract; all valves closed | Left ventricular pressure climbs from 8 to 80 mmHg |
| Rapid ejection | 0.15 s | Aortic and pulmonary valves open; blood exits | Left ventricle peaks near 120 mmHg |
| Reduced ejection | 0.15 s | Flow slows as ventricles begin to relax | Aortic pressure begins to fall |
| Isovolumetric relaxation | 0.08 s | Ventricles relax with valves closed | Pressure drops from 80 to 8 mmHg |
| Rapid filling | 0.15 s | Mitral and tricuspid valves open; ventricles fill passively | About 70% of ventricular filling occurs here |
| Diastasis | 0.12 s | Slow filling; atria and ventricles at rest | Ventricular volume reaches about 120 mL |
The two isovolumetric phases are the key to the whole cycle. During these brief windows, all four valves are closed and the ventricular volume does not change, yet pressure swings dramatically. The heart is doing work without moving blood, which is why these phases are sometimes called the "isometric" moments of the cardiac cycle. The first and second heart sounds, the familiar "lub-dub," are produced at the boundaries of these phases: the closure of the mitral and tricuspid valves makes the first sound, and the closure of the aortic and pulmonary valves makes the second.
This cycle scales with demand. At a heart rate of 150 beats per minute, the whole cycle compresses to about 0.4 seconds, and most of that loss comes out of diastole, the filling time. That is why very fast heart rates are dangerous: the ventricles get less time to fill, stroke volume falls, and blood pressure can drop even though the heart is beating faster.
How Much Blood the Heart Moves: The Numbers
Cardiac output is the product of heart rate and stroke volume, and it is the single best measure of how well the heart is doing its job. At rest, a healthy adult ejects about 70 mL per beat at 70 beats per minute, giving a cardiac output of roughly 4.9 liters per minute, close to the 5 quarts mentioned earlier. During maximal exercise, output can climb to 20 to 25 liters per minute in a trained athlete through a combination of a higher heart rate and a larger stroke volume.
The heart also has a built-in reserve that most people never use. Ejection fraction, the percentage of ventricular volume pushed out with each beat, normally sits between 55% and 70%. A person with an ejection fraction of 30% is moving far less blood per beat, and the body compensates by raising heart rate and constricting blood vessels. This compensation can mask heart failure for months, which is why ejection fraction is measured directly with echocardiography rather than inferred from symptoms.
Per the 2021 ESC guidelines, a left ventricular ejection fraction of 40% or below defines heart failure with reduced ejection fraction, while 41% to 49% is mildly reduced, and 50% or above is preserved. These cutoffs are not arbitrary. They reflect where treatments proven in trials begin to lose their benefit. A patient at 45% falls into a gray zone where the evidence for many therapies is weaker, and clinicians must weigh risks individually.
Coronary Supply and the Heart's Own Blood Flow
The heart cannot use the blood inside its chambers for its own nourishment; it needs its own supply through the coronary arteries. The left main coronary artery branches into the left anterior descending and circumflex arteries, which together supply the left ventricle and much of the septum. The right coronary artery supplies the right ventricle and, in about 60% of people, the sinoatrial node, the heart's natural pacemaker. Blockage of the left anterior descending artery is sometimes called the widowmaker because it can starve a large portion of the left ventricle at once.
Coronary blood flow is unusual because most of it happens during diastole, when the heart muscle is relaxed. During systole, the contracting muscle squeezes the small vessels and restricts flow. This is why a fast heart rate, which shortens diastole, can tip a patient with narrowed coronary arteries into chest pain even at modest exertion. The supply-demand balance is the core of ischemic heart disease, and every treatment for angina works by improving one side of that equation.
The heart extracts about 70% to 80% of the oxygen delivered to it, far more than most organs, which extract 25% to 30%. That high extraction rate means the heart has little reserve to pull more oxygen from the same blood flow. When demand rises, the only reliable response is to increase coronary flow. This is a corrected misconception: the heart does not mainly adapt by extracting more oxygen per unit of blood, because it is already near its limit. It adapts by widening its coronary arteries.
What Can Go Wrong: Common Conditions and Red Flags
The map of the heart becomes most useful when something fails, and the failure patterns follow directly from the anatomy. Narrowed coronary arteries cause angina or heart attack. Leaky or narrowed valves cause murmurs and eventually heart failure. A weakened left ventricle causes fluid to back up into the lungs. An electrical fault causes arrhythmias, from harmless skipped beats to life-threatening ventricular fibrillation.
Some warning signs demand immediate emergency care rather than a routine appointment. Seek emergency help for any of the following:
- Chest pressure, squeezing, or pain lasting more than a few minutes, especially with sweating or nausea.
- Sudden shortness of breath at rest, or breathlessness that wakes you from sleep.
- Fainting, near-fainting, or a racing heartbeat that does not settle.
- Swelling in both legs with rapid weight gain of 2 to 3 pounds in a day.
- Pain radiating to the jaw, neck, or left arm, particularly in women and older adults.
These red flags matter because time is muscle. In a heart attack, roughly 2 million heart muscle cells die every minute that flow is blocked. Per the 2013 ACC/AHA guidelines, the goal is to restore blood flow within 90 minutes of first medical contact for the most severe type of heart attack, which is why calling emergency services rather than driving yourself saves lives.
How to Keep the Map Intact: Practical Actions
The heart's structure is largely fixed, but its workload and the health of its arteries are not. Concrete actions with measurable effects include keeping blood pressure below 130/80 mmHg, since each 20 mmHg rise in systolic pressure roughly doubles cardiovascular risk. Aim for at least 150 minutes of moderate aerobic activity per week, which can lower resting heart rate by 5 to 10 beats per minute over months. Keep LDL cholesterol in a range your clinician sets based on your risk, often below 100 mg/dL for moderate risk and below 70 mg/dL for high risk. Avoid smoking entirely, because a single cigarette raises heart rate and blood pressure for up to 30 minutes and chronic smoking accelerates plaque formation. Limit alcohol to no more than two drinks per day for men and one for women. Get 7 to 9 hours of sleep, since short sleep raises blood pressure and inflammation.
These targets overlap and reinforce each other. A person who exercises regularly, sleeps well, and keeps blood pressure controlled is not just lowering one risk factor but improving several at once, and the combined effect is larger than the sum of the individual changes.
Limits of the Map: What We Still Do Not Know
A detailed map of the heart is a model, and every model has edges. We can measure ejection fraction, valve areas, and coronary flow with good precision, but we cannot yet predict exactly when a stable plaque will rupture, which is the event that triggers most heart attacks. Most heart attacks occur in arteries that were not severely narrowed on prior imaging, a finding that complicates the idea that blockages alone predict risk.
Some mechanisms remain hypotheses rather than established science. The role of chronic low-grade inflammation in coronary disease is well supported by trials of anti-inflammatory therapy, but the precise triggers that start plaque instability in an individual patient are still under investigation. The heart's electrical system is mapped in detail, yet why some scarred hearts generate dangerous rhythms and others do not is not fully explained. What is certain is that the structure described here, the chambers, valves, and coronary arteries, is the foundation every clinician works from, and understanding it changes how you read your own symptoms and test results.
Keeping the map in mind also changes how you interpret numbers. A normal ejection fraction does not rule out heart failure, and a clean coronary angiogram does not rule out a future event. The best use of this anatomy is as a framework for asking better questions, not as a guarantee.