How the Cardiovascular System Works

The Heart: Structure and Chambers

The human heart is roughly the size of a closed fist, weighing between 250 and 350 grams in adults. It sits in the mediastinum, the central compartment of the thoracic cavity, slightly left of center and enclosed in a protective double-walled sac called the pericardium. The pericardial fluid between the inner and outer layers reduces friction as the heart beats.

Internally, the heart divides into four chambers. The two upper chambers, the right atrium and left atrium, receive blood returning to the heart. The two lower chambers, the right ventricle and left ventricle, pump blood out of the heart. A muscular wall called the septum separates the right and left sides completely, preventing oxygenated and deoxygenated blood from mixing. The left ventricle has the thickest walls because it must generate enough pressure to push blood through the entire systemic circulation, while the right ventricle only needs to push blood the shorter distance to the lungs.

Four valves ensure blood flows in one direction. The tricuspid valve separates the right atrium from the right ventricle. The mitral (bicuspid) valve separates the left atrium from the left ventricle. The pulmonary valve guards the exit from the right ventricle into the pulmonary artery. The aortic valve guards the exit from the left ventricle into the aorta. When doctors listen to a heartbeat with a stethoscope, the "lub-dub" sound comes from these valves closing: the first sound from the tricuspid and mitral valves, the second from the pulmonary and aortic valves.

The Cardiac Cycle

Each heartbeat consists of a coordinated sequence of contraction (systole) and relaxation (diastole). During atrial systole, both atria contract simultaneously, pushing blood through the open tricuspid and mitral valves into the ventricles below. This tops off the ventricles, which have already filled passively during the previous diastole. Atrial contraction contributes about 20% of ventricular filling; the remaining 80% flows in by gravity and pressure gradients.

During ventricular systole, both ventricles contract with enough force to close the tricuspid and mitral valves and open the pulmonary and aortic valves. The right ventricle pushes blood into the pulmonary trunk toward the lungs at a systolic pressure of about 25 mmHg. The left ventricle pushes blood into the aorta at a systolic pressure of about 120 mmHg, roughly five times higher. During diastole, the entire heart relaxes, the pulmonary and aortic valves close to prevent backflow, and blood begins filling the atria and ventricles again.

A complete cardiac cycle at a resting heart rate of 75 beats per minute lasts about 0.8 seconds: 0.1 seconds for atrial systole, 0.3 seconds for ventricular systole, and 0.4 seconds for complete diastole. At rest, each ventricle ejects roughly 70 milliliters of blood per beat (stroke volume), giving a cardiac output of about 5.25 liters per minute. During intense exercise, cardiac output can increase to 20 to 25 liters per minute through increases in both heart rate and stroke volume.

The Electrical Conduction System

The heart generates its own electrical impulses, which is why it can continue beating even when removed from the body or after the nerves supplying it are cut. The sinoatrial (SA) node, a cluster of specialized cells in the upper wall of the right atrium, acts as the natural pacemaker. It spontaneously depolarizes 60 to 100 times per minute, sending electrical waves across both atria and triggering atrial contraction.

The signal reaches the atrioventricular (AV) node at the junction between the atria and ventricles. The AV node delays the signal by about 0.1 seconds, ensuring the atria finish contracting and emptying before the ventricles begin. The impulse then travels rapidly down the bundle of His, splits into the right and left bundle branches, and spreads through the Purkinje fibers, which distribute the signal to the ventricular muscle cells. This rapid conduction ensures the ventricles contract from the apex upward, squeezing blood efficiently toward the outflow valves.

An electrocardiogram (ECG or EKG) records this electrical activity from the body surface. The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. Abnormalities in these waveforms help diagnose arrhythmias, heart blocks, ischemia, and other cardiac conditions. A standard 12-lead ECG takes about 10 seconds to record and remains one of the most important diagnostic tools in medicine.

Blood Vessels: Arteries, Veins, and Capillaries

Arteries carry blood away from the heart. They have thick, muscular, elastic walls designed to withstand and smooth out the high-pressure pulses generated by ventricular contraction. The aorta, the largest artery, has a diameter of about 2.5 centimeters and branches into progressively smaller arteries. Arterioles, the smallest arteries, have diameters between 10 and 100 micrometers and regulate blood flow to specific tissues by constricting or dilating their smooth muscle walls. This vasomotor control is how the body redirects blood, for example, from the digestive organs to skeletal muscles during exercise.

Capillaries are the smallest vessels, with diameters of 5 to 10 micrometers, barely wide enough for red blood cells to pass through single file. Capillary walls are only one endothelial cell thick, allowing rapid exchange of oxygen, carbon dioxide, nutrients, and waste between blood and interstitial fluid. The estimated 10 billion capillaries in the human body provide an exchange surface area of roughly 500 to 700 square meters. No cell in the body is more than a few cell widths from a capillary.

Veins carry blood back to the heart. They have thinner walls and lower pressure than arteries, and many veins contain one-way valves to prevent blood from flowing backward under gravity. The venous return is assisted by skeletal muscle contractions (the "muscle pump"), respiratory pressure changes (the "respiratory pump"), and the slight suction created by the expanding atria during diastole. Deep vein thrombosis (DVT) occurs when blood clots form in the deep veins, usually in the legs, often due to prolonged immobility. A clot that breaks free and travels to the lungs causes a pulmonary embolism, a potentially fatal blockage of pulmonary blood flow.

Pulmonary and Systemic Circulation

The cardiovascular system operates two circulation loops simultaneously. Pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs through the pulmonary arteries (the only arteries that carry deoxygenated blood), where carbon dioxide diffuses out and oxygen diffuses in across the alveolar-capillary membrane. Oxygenated blood returns to the left atrium through the pulmonary veins (the only veins that carry oxygenated blood). The entire pulmonary circuit operates at low pressure, about one-fifth of systemic pressure, to protect the delicate alveolar capillaries.

Systemic circulation carries oxygenated blood from the left ventricle through the aorta to every tissue in the body. Blood delivers oxygen and nutrients at the capillary level, picks up carbon dioxide and metabolic waste, and returns to the right atrium through the superior vena cava (from above the heart) and inferior vena cava (from below). A subset of systemic circulation, the coronary circulation, supplies blood to the heart muscle itself. The left and right coronary arteries branch from the aorta just above the aortic valve and deliver approximately 250 milliliters of blood per minute to the myocardium. Blockage of a coronary artery causes a myocardial infarction (heart attack).

Blood Pressure Regulation

Blood pressure is the force blood exerts against arterial walls. It is measured in millimeters of mercury (mmHg) and expressed as systolic pressure over diastolic pressure. Normal resting blood pressure in adults is about 120/80 mmHg. Hypertension, defined as sustained readings above 130/80 mmHg, affects roughly 1.3 billion people worldwide and is the leading modifiable risk factor for cardiovascular disease, stroke, and kidney failure.

Short-term blood pressure regulation involves the baroreceptor reflex. Baroreceptors, pressure-sensitive nerve endings in the carotid sinus and aortic arch, detect changes in arterial pressure and send signals to the cardiovascular center in the medulla oblongata. If pressure rises, the medulla reduces sympathetic output and increases parasympathetic output, slowing the heart rate and dilating blood vessels. If pressure drops, the reverse occurs. This reflex operates within seconds, which is why standing up quickly can cause brief dizziness: baroreceptors need a moment to adjust.

Long-term blood pressure regulation involves the kidneys. The renin-angiotensin-aldosterone system (RAAS) activates when kidney blood flow drops. The kidneys release renin, which catalyzes production of angiotensin II, a potent vasoconstrictor that also stimulates the adrenal glands to release aldosterone. Aldosterone causes the kidneys to retain sodium and water, increasing blood volume and raising pressure. Most classes of blood pressure medications target components of this pathway: ACE inhibitors block angiotensin II formation, ARBs block its receptors, and diuretics reduce blood volume by increasing urine output.

Common Cardiovascular Conditions

Atherosclerosis, the buildup of fatty plaques inside arterial walls, is the underlying cause of most cardiovascular disease. Plaques form when LDL cholesterol accumulates in the arterial intima, triggering an inflammatory response that recruits immune cells and produces fibrous caps over fatty deposits. Over decades, plaques narrow the arterial lumen and restrict blood flow. If a plaque ruptures, it triggers rapid clot formation that can completely block the artery. In coronary arteries, this causes a heart attack. In cerebral arteries, it causes an ischemic stroke.

Heart failure occurs when the heart cannot pump enough blood to meet the body's needs. It is not a sudden event but a progressive condition, often developing over years after damage from heart attacks, chronic hypertension, valve disease, or cardiomyopathy. The body compensates by retaining fluid (causing edema), increasing heart rate, and enlarging the heart muscle, but these compensations eventually fail. Heart failure affects approximately 64 million people globally. Treatment includes ACE inhibitors, beta-blockers, diuretics, and in severe cases, ventricular assist devices or heart transplantation.

Arrhythmias are abnormal heart rhythms caused by disruptions in the electrical conduction system. Atrial fibrillation, the most common sustained arrhythmia, affects the atria with rapid, chaotic electrical signals that cause inefficient quivering instead of coordinated contraction. It affects 33 to 46 million people worldwide and increases stroke risk fivefold because blood can pool and clot in the fibrillating atria. Ventricular fibrillation is far more dangerous: chaotic ventricular activity produces no effective pumping and causes death within minutes unless defibrillation restores a normal rhythm.