The cardiovascular system functions primarily as the transport network of the human body. It consists of a central muscular pump, the heart, and an extensive network of blood vessels that reach every living tissue. Nutrients absorbed from the gastrointestinal tract, oxygen inhaled into the lungs, hormones secreted by endocrine glands, and cellular waste products all rely on this system for movement. Without a continuous supply of blood, cells quickly run out of energy and accumulate toxic metabolites. In a clinical setting, assessing this system involves measuring vital signs like pulse rate and blood pressure, which offer immediate windows into how well perfusion is maintained throughout the body.
The human heart sits securely within the mediastinum of the thoracic cavity, roughly the size of a clenched fist. Its wall is composed of three distinct tissue layers:
The heart features four distinct chambers: two upper atria that receive blood and two lower ventricles that pump blood outward. Deoxygenated blood enters the right atrium from the body via the superior and inferior vena cava, moves through the tricuspid valve into the right ventricle, and travels through the pulmonary semilunar valve into the pulmonary trunk toward the lungs. After oxygenation, blood returns via the pulmonary veins into the left atrium, passes through the bicuspid or mitral valve into the powerful left ventricle, and is forced through the aortic semilunar valve into the aorta for systemic distribution.
Although the heart chambers are constantly filled with blood, the myocardium is far too thick to absorb nutrients directly from that internal source. Instead, the heart relies on its own dedicated vascular network known as coronary circulation. The left and right coronary arteries branch directly from the base of the aorta just above the aortic valve, supplying oxygen-rich blood to the heart muscle during diastole. Deoxygenated blood is collected by cardiac veins, which empty into the coronary sinus and finally drain directly into the right atrium. Occlusion of these coronary vessels leads to ischemia and potential myocardial infarction.
The heart beats rhythmically without direct nervous system stimulation due to its autorhythmic cardiac conduction fibers. The electrical impulse typically originates at the sinoatrial node, known as the natural pacemaker, located in the right atrial wall. From there, the wave of depolarization spreads across both atria, causing them to contract, before converging at the atrioventricular node. The impulse then travels down the bundle of His, splits into right and left bundle branches, and surges through the subendocardial conducting network, known as Purkinje fibers, triggering a coordinated ventricular contraction from the apex upward.
The cardiac cycle encompasses all mechanical and electrical events occurring during a single heartbeat, divided into systole (contraction) and diastole (relaxation). During atrial systole, blood is actively pushed into the ventricles. As ventricles contract during ventricular systole, rising pressure snaps the atrioventricular valves shut, producing the first heart sound, often described as 'lub'. When ventricular pressure drops below aortic and pulmonary pressures at the end of systole, the semilunar valves snap shut, generating the second heart sound, or 'dub'.
Cardiac output represents the total volume of blood pumped by each ventricle per minute, calculated as heart rate multiplied by stroke volume. Stroke volume itself depends on venous return, myocardial contractility, and peripheral resistance. Blood pressure is the force exerted by circulating blood against the walls of blood vessels, determined by cardiac output and systemic vascular resistance. Short-term regulation relies on baroreceptors in the carotid sinuses and aortic arch that instantly adjust heart rate and vessel tone, while long-term control is managed primarily by the kidneys through fluid balance and the renin-angiotensin-aldosterone system.
The vascular tree is composed of specialized vessels that manage blood distribution. Arteries feature thick, muscular, and elastic walls designed to withstand high pressure as blood surges away from the heart. Arterioles act as primary control valves that adjust local resistance and direct blood flow into capillary beds. Capillaries are microscopic vessels consisting of a single layer of endothelial cells, permitting the rapid exchange of gases, nutrients, and waste between blood and tissue fluid. Veins possess thinner walls and larger lumens, acting as capacitance vessels that return blood to the heart, aided by one-way valves and skeletal muscle contractions.