A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
The cardiovascular system is a closed double-circuit transport network consisting of the heart, blood vessels, and blood. The right heart pumps deoxygenated blood through the pulmonary circuit to the lungs for gas exchange; the left heart pumps oxygenated blood through the systemic circuit to all body tissues. Blood vessels are classified by function and structure: arteries carry blood away from the heart under high pressure; capillaries are the thin-walled sites of exchange between blood and interstitial fluid; veins return blood to the heart under low pressure via one-way valves. The system transports O2, CO2, nutrients, hormones, heat, and immune cells, and is central to blood pressure regulation and thermoregulation.
Trace the complete circuit of a single red blood cell: left ventricle → aorta → systemic arteries → capillaries (O2 offload, CO2 pickup) → veins → right atrium → right ventricle → pulmonary artery → pulmonary capillaries (CO2 offload, O2 pickup) → pulmonary veins → left atrium → left ventricle. Label the pressure at each major point.
The cardiovascular system's core function is transport — moving oxygen, carbon dioxide, nutrients, hormones, and heat between different parts of the body. To appreciate why the system is built the way it is, it helps to start with the problem it solves: your cells constantly consume O₂ and produce CO₂, but gas exchange with the outside world happens only in the lungs. The cardiovascular system is the logistics network that connects every cell to the lungs (and to the digestive tract, kidneys, and endocrine glands).
The structural answer to this problem is a double circuit driven by a four-chambered pump. The right side of the heart handles the pulmonary circuit: the right atrium receives deoxygenated blood returning from the body via the venae cavae, passes it to the right ventricle, which pumps it through the pulmonary artery to the lungs. In the pulmonary capillaries, CO₂ diffuses out and O₂ diffuses in. Freshly oxygenated blood then returns via the pulmonary veins to the left atrium. The left side handles the systemic circuit: the left ventricle (the most muscular chamber) pumps oxygenated blood through the aorta to systemic arteries, which branch into capillaries throughout the body. At those capillaries, O₂ is delivered to tissues and CO₂ is picked up. Deoxygenated blood returns through veins to the right atrium, completing the loop.
The blood vessel types reflect their functional roles. Arteries carry blood away from the heart under high pressure and have thick, elastic walls to withstand that pressure. Capillaries are the sites of actual exchange — their walls are only one cell thick, allowing rapid diffusion of gases, nutrients, and waste. Veins return blood to the heart under low pressure; they have thinner walls and one-way valves that prevent backflow, relying partly on skeletal muscle contractions during movement to push blood upward against gravity.
A critical anatomical distinction that confuses many students: arteries are defined by the direction they carry blood (away from the heart), not by whether that blood is oxygenated. The pulmonary artery is an artery — it carries blood away from the heart — but it carries deoxygenated blood. Similarly, the pulmonary veins carry oxygenated blood toward the heart. The shortcut "arteries = oxygenated, veins = deoxygenated" works for the systemic circuit but fails for the pulmonary circuit. Always reason from anatomy and function rather than from the shortcut.
Finally, the heart itself needs a blood supply. It does not receive oxygen from the blood passing through its chambers — the muscle walls are too thick for diffusion across the chamber lining to be sufficient. Instead, the coronary arteries branch off the aorta immediately above the aortic valve, wrapping around the outside of the heart and penetrating the muscle. Blockage of a coronary artery — typically by atherosclerotic plaque — starves heart muscle of oxygen, causing the cell death we call a myocardial infarction (heart attack). This is why the coronary arteries are among the most clinically important vessels in the body.