The cardiovascular system is a complex network that transports blood, nutrients, oxygen, and waste products throughout the body. At its center is the heart, a remarkable four-chambered muscular pump. The heart consists of two atria that receive blood and two ventricles that pump blood out. The right side handles deoxygenated blood shown in blue, while the left side handles oxygenated blood shown in red. Four valves ensure blood flows in only one direction: the tricuspid and mitral valves between atria and ventricles, and the pulmonary and aortic valves at the ventricle exits.
The cardiovascular system operates through two interconnected circulation pathways. Pulmonary circulation begins when the right ventricle pumps deoxygenated blood through the pulmonary arteries to the lungs. In the lungs, carbon dioxide is released and oxygen is absorbed. The oxygen-rich blood then returns to the left atrium via pulmonary veins. Systemic circulation starts when the left ventricle pumps oxygenated blood through the aorta to supply all body tissues with nutrients and oxygen. After delivering oxygen and collecting waste products, the deoxygenated blood returns to the right atrium through the vena cava, completing the circuit.
The cardiac cycle is a precisely coordinated sequence of events that repeats with each heartbeat. During diastole, the heart muscle relaxes, allowing the ventricles to fill with blood as the atrioventricular valves open. The atria contract to complete ventricular filling. During systole, the ventricles contract forcefully, closing the AV valves and opening the semilunar valves to pump blood into the arteries. This mechanical activity is triggered by electrical impulses shown in the ECG, with the P wave representing atrial depolarization, the QRS complex showing ventricular depolarization, and the T wave indicating ventricular repolarization. The pressure changes throughout the cycle drive blood flow through the cardiovascular system.
The blood vessel network forms a sophisticated transportation system with distinct structural adaptations for different functions. Arteries have thick muscular walls to withstand high pressure from the heart's pumping action, starting at about 120 millimeters of mercury. As arteries branch into smaller arterioles, the muscle layer helps regulate blood flow to different tissues. Capillaries are microscopic vessels with walls only one cell thick, allowing efficient exchange of oxygen, nutrients, carbon dioxide, and waste products between blood and tissues. The pressure drops significantly to about 30 millimeters of mercury in capillaries. Blood then collects into venules and larger veins, which have thinner walls and one-way valves to prevent backflow as blood returns to the heart at low pressure. This hierarchical organization ensures efficient circulation and precise control of blood distribution throughout the body.
Blood is a remarkable fluid tissue composed of plasma and cellular components. When blood is separated, plasma makes up about 55 percent and contains water, proteins, nutrients, hormones, and waste products. The cellular portion includes three main types of cells. Red blood cells, or erythrocytes, are the most numerous and contain hemoglobin molecules that bind oxygen for transport throughout the body. White blood cells, or leukocytes, are part of the immune system and defend against infections and foreign substances. Platelets, or thrombocytes, are cell fragments that initiate blood clotting when blood vessels are damaged. The clotting process involves platelet aggregation and the formation of fibrin mesh to seal wounds. Each component plays a crucial role in maintaining cardiovascular function and overall health.