The cardiovascular system circulates blood throughout the body. Blood carries a wide range of molecules necessary for the body to function, including red and white blood cells, nutrients, wastes, hormones, and gases. Red blood cells contain hemoglobin, an iron-containing protein that carries oxygen. White blood cells circulate as part of the immune system. Both red and white blood cells are suspended in a fluid called plasma.
Blood is circulated by a muscular organ called the heart. The circulatory system includes two closed loops. In the pulmonary loop, deoxygenated blood leaves the heart and travels to the lungs, where it loses carbon dioxide and becomes rich in oxygen. The oxygenated blood then returns to the heart, which pumps it through the systemic loop. The systemic loop delivers oxygen to the rest of the body and returns deoxygenated blood to the heart. The pumping action of the heart is regulated the sinoatrial (SA) and atrioventricular (AV) nodes, whose electrical activity sets the rhythm of the heart.
Deoxygenated blood from the body enters the heart via the right atrium. It then passes through the tricuspid valve into the right ventricle and is pumped out to the lungs. Oxygenated blood returns from the lungs into the left atrium. It then passes through the mitral valve into the left ventricle and is pumped out to the body through the aorta. The contraction of the heart during this process is called systole, and the relaxation of the heart is diastole.
Blood is carried through the body in a system of blood vessels. Oxygenated blood leaves the heart in large vessels called arteries, which branch into smaller and smaller vessels. The smallest vessels, capillaries, are where the exchange of molecules between blood and cells takes place. Deoxygenated blood returns to the heart in veins.
Blood leaves the heart to travel to the body through the aorta; in the lower body, the aorta branches into the iliac arteries. Deoxygenated blood returns to the heart from the body via the superior vena cava (upper body) and inferior vena cava (lower body). Blood then leaves the heart again to travel to the lungs through the pulmonary arteries, and returns from the lungs via the pulmonary veins.
The lymphatic system is an open circulatory system that functions alongside the cardiovascular system. It facilitates the movement of substances between cells and the blood by removing interstitial fluid (the fluid between cells). It also plays an important role in the immune system by circulating white blood cells. The system is composed of lymphatic vessels that carry lymph, a clear fluid containing lymphocytes and waste products. Lymph passes through lymph nodes, which are collections of tissue rich in white blood cells that filter out harmful substances such as pathogens and cell waste. The lymph is then returned to the circulatory system through the veins near the heart.
Lymph nodes can become inflamed during infections when they contain a higher than normal number of lymphocytes.
Heart disease is the most common cause of death in the United States. This umbrella term includes a number of different pathologies that cause the heart to weaken or stop:
Many cardiac medications work by relaxing blood vessels (to decrease blood pressure) or constricting blood vessels (to increase blood pressure).