Circulatory System

The circulatory system carries blood around the body. It delivers oxygen and nutrients to the tissues and removes waste products such as carbon dioxide. It is made of the heart, the blood vessels, and the blood. The circulation of blood in a closed system was first described by William Harvey in his 1628 work “De Motu Cordis.”

The human heart has four chambers: two upper chambers called atria and two lower chambers called ventricles. The right side of the heart receives deoxygenated blood and the left side handles oxygenated blood. The left ventricle is the chamber that pumps oxygenated blood to the whole body, which is why its wall is the thickest. At rest, a healthy heart beats about 72 times per minute (the normal range is 60 to 100).

The heart beats because of electrical signals. The sinoatrial (SA) node is the natural pacemaker of the heart because it starts each heartbeat. The signal then reaches the atrioventricular (AV) node, whose main role is to delay the impulse briefly so the atria can finish contracting and empty into the ventricles before the ventricles contract. After leaving the AV node, the impulses travel through the bundle of His, the bundle branches, and finally the Purkinje fibers, which spread the signal quickly through the ventricular muscle.

There are three main types of blood vessels. Arteries carry blood away from the heart, and the largest artery in the body is the aorta. Veins carry blood back to the heart, and they have valves that prevent the backflow of blood. Capillaries are very thin vessels where the exchange of gases and nutrients between blood and body tissues takes place. The pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart. These are special cases because the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.

Blood pressure is written as two numbers. In a reading of 120/80 mmHg, 120 is the systolic pressure, which is the pressure when the heart contracts, and 80 is the diastolic pressure, which is the pressure in the arteries when the heart relaxes between beats.

Blood is made of plasma and blood cells. Plasma is about 90 percent water and also carries proteins, salts, and nutrients. The most abundant protein in plasma is albumin. Red blood cells carry oxygen with the help of hemoglobin. One adult hemoglobin molecule is made of four polypeptide (globin) subunits. Oxygen binding to hemoglobin is called cooperative because when one subunit binds oxygen, it changes the shape of the molecule and makes it easier for the other subunits to bind oxygen. White blood cells fight infection. Platelets help the blood to clot, which stops bleeding after an injury.

Blood type O negative is known as the universal donor because its red cells have no A, B, or Rh antigens, so it can be given to almost anyone in an emergency.

The circulatory system also works with the lymphatic system. Dietary fats absorbed in the small intestine first enter the lymphatic system through small structures called lacteals, which are found in the villi of the intestine.

Atherosclerosis is a disease in which fatty deposits (plaque) build up inside the walls of the arteries, making them narrow and hard. This reduces blood flow and can lead to heart attack and stroke.