The circulatory system consists of the heart and the blood vessels. The heart is a muscular, double pump with four chambers. There are right and left atria, and right and left ventricles. The right atrium receives low oxygen blood from the body and pumps it to the right ventricle, which pumps it to the lungs. The left atrium receives oxygenated blood from the lungs and pumps it to the left ventricle, which pumps the blood to the body tissues. Four different heart valves prevent backflow of blood in the heart and a cardiac conduction system, consisting of fibers with both muscular and nervous characteristics, conducts impulses through the myocardium thus producing muscular contractions. These muscular contractions comprise the cardiac cycle. During the cardiac cycle, both atria contract simultaneously, then both ventricles contract simultaneously. Systole refers to the period when a chamber is contracting. Diastole refers to the period when a chamber is relaxing. The production of heart sounds during the cardiac cycle is the result of the closing of the heart valves, and the characteristic electrocardiogram tracing is due to depolarization and repolarization of the heart chambers. The heartbeat is intrinsic and regulated by a part of the cardiac conduction system called the SA node, but the rate of the heartbeat is also controlled by the nervous system. Blood vessels comprise the vascular system. These vessels are the arteries, which carry blood away from the heart; the smaller arterioles, which branch to form the capillaries; the venules, which carry blood away from capillary beds; and the veins, which carry blood back to the heart. Arteries are the most muscular blood vessels and have the thickest walls, capillaries have walls that are only one cell thick, and veins contain valves that prevent the backflow of blood. The exchange of gases, nutrients, and wastes between the blood and the body cells occurs across the walls of the capillaries. There are two major paths of circulation, the pulmonary circuit, which circulates blood through the lungs; and the systemic circuit, which serves the needs of the body tissues. The major vessels of these circuits are discussed in the text, as are the vessels involved in supplying blood to the brain, heart, and liver. A pulse results when the blood entering the arteries causes the arterial walls to swell, and blood pressure is the force of the blood against a blood vessel wall. Blood pressure depends on cardiac output and peripheral resistance and is regulated by the central nervous system and various hormones. Blood pressure is responsible for the flow of blood in the arteries, but skeletal muscle contraction causes the flow of blood in the veins. Fetal circulation is discussed and the features of fetal circulation that are not present in adults are described.