The Heart
The heart, a hollow muscular organ, is located in the center of the chest. The right and left sides of the heart each have an upper chamber (atrium), which collects blood and pumps it into a lower chamber (ventricle), which pumps blood out.
To ensure that blood flows in only one direction, each ventricle has an "in" (inlet) valve and an "out" (outlet) valve. In the left ventricle, the inlet valve is the mitral valve, and the outlet valve is the aortic valve. In the right ventricle, the inlet valve is the tricuspid valve, and the outlet valve is the pulmonary (pulmonic) valve. Each valve consists of flaps (cusps or leaflets), which open and close like one-way swinging doors. The mitral valve has two cusps; the others (tricuspid, aortic, and pulmonary) have three. The large inlet valves (mitral and tricuspid) have tethers--consisting of the papillary muscles and cords of tissue--which prevent the valves from swinging backward into the atria. If a papillary muscle is damaged (for example, by a heart attack), the valve may then swing backwards and start leaking. If a valve opening is narrowed, blood flow through the valve is reduced. A valve may have both problems.
The heartbeats are evidence that the heart is pumping. The first sound (the lub of lub-dub) is the sound of the mitral and tricuspid valves closing. The second sound (the dub) is the sound of the aortic and pulmonary valves closing. Each heartbeat has two parts: diastole and systole. During diastole, the ventricles relax and fill with blood; then the atria contract, forcing more blood into the ventricles. During systole, the ventricles contract and pump blood, and the atria relax and begin filling with blood again.
Function of the Heart
The heart's only function is to pump blood. The right side of the heart pumps blood to the lungs, where oxygen is added to the blood and carbon dioxide is removed from it. The left side pumps blood to the rest of the body, where oxygen and nutrients are delivered to tissues and waste products (such as carbon dioxide) are transferred to the blood for removal by other organs (such as the lungs and kidneys).
 See the figure A Look Into the Heart.
Blood travels the following circuit: Blood from the body, which is depleted of oxygen and laden with carbon dioxide, flows through the two largest veins (the venae cavae) into the right atrium. When the right ventricle relaxes, blood in the right atrium pours through the tricuspid valve into the right ventricle. When the right ventricle is nearly full, the right atrium contracts, propelling additional blood into the right ventricle, which then contracts. This contraction propels blood through the pulmonary valve into the pulmonary arteries, which supply the lungs. In the lungs, blood flows through the tiny capillaries that surround the air sacs. Here, the blood absorbs oxygen and gives up carbon dioxide, which is then exhaled.
Blood from the lungs, which is now oxygen-rich, flows through the pulmonary veins into the left atrium. When the left ventricle relaxes, the blood in the left atrium pours through the mitral valve into the left ventricle. When the left ventricle is nearly full, the left atrium contracts, propelling additional blood into the left ventricle, which then contracts. (In older people, the left ventricle does not fill well before the left atrium contracts, making this contraction of the left atrium especially important.) The contraction of the left ventricle propels blood through the aortic valve into the aorta, the largest artery in the body. This blood carries oxygen to all of the body except to the lungs.
The circuit between the right side of the heart, the lungs, and the left atrium is called the pulmonary circulation. The circuit between the left side of the heart, most of the body, and the right atrium is called the systemic circulation.
Blood Supply of the Heart
 See the figure Supplying the Heart With Blood.
Like all organs, the heart needs a constant supply of oxygen-rich blood. A system of arteries and veins called the coronary circulation supplies the heart muscle (myocardium) with oxygen-rich blood and then returns oxygen-depleted blood to the right atrium. The right coronary artery and the left coronary artery branch off the aorta (just after it leaves the heart) to deliver oxygen-rich blood to the heart muscle. These two arteries branch into other arteries, including the circumflex artery, that also supply blood to the heart. The cardiac veins collect blood from the heart muscle and empty it into a large vein on the back surface of the heart called the coronary sinus, which returns the blood to the right atrium. Because of the great pressure exerted in the heart as it contracts, most blood flows through the coronary circulation only while the heart is relaxing between beats (during diastole).
Regulation of the Heart
The contraction of the muscle fibers in the heart is very organized and highly controlled. Rhythmic electrical impulses (discharges) flow through the heart in a precise manner along distinct pathways and at a controlled speed. The impulses originate in the heart's pacemaker (the sinus or sinoatrial node--a small mass of tissue in the wall of the right atrium), which generates a tiny electrical current (see Section 3, Chapter 27).
The rate at which the pacemaker sends out its impulses (and thus governs the heart rate) is determined by two opposing systems--one to speed the heart rate up (the sympathetic division of the nervous system) and one to slow it down (the parasympathetic division (see Section 6, Chapter 76)). The sympathetic division works through a network of nerves called the sympathetic plexus and through the hormones epinephrine (adrenaline) and norepinephrine (noradrenaline), which are released by the adrenal glands and the nerve endings. The parasympathetic division works through a single nerve--the vagus nerve--which releases the neurotransmitter acetylcholine.
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