Heart

The heart is a two-sided, double pump. It weighs less than a pound and is slightly larger than a fist. The heart is located between the lungs in the thoracic cavity, positioned partially to the left of the sternum. The base or topmost (superior) part of the heart has a flatter shape than the tapered apex or lower (inferior) portion.

The right side of the heart pumps oxygen-poor (deoxygenated) blood to the lungs, where carbon dioxide is exchanged for oxygen. This is referred to as pulmonary circulation. The left side pumps the oxygen-rich (oxygenated) blood to the rest of the body. This is referred to as systemic circulation. The blood returns to the right side of the heart from the body to complete the cycle (Fig. 11-1).

Hepatic circulation refers to the path of the blood from the intestines, gallbladder, pancreas, stomach, and spleen through the liver. The liver stores and modifies nutrients in the blood for use by the body. It also removes or alters toxic substances so that they can be eliminated by the urinary system. The nutrient-rich blood, which has been filtered by the liver, is returned to the heart through the inferior vena cava for use throughout the body.

The heart has four chambers (Fig. 11-2). The top chambers are called atria. The lower chambers are called ventricles. The blood enters the heart through the atria and leaves the heart from the ventricles. The septum divides the right and left sides of the heart. Four valves prevent the blood from flowing backward through the system. Two of these valves are called atrioventricular (AV) valves. They separate the atria and ventricles on each side of the heart. The semilunar valves separate the ventricles from the outgoing vessels (pulmonary artery and aorta). The valves are named according to their structure (semilunar) and location (pulmonary or aortic).

The heart has three layers of tissue. The endocardium is a smooth layer of cells lining the inside of the heart and forming the valves. The smoothness of the endocardial tissue helps prevent damage to blood cells circulating through the system. The myocardium is the thickest layer, consisting of muscle tissue. This part of the heart pumps blood through the system. The pericardium is a double membrane that covers the outside of the heart, providing lubrication between the heart and surrounding structures to prevent tissue damage. The pericardial sac is made up of the inner serous (watery) and outer fibrous layers.

The activity of the heart muscle is controlled largely by the nervous system but is affected also by the action of hormones and other mechanisms such as fluid balance. Additionally, the heart contains the only muscle tissue that can stimulate its own contractions. Specialized sinoatrial (SA) cells in the right atrium (SA node) act as a pacemaker to start a heart contraction (Fig. 11-3). The change in the electrical potential of these cells stimulates another group of cells, called the atrioventricular node (AV node), to send the impulse into the lower portions of the heart. The impulse of the AV node stimulates specialized bundles of muscle called the AV bundle or bundle of His. These fibers then stimulate the Purkinje fibers, which surround the lower portions of the ventricles. The Purkinje fibers cause the ventricles to contract. Another unique property of the heart is the ability to adjust the strength of the contractions on the basis of the amount of blood in its chambers. Without the influence of the nervous system and other controls, the heart would contract only 40 times each minute instead of the normal 60 to 90 times. The adult heart beats 10,000 times a day.

Blood Vessels

The body has three main types of blood vessels (Fig. 11-4):

Blood is pumped from the heart to the body by the largest artery in the body, the aorta. Fig. 11-5 shows the principal arteries of the body. The aorta branches into other arteries, which in turn branch into smaller vessels called arterioles. The blood moves from arterioles to microscopic capillaries. Gases, nutrients, and wastes are exchanged through the thin walls of the capillaries. The blood, which has now given up its oxygen, flows from the capillaries into tiny veins called venules. Venules branch together to form larger veins (Fig. 11-5). The blood is returned to the heart in the body’s largest veins, the superior vena cava and inferior vena cava. With the exception of the pulmonary artery, blood in the arteries is oxygenated. Except for the pulmonary vein, blood in veins is deoxygenated.

Arteries have a muscular layer of tissue that helps pump blood out to the body. Veins have a much thinner muscular layer. Gravity and the movement of the muscles surrounding the veins help deliver blood back to the heart. Veins also have valves that prevent blood from flowing back, away from the heart, once it has moved forward.

Path of the Blood Through the Heart

Tracing the path of a blood cell through the heart is one way to learn the heart’s structures and understand its functions (Table 11-1). Although the heart is considered a two-sided pump to differentiate the systemic and pulmonary circulation, the two atria contract at the same time, then the ventricles contract. Deoxygenated blood enters the right atrium of the heart from the body through the inferior and superior vena cavae. Additionally, blood from the heart muscle itself returns through a structure called the coronary sinus. The blood then passes through the tricuspid valve into the right ventricle. This valve closes as the pulmonary valve opens, allowing the passage of blood from the right ventricle to the pulmonary arteries. The pulmonary valve closes as the blood enters the lungs for the diffusion of oxygen and carbon dioxide. The oxygenated blood then travels through the pulmonary veins to the left atrium. From the left atrium the blood travels through the bicuspid or mitral valve to the left ventricle. The mitral valve closes as the blood leaves the left ventricle through the aortic valve. The blood then travels through the aorta to the rest of the body. As the ascending aorta leaves the heart, it branches in three directions to supply blood to the head and upper limbs. Two coronary arteries, which supply blood to the heart, branch off of the ascending aorta. The descending portion of the aorta supplies blood to the abdominal area and lower extremities. Deoxygenated blood is returned to the heart through the inferior and superior vena cavae from the body to complete the path.

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