The cardiovascular system is composed of the heart and blood vessels. The heart pumps blood through blood vessels in a closed system. Arteries carry blood from the heart to capillaries in the body tissues, whereas veins carry blood back to the heart. The heart and blood vessels function to transport oxygen, nutrients, and hormones to cells and to remove waste products and carbon dioxide from cells. The heart and blood vessels work together to pump and circulate the equivalent of 7200 quarts of blood through the heart every 24 hours.
FIGURE. (A) Anterior view of the heart and major blood vessels. (B) Interior view of the heart. (Adapted from Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 276, with permission.)
The heart, about the size of a human fist, is made of muscle and valves. There are the four chambers—two atria and two ventricles. The walls of the heart chambers have a thick cardiac muscular tissue called the myocardium that contracts and provides the force behind the pumping action of the heart. The endocardium, a single layer of epithelium, lines the chambers of the heart and covers the heart valves. The epicardium, a thin membrane, attaches to the exterior surface of the myocardium. The atria walls are separated by the interatrial septum, and the ventrical walls are separated by the interventricular septum.
The four valves of the heart—the tricuspid or right atrioventricular, pulmonary, bicuspid or mitral, and aortic valves—allow the blood to flow in only one direction. Contraction of the heart is called systole, and relaxation of the heart is called diastole. When the atria contract (atrial systole), the ventricles relax (ventricular diastole). Then the ventricles contract (ventricular systole), and the atria relax (atrial diastole). There is a short interval when both atria and ventricles relax. This whole process constitutes the cardiac cycle.
Blood and Blood Vessels
FIGURE. Pulmonary circulation circuit showing oxygen and carbon dioxide exchange to and from the lungs.
The blood vessel network of the body is composed of arteries, arterioles (small arteries), veins, venules (small veins), and capillaries. Arteries carry blood from the heart; veins carry blood to the heart; capillaries link the arteries and the veins. Blood transports hormones, nutrients, and waste products; it defends the body against infection; and its ability to clot prevents blood loss. Blood circulation is either pulmonary or systemic. Pulmonary circulation pumps blood from the heart into the lungs; systemic circulation pumps blood from the heart to the remainder of the body.
FIGURE. Systemic arteries. The aorta and its major branches are shown in anterior view. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 297, with permission.)
FIGURE. Systemic veins shown in anterior view. (From Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 298, with permission.)
Blood is composed of plasma and blood cells. Although thicker than water, the blood is about 92% water. Plasma is the transportation system for the hormones, nutrients, and waste products. It also consists of important proteins. They include prothrombin and fibrinogen, important in blood clotting; albumin, which helps to keep blood from leaking out of the blood vessels; and three types of globulins that assist in the immune response. The blood’s cells are the erythrocytes, leukocytes, and platelets. Erythrocytes, or red blood cells (RBCs), carry oxygen on the protein hemoglobin. The leukocytes, or white blood cells (WBCs), protect the body from infection and provide immunity against some diseases. The five types of WBCs are neutrophils, eosinophils, basophils, lymphocytes, and monocytes. The platelets or thrombocytes are pieces of cells that contribute to blood clotting and hemostasis.
Lymph and Lymph Vessels
FIGURE. System of lymph vessels and major groups of lymph nodes, showing spleen and thymus glands.
The lymphatic system is composed of lymph capillaries, lacteals (capillaries in the villus of the small intestine), nodes, vessels, and ducts. The lymphatic system transports fluids, nutrients, and wastes exuded from tissues back to the bloodstream through connections with major veins. See Figure for the relationship between lymphatic vessels and the cardiovascular system. Without the lymphatic system, fluid would accumulate in tissue spaces, and “foreign” particles, such as infection, microorganisms, and viruses, would cause disease. The lymphatic system also functions to attack toxins and cancer cells. The thymus and spleen are two important lymphatic organs whose function is similar to the lymphatic system.
Lymph nodes are masses of lymphatic tissue found singly or in groups along the route of lymph vessels. They serve as a filter by destroying microorganisms and abnormal cells harmful to the body through a process called phagocytosis. An increase in the size of the nodes usually indicates a high level of phagocytosis. Three pairs of lymph nodes that are relatively easy to palpate are the cervical nodes (side of the neck), axillary nodes (in the armpit), and inguinal nodes (in the groin region).
The spleen, usually considered a part of the lymphatic system, helps to store platelets and destroys platelets and old RBCs when they are no longer necessary. It also destroys some foreign microorganisms and produces antibodies to foreign antigens. The thymus gland produces T lymphocytes in its stem cells. These cells move to the spleen, lymph nodes, and lymph tissue where they help control cell-mediated immunity.