Chapter Summary

Internal Affairs (110.0K)

Functions and Major Components of the Circulatory System

1. The circulatory system transports oxygen and nutritive molecules to the tissue cells and carbon dioxide and other wastes away from tissue cells; it also carries hormones and other regulatory molecules to their target organs.
2. Leukocytes and their products help to protect the body from infection, and platelets function in blood clotting.
3. The components of the circulatory system are the heart, blood vessels, and blood, which constitute the cardiovascular system, and the lymphatic vessels and lymphoid tissue and organs of the lymphatic system.

Blood

1. Blood, a highly specialized connective tissue, consists of formed elements (erythrocytes, leukocytes, and platelets) suspended in a watery fluid called plasma.
2. Erythrocytes are disc-shaped cells that lack nuclei but contain hemoglobin. There are approximately 4 million to 6 million erythrocytes per cubic millimeter of blood, and they are functional for about 120 days.
3. Leukocytes have nuclei and are classified as granular (eosinophils, basophils, and neutrophils) or agranular (monocytes and lymphocytes). Leukocytes defend the body against infections by microorganisms.
4. Platelets, or thrombocytes, are cytoplasmic fragments that assist in the formation of clots to prevent blood loss.
5. Erythrocytes are formed through a process called erythropoiesis; leukocytes are formed through leukopoiesis.
6. Prenatal hemopoietic centers are the yolk sac, liver, and spleen. In the adult, bone marrow and lymphoid tissues perform this function.

Heart

1. The heart is enclosed within a pericardial sac. The wall of the heart consists of the epicardium, myocardium, and endocardium.
1. The right atrium receives blood from the superior and inferior venae cavae, and the right ventricle pumps blood through the pulmonary trunk into the pulmonary arteries.
2. The left atrium receives blood from the pulmonary veins, and the left ventricle pumps blood into the ascending aorta.
3. The heart contains right and left atrioventricular valves (the tricuspid and bicuspid valves, respectively); a pulmonary semilunar valve; and an aortic (semilunar) valve.

2. The two principal circulatory divisions are the pulmonary and the systemic; in addition, the coronary system serves the heart.
1. The pulmonary circulation includes the vessels that carry blood from the right ventricle through the lungs, and from there to the left atrium.
2. The systemic circulation includes all other arteries, capillaries, and veins in the body. These vessels carry blood from the left ventricle through the body and return blood to the right atrium.
3. The myocardium of the heart is served by right and left coronary arteries that branch from the ascending portion of the aorta. The coronary sinus collects and empties the blood into the right atrium.

3. Contraction of the atria and ventricles is produced by action potentials that originate in the sinoatrial (SA) node.
1. These electrical waves spread over the atria and then enter the atrioventricular (AV) node.
2. From here, the impulses are conducted by the atrioventricular bundle and conduction myofibers into the ventricular walls.

4. During contraction of the ventricles, the intraventricular pressure rises and causes the AV valves to close; during relaxation, the pulmonary and aortic valves close because the pressure is greater in the arteries than in the ventricles.
5. Closing of the AV valves causes the first sound (lub); closing of the pulmonary and aortic valves causes the second sound (dub). Heart murmurs are commonly caused by abnormal valves or by septal defects.
6. A recording of the pattern of electrical conduction is called an electrocardiogram (ECG or EKG).

Blood Vessels

1. Arteries and veins have a tunica externa, tunica media, and tunica interna.
1. Arteries have thicker muscle layers in proportion to their diameters than do veins because arteries must withstand a higher blood pressure.
2. Veins have venous valves that direct blood to the heart when the veins are compressed by the skeletal muscle pumps.

2. Capillaries are composed of endothelial cells only. They are the basic functional units of the circulatory system.

Principal Arteries of the Body

1. Three arteries arise from the aortic arch: the brachiocephalic trunk, the left common carotid artery, and the left subclavian artery. The brachiocephalic trunk divides into the right common carotid artery and the right subclavian artery.
2. The head and neck receive an arterial supply from branches of the internal and external carotid arteries and the vertebral arteries.
1. The brain receives blood from the paired internal carotid arteries and the paired vertebral arteries, which form the cerebral arterial circle surrounding the pituitary gland.
2. The external carotid artery gives off numerous branches that supply the head and neck.

3. The upper extremity is served by the subclavian artery and its derivatives.
1. The subclavian artery becomes first the axillary artery and then the brachial artery as it enters the arm.
2. The brachial artery bifurcates to form the radial and ulnar arteries, which supply blood to the forearm and hand.

4. The abdominal portion of the aorta has the following branches: the inferior phrenic, celiac trunk, superior mesenteric, renal, suprarenal, testicular (or ovarian), and inferior mesenteric arteries.
5. The common iliac arteries divide into the internal and external iliac arteries, which supply branches to the pelvis and lower extremities.

Principal Veins of the Body

1. Blood from the head and neck is drained by the external and internal jugular veins; blood from the brain is drained by the internal jugular veins.
2. The upper extremity is drained by both superficial and deep veins.
3. In the thorax, the superior vena cava is formed by the union of the two brachiocephalic veins and also collects blood from the azygos system of veins.
4. The lower extremity is drained by both superficial and deep veins. At the level of the fifth lumbar vertebra, the right and left common iliac veins unite to form the inferior vena cava.
5. Blood from capillaries in the GI tract is drained via the hepatic portal vein to the liver.
1. This venous blood then passes through hepatic sinusoids and is drained from the liver in the hepatic veins.
2. The pattern of circulation characterized by two capillary beds in a series is called a portal system.

Fetal Circulation

1. Structural adaptations in the fetal cardiovascular system reflect the fact that oxygen and nutrients are obtained from the placenta rather than from the fetal lungs and GI tract.
2. Fully oxygenated blood is carried only in the umbilical vein, which drains the placenta. This blood is carried via the ductus venosus to the inferior vena cava of the fetus.
3. Partially oxygenated blood is shunted from the right to the left atrium via the foramen ovale and from the pulmonary trunk to the aorta via the ductus arteriosus.

Lymphatic System

1. The lymphatic system returns excess interstitial fluid to the venous system and helps to protect the body from disease; it also transports fats from the small intestine to the blood.
2. Lymphatic capillaries drain interstitial fluid, which is formed from blood plasma; when this fluid enters lymphatic capillaries, it is called lymph.
3. Lymph is returned to the venous system via two large lymph ducts-the thoracic duct and the right lymphatic duct.
4. Lymph filters through lymph nodes, which contain phagocytic cells and lymphatic nodules that produce lymphocytes.
5. Lymphoid organs include the lymph nodes, tonsils, spleen, and thymus.