Introduction to the Cardiovascular and Lymphatic Systems PDF

From Moore's Essential Clinical Anatomy, 7th Edition, pgs. 20‐26 The circulatory system transports fluids throughout the body; it consists of the cardiovascular and lymphatic systems. The heart and blood vessels make up the blood transportation network, the cardiovascular system. Through this sys...

From Moore's Essential Clinical Anatomy, 7th Edition, pgs. 20‐26 The circulatory system transports fluids throughout the body; it consists of the cardiovascular and lymphatic systems. The heart and blood vessels make up the blood transportation network, the cardiovascular system. Through this system, the heart pumps blood through the body’s vast system of blood vessels. The blood carries nutrients, oxygen, and waste products to and from the cells. The heart consists of two muscular pumps that, although adjacently located, act in series, dividing the circulation into two components: the pulmonary and systemic circulations or circuits There are three types of blood vessels: arteries, veins, and capillaries. Blood under high pressure leaves the heart and is distributed to the body by a branching system of thicker‐walled (more muscular) arteries. The final distributing vessels, arterioles, deliver oxygenated blood to capillaries. Minute but numerous thin‐walled capillaries form a capillary bed, where the interchange of oxygen, nutrients, waste products, and other substances with the extracellular fluid occurs. Blood from the capillary bed passes into venules, which resemble wide capillaries. Venules drain into small veins that open into larger veins. The largest veins, the superior vena cava (SVC) and inferior vena cava (IVC), return poorly oxygenated blood to the heart. Arteries carry blood away from the heart and distribute it to the body. Large elastic arteries (conducting arteries) have many elastic layers in their walls; examples are the aorta and its branches from the arch of the aorta. The maintenance of blood pressure in the arterial system between contractions of the heart results from the elasticity of these arteries. This quality allows them to expand when the heart contracts and to return to normal between cardiac contractions. Medium muscular arteries (distributing arteries) have walls that consist mainly of smooth muscle circularly arranged; one example is the femoral artery. The ability of these arteries to decrease their diameter (vasoconstrict) regulates the flow of blood to different parts of the body as required. Small arteries and arterioles have relatively narrow lumina and thick muscular walls. The degree of arterial pressure within the vascular system is mainly regulated by the degree of tonus (firmness) in the smooth muscle of the arteriolar walls. If the tonus of muscle in the arteriolar wall is above normal, hypertension (high blood pressure) results. Anastomoses (communications) between multiple branches of an artery provide numerous potential detours (collateral circulation) for blood flow in case the usual pathway is obstructed by compression due to the position of a joint, pathology, or surgical ligation. Arteries that do not anastomose with adjacent arteries are true (anatomic) terminal arteries (end arteries). Occlusion of an end artery interrupts the blood supply to the structure or segment of an organ it supplies (e.g. occlusion of terminal arteries to the retina will result in blindness). Veins return poorly oxygenated blood to the heart from the capillary beds. The large pulmonary veins are atypical in that they carry well‐oxygenated blood from the lungs to the heart. Because of the lower blood pressure in the venous system, the walls of veins are thinner than those of their companion arteries. The smallest veins, venules, unite to form larger veins that usually form venous plexuses, such as the dorsal venous arch of the foot. Medium veins in the limbs, where the flow of blood is opposed by the pull of gravity, and other locations (such as the neck) have valves that permit blood to flow toward the heart but not in the reverse direction. Large veins, such as the SVC and IVC, are characterized by wide bundles of longitudinal smooth muscle and a well‐developed tunica adventitia. Veins are more variable than the arteries and more frequently form anastomoses. Although often depicted as single vessels, veins usually consist of two or more vessels. The veins that accompany deep arteries (accompanying veins) surround them in a branching network and occupy a relatively unyielding vascular sheath with the artery they accompany. As a result, they are stretched and flattened as the artery expands during contraction of the heart, which assists in driving the venous blood toward the heart. The outward expansion of the bellies of contracting skeletal muscles in the legs, for example, compresses the veins, “milking” the blood superiorly toward the heart; this is known as the musculovenous pump. Capillaries are simple endothelial tubes connecting the arterial and venous sides of the circulation. They are generally arranged in networks (capillary beds) between the arterioles and venules. The blood flowing through capillaries is brought to them by arterioles and carried away from them by venules. As the hydrostatic pressure in the arterioles forces blood through the capillary bed, oxygen, nutrients, and other cellular materials are exchanged with the surrounding tissue. In some regions, such as in the fingers, there are direct connections between the small arteries and veins proximal to the capillary beds they supply and drain, arteriovenous anastomoses (AV shunts) that permit blood to pass directly from the arterial to the venous side of the circulation without passing through capillaries. In some situations, blood passes through two capillary beds before returning to the heart; a venous system linking two capillary beds constitutes a portal venous system. The venous system by which nutrient‐rich blood passes from the capillary beds of the alimentary tract to the capillary beds or sinusoids of the liver—the hepatic portal system—is the major example. The lymphatic system provides for the drainage of surplus tissue fluid (lymph) and leaked plasma proteins to the bloodstream and for the removal of cellular debris and infection. The lymphoid system consists of the following structures: Lymphatic plexuses, networks of small lymphatic vessels; lymphatic capillaries that originate in the extracellular spaces of most tissues (Fig. 1.21B) Lymphatic vessels (lymphatics), a nearly body‐wide network of thin‐walled vessels with abundant valves originating from lymphatic plexuses along which lymph nodes are located. Lymphatic vessels occur almost everywhere blood capillaries are found, except, for example, teeth, bone, bone marrow, and the entire central nervous system (excess fluid here drains into the cerebrospinal fluid). Lymph nodes, small masses of lymphatic tissue through which lymph is filtered on its way to the venous system Lymphocytes, circulating cells of the immune system that react against foreign materials Lymphoid organs, sites that produce lymphocytes, such as that found in the walls of the digestive tract; in the spleen, thymus, and lymph nodes; and in myeloid tissue in red bone marrow After traversing one or more lymph nodes, lymph enters larger lymphatic vessels, called lymphatic trunks, which unite to form either the right lymphatic duct or the thoracic duct: The right lymphatic duct drains lymph from the body’s right upper quadrant (right side of head, neck, and thorax and the entire right upper limb). The duct ends in the angular junction of the right subclavian and internal jugular veins, called the right venous angle. The thoracic duct drains lymph from the remainder of the body. This duct begins in the abdomen as a dilatation, the cisterna chyli, and ascends through the thorax and enters the junction of the left internal jugular and left subclavian veins, called the left venous angle. Superficial lymphatic vessels in the skin and subcutaneous tissue eventually drain into a deep lymphatic vessel. The deep vessels accompany the major blood vessels. Be sure to note the locations of the major groups of lymph nodes in the diagram above. The most common acquired disease of arteries is arteriosclerosis (hardening of arteries), a group of diseases characterized by thickening and loss of elasticity of arterial walls. Atherosclerosis, a common form of arteriosclerosis, is associated with the buildup of fat (mainly cholesterol) in the arterial walls. Calcium deposits then form an atheromatous plaque, resulting in arterial narrowing and irregularity. This may result in thrombosis (formation of a local thrombus [clot]), which may occlude the artery or be flushed into the bloodstream, resulting in ischemia (reduction of blood supply to an organ or region) and infarction (local death of an organ or tissue). Among the consequences of a thrombus are myocardial infarction (heart attack), stroke, and gangrene (necrosis in parts of the limbs). When the walls of veins lose their elasticity or deep fascia becomes incompetent in sustaining the musculovenous pump, the veins become weak and dilate under the pressure of supporting a column of blood against gravity. This results in varicose veins, abnormally swollen, twisted veins, most often seen in the legs. Varicose veins have a caliber greater than normal, and their valve cusps do not meet or have been destroyed by inflammation. These veins have incompetent valves; thus, the column of blood ascending toward the heart is unbroken, placing increased pressure on the weakened walls of the veins and exacerbating their varicosities. Lymphogenous spread of cancer is the most common route for the initial dissemination of carcinomas (epithelial tumors), the most common type of cancer. Cells loosened from the primary cancer site enter and travel via lymphatics. The lymph‐borne cells are filtered through and trapped by lymph nodes, which thus become secondary (metastatic) cancer sites.