Zoology Notes: 014 Chapter 10

Chapter 10. The Circulatory System The circulatory system nourishes every part of the body. It has three main parts: the heart, blood vessels and the blood itself. The fluid bathing the body tissues is derived from the blood; the pump circulating the blood is the heart; the tubes through which the blood flows are the blood vessels. Blood If the blood is allowed to stand, it separates into two distinct fractions. Less than half the blood consists of the solid part: the RBC, WBC, and the platelets. The remainder is a straw colored fluid, the plasma. Erythrocytes. Red blood cell formation (erythropoiesis) is stimulated by any factor lowering the oxygen available to the body tissues. Therefore, high altitudes, hemorrhage, nutritional deficiencies, and endocrine disturbances may stimulate the production of red blood cells. The presence of hemoglobin within the rbc is responsible for its ability to transport oxygen. Hemoglobin combines with oxygen from the air of the lungs to form oxyhemoglobin, which in turn, readily gives up its oxygen to tissue cells within the body. Chemically, hemoglobin is a complex organic compound composed of four porphyrin pigments (heme), each of which contains an atom of iron, plus globin, which is a globular protein consisting of four amino acid chains. The life span of erythrocytes is approximately two to four months. The red blood cells disintegrate, releasing Hb into the blood, and the broken cell debris is removed from the circulation by the reticuloendothelial system, which consists of special cells in the liver, spleen, bone marrow, and lymph nodes. The reticuloendothelial cells phagocytize the debris. The fragments are digested and released into the blood. The Hb disintegrates into globin and heme. The globular protein fraction is degraded into amino acids. Heme is broken down into bilirubin and iron. The iron is picked up by the globulin transferin and is then either deposited in bone marrow for use again, combined and stored in the liver as ferritin for future use, contributed to form myoglobin in muscle or stored in tissue cells as hemosiderin. Bilirubin is carried to the liver and excreted with the bile. Two to ten million cells are destroyed each second yet because of replacement, the number of circulating cells remains remarkably constant. Reduction in the total number of rbc in the body results in anemia. Anemia is characterized by the diminished capacity of the blood to transport oxygen to the tissues. Anemia results if either the number of functional red blood cells or the quantity of hemoglobin is decreased much below normal. Anemia may be due to deficient blood formation because of poor nutrition, including dietary deficiency of iron, copper, vitamins, or amino acids. Anemia may also be caused by loss of blood due to hemorrhage from wounds or because of parasites. It is also caused by increased cell destruction, or decreased formation of red blood cells. Leukocytes. Leukocytes or white blood cells differ considerably from erythrocytes in that they are nucleated and are capable of independent movement. They are classified as granulocytes and agranulocytes. Granulocytes contain granules within the cytoplasm that stain with common blood stains, such as Wright’s stain. These cells include neutrophils, eosinophils, and basophils. Agranulocytes usually show little or no granules in their cytoplasm. They include lymphocytes and monocytes. Neutrophils, the most numerous of the granulocytes, contain granules that stain indifferently and are not notably red or blue. They constitute the first line of defense against infection by migrating to any area invaded by bacteria, passing through the vessel walls and engulfing the bacteria to destroy them. In the process, many neutrophils also degrade dead (necrotic) tissue in the area, and the resulting 47

semiliquid material is known as pus. Eosinophils, also known as acidophils show red staining granules in the cytoplasm. These cells, which normally are scarce, increase in number in certain chronic diseases, such as infection with parasites. Basophils, which are blue staining granules, are also rare in normal blood. They contain heparin and may be involved in preventing blood from clotting in areas of inflammation. They also contain some histamine and may possibly be the precursors of mast cells. Lymphocytes are important in the process of immunity, producing antibodies in response to antigens. Monocytes possess a relatively large amount of cytoplasm and function as phagocytes, becoming transformed into macrophages after invading infected sites, where numbers reach a peak in 48 hours. Many diseases are characterized by a change in the number of circulating leukocytes. An increase in the wbc count, generally indicating an acute infection, is called leukocytosis. Leukopenia, a reduction in the number of wbc occurs occasionally in viral diseases. Leukemia is characterized by uncontrolled proliferation of leukocytes which generally resemble immature cells and are usually nonfunctional. Thrombocytes. Platelets or thrombocytes are cytoplasmic fragments of giant, multinucleated red blood cells called megakaryocytes and play an important role in hemostasis, the process of blood clotting. Platelets function chiefly to reduce loss of blood from injured vessels. Platelets clump together to form a plug in the initial phase of controlling bleeding. Clumping is followed by the retraction of platelet pseudopods with enmeshed fibrin and blood cells to produce hard clot. A deficiency in platelets causes a tendency to bleed. Plasma. Blood plasma is a straw colored liquid composed of about 90% water and about 10% chemical compounds, mainly proteins. Plasma can only be maintained if an anticoagulant is added to keep coagulation from occurring. After coagulation, the fibrinogen separates from plasma, leaving serum. The four major plasma proteins are albumin, globulin, fibrinogen, and prothrombin. Blood Groupings. The safe administration of blood from donor to recipient requires typing and cross-matching. These procedures are necessary since a patient receiving blood incompatible with his own blood can experience a serious or fatal reaction. The system of classification is based on the presence of specific antigens (agglutinogens) in the red blood cells and the presence of antibodies (agglutinins) that react to the antigens. ABO Groupings. Blood groups are named for the antigens present in the blood. In each case, the blood contains antibodies to antigens not present in the recipient’s blood. Thus, individuals with type A blood cannot receive blood from blood type B and AB individuals; type B individuals cannot receive blood from type A or AB individuals; type O individuals cannot accept blood from type A, B, or AB individuals; and type AB individuals can accept from all the other blood types. Individuals with type AB blood are universal recipients and those with type O blood are universal donors. Blood type A B O AB

Antigen present A B AB

Antibody present Anti-B Anti-A Anti-A, Anti-B -

Can give blood to A, AB B, AB A, B, O, AB AB

Can receive blood from A, O B, O O A, B, O, AB

Table 10.1. Compatibility between the different blood types.

Rh Factor. The Rh factor was first found in the blood of the rhesus monkey. It is a system consisting of at least 8 antigens. Reaction to one of the antigens determines whether a person is Rh 48

positive or negative. The antigen is usually designated as “D”. This classification is most significant in the condition erythroblastosis fetalis wherein a mother is Rh negative while her child (particularly a second child) is Rh positive. The Heart The heart is a four-chambered, hollow, muscular organ lying between the lungs in the middle mediastinum. The pericardium is the sac enclosing the heart. It is an invaginated sac consisting of an external fibrous coat and an internal serous membrane.

The outer, or parietal, layer of the serous membrane (parietal pericardium) lines the fibrous coat. The inner, or visceral, layer of the serous membrane (visceral pericardium) adheres to the heart and becomes the outermost layer of the heart, the epicardium. The wall of the heart consists of three distinct layers: The epicardium (external layer), the myocardium (middle muscular layer), and the endocardium (inner layer of endothelium). The heart is divided into the right and left halves, each half divided into two chambers, the atria (upper chamber), and the ventricle (lower chamber). The atria are separated by the intra-atrial septum, while the ventricles are separated by the intra-ventricular septum. The atria serve as receiving chambers from the lungs and various parts of the body and pump blood into the ventricles. The ventricles, in turn, pump blood to the lungs and the remainder of the body. The primary function of the Fig. 10.1. The heart and its parts.

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heart is to serve as a muscular pump propelling blood into and through vessels to and from all parts of the body. There are two types of valves located in the heart. The atrio-ventricular valves (the tricuspid valve on the right and the bicuspid (mitral) valve on the left), and the semilunar valves (the pulmonary and the aortic valves). The Blood Vessels The blood vessels consist of a closed system of tubes functioning to transport blood to all parts of then body and back to the heart. Arteries transport blood to various body tissues under high pressure exerted by the pumping action of the heart. Arterioles, the last branches of the arterial system, act as control valves through which blood is released into the capillaries. The focal point of the entire cardiovascular system is the network of about 10 billion microscopic capillaries functioning to provide a method whereby fluids, nutrients, oxygen, carbon dioxide, and wastes are exchanged between the blood and interstitial spaces. Veins function to conduct blood from the body tissues to the heart. Fig. 10.2. Blood circulation.

Two Closed Circuits of the Circulatory System

The circulatory system has two closed circuits: the pulmonary circulation, carrying blood from the right ventricle to the respiratory surfaces of the lungs and back to the left atrium, and the systemic circulation, carrying blood from the left ventricle to the remaining parts of the body and back to the right atrium.

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