Circulatory System in Animals PDF

Circulatory system in animals Single-celled organisms that live in seawater have an external environment that provides all of the needs of the organisms, such as food, disposal of excreted wastes, and relatively constant conditions for maintenance of life. As the co...

Circulatory system in animals Single-celled organisms that live in seawater have an external environment that provides all of the needs of the organisms, such as food, disposal of excreted wastes, and relatively constant conditions for maintenance of life. As the complexity of organisms increases, the problem of supplying each cell with a proper environment becomes more acute. Higher forms of animals have developed circulating blood, and the fluids derived from it, as a means of maintaining a relatively constant environment for all cells. (1) (2) Animals without circulatory systems: Simple animals consisting of a single cell Examples of animal circulatory systems: (a) Fish have the simplest layer, such as the (a) sponge, or only a few circulatory systems of the vertebrates: blood flows unidirectionally cell layers, such as the (b) jellyfish, do not from the two-chambered heart through the gills and then to the rest of have a circulatory system. Instead, gases, the body. (b) Amphibians have two circulatory routes: one for nutrients, and wastes are exchanged by diffusion. oxygenation of the blood through the lungs and skin, and the other to take oxygen to the rest of the body. The blood is pumped from a three-chambered heart with two atria and a single ventricle. (c) Reptiles also have two circulatory routes; however, blood is only oxygenated through the lungs. The heart is three chambered, but the ventricles are partially separated so some mixing of oxygenated and deoxygenated blood occurs, except in crocodilians and birds. (d) Mammals and birds have the most efficient heart with four chambers that completely separate the oxygenated and deoxygenated blood; it pumps only oxygenated blood through the body and deoxygenated blood to the lungs. 1 Open and closed circulation systems in animals: There are two types of circulatory systems found in animals: open and closed circulatory systems:  A closed circulatory system, found in all vertebrates and some invertebrates, circulates blood unidirectionally from the heart, around the body, and back to the heart.  An open circulatory system, found in arthropods, pumps blood into a cavity called a hemocoel where it surrounds the organs and then returns to the heart(s) through ostia (openings). The blood found in arthropods, a mix of blood and interstitial fluid, is called hemolymph.  The circulatory system of homeotherms with the double closed circulation consists of two subdivisions: 1. The cardiovascular system It consists of the heart and blood vessels. 2. Lymphatic system It includes lymphatic vessels and lymphoid tissues within the spleen, thymus, tonsils, and lymph nodes. Functions of the Circulatory System 1. Transportation  In the lungs, oxygen from the inhaled air attaches to hemoglobin molecules within the erythrocytes and is transported to the cells for aerobic respiration. Carbon dioxide produced by cell respiration is carried by the blood to the lungs for elimination in the exhaled air.  The digestive system is responsible for the mechanical and chemical breakdown of food so that it can be absorbed through the intestinal wall into the blood and lymphatic vessels. The blood then carries these absorbed products of digestion through the liver to the cells of the body.  Metabolic wastes (such as urea), excess water and ions, and other molecules not needed by the body are carried by the blood to the kidneys and excreted in the urine. 2. Regulation  The blood carries hormones from their site of origin to distant target tissues where they perform a variety of regulatory functions.  Temperature regulation is aided by the diversion of blood from deeper to more superficial cutaneous vessels or vice versa. When the ambient temperature is high, diversion of blood from deep to superficial vessels helps cool the body, and when 2  the ambient temperature is low, the diversion of blood from superficial to deeper vessels helps keep the body warm. 3. Protection  The clotting mechanism protects against blood loss when vessels are damaged.  The immune function of the blood is performed by the leukocytes (white blood cells) that protect against many disease-causing agents (pathogens). The cardiovascular system Blood Blood accounts for between 6-10% of the body weight of animals, varying with the species and the stage of life. Animals cannot tolerate losses of greater than 3% of the total volume when the condition known as shock occurs. Blood is a circulating tissue composed of fluid plasma and cells (red blood cells, white blood cells, platelets). Anatomically, blood is considered a connective tissue, due to its origin in the bones and its function. Blood is the means and transport system of the body used in carrying elements (e.g. nutrition, waste, heat) from one location in the body to another, by way of blood vessels. Composition of the blood: Cellular portion constitutes approximately 45% of the total blood volume, and the plasma accounts for the remaining 55%. Plasma makeup Plasma is made up of 90% water, 7-8% soluble proteins (albumins, globulins, and clotting proteins). Cellular portion 1. Red Blood Cells Red blood cell (erythrocyte) also known as "RBC's". Red blood cells are biconcave, disc, lose nuclei upon maturation [Mature erythrocytes of mammals have no nuclei. Immature forms in the bone marrow do have nuclei, but these are lost during the latter stages of development. In birds and reptiles, nuclei normally persist in the red cells throughout the life of the cells]. They are about 7-8 micrometers in diameter, and formed red bone marrow. Total erythrocyte counts are expressed as number of cells per microliter of whole blood, and most domestic animals have about 7 million per microliter RBC's live about 120 days. RBC's contain hemoglobin which transports oxygen from the lungs to the rest of the body, such as to the muscles, where it releases the oxygen load. The hemoglobin gets it's red color from their respiratory pigments. 2. White Blood Cells White blood cells (leukocytes) are also known as "WBC's". White blood cells also have nucleii that are somewhat segmented. White blood cells are made in the bone marrow but they also divide in the blood and lymphatic systems. 3  The different types of WBC's 1. Granular leukocytes [Neutrophils, Eosinophils, and Basophils]. 2. Agranular leukocytes [Lymphocytes and Monocytes]. 3. Platelets Platelets, or thrombocytes, are the smallest of the formed elements and are actually fragments of large cells called megakaryocytes, which are found in bone marrow. Platelets play an important role in blood clotting. Structure of the heart The pericardium, or pericardial sac, is a covering that encloses the heart and the proximal ends of the large blood vessels to which it attaches. Heart Chambers and Valves (Fig.1) The heart is a hollow, cone-shaped, muscular pump. The heart contains four chambers: two atria, which receive venous blood, and two ventricles, which eject blood into arteries. A structure called the interatrial septum separates the right from the left atrium. An interventricular septum separates the two ventricles. These septa normally prevent mixture of the blood from the two sides of the heart. The atrium on each side communicates with its corresponding ventricle through an opening called the atrioventricular orifice, guarded by an atrioventricular valve (AV valve). Heart Chambers The heart has four chambers, two atria and two ventricles. The atriums are smaller with thin walls, while the ventricles are larger and much stronger. Atrium There are two atria on either side of the heart. On the right side is the atrium that holds blood that needs oxygen. The left atrium holds that blood that has been oxygenated and is ready to be sent to the body. Ventricles The ventricle is a heart chamber which collects blood from an atrium and pumps it out of the heart. There are two ventricles: the right ventricle pumps blood into the pulmonary circulation for the lungs, and the left ventricle pumps blood into the systemic circulation for the rest of the body. Ventricles have thicker walls than the atria, and thus can create the higher blood pressure. 4  (Fig.1) Heart Chambers and Valves Valves are structure with the heart that prevent the blood form regurgitation in the opposite direction during contraction. The AV valves allow blood to flow from the atria to the ventricles, but they normally prevent the backflow of blood into the atria. Opening and closing of these valves occur as a result of pressure differences between the atria and ventricles. valve Function Tricuspid Right atrioventricular orifice Prevents blood from moving from valve right ventricle into right atrium during ventricular contraction Mitral valve Left atrioventricular orifice Prevents blood from moving from left ventricle into left atrium during ventricular contraction Pulmonary Entrance to pulmonary trunk Prevents blood from moving from valve pulmonary trunk into right ventricle during ventricular relaxation Aortic valve Entrance to aorta Prevents blood from moving from aorta into left ventricle during ventricular relaxation 5 The vascular system (blood vessels) The blood vessels form a closed circuit of tubes that transports blood between the heart and body cells. The tubes include arteries, arterioles, capillaries, venules, and veins. vessel Type of wall Function Artery Thick, strong wall with three layers—an Carries blood under relatively endothelial lining, a middle layer of smooth high pressure from the heart muscle and elastic tissue, and an outer layer to arterioles. of connective tissue. Arteriole Thinner wall than an artery but with three Connects an artery to a layers; smaller arterioles have an endothelial capillary, helps control the lining, some smooth muscle tissue, and a blood flow into a capillary by small amount of connective tissue. vasoconstricting or vasodilating. Vessel Type of wall Function Capillary Single layer of squamous epithelium Provides a membrane through which nutrients, gases, and wastes are exchanged between the blood and tissue fluid; connects an arteriole to a venule. Venule Thinner wall than an arteriole, less smooth Connects a capillary to a vein. muscle and elastic tissue. Vein Thinner wall than an artery but with similar Carries blood under relatively layers; the middle layer is more poorly low pressure from a venule to developed; some have flap-like valves. the heart; valves prevent a backflow of blood; serves as blood reservoir. Lymphatic system Lymphatic system: The tissues and organs, including the bone marrow, spleen, thymus, and lymph nodes, that produce and store cells that fight infection and disease. The channels that carry lymph are also part of this system.  Lymph: The almost colorless fluid that travels through the lymphatic system, carrying cells that help fight infection and disease. Lymphatic vessels absorb excess interstitial fluid and transport this fluid—now called lymph—to ducts that drain into veins. Lymph nodes, and lymphoid tissue in the thymus, spleen, and tonsils, produce lymphocytes, which are white blood cells involved in immunity. The lymphatic system has three basic functions: (1) It transports interstitial (tissue) fluid, initially formed as a blood filtrate, back to the blood. 6  (2) It transports absorbed fat from the small intestine to the blood. (3) Its cells—called lymphocytes —help provide immunological defenses against disease-causing agents (pathogens). Lymphatic capillaries are the smallest vessels of the lymphatic system. Lymphatic capillaries are microscopic closed-ended tubes that form vast networks in the intercellular spaces within most organs. Because the walls of lymphatic capillaries are composed of endothelial cells with porous junctions, interstitial fluid, proteins, white blood cells, microorganisms, and absorbed fat (in the intestine) can easily enter. Once fluid enters the lymphatic capillaries, it is referred to as lymph. The lymph is carried into larger lymphatic vessels called lymph ducts. Fluid movement within these vessels occurs as a result of peristaltic waves of contraction. The smooth muscle within the lymph ducts contains a pacemaker that initiates action potentials associated with the entry of Ca2+, which stimulates contraction. The lymph ducts eventually empty into one of two principal vessels: the thoracic duct or the right lymphatic duct. These ducts drain the lymph into the left and right subclavian veins, respectively. Thus interstitial fluid, which is formed by filtration of plasma out of blood capillaries is ultimately returned to the cardiovascular system (fig.2). Before the lymph is returned to the cardiovascular system, it is filtered through lymph nodes. Lymph nodes contain phagocytic cells, which help remove pathogens, and germinal centers, which are sites of lymphocyte production. The tonsils, thymus, and spleen—together called lymphoid organs —likewise contain germinal centers and are sites of lymphocyte production for the immune system. 7 (Fig.2) Schematic representation of lymphatic vessels transporting fluid from interstitial spaces to the bloodstream. 8