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Circulatory Systems (Chpt 46, 5th Ed; Chpt 49, 6th Ed) Gastrovascular Cavities: • In primitive animals, where each cell is close enough to its environment to exchange gases by diffusion, a circulatory system is unnecessary. • In animals like the hydra (Fig. 49.1), the entire middle of the animal is a large sac that serves for digestion and transport. • Planarians have a slightly more elaborate gastrovascular cavity, but the principle remains the same. 1 Open Circulatory Systems: • These are more common in animals like insects, where the blood does not serve a major gas transport function. • The blood and interstitial fluid are collectively know as hemolymph and are circulated by a heart. • Fluid enters the heart through pores called ostia and then is pumped back to the hemocoel (Fig. 49.2). Closed Circulatory Systems: • In closed circulatory systems, the blood remains within blood vessels at all times, separating the blood from the interstitial (lymphatic) fluid. The advantages of a closed system are: 1) delivery of gasses and nutrients to the tissues is more rapid. 2) blood flow can be directed to specific tissues as needed. 3) blood cells and large molecules are confined to a restricted compartment. 2 • In vertebrates the flow of blood is from the heart => arteries => arterioles => capillaries => venules => veins => heart. Circulatory Systems of Fishes: • Fishes have a two-chambered heart. The atrium receives systemic blood and pumps it into the more muscular ventricle. • Blood is pumped through the gills for gas exchange and then through the large aorta which distributes it to the tissues after which it returns to the heart (page 868). • The high resistance of the gills dissipates most of the blood pressure, so the systemic blood pressure is very low. • Lung fish have modified this system somewhat in that part of the gill circulation goes through the air bladder (rather than the aorta) and returns directly to the heart, which has two atria. • This may be how two separate circulatory systems evolved (page 869). 3 Circulatory Systems of Amphibians: • Amphibians have a three-chambered heart and separation of the pulmonary (lung) and systemic (body) circulation. • The left atrium receives oxygenated blood from the lungs and the right atrium receives deoxygenated blood from the tissues. Both pump into the single ventricle which pumps blood to both the lungs and the tissues (page 869). • This mixing of the oxygenated and deoxygenated blood sounds inefficient, but in fact the spiral valve inside the aorta effectively separates pulmonary and systemic blood under most conditions. • Keeping pulmonary separate from systemic circulation permits higher blood pressures and better circulation. Circulatory Systems of Reptiles: •Reptiles have gone a little farther in that while they have three-chambered hearts, there is a partial septum, or wall, (complete in the crocodilians) that separates pulmonary and systemic blood flow (page 870). 4 Circulatory Systems of Birds and Mammals: • Birds and mammals have complete four-chambered hearts. • In essence there are two separate hearts joined together. The right heart pumps blood to the lungs, the left heart takes the oxygenated blood and pumps it out to the body (page 870). Fig. 46.4 • The left heart is generally larger because it has a larger work load. • Pulmonary resistance is high, but high blood pressures would tend to make the lungs leaky, which is not desirable. • The two circulations operate at different pressures and keeping them separate means that fully oxygenated blood is available at high pressures to the body. This is important for sustained rapid activity. 5 The Human Heart: Two Pumps in One: • The human heart is really two pumps, one for pulmonary circulation, one for systemic. • A series of one-way valves prevent backflow and keep the circulation unidirectional. • The atrioventricular valves lie between the atria and the ventricles, the pulmonary valve sits between the right ventricle and the pulmonary artery and the aortic valve seals the aorta from the left ventricle (Fig. 49.4). • Normally the two hearts contract together in the cardiac cycle, which consists of contraction (systole) followed by relaxation (diastole, Fig. 49.5). 6 • Normal blood pressures (systolic/diastolic, measured as mm Hg) are 120/80 in %%, lower in &&. • Medically, the major concern is the diastolic pressure; values above 95 cause long-term cardiac damage, values above 105 pose immediate risk for heart failure or stroke. Cardiac Muscle and the Heartbeat: • Cardiac muscle is myogenic, it can beat without nervous input. • Near the junction of the superior vena cava with the right atrium lies the sinoatrial node, the pacemaker for the heart. • At regular intervals the node fires (initiates an action potential). • Because the cells in the two atria are linked by intercalated disks, a wave of contraction spreads through the atria and they contract, forcing blood into their respective ventricles. • Electrical resistance between atria and ventricles is high, so only the atria contract. • However, another specialized region, the atrioventricular node, picks up the AP, and after a slight delay sends an AP to the bundle of His and the Purkinje fibers. 7 • The Purkinje fibers are arranged so that the ventricles contract from the tip upwards, producing the maximum force into the pulmonary artery and the aorta (Fig. 49.8). • This intense electrical activity can be detected even at the skin surface to produce a recording called an electrocardiogram, or EKG, as shown in Fig. 49.8. The Vascular System: Arteries and arterioles: • Blood pressure is highest in those vessels that carry blood away from the heart. • The walls of these vessels have elastic fibers and smooth muscle. • The elastic fibers allow them to withstand high pressures, as well as to expand during systole, and contract during diastole. • This dampens (smooths out) the pulses in the blood flow and maintains arterial pressure high. • Arteries also have smooth muscle, which can contract or relax to direct blood flow. 8 Capillaries: • Blood moves from arteries to veins through a series of ever smaller vessels until the level of the capillary bed, where the blood vessels are so small that the red cells must move through them in single file (Fig. 49.11). • There are so many branches that the total crosssectional area of the capillaries is greater than any other class of blood vessel and no cell in the body is more than two cells from a capillary (Fig. 49.12). Fig. 46.11 9 Exchange in Capillary Beds: • Capillary walls are permeable to water and small molecules, so hydrostatic pressure (blood pressure) tends to force these out of the vessels into the interstitial spaces (interstitial fluid is called lymph). • The hydrostatic pressure is opposed by the osmotic pressure caused by the large molecules in the blood. • The high resistance of the capillary beds causes a large drop in blood pressure across the capillary bed, so that there is in fact a negative pressure on the venous side (Fig. 46.13). • This drains the interstitial space, preventing the accumulation of fluid. • If the blood vessels become more leaky, perhaps in response to histamine, the balance shifts and the tissue swells, a condition called edema. Edema can also be caused by high blood pressure or poor venous return. 10 Veins and venules: • The venous system operates at very low pressure and blood flow is slower, so that the blood tends to accumulate in the veins. • Veins have expandable walls and as much as 80% of the blood may be in the veins at any one time. Because of their capacity to store blood, veins are known as capacitance vessels. • The blood pressure in the veins is insufficient to overcome the force of gravity, and so veins have pocket valves to ensure the one-way flow of blood (Fig. 49.13). • Venous flow is also assisted by the contraction of the skeletal muscles which squeeze the veins, pushing the blood towards the heart. 11 • Absence of muscular activity is why your feet swell on a plane trip; the prolonged inactivity results in the pooling of blood in the lower extremities. Blood: A Fluid Tissue: • Blood has two primary components -- cellular and plasma. • The cellular component, called the packed cell volume or hematocrit, consists of erythrocytes (red blood cells; RBCs), leukocytes (white blood cells) and platelets. • Hematocrit runs about 40% under normal conditions in humans. • The plasma components are primarily water with some salts and plasma proteins (Fig. 49.15). 12 Red Blood Cells: • There are about 5 million (5 x 106) erythrocytes per ml of blood, whose main function is to transport respiratory gasses between tissues and lungs. • Erythrocytes are made in the bone marrow by stem cells and survive only about 120 days once they enter the circulation. • The rate of erythrocyte formation is controlled by the hormone, erythropoietin. White Blood Cells: •There are only 5-10,000 leukocytes, or white cell, per ml of blood. • Leukocytes are capable of independent movement using ameboid motion, and will squeeze through the walls of blood vessels in response to chemical signals in order to attack bacteria or other pathogenic organisms. 13