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Chapter 21

Chapter 21 (22).doc

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Western University
Biology 1225
Michael Butler

Chapter 21- Circulation and Respiration (22) *Sections 21.1, 21.6, 21.9 will not be tested This Chapter helps you to bring together ideas about the biochemistry of cellular respiration and the diffusion of gases across membranes in the context of the familiar organs and tissues of a multicellular organism. Note the direction of blood flow through a mammalian heart (such as ours) - from ventricles into arteries then into arterioles (small arteries) then into capillaries (where the real exchange of gases, wastes, nutrients occurs from body tissues to the blood and vice versa) then from capillaries into venules (small veins) then into veins and back to the heart. Since the left side of the heart delivers oxygenated blood to the systemic circulation (all the blood vessels in the body other than the lungs), this means that the left ventricle has to be a lot stronger and larger and more muscular than the right ventricle, which pumps blood through the much less extensive blood system that is in the lungs. The lungs are also intimately involved with the functions of the blood, transferring oxygen into the blood and removing carbon dioxide. The lungs can precisely match their operation to the need for oxygen by a feedback mechanism that measures carbon dioxide levels in the blood, in special receptors in blood vessels in the medulla region of the brain. Responsibilities All of this Chapter is relevant, Focus on the structure of blood as a tissue and the flow of blood through the heart and body. Figure 21.3 c and 21.4 are useful for your understanding of the way blood flows in the body, you do not need to memorize them, but you should coordinate your reading of the relevant material in the chapter on blood flow through the body with these figures so that you can gain a better understanding. Blood and Circulation. Blood When a test tube full of blood is centrifuged at the appropriate speed a number of layers form, at bottom 40-50% of the tube is a packed layer of red blood cells, then a thin tissue paper-like white layer of white cells which comprise much of the immune system, and an upper layer of a pale yellowish fluid called plasma which consists of water packed with protein and salt solutes, and many other nutrients as, well as wastes. The bone marrow contains stem cells, that generate the cells of the blood. Red blood cells (aka erythrocytes), in which the nucleus degenerates and disappears. Red blood cells are red because they contain large amounts of hemoglobin, a complex protein that has heme groups which contain the iron that binds oxygen for transport around the body. Very little of the oxygen in the blood is in solution, nearly all of it is chemically bound to hemoglobin ( a single hemoglobin protein molecule can bind 4 oxygen atoms), which means that a steep gradient of oxygen concentration is maintained across the lungs Megakaryocytes are cells that fragment to produce tiny membrane bound platelets that are involved in blood clot formation. Stem cells also produce two groups of white cells (leukocytes) – components of the immune system, dealt with later. Some animals, such as insects, have an open circulatory system, it is not a closed system of heart veins, arteries etc, the heart pumps blood into a body cavity where it directly bathes organs and then returns to blood vessels that collect the blood and return it to the heart. Fishes have a two chambered heart, a single atrium that collects blood from the body, passes it to the single ventricle where it passes on through the blood vessels of the gills and the body. Amphibians and most reptiles have a three chambered heart, they have separate capillary beds for the lungs and the body and these are collected by separate atria, but these atria pump into a single ventricle. This means that oxygen poor blood from the body and oxygen rich blood returning from the lungs is mixed in the single ventricle, which is inefficient when compared to the four chambered heart, below. Birds, mammals and a few reptiles (crocodiles for instance) have a four chambered heart. In a four chambered heart one as essentially two separate pumps, each consisting of an atrium and a ventricle, that are interconnected so that when one pump collects and pumps out oxygen poor blood from the body, into the lungs, the blood from the lungs is collected by the other pump and pumped out into the body. The Human cardiovascular system One can regard the fluid balance of the body as existing in three places, the cardiovascular system - which transports the blood around the body, the lymphatic system which drains fluids back from the tissues into the cardiovascular system, and the fluids in and around the tissues and organs. The cardiovascular system transports oxygen to and carbon dioxide away from body tissues, and transports nutrients from the digestive system to the tissues and wastes from the tissues to the kidneys. Blood circulation is closed, it proceeds in from the heart in arteries, into smaller arterioles, from these into single cell layered capillaries, on in to venules and then into veins for transport back to the heart. Arteries and veins are complex layered tubes. From the outermost layer inwards one finds an outer coat, elastic tissue, smooth muscle, and a basement membrane that surrounds and supports the innermost endothelium that is in contact with the blood. Arteries, are thicker than veins, they generally have more substantial smooth muscle layers that add strength, and the ability to actively resist blood pressure as the heart beats. The elastic nature of arteries is a key feature, it maintains a small amount of blood pressure between contractions of the heart; during a heartbeat (systole) the high pressure in the artery causes the wall to bulge and thus retain some blood, when the heart is not beating (diastole) the elastic artery wall which has bulged during systole now relaxes (the so called elastic recoil effect) and in so doing exerts a pressure that serves to continue pumping of the blood, arteries are thus described as "pressure reservoirs". Arterioles are smaller arteries - with some differences in wall construction, and they conduct blood from the arteries to the capillaries. Arterioles can respond to hormonal and nervous signals that command them to reduce in diameter (vasoconstrict) which increases blood pressure, or to increase their diameter (vasodilate) and this decreases blood pressure. Arterioles thus play a key role in control of blood pressure and the rate of blood flow to particular tissues. Arterioles conduct blood to the capillaries and onwards to the veins Capillaries are a single cell layer thick, they consist of an outermost basement membrane and the single cell layer thick innermost endothelium. All cells in the body are close to a capillary. Capillaries are “leaky”, there are tiny gaps in the walls, so that they can release nutrients and gases to tissues and accept waste fluids and gases from tissues. Capillaries in the brain are an exception, they are much less leaky, they bar many molecules from crossing into the brain fluids - this is the so-called “blood-brain” barrier. The blood is under high hydrostatic pressure when it has just entered a capillary from an arteriole and this forces fluids into the tissue space from the capillary so that nutrients and oxygen are delivered, further along the capillary towards its venule end, osmotic forces and hydrostatic forces begin to dominate in a way which forces fluids into the capillary, which is how wastes and carbon dioxide are taken away from the tissues. Venules are similar to veins but smaller in diameter, they pass the blood on from the capillaries to the veins. Veins are smaller and thinner than arteries in cross section and also react to blood pressure by expanding, this is because veins are thinner walled than arteries and “stretch” more in response to blood pressure but without the strong recoil of arteries. Veins are blood reservoirs, about 60% of the blood is in the veins. During times of exercise when blood pressure needs to be increased the smooth muscle layer in the walls of veins contracts somewhat to exert pressure on the blood, but in fact there is very little involvement of veins in actually moving blood back to the heart. Veins return blood to the heart, and to prevent backflow they contain one way valves. Structure and function of the human heart The human heart is composed mostly of myocardium (cardiac muscle cells) that contract is response to electrical stimulation, the myocardium cells are embedded within elastin and collagen fibers. Myocardium cells are joined by adherent junctions, and possess gap junctions that allow the electrical excitation that causes the heartbeat to spread rapidly. The inner part of the heart that is contacted by blood is lined with endothelium. The human heart is a double pump which connects two separate but interlinked circulatory loops, the systemic circuit in which the left side of the heart pumps oxygen rich blood around the body, and the pulmonary circuit in which the right side of the heart collects oxygen depleted blood from the body which is high in carbon dioxide, and pumps it into lungs where it picks up oxygen and offloads carbon dioxide before “re- connecting” with the heart once again at the left side for transport to the body once more. The cardiac cycle Each half of the heart (each “pump”) consists of two chambers, the atrium and the ventricle. The atrium is a collecting chamber, it receives blood and then passes it to the ventricle which is a pumping chamber. The heart receives oxygen depleted and low pressure blood from the body through essentially, two large veins, the superior and inferior vena cava. These veins connect with the right atrium. When the right atrium is full it passes the blood to the right ventricle, which then contracts forcefully and pumps the blood through the pulmonary arteries to both lung, where it picks up oxygen and offloads carbon dioxide. From the lungs, the blood, now oxygen rich and at lowered pressure (because it lost pressure making its way through the lungs) passes to the left atrium through the pulmonary veins. The left ventricle is much thicker and more muscular than the right ventricle because it has to develop sufficient pressure to force the blood through the extensive systemic circulation which has a much larger capillary bed than in the lung system. The left ventricle passes the blood into the aortic arch from which issue a number of arteries to the body, including smaller coronary arteries that supply the heart itself with blood. There are mitral valves between the atria and the ventricles that prevent back-flow of blood from the ventricles into the atria, and also semilunar valves in the arteries leaving the heart, which also prevent backflow between heart beats. When the ventricles are contracting (and the atria are relaxed), the blood is at its highest pressure in the body and this is called systole, when the heart is not pumping, in that very short interval between beats, the blood pressure is lowest and this is diastole. This process of systole and diastole is what creates the pulse that can be felt at the wrist. When a blood pressure is recorded as 80/120 the first figure is diastole, the second is systole. The heart is under electrical control (the cardiac conduction system). Nerves in the spinal cord and the brain can adjust the rate of the heartbeat, but the intrinsic mechanism of control of the cardiac system itself, the cycle of atrial and ventricular contraction and relaxation, is precisely controlled by nerve fibres and bundles in the heart, the heart can beat entirely on its own, controlled by electrical signals generated and delivered in the heart itself. A small clump of cells in the wall of the right atrium called the sinoatrial node (SA node, aka the cardiac pacemaker) generates waves of electrical signals, each wave triggers both atria to contract simultaneously and empty blood into the ventricles. The signal next spreads to the atrioventricular (AV) node, which is located in the septum dividing the two atria. The AV node then conducts the signal along long fibres to each ventricle and causes them to contract. A slight but significant pause at the AV node allows the atria to finish contracting before the ventricles receive the signal to contract. The SA node is the pacemaker of the heart. The lymphatic system is a widespread network of lymph capillaries and lymph vessels that drain excess fluids from the tissues and place them back into the blood circulation. The lymphatic system also contains numerous lymph nodes where infection fighting white cells are in residence, and these react against infectious agents, which is why lymph nodes swell during infections. The spleen is considered to be a part of the lymphatic system. Hypertension (high blood pressure) forces the heart to work harder and may eventually cause heart attacks and heart failure, but can be treated with drugs, dietary measures and exercise. Atherosclerosis is the accumulation of cholesterol and other lipids in an artery that both restricts blood flow and causes the artery wall to harden. A fat-laden diet is a factor, as is smoking. Irregular electrical activity of the heart can also cause various problems including an inefficient pumping action. Human Lungs; Respiration The human respiratory system begins with the oral cavity, and the nasal cavity, where the nose warms, filters, and humidifies the air before it enters the pharynx
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