lecture 1

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Biological Sciences
Ivana Stehlik

Overview of the Cardiovascular System Figure 1The cardiovascular system is composed of the heart blood vessels and bloodIn simple terms its main functions are1 Distribution of 0 and nutrients eg glucose amino acids to all body tissues 22 Transportation of CO and metabolic waste products eg urea from the 2tissues to the lungs and excretory organs 3 Distribution of water electrolytes and hormones throughout the body 4 Contributing to the infrastructure of the immune system 5 ThermoregulationFigure 1 illustrates the plumbing of the cardiovascular systemBlood is driven through the cardiovascular system by heart a muscular pump divided into left and right sidesEach side contains two chambers an atrium and a ventricle composed mainly of cardiac muscle cellsThe thinwalled atria served to fill or prime the thickwalled ventricles which when full constrict forcefully creating a pressure head that drives the blood out into the bodyBlood enters and leaves each chamber of the heart through separate oneway valves which open and close reciprocally ie one closes before the other opens to ensure that flow is unidirectionalConsider the flow of blood starting with its exit from the left ventricleWhen the ventricles contract the left ventricular internal pressure rises from 0 to 120 mmHg atmospheric pressure0As the pressure rises the aortic valve opens and blood is expelled into the aorta the first and largest artery of the systematic circulationThis period of ventricular contraction is termed systoleThe maximal pressure during systole is called the systolic pressure and it serves both to drive blood through the aorta and to distend the aorta which is quite elasticThe aortic valve then closes and the left ventricle relaxes so that it can be refilled with blood from the left atrium via the mitral valveThe period of relaxation is called diastoleDuring diastole aortic blood flow and pressure diminish but do not fall to zero because elastic recoil of the aorta continues to exert a diastolic pressure on the blood which gradually falls to a minimum level of about 80 mmHgThe differences between systolic and diastolic pressure is termed the pulse pressureMean aortic pressure is pressure averaged over the entire cardiac cycleBecause the heart spends approximately 60 of the cardiac cycle in diastole the mean aortic pressure is approximately equal to the diastolic pressureonethird of the pulse pressure rather than to the arithmetic average of the systolic and diastolic pressuresThe blood flows from the aorta into the major arteries each of which supplies blood to an organ or body regionThese arteries divide and subdivide into smaller muscular arteries which eventually give rise to the arteriolesarteries with diameters of100 um Blood enters the arterioles at a mean pressure of about 6070 mmHgThe walls of the arteries and arterioles have circumferentially arranged layers of smooth muscle cellsThe lumen of the entire vascular system is lined by a monolayer of endothelial cellsThese cells secrete vasoactive substances and serve as a barrier restricting and controlling the movement of fluid molecules and cells into and out of the vascultureThe arterioles lead to the smallest vessels the capillaries which from a dense network within all body tissuesThe capillary wall is a layer of overlapping endothelial cells with no smooth muscle cellsThe pressure in the capillaries ranges from about 25 mmHg on the arterial side to 15 mmHg at the venous endThe capillaries converge into small venules which also have thin walls of many endothelial cells The venules merge into larger venules with an increasing content of smooth muscle cells as they widenThese then converge to become veins which progressively join to give rise to the superior and inferior venae cavae through which blood returns to the right side of the heartVeins have a larger diameter than arteries and thus offer relatively little resistance to flowThe small pressure gradient between venules 15 mmHg and the venae cavae 0 mmHg is therefore sufficient to drive blood back to the heartBlood from the venae cavae enters the right atrium and then the right ventricle through the tricuspid valveContraction of the right ventricle simultaneous with that of the left ventricle forces blood through the pulmonary valve into the pulmonary artery which progressively subdivides to form the arteries arterioles and capillaries of the pulmonary circulationThe pulmonary circulation is shorter and has a much lower pressure than the systemic circulation with systolic and diastolic pressures of about 25 and 10mmHg respectivelyThe pulmonary capillary network within the lungs surrounds the alveoli of the lungs allowing exchange of CO for OOxygenated blood enters pulmonary 22venules and veins and then the left atrium which pumps it into the left ventricle for the next systemic cycleBlood vessel functionsEach vessel has important functions in addition to being a conduit for bloodThe branching system of elastic and muscular arteries progressively reduces the pulsations in blood pressure and flow imposed by the intermittent ventricular contractionsThe smallest arteries and arterioles play a crucial role in regulating the amount of blood flowing to the tissues by dilating or constrictingThis function is regulated by the sympathetic nervous system and factors generated locally in tissuesThese vessels are referred to as resistance arteries because their contriction resists the flow of bloodCapillaries and small venules are the exchange vesselsThrough their walls gases fluids and molecules are transferred between blood and tissuesWhite blood cells can also pass through the venule walls to fight infection in the tissuesVenules can constrict to offer resistance to the bloodflow and the ratio of arteriolar and venular resistance exerts an important influence on the movement of fluid between capillaries and tissues thereby affecting blood volumeThe veins are thin walled and very distensible and therefore contain about 70 of all blood in the cardiovascular systemThe arteries contain just 17 of total blood volumeVeins and venules thus serve as volume reservoirs which can shift blood from the peripheral circulation into the heart and arteries by constricting In doing so they are able to increase the cardiac output volume of blood pumped by the heart per unit time and they are also able to maintain the blood pressure and tissue perfusion in essential organsif hemorrhage blood loss occurs
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