lecture 5

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University of Toronto Scarborough
Biological Sciences
Ivana Stehlik

1 Physiology IThe Cardiovascular System Overview of the Cardiovascular System The 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 system2Blood 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
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