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Lecture 9

lecture 9

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University of Toronto Scarborough
Biological Sciences
Ivana Stehlik

The Venous SystemThe venules and veins return the blood from the microcirculation to the right atrium of the heart They do not however serve merely as passive conduits Instead they have a crucial active role in stabilizing and regulating the venous return of blood to the heartThe venous system differs from the arterial system in two important respects First the total volume and crosssectional area of the venous system is much greater that that if the arterial system This is because there are many more venules than arterioles venules also tend to have larger internal diameters than arterioles Second the veins are quite thin walled and can therefore expand greatly to hold more blood if their internal pressure rises As a result of its large crosssectional area the venous system offers much less resistance to flow compared to the arterial system The pressure gradient require to drive the blood through the venous system is therefore much smaller than the pressure needed in the arterial system The average pressure in the vena cava the central venous pressure is usually close to 0 mmHg ie atmospheric pressure The Veins as Capacitance Vessels Because of their large volumes and high compliance the veinsvenules accommodate a much larger volume of the blood 70 of the total than do the arteriesarterioles 12 They are therefore termed capacitance vessels and are able to serve blood volume reservoirs During exercise and in hypotensive states eg during hemorrhage sympathetically mediated constriction of the veinsvenules notably in the splanchnic including the gastrointestinal tract and liver and cutaneous circulations displaces blood to other essential vascular beds while also helping to maintain the blood pressure At the same time the resulting reduction of the venous volume increases the volume of the blood to the heart thereby boosting cardiac output Venous Capacity The volume of blood that veins can accommodate depends on the distensibility of the walls of the vein how much they can stretch to hold blood and the influence of any externally applied pressure squeezing inwardly on the veins At a constant blood volume as venous capacity increases more blood remains in the veins instead of being returned to the heart Such venous storage decreases the effective circulating stressed volume Conversely when venous capacity decreases more blood is returned to the heart and continues circulating Thus changes in venous capacity directly influence the magnitude of venous return which in turn is an important determinant of effective circulating stressed blood volume The magnitude of total blood volume is also influenced on a short term basis by passive shifts in bulk flow between the vascular and interstitial fluid compartments and on a longterm basis by factors that control total ECF volume such as salt and water balance Venous ReturnVenous return refers to the volume of blood entering each atrium per minute from the veins It is dependent on the pressure gradient between the arterial and venous sides of the heart
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