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Biological Sciences
Stephen Reid

1Lecture 7 Blood Flow Regulation continued1 Calculating Blood FlowBlood flow is calculated by dividing the pressure gradient by the resistance to flow The pressure gradient in the case of a blood vessel is the pressure at the start of the vessel minus the pressure at the end of the vessel2 Distribution of Blood in Circulation and Changes in Pressure Under healthy conditions approximately 61 of the five litres of blood in the body is found in the systemic veins and venules with 9 in the heart 7 in the arterioles and capillaries 11 in the systemic arteries and 12 in the pulmonary circulation However the vast majority of the blood pressure in the circulatory system is found on the arterial side The greatest pressure differential or pressure decrease in the systemic circulation is between the arteries and the arterioles and to some extent the capillaries This occurs because the arterioles and capillaries are the two greatest sites of resistance to blood flow in the systemic circulationIn the pulmonary circuit the same is true the greatest pressure drops are in the arterioles and the capillariesHowever the pressure is far less in the pulmonary circuit than it is in the systemic circuit This is due to the need to protect delicate lung tissue from high pressures High blood pressure in the lungs would force fluid from blood vessels into the alveoli and cause pulmonary edema However blood flow between these two circuits is equaland this means that resistance to flow in the pulmonary circuit must be lower than it is in systemic circuit see below3 The Driving Forces for Blood FlowMean arterial pressure MAP is the driving force for blood flow in the systemic circulation Systemic blood flow is driven by the difference between mean arterial pressure ie pressure in the aorta and central venous pressure pressure in the vessels emptying into the right atria However given that central venous pressure is very close to zero and we approximate it to be zero for these purposes we only really need to consider mean arterial pressure when calculating blood flow in the systemic circuit In other words MAP is the driving force for blood in the systemic circulationThe driving force for blood flow in the pulmonary circulation follows the same principle although the pressure in this circuit is far lower The pressure gradient driving blood flow here is the pressure in the pulmonary artery leaving the right ventricle and heading to the lungs minus pressure in the pulmonary vein emptying into the left ventricle Pulmonary artery pressure is approximately 15 mmHg while pulmonary venous pressure in the pulmonary vein can be approximated to be zero
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