Topic 3 Circulatory System: Blood Flow

Question 1

• Cross sectional area of veins is about 4x higher than that of arteries and the total cross-sectional area of capillaries is far greater than that of arteries of veins
• Why? Well, while capillaries are the narrowest vessels, there are far more capillaries -> total cross-sectional area of all of them put together is higher than any cross-sectional area
• Because blood volume flow rate is constant, blood velocity is inversely proportional to total cross-sectional area
• Blood pressure drops as we go from aorta -> capillaries because of energy loss due to increased resistance and decreased vessel diameter
• This seems contradictory since blood pressure = cardiac output x systematic vascular resistance (resistance controlled by vasoconstriction/dilation)
• If resistance increases, why does pressure decrease? The blood pressure formula above applies to MAP (mean arterial pressure) , which is measured at the arteries by a sphygmomanometer. When a blood vessel constricts (increased resistance), the blood pressure is indeed higher in the part of the tube before the constriction (which is presumably where we measure blood pressure)
• The pressure after is the constriction is what is lowered, hence why blood pressure effectively decreases as we go through smaller diameter vessels. By the time we hit venules/veins, the original source of the blood pressure/flow (the beating of the heart) is virtually gone, which is why the pressure continues to decrease further.
• Arterioles have the greatest resistance to flow (highest ability to constrict). At any given time, most blood is in the veins/venules/venus sinuses

Answer 1

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