Question

i) Explain with a diagram how blood flow through a capillary bed is regulated. (30%) (ii) What are the main pressures driving exchange of nutrients in a capillary bed (30%), how is this affected by decreased plasma osmolality? (20%), how is this affected by decreased blood volume? (20%).

physiology question

Answer 1

Blood flow through a capillary bed is regulated via precapillary

sphincter which when relaxed allows the flow of blood through the capillaries

When constructed it restricts the blood flow through the capillaries

Capillary perfusion pressure .

perfusion pressure provides the energy required to overcome the blood flow resistance associated with vessel diameter, hydrodynamic effects at bifurcations, RBC deformability, capillary lumen obstructions, and viscosity. Capillary perfusion pressure is determined by central blood pressure or MAP. In general, hemodilution improves the transmission of central pressure to the capillary level owing to decreases in flow resistance due to viscosity. vasoconstriction reduces capillary perfusion pressure . At moderate levels of hemodilution, increases in flow rate generate sufficient shear stress to ensure arteriolar vasodilation and the transmission of perfusion pressure to the level of the capillaries. With greater levels of hemodilution (> 50%), the loss of shear stress-mediated vasodilation, reduces capillary perfusion pressure . Increasing plasma viscosity in extreme hemodilution (75% normal HCT) have been shown to maintain arteriole vasodilation, capillary perfusion pressure, maintaining central perfusion pressure with vasoconstrictors may worsen complications associated with hypoperfusion.

Decreased plasma volume is associated with reduced MAP which is associated with reduced capillary perfusion pressure which inturn affects the nutrient exchange