How does heat affect capillary permeability?

Heat application leads to vasodilation, which is the widening of blood vessels. As blood vessels dilate, there is an increase in blood flow to the area, resulting in elevated hydrostatic pressure within the capillaries. This increase in capillary pressure enhances capillary permeability, allowing for greater movement of fluids and solutes from the bloodstream into the interstitial space. This physiological response is beneficial for healing as it facilitates the delivery of nutrients and inflammatory cells to damaged tissues.

The other options are less accurate in describing the effects of heat on capillary permeability. While decreasing or having no effect on capillary pressure would imply limited movement of fluids, heat's primary action promotes increased circulation and permeability, rather than restricting it. Additionally, the thickening of capillary walls does not typically follow from increased heat; instead, heat primarily influences the function of existing capillaries rather than their structural characteristics.