Vasoconstriction & Vasodilation (Edexcel GCSE Biology)

Higher tier only

Vasodilation

• Heat exchange (both during warming and cooling) occurs at the body's surface as this is where the blood comes into closest proximity to the environment
• One way to increase heat loss is to supply the capillaries in the skin with a greater volume of blood, which then loses heat to the environment via radiation
• Arterioles (small vessels that connect arteries to capillaries) have muscles in their walls that can relax or contract to allow more or less blood to flow through them
• During vasodilation these muscles relax, causing the arterioles near the skin to dilate and allowing more blood to flow through capillaries
  • Note that it is the arterioles that supply the skin capillaries that vasodilate, and not the capillaries themselves; capillary walls are only one cell thick and contain no muscle that is capable of relaxing or contracting
• This is why pale-skinned people go red when they are hot

Vasoconstriction

• One way to decrease heat loss is to supply the capillaries in the skin with a smaller volume of blood, minimising the loss of heat to the environment via radiation
• During vasoconstriction the muscles in the arteriole walls contract, causing the arterioles near the skin to constrict and allowing less blood to flow through capillaries
  • Again, remember that it is the arterioles that supply the skin capillaries that vasoconstrict and not the capillaries themselves
• Instead, the blood is diverted through shunt vessels, which are further down in the skin and therefore do not lose heat to the environment
• Vasoconstriction is not, strictly speaking, a 'warming' mechanism as it does not raise the temperature of the blood but instead reduces heat loss from the blood as it flows through the skin

The mechanisms of vasodilation and vasoconstriction