Vasodilation is the widening of blood vessels, increasing blood flow near the skin and promoting heat loss. Vasoconstriction is the narrowing of blood vessels, reducing skin blood flow and conserving heat.
The Mechanism
Both responses maintain homeostasis, the regulation of internal conditions within narrow limits. In thermoregulation, core body temperature is controlled near its set point by negative feedback.
Temperature receptors detect changes in blood or skin temperature and signal the hypothalamus, the body's thermoregulatory centre. It coordinates effectors, including smooth muscle in arterioles supplying the skin.
| Feature | Vasodilation | Vasoconstriction |
|---|---|---|
| Change in blood vessel diameter | Arterioles widen as smooth muscle relaxes | Arterioles narrow as smooth muscle contracts |
| Skin blood flow | Increases | Decreases |
| Heat transfer at the skin | Increases | Decreases |
| Typical stimulus | Core temperature rises | Core temperature falls |
| Homeostatic effect | Promotes heat loss and lowers body temperature | Conserves heat and limits further cooling |
During exercise, increased metabolic activity generates heat. If core temperature rises, vasodilation directs more warm blood towards the skin. This increases the temperature gradient between skin and environment, promoting radiation and convection. Sweating additionally permits heat loss through evaporation.
In cold conditions, vasoconstriction reduces blood flow near the skin surface. Less heat transfers from blood to the environment, helping preserve core temperature and protect vital organs.
A common misconception is that vasodilation directly cools the body. It does not remove heat; it increases skin blood flow so heat can transfer to the environment. When environmental temperature exceeds skin temperature, evaporation becomes the main avenue of heat loss.
Exam Technique
For IB SEHS A.1.2, link the stimulus, hypothalamic control, effector response and restoration of core temperature. Identify smooth muscle relaxation or contraction, then explain the change in skin blood flow and heat transfer. Avoid saying only that vessels become “bigger” or “smaller”.
