Vasodilation: The smooth muscles in the walls of the arterioles near the skin surface relax, causing the blood vessels to widen. This increases the volume of blood flowing near the skin surface, allowing more heat to be lost to the environment via radiation.
Sweat Production: Sweat glands secrete liquid (mostly water) onto the skin surface. As this water evaporates, it absorbs heat energy from the skin (the latent heat of vaporization), effectively cooling the body down.
These mechanisms work together to transfer excess thermal energy from the core to the external environment.
Vasoconstriction: Arterioles near the skin surface constrict (narrow), which reduces the volume of blood flowing through the capillaries near the skin. This minimizes heat loss by radiation, keeping the warmth in the body's core.
Shivering: Skeletal muscles undergo rapid, involuntary contractions. These contractions require energy from respiration, a metabolic process that is exothermic and releases heat as a byproduct to warm the blood.
Cessation of Sweating: The body stops producing sweat to prevent further evaporative cooling.
| Feature | Vasodilation | Vasoconstriction |
|---|---|---|
| Trigger | Body temperature is too high | Body temperature is too low |
| Vessel Action | Arterioles widen (dilate) | Arterioles narrow (constrict) |
| Blood Flow | Increases near skin surface | Decreases near skin surface |
| Heat Effect | Increases heat loss via radiation | Conserves heat in the core |
It is a common misconception that blood vessels 'move' up or down in the skin; they actually stay in the same place but change their diameter to control flow volume.
Shivering is not just 'movement'; it is a physiological strategy to increase the rate of exothermic chemical reactions (respiration) in muscle cells.
Identify the Coordinator: Always state that the thermoregulatory center in the brain is the coordinator that receives impulses and sends signals to effectors.
Explain the 'Why' of Sweating: When discussing sweat, use the term evaporation. It is the change of state from liquid to gas that removes heat energy from the skin, not the mere presence of the liquid.
Respiration Connection: If asked about shivering, you must mention that muscle contraction requires respiration and that respiration releases heat energy.
Negative Feedback: Be prepared to explain that thermoregulation is a 'negative feedback' process, meaning the body's response works to reverse the initial change (e.g., if temp goes up, the body acts to bring it down).
