Feedback, dynamic equilibrium and thermoregulation

What you’ll learn

• Distinguish negative and positive feedback.
• Explain hypothalamic control of body temperature.
• Link homeostasis to changing demands during exercise.

Understand the idea

Homeostasis maintains internal conditions within a suitable range despite external changes and internal activity. It is a dynamic equilibrium: inputs and outputs continue, and regulated variables fluctuate. In negative feedback, receptors detect a deviation, coordination centres process information, and effectors act to oppose the deviation. Positive feedback reinforces the original change and usually needs a terminating event rather than maintaining a stable range.

The hypothalamus integrates information about blood temperature and signals from skin thermoreceptors. When the body becomes too warm, increased skin blood flow transfers more heat towards the surface and sweat evaporation removes latent heat. When too cold, reduced skin blood flow limits heat transfer and shivering increases muscle respiration and heat production. Behaviour, such as seeking shade or adding clothing, also changes heat exchange.

Exercise increases metabolic heat production while sweating can reduce water content and alter plasma concentration. Thermoregulation therefore interacts with cardiovascular and water-balance control. Sweating is most effective when sweat evaporates; high humidity limits the gradient for evaporation. Skin colour changes or sweat volume alone do not quantify heat loss, and a stable body temperature can conceal substantial simultaneous heat production and loss.

Worked example

A runner’s core temperature rises in hot weather. Describe a negative-feedback response and explain why high humidity can weaken it.

1. Thermoreceptors and the hypothalamus detect an increase in temperature.
2. Effectors increase sweating and skin blood flow, promoting heat transfer and evaporation.
3. High atmospheric water-vapour content reduces evaporation, so a smaller fraction of the sweat removes latent heat.

Answer: The response opposes the temperature rise, but its effectiveness is limited when sweat cannot evaporate readily.