Control of Body Temperature

Overview

Thermoregulation is a fundamental concept in biology, representing a key example of homeostasis – the maintenance of a stable internal environment. For your OCR GCSE exam, understanding how the human body controls its temperature is not just about knowing facts; it’s about applying principles to explain physiological responses. This topic, specification point 5.5, frequently appears in exams, often as challenging 6-mark, level-of-response questions. A solid grasp of thermoregulation connects to other key areas like enzymes (their optimal temperature), the nervous system (receptors, coordinators, effectors), and respiration (heat release). This guide will equip you with the precise language and conceptual understanding required to earn maximum credit.

Key Concepts

1. The Thermoregulatory Centre: The Hypothalamus

The control of body temperature is coordinated by the hypothalamus, a small but vital part of the brain. It acts as the body’s thermostat. The hypothalamus contains thermoreceptors, which are specialised cells that constantly monitor the temperature of the blood flowing through them. If the blood temperature deviates from the set point of approximately 37°C, the hypothalamus initiates a coordinated response by sending nerve impulses to effectors around the body. It is critical for candidates to identify the hypothalamus as both the primary receptor and the coordinator.

2. Negative Feedback: The Control System

Thermoregulation operates on a principle of negative feedback. This is a self-regulating mechanism where the response counteracts the initial stimulus, bringing the system back to its normal set point.

• If temperature rises: The hypothalamus detects the change and triggers responses to cool the body down.
• If temperature falls: The hypothalamus detects the change and triggers responses to warm the body up.

Once the temperature returns to 37°C, the hypothalamus detects this and switches off the corrective mechanisms. This prevents over-correction and maintains a stable temperature.

3. Responses to High Temperature

When the body gets too hot, the hypothalamus triggers two main responses to lose heat:

• Vasodilation: The arterioles (small arteries) leading to the skin capillaries dilate (widen). This increases blood flow through the capillaries near the skin surface. As the warm blood is closer to the cooler external environment, more heat is lost from the blood to the air by radiation. This is why a person may appear flushed or red when hot.
• Sweating: Sweat glands are stimulated to secrete sweat onto the skin surface. For sweating to cool the body, the sweat must evaporate. This process of changing from a liquid to a gas requires energy, which is taken from the skin as heat. It is crucial to explain that evaporation transfers thermal energy from the skin to the surroundings.

4. Responses to Low Temperature

When the body gets too cold, the hypothalamus triggers responses to conserve heat and generate more:

• Vasoconstriction: The arterioles leading to the skin capillaries constrict (narrow). This reduces blood flow to the skin surface, keeping warm blood deeper within the body and minimising heat loss by radiation. The skin may appear pale.
• Shivering: The skeletal muscles contract and relax rapidly and involuntarily. This process requires energy from respiration. Respiration is an exothermic reaction, meaning it releases heat. This released heat warms the body. Candidates must link muscle contraction to respiration to heat release to gain full marks.
• Piloerection: The hair erector muscles contract, pulling the body hairs upright. In animals with thick fur, this traps a layer of air which acts as an insulator. In humans, this response (goosebumps) is not very effective due to our lack of dense body hair, but it is still a recognised mechanism.

Mathematical/Scientific Relationships

There are no specific mathematical formulas to memorise for this topic at GCSE level. However, understanding the relationship between surface area and heat loss is important. A larger surface area to volume ratio leads to faster heat loss, which is why smaller animals lose heat more quickly than larger ones. This principle also applies to the increased surface area of blood capillaries during vasodilation, facilitating greater heat loss.

Practical Applications

This topic does not have a specific required practical. However, the principles of thermoregulation are applied in many real-world contexts:

• Survival in extreme environments: Understanding thermoregulation is critical for survival in deserts or polar regions.
• Sports science: Athletes need to manage their body temperature to perform optimally and avoid heatstroke.
• Medicine: Fever is a controlled raising of the body temperature set point by the hypothalamus to help fight infection. Hypothermia is a dangerous drop in body temperature.