explain the principles of homeostasis in terms of internal and external stimuli, receptors, coordination systems (nervous system and endocrine system), effectors (muscles and glands) and negative feedback

Homeostasis in Mammals

Homeostasis is the process by which living organisms maintain a relatively stable internal environment despite changes in the external environment. In mammals this involves a series of linked components that detect a change (stimulus), process the information and generate a response that restores the variable to its set point.

1. Stimuli

Stimuli are any changes that can disturb the internal balance. They are classified as:

• Internal stimuli – e.g., rise in blood glucose, low blood oxygen, changes in blood pH.
• External stimuli – e.g., ambient temperature, light intensity, availability of food or water.

2. Receptors (Sensors)

Receptors detect the stimulus and convert it into an electrical or chemical signal (a nerve impulse or hormone).

• Thermoreceptors – detect changes in temperature.
• Chemoreceptors – monitor concentrations of O₂, CO₂, glucose, and pH.
• Baroreceptors – sense blood pressure in arterial walls.
• Mechanoreceptors – respond to stretch or pressure (e.g., in the bladder).

3. Coordination Systems

The information from receptors is transmitted to control centres, which then coordinate an appropriate response. Two major systems operate in mammals:

3.1 Nervous System

• Fast, short‑term control.
• Signal transmission via action potentials along neurons.
• Control centres are specialised regions of the brain (e.g., hypothalamus for temperature regulation).
• Responses are transmitted to effectors through motor neurons.

3.2 Endocrine System

• Slower, long‑term control.
• Control centres (e.g., hypothalamus, pituitary) release hormones into the bloodstream.
• Hormones travel to distant target cells, altering their activity.
• Key hormones in homeostasis include insulin, glucagon, adrenaline, antidiuretic hormone (ADH) and thyroid hormones.

4. Effectors

Effectors are organs, tissues or cells that act to correct the deviation from the set point.

• Muscles – e.g., shivering muscles generate heat when body temperature falls.
• Glands – e.g., sweat glands release sweat to dissipate heat; pancreatic islets secrete insulin or glucagon to regulate blood glucose.
• Kidneys – adjust water reabsorption under the influence of ADH.
• Blood vessels – dilate or constrict to regulate heat loss and blood pressure.

5. Negative Feedback

Negative feedback is the dominant control mechanism in homeostasis. The response reduces the original stimulus, bringing the variable back toward its set point.

1. Stimulus deviates a variable from its set point.
2. Receptor detects the deviation.
3. Signal is sent to a control centre.
4. Control centre activates an effector.
5. Effector action counteracts the original change.
6. Variable returns to set point; the signal diminishes.

6. Example: Regulation of Body Temperature

When ambient temperature falls below the set point (\overline{37} °C):

1. Thermoreceptors in the skin and hypothalamus detect the drop.
2. Signals travel via sensory neurons to the hypothalamic thermoregulatory centre.
3. The hypothalamus activates sympathetic nerves that stimulate:

   • Shivering of skeletal muscles (heat production).
   • Constriction of cutaneous blood vessels (reduces heat loss).
   • Release of thyroid‑stimulating hormone (TSH) → thyroid hormones increase basal metabolic rate.

4. As core temperature rises, thermoreceptors reduce their firing, diminishing the stimulus to the hypothalamus – the system stabilises.

7. Summary Table of Homeostatic Components