Key internal conditions controlled by homeostasis include:

• Core body temperature (around 37°C).
• Blood glucose concentration (approximately 4–7 mmol/L).
• Water potential of blood (to prevent cells from swelling or shrinking).
• pressure and oxygen levels (for sufficient nutrient and gas exchange).

Components of Homeostasis

Homeostasis relies on a series of systems that constantly monitor and adjust bodily functions. These systems involve:

1. Receptors (Sensors)

   Receptors are specialized cells or proteins that detect changes (stimuli) in specific conditions, such as temperature, pressure, or chemical concentration. Each type of receptor is highly specific to the stimulus it detects.
   

   Examples of receptors include:
   • Thermoreceptors: in the hypothalamus and skin, these detect changes in temperature.
   • Chemoreceptors: Found in the carotid arteries and aortic arch, these detect changes in oxygen, carbon dioxide, and pH levels in the blood.
   • Baroreceptors: Found in the walls of blood vessels, these detect changes in blood pressure.
   • Osmoreceptors: Located in the hypothalamus, these detect changes in the osmotic pressure of blood, which indicates hydration levels.
   These receptors constantly monitor the environment and send information to coordination centres via sensory neurons.

2. Coordination Centres

   The coordination centre receives information from receptors and determines the appropriate response. It compares the incoming data to a set point (the desired level for that variable) and sends signals to effectors to restore balance.
   

   Examples include:
   • The Hypothalamus: Controls thermoregulation, osmoregulation, and hormone release.
   • The Pancreas: Monitors blood glucose levels and regulates them by releasing insulin and glucagon.
   • The Medulla Oblongata: Regulates breathing and heart rate based on signals from chemoreceptors and baroreceptors.
   The coordination centre integrates multiple signals to ensure an appropriate response is carried out efficiently.

3. Effectors (Responders)

   Effectors are muscles or glands that carry out the response directed by the coordination centre to restore homeostasis.
   

   Examples of effectors include:
   • Sweat glands: Secrete sweat during thermoregulation.
   • Smooth muscle in blood vessels: Contracts (vasoconstriction) or relaxes (vasodilation) to regulate heat loss or retention.
   • Liver cells (hepatocytes): Store or release glucose based on insulin or glucagon signals.
   Effectors perform the necessary actions to reverse the detected changes.

Negative Feedback: The Mechanism of Balance

Negative feedback is a regulatory mechanism in which a change in a condition triggers a response that counteracts the initial change. This is the key process that allows homeostasis to maintain stability.

How Negative Feedback Works

1. Stimulus: A change in the environment (internal or external) that deviates from the normal set point.
2. Receptor Activation: Specialized receptors detect this change and send information to the coordination centre.
3. Processing by Coordination Centre: The coordination centre compares the data to the normal set point and sends instructions to effectors.
4. Effector Response: Effectors act to reverse the change and get back to balance.
5. Restoration of Normal State: Once the set point is reached, the system stops the response.

This loop ensures that internal conditions remain stable over time.

Detailed Examples of Homeostasis

1. Thermoregulation (Body Temperature Control)

   Maintaining a core temperature of around 37°C is essential because enzymes work optimally at this temperature. Too high, and enzymes denature; too low, and reaction rates slow down.

   Key Components Involved:

   • Receptors:
     • Peripheral thermoreceptors in the skin detect external temperature changes.
     • Central thermoreceptors in the hypothalamus detect changes in blood temperature.
   • Coordination Centre: The hypothalamus integrates data from thermoreceptors and initiates responses.
   • Effectors:
     • Sweat glands: Release sweat for evaporative cooling.
     • Arterioles in the skin: Vasodilation increases heat loss; vasoconstriction reduces heat loss.
     • Skeletal muscles: Shiver to generate heat through rapid, involuntary contractions.

   Example Process (When Body is Too Hot):

   1. Stimulus: Blood temperature rises above the set point.
   2. Receptor Activation: Central thermoreceptors in the hypothalamus detect the change.
   3. Signal to Coordination Centre: The hypothalamus processes the information and sends nerve impulses to effectors.
   4. Effector Response:
      • Sweat glands secrete sweat.
      • Arterioles dilate to increase blood flow to the skin, allowing more heat to escape.
   5. Outcome: Body temperature decreases, and the hypothalamus stops signaling the effectors.

2. Blood Glucose Regulation

   The concentration of glucose in the blood must be carefully controlled to ensure that cells receive enough energy while avoiding damage from excessive glucose.

   Key Components Involved:

   • Receptors:
     • Specialized beta cells in the pancreas detect high blood glucose levels.
     • Alpha cells in the pancreas detect low blood glucose levels.
   • Coordination Centre: The pancreas acts as both the receptor and coordination centre.
   • Effectors:
     • Liver cells (hepatocytes): Store or release glucose.
     • Muscle cells: Absorb glucose for immediate use or storage as glycogen.

   Example Process (When Blood Glucose is High):

   1. Stimulus: Blood glucose rises after eating.
   2. Receptor Activation: Beta cells in the pancreas detect the increase.
   3. Signal to Coordination Centre:Beta cells release the hormone insulin into the blood.
   4. Effector Response:
      • Liver cells absorb glucose and convert it to glycogen (glycogenesis).
      • Muscle cells absorb glucose for storage or use in respiration.
   5. Outcome: Blood glucose levels fall back to normal, and insulin release decreases.

Homeostasis Beyond Negative Feedback

While negative feedback is the most common mechanism for homeostasis, there are exceptions where positive feedback is used to amplify a response. Examples include:

• Blood Clotting: Platelets release chemicals that attract more platelets to form a clot.
• Childbirth: The hormone oxytocin causes uterine contractions, which stimulate the release of more oxytocin.

However, positive feedback is usually short-term and occurs in specific situations.