Principles of Homeostasis

Homeostasis is the maintenance of a stable internal environment (within restricted limits) despite changes in internal and external conditions.

This means keeping internal blood glucose levels within certain limits. For example, if you eat 5 chocolate bars, we need internal mechanisms to bring sugar levels down. If we have been for a run, we need internal mechanisms to bring sugar levels back up.

Why is homeostasis important?

This control of the internal environment is important for:

• Ensuring enzymes (which are involved in many processes) can function optimally
• Preventing cell damage (e.g. preventing cell lysis from changes in water potential)

Temperature and Blood pH

Enzymes control virtually every metabolic reaction in the body. Their activity depends on:

• Temperature — enzymes have an optimum temperature they work best at.
  • Too high — even small rises above the optimum cause bonds in the tertiary structure (e.g. hydrogen bonds) to break. This means the shape of the active site no longer fits the substrate, so the enzyme won’t function effectively (it is denatured). See Properties of Proteins for more detail.
  • Too low — reduces kinetic energy and slows the rate of metabolic reactions
• Blood pH — similarly, changes in pH (too high or too low) disrupt bonding (e.g. ionic, hydrogen) in the tertiary structure, changing the active site shape

Stable Blood Glucose Concentration

Blood glucose must be kept within narrow limits for two reasons:

• (Too low) Glucose is a respiratory substrate — glucose is the primary substrate for cellular respiration. Too little means cells cannot produce sufficient ATP for energy-requiring processes.
• (Too high) Water potential of blood —
  • Glucose dissolved in blood lowers water potential. If there is too high a concentration of glucose, this creates a water potential gradient that draws water out of cells by osmosis, potentially causing cell damage (shrinkage)

How do systems detect change?

Homeostasis in mammals involves ‘physiological control systems’ (the terminology from the specification). These are the systems inside your body that detect deviations from a set point and trigger corrective responses to return the internal environment to within restricted limits.

These systems share a common structure:

Negative Feedback

Negative feedback is the mechanism by which homeostatic systems restore a variable to its set point. When the variable deviates, the response opposes the change and returns it toward the original level.

For example: if blood glucose rises → insulin is secreted → glucose is removed from blood → blood glucose falls back to set point.

Separate Mechanisms for Each Direction

Greater control is achieved by having two separate negative feedback mechanisms, one for each direction of departure from the set point. Each mechanism acts independently, allowing precise correction regardless of which direction the variable moves.

For instance, blood glucose is controlled by:

• Insulin (from β cells) — lowers blood glucose when it rises too high
• Glucagon (from α cells) — raises blood glucose when it falls too low

Negative vs Positive Feedback

When interpreting feedback diagrams or data, identify whether the response is moving the variable back toward the original level (negative) or away from it (positive).