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, e.g if you eat 5 chocolate bars, we need internal mehcnaisms to bring down sugar levels. If we habe been for a run we need to ensure we have internal mechnaisms to bring the sugar levels back up

Why is homeostasis important?

This control of the internal environment is important for:

• Ensuring enzymes (which are invovled in many processes) can function optimumally
• Preventing cell damage (e.g preventing cell lysis from water potential control)

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 temp - Even small rises above this cause bonds in the tertiary structure (e.g hydrogen bonds) to break. Note for claude - add a link to protein properties (unit 1) This means the shape of the active site no longer fits the substrate, so the enzyme won’t function effectively (it is denatured)
  • Too low - reduce kinetic energy and slow rate of metabolic reactions
• Blood pH — Similarly changes in pH (too high or too low, disrupting bodning (e.g ionic, hyodrgen) in tertiarty structue, this changes 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 a 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’. (This is is the terminlogy from the specification). This means the systems inside your body that detect deviations from a set point and trigger corrective responses to return the internal environment 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.

For example, 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

Each mechanism acts independently, allowing precise correction regardless of which direction the variable moves.

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).