explain what is meant by homeostasis and the importance of homeostasis in mammals

Homeostasis in Mammals – Cambridge A‑Level Biology (9700)

Learning‑Outcome Checklist (AO1‑AO3)

1. What is Homeostasis?

• Regulated variable: any physiological parameter that must be kept within a narrow range (the set‑point) for normal function.
• Set‑point: the ideal value (or range) of a variable determined by genetics and modified by long‑term adaptation.
• Homeostatic imbalance: deviation of a variable from its set‑point that triggers corrective mechanisms.
• In mammals, continuous monitoring (receptors) and coordinated responses (neural and hormonal) restore the variable to its normal range.

2. Why is Homeostasis Essential?

• Enzyme efficiency: enzymes have optimal temperature, pH and ion concentrations; deviations reduce catalytic rates or cause denaturation.
• Cellular integrity: osmotic and ionic balance prevents swelling, shrinkage or lysis of cells.
• Energy supply: stable blood glucose ensures a constant supply of ATP to the brain, muscles and other tissues.
• Nervous system function: precise extracellular Na⁺/K⁺ ratios are required for generation and propagation of action potentials.
• Survival & reproduction: ability to cope with temperature extremes, dehydration or acid‑base disturbances determines fitness.

3. Key Variables Regulated in Mammals

4. Control Mechanisms – Primary (Neural) vs Secondary (Hormonal)

• Primary (Neural) control – rapid (seconds), uses electrical impulses.
  • Thermoregulation: hypothalamic‑sympathetic pathways → cutaneous vasomotion, shivering.
  • Blood‑gas regulation: carotid body → medullary respiratory centre → ventilation adjustments.
  • Osmoregulation: osmoreceptors → ADH‑secreting neurons (indirect hormonal output).
• Secondary (Hormonal) control – slower (minutes–hours), uses circulating messengers.
  • Pancreas: insulin & glucagon for glucose homeostasis.
  • Posterior pituitary: ADH for water balance.
  • Adrenal cortex: aldosterone (Na⁺/K⁺ balance) and cortisol (gluconeogenesis, vascular tone).
  • Parathyroid glands: PTH for Ca²⁺ regulation.
  • Thyroid: T₃/T₄ for basal metabolic rate & thermogenesis.

5. Feedback Loops

5.1 Negative Feedback (most common)

1. Stimulus: Blood glucose rises after a carbohydrate‑rich meal.
2. Receptor: β‑cells of the pancreas detect the increase.
3. Control centre: Pancreas releases insulin into the bloodstream.
4. Effector: Insulin promotes glucose uptake (muscle, adipose) and glycogen synthesis (liver).
5. Feedback: Blood glucose falls toward the set‑point; insulin secretion diminishes.

Diagram (textual): Stimulus → Receptor → Control centre → Effector → Response → (negative) → Receptor

5.2 Positive Feedback (used for rapid, self‑reinforcing processes)

1. Stimulus: Stretch of the cervix during labour.
2. Receptor: Mechanoreceptors in the uterine wall.
3. Control centre: Hypothalamus triggers oxytocin release from the posterior pituitary.
4. Effector: Oxytocin intensifies uterine contractions, further stretching the cervix.
5. Feedback: Loop continues until delivery removes the stimulus.

6. Mathematical Representation of a Simple Negative‑Feedback System

A proportional feedback model:

$$\frac{dx}{dt} = -k\,(x - x_s)$$

where x = variable, xₛ = set‑point, k = feedback constant (min⁻¹). Solution:

$$x(t) = x_s + (x_0 - x_s)\,e^{-kt}$$

Worked example – temperature recovery in a mouse

• Stimulus: core temperature falls to 35 °C; set‑point = 37 °C.
• Given: k = 0.12 min⁻¹.
• Time to be within 0.5 °C of set‑point:

$$0.5 = (37-35)\,e^{-0.12t}\;\Rightarrow\;e^{-0.12t}=0.25\;\Rightarrow\;-0.12t=\ln0.25$$

$$t = \frac{-\ln0.25}{0.12}\approx\frac{1.386}{0.12}\approx 11.6\ \text{min}$$

≈ 12 minutes for the mouse to regain normal temperature.

7. Suggested Practical Investigation – Thermoregulation

| Time (min) | Core Temp (°C) | Shivering (Y/N) |
|------------|----------------|-----------------|
| 0          | 37.2           | N               |
| 2          | 36.5           | N               |
| 4          | 35.8           | Y               |
| 6          | 35.4           | Y               |
| 8          | 35.2           | Y               |
| 10         | 35.1           | Y               |
| 12         | 35.0           | Y               |
| 14         | 35.0           | Y               |
| 16         | 35.3           | N               |
| 18         | 35.8           | N               |
| 20         | 36.4           | N               |
| 22         | 36.9           | N               |
| 24         | 37.1           | N               |
    

8. Links to Other Syllabus Topics

• Respiration: Ventilation controls CO₂ removal, directly influencing blood pH (acid‑base homeostasis).
• Transport: Circulatory system delivers O₂ and nutrients needed for cellular metabolism, the source of heat for thermoregulation.
• Enzymes: Temperature and pH homeostasis preserve enzyme active‑site geometry, ensuring optimal reaction rates.
• Hormones: Insulin, glucagon, ADH, aldosterone, PTH, cortisol and thyroid hormones illustrate integration of endocrine signalling with neural pathways.
• Energy metabolism: Glucose regulation links to glycolysis, the citric‑acid cycle and oxidative phosphorylation, providing ATP and heat.

9. Health & Disease Case Studies (Brief)

1. Type 1 Diabetes Mellitus – Auto‑immune destruction of pancreatic β‑cells → no insulin → chronic hyperglycaemia, ketoacidosis, polyuria, dehydration.
2. Heat‑stroke – Failure of sweating and cutaneous vasodilation → core temperature > 40 °C → protein denaturation, CNS damage.
3. Hyponatraemia (SIADH) – Excess ADH secretion → water retention → dilutional low Na⁺, cellular oedema, neurological symptoms.
4. Respiratory acidosis (COPD) – Reduced ventilation → ↑pCO₂ → ↓pH; renal compensation by increased HCO₃⁻ re‑absorption.
5. Hypocalcaemia – Parathyroid failure → ↓PTH → reduced bone resorption & renal Ca²⁺ re‑absorption → tetany, paresthesia.

10. Summary

Homeostasis is the cornerstone of mammalian physiology. By constantly monitoring variables such as temperature, glucose, osmolality, pH, calcium and blood gases, and by employing rapid neural pathways together with slower hormonal cascades, mammals maintain the internal conditions required for optimal enzyme activity, cellular integrity and overall survival. Mastery of these mechanisms underpins understanding of normal physiology, experimental design, and the pathological states that arise when homeostatic control fails.