2.1 - Homeostasis

Core Definition & History

Define homeostasis.

The control or stabilisation of the internal environment.

Who coined the term “homeostasis”?

Walter B. Cannon (1929)

What did Claude Bernard contribute?

Constancy of the internal environment is essential for life.

What is the central idea of homeostasis?

The internal environment is regulated within narrow limits.

Internal Environment

What is the “internal environment”?

The extracellular fluid (ECF).

What separates internal from external environment?

The cell membrane.

Why is the ECF more important than the ICF for homeostasis ?

• ECF interacts with external environment

• ICF is protected by cell membrane

• Changes in ICF follow after ECF changes

Body Fluid Compartments

Total body water percentage ?

60%

Breakdown of body fluids ?

• ICF - 67%

• ECF - 33%

Components of ECF ?

• Plasma

• Interstitial fluid

• Trans cellular fluid

Examples of trans cellular fluid ?

• CSF

• Synovial fluid

• Ocular fluid

REGULATORS VS CONFORMERS

what are conformers ?

Organisms that match their environment,

What are regulators ?

Organisms that maintain stable conditions

CONTROLLED VARIABLES

What variables are tightly regulated?

• O₂

• CO₂

• Na⁺

• K⁺

• Ca²⁺

• Glucose

• pH

• Temperature

Normal blood pH range?

7.35 – 7.45

Normal body temperature?

37

Why must these be tightly controlled?

Small deviations → disease or death

Consequences of Loss of Homeostasis

What happens if homeostasis fails?

• Ill health

• Cell death

• Threat to survival

Example: loss of blood supply?

• Cell death in ~30 seconds

• Brain death in 3–4 minutes

Example: cholera toxin effect?

• Cl⁻ secretion → Na⁺ follows → water follows

• Causes diarrhoea → dehydration → death

Examples of Homeostatic Control

Oxygen Regulation

How is oxygen regulated?

Via haemoglobin binding/release.

Key feature of haemoglobin?

Releases O₂ when local concentration is low.

CO₂ Regulation

Source of CO₂?

Cellular respiration

Equation for respiration?

C₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂ + energy

Effect of hyperventilation?

↓ CO₂

Effect of hypoventilation?

↑ CO₂

Electrolytes

What are electrolytes?

Charged ions in solution (Na⁺, K⁺, Ca²⁺ etc.)

Blood Glucose

What controls blood glucose?

Hormones (insulin & glucagon)

Insulin effect?

Lowers blood glucose

Glucagon effect?

Raises blood glucose

Temperature Regulation

Where is the body’s thermostat?

Hypothalamus

Temperature responses include?

• Sweating

• Shivering

• Blood vessel changes

Other homeostatic systems?

• Blood pressure

• Acid-base balance (pH)

Feedback Systems

Negative Feedback

Define negative feedback.

Response opposes the initial change.

Why is it most common?

Maintains stability.

Key components of negative feedback?

1. Set point

2. Sensor

3. Comparator

4. Effector

Example: Blood Pressure Regulation

Sensors?

Baroreceptors

Location?

Aortic arch & carotid sinus

Comparator?

Medulla

Effectors?

Heart rate & blood vessels

Example: Temperature Regulation

Sensor?

Thermoreceptors

Comparator?

Hypothalamus

Effector?

Sweat glands, muscles, vessels

Example: Blood Glucose

What happens after glucose intake?

• Glucose rises

• Insulin released

• Glucose falls back to set point

Can it overshoot?

Yes (drops below set point temporarily)

Positive Feedback

Positive Feedback

Response amplifies initial change.

Why is it dangerous?

Can become uncontrolled.

Examples?

• Blood clotting

• Labour (oxytocin)