D3.3 Homeostasis

IB Biology (2025 Syllabus)

Maintaining the Internal Environment

1. Principles of Homeostasis

Homeostasis is the maintenance of a stable internal environment within physiological limits despite changes in the external environment. This involves complex interactions of various regulatory systems.

Key Parameters Controlled:
  • Core body temperature
  • Blood pH (e.g., buffering with H₂CO₃ ⇌ H⁺ + HCO₃⁻)
  • Blood glucose concentration
  • Water balance (Osmoregulation)
Balance System

Key Vocabulary

Negative Feedback
A mechanism where a deviation from the set point triggers a response that reverses the change.
Set Point
The target value or range at which a physiological parameter is maintained.

2. Thermoregulation

Endotherms maintain a constant core body temperature, essential for optimal enzyme activity. The hypothalamus acts as the coordinating center.

Responses to Heat:
  • Vasodilation: Arterioles dilate, increasing blood flow to the skin surface for heat loss via radiation.
  • Sweating: Evaporation of water from sweat glands absorbs latent heat of vaporization.
Responses to Cold:
  • Vasoconstriction: Reduces heat loss.
  • Shivering thermogenesis: Involuntary muscle contractions generate heat.
  • Uncoupled respiration in brown adipose tissue (BAT).

3. Osmoregulation

Maintaining the balance of water and solutes (electrolytes) in the body fluids. This is crucial to prevent cells from lysing or crenating due to osmotic pressure.

Water Cells

Water moves by osmosis from a region of lower solute concentration to a region of higher solute concentration.

4. The Kidney and Nephron (HL)

The kidney functions in both excretion of nitrogenous waste (urea, $CO(NH_2)_2$) and osmoregulation.

Key Nephron Processes:
  1. Ultrafiltration: Occurs in the Bowman's capsule. High hydrostatic pressure forces water and small solutes out of the glomerulus.
  2. Selective Reabsorption: Proximal convoluted tubule (PCT) reabsorbs all glucose, amino acids, and most $Na^+$ and $H_2O$.
  3. Loop of Henle: Creates a hypertonic gradient in the medulla. The descending limb is permeable to water, ascending limb to salts.
  4. Osmoregulation (Collecting Duct): ADH (Antidiuretic Hormone) increases the permeability of the collecting duct to water by inserting aquaporins.
Mathematical Application:
Calculating solute concentration: $C = \frac{n}{V}$ where $C$ is concentration, $n$ is moles of solute, and $V$ is volume.

Check Your Understanding

Question 1: Explain the role of the Loop of Henle in osmoregulation.

The Loop of Henle functions as a countercurrent multiplier. It actively transports salts out of the ascending limb into the medulla, creating a hypertonic environment. This allows water to be passively reabsorbed from the descending limb and later from the collecting duct (if ADH is present), concentrating the urine.

Question 2: What is the effect of ADH on the collecting duct?

ADH binds to receptors on the collecting duct cells, triggering a cascade that causes vesicles containing aquaporins to fuse with the apical membrane. This increases water permeability, leading to more water reabsorption and a smaller volume of concentrated urine.

Question 3: How is blood pH regulated through homeostasis?

Blood pH is regulated via buffers (like the bicarbonate buffer system), respiratory compensation (altering ventilation rate to expire $CO_2$), and renal compensation (excretion or reabsorption of $H^+$ and $HCO_3^-$).