Lecture Notes: Osmoregulation and Role of ADH in the Collecting Duct

Key Concepts

Selective Reabsorption: Focus mainly on the Proximal Convoluted Tubule (PCT), not the Loop of Henle or Distal Convoluted Tubule (DCT).
Collecting Duct: Responsible for reabsorption of water based on body's water levels.
Osmoregulation: Control of water potential and homeostasis in the body.
Homeostasis: Involves stimulus detection, response by effector, and negative feedback mechanism.

Definition: Control of the water potential of blood and tissue fluid, ensuring balance.
Importance of Water Balance:
- Excess Water: High water potential in blood, cells may burst due to osmosis.
- Deficit of Water: Low water potential in blood, leads to dehydration of cells.
- Optimum Condition (Goldilocks Situation): Balanced water potential, cells function normally.

Stimulus Detection:
- Changes in blood water potential detected by osmoreceptors in the hypothalamus.
Control Center:
- Hypothalamus: Detects changes and sends signals to the posterior pituitary gland.

Function: Hormone that reduces urine production.
Release Mechanics:
- Signal from hypothalamus prompts posterior pituitary gland to release or withhold ADH.
Effect on Collecting Duct:
- Increased ADH:
  - Collecting duct more permeable to water.
  - More water reabsorbed, less urine produced.
  - Urine becomes more concentrated.
- Decreased/No ADH:
  - Collecting duct less permeable to water.
  - Less water reabsorbed, more urine produced.
  - Urine becomes more dilute.

ADH Receptors: Present on the collecting duct cells, not other parts of nephron.
Aquaporins: Channel proteins for water on vesicle membranes.
ADH Binding: Triggers vesicles to fuse with the cell surface membrane increasing permeability to water.

Too Much Water:
- Osmoreceptors detect, less ADH released.
- Collecting duct less permeable, more urine produced.
Too Little Water:
- Osmoreceptors detect, more ADH released.
- Collecting duct more permeable, less urine produced.

Understanding the role of ADH in osmoregulation is crucial for maintaining water balance in the body.
This mechanism illustrates a classic example of homeostasis in action.

With this understanding, we have completed the section on kidneys and osmoregulation.