Kidney and Osmoregulation

What is the amount of water lost in urine controlled by?

ADH (anti diuretic hormone) in a negative feedback system

Where is ADH produced and stored

Produced: hypothalamus

Stored and secreted from: posterior pituitary gland

When is ADH released?

When stimulated by neurones from osmoreceptors

What does ADH do to parts of the kidney?

Increases permeability of DCT and collecting duct to water

How does ADH act on cells?

Binds to receptors on cell membrane of tubule cells

Mechanism of ADH action

1. Binds to receptors on cell membrane of DCT, collecting duct 

2. Triggers formation of cAMP as 2nd messenger in cell

Second messenger

Molecule which relays signals received at a cell surface receptors to molecules inside cell

What is the cascade of events cAMP causes?

1. Vesicles in cells lining CD fuse w. cell surface membranes on side of cell in contact w. tissue fluid of medulla 

   1. Vesicle contains aquaporins

2. Membrane has increased aquaporins, making it more permeable to water

More water able to move out of tubule cells into medulla’s tissue and into blood capillaries by osmosis

= increased water reabsorption , small conc. urine, maintains water potential of blud

What happens when ADH levels fall?

1. Levels of cAMP fall

2. Aquaporins removed from tubule cell membranes & re-enclosed in vesicles (endocytosis)

3. CD becomes impermeable to water (can’t leave)

4. Large amount of v. dilute urine

Water potential of blood and tissue fluid maintained

osmoreceptors

• Monitors water potential of blood

• Are sensitive to conc. of inorganic ions in blood

• Linked to release of ADH

What happens when body in short supply of water?

1. Blood water potential drops

2. Detected by osmoreceptors which send nerve impulses to posterior pituitary 

3. Releases more ADH into blood

4. Binds to receptors in cells of CD, and increases the permeability of the tubules to water

   1. More water reabsorbed from nephron

5. Small vol. of conc. urine produced

What happens when body has excess of water?

1. Water potential of blood rises

2. Detected by stretched osmoreceptors in hypothalamus

3. Nerve impulses to p.p. are reduced or stopped, so less ADH released or is inhibited

4. Less water reabsorbed from nephron

   1. Endocytosis of aquaporins

5. Large vol. of dilute urine produced