D3.3.10 Osmoregulation by water reabsorption in the collecting ducts (HL) Notes

Osmoregulation Balances Water and Solutes

1. This process ensures that cells remain functional by preventing swelling or shrinking caused by water imbalances.
2. The kidneys play a pivotal role in osmoregulation by regulating how much water is reabsorbed into the bloodstream from urine.

The Collecting Ducts Adjust Water Reabsorption

1. The collecting ducts are the final section of the nephron and play a critical role in osmoregulation.
   1. They determine the volume and concentration of water reabsorbed or excreted.
   2. This regulation is what allows the body to adjust urine concentration based on hydration status.

Water Leaves the Collecting Ducts by Osmosis

1. Filtrate Composition: The filtrate entering the collecting ducts is hypotonic (low solute concentration).
2. Medullary Osmotic Gradient: The kidney’s medulla contains a steep osmotic gradient, created by the loop of Henle, that draws water out of the filtrate by osmosis.
3. Water Movement: If the collecting ducts are permeable to water, it moves out into the medulla, concentrating the urine.

ADH Increases Water Reabsorption

The hormone antidiuretic hormone (ADH) regulates the permeability of the collecting ducts, determining whether water is reabsorbed or excreted.

How ADH Works

1. Production: ADH is synthesized in the hypothalamus and stored in the pituitary gland.
2. Release: ADH is secreted when blood osmolarity (solute concentration) increases.
3. Action: ADH binds to receptors on collecting duct cells, triggering the insertion of aquaporins (water channels) into their membranes.

Aquaporins Allow Efficient Water Reabsorption

1. With ADH: Aquaporins increase water permeability, allowing the body to reabsorb water efficiently.
2. Without ADH: Aquaporins are absent, and the collecting ducts remain impermeable to water, resulting in dilute urine.

The Hypothalamus Detects Changes in Osmolarity

1. The hypothalamus serves as the control center for osmoregulation, detecting changes in blood osmolarity via osmoreceptors.

How Osmoreceptors Work

1. High Osmolarity: During dehydration, osmoreceptors shrink, signaling the release of ADH to conserve water.
2. Low Osmolarity: After consuming excess water, osmoreceptors swell, suppressing ADH release and promoting urine excretion.

Negative Feedback Restores Balance

1. Osmoregulation operates through a negative feedback loop:
   1. Stimulus: Changes in blood osmolarity (solute concentration).
   2. Receptor: Osmoreceptors in the hypothalamus detect these changes.
   3. Control Center: The hypothalamus signals the pituitary gland to adjust ADH secretion.
   4. Effector: The collecting ducts respond to ADH levels by altering water reabsorption.
   5. Response: Blood osmolarity is restored to normal.

Disorders Highlight the Importance of Osmoregulation

1. Diabetes Insipidus
   1. A condition where ADH production or response is impaired.
   2. Symptoms include excessive urination and dehydration.
2. Overhydration
   1. Consuming excessive water dilutes blood osmolarity, suppressing ADH.
   2. This can lead to hyponatremia, a dangerous condition caused by low sodium levels.

Why Osmoregulation Matters

1. Prevents Dehydration: By conserving water during deficits, osmoregulation ensures survival in challenging conditions.
2. Maintains Cellular Function: Stable water and solute levels protect cells from osmotic damage.
3. Reflects Biological Trade-offs: Balancing water conservation with waste excretion highlights the adaptive strategies organisms use to survive.