Osmoregulation Balances Water and Solutes
Definition
Osmoregulation
Osmoregulation is the process of maintaining the balance of water and solutes in the body.
- This process ensures that cells remain functional by preventing swelling or shrinking caused by water imbalances.
- 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
- The collecting ducts are the final section of the nephron and play a critical role in osmoregulation.
- They determine the volume and concentration of water reabsorbed or excreted.
- This regulation is what allows the body to adjust urine concentration based on hydration status.
Water Leaves the Collecting Ducts by Osmosis
- Filtrate Composition: The filtrate entering the collecting ducts is hypotonic (low solute concentration).
- 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.
- Water Movement: If the collecting ducts are permeable to water, it moves out into the medulla, concentrating the urine.
When you're dehydrated, your body produces small amounts of concentrated urine. Conversely, when you're well-hydrated, urine is more dilute.
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
- Production: ADH is synthesized in the hypothalamus and stored in the pituitary gland.
- Release: ADH is secreted when blood osmolarity (solute concentration) increases.
- Action: ADH binds to receptors on collecting duct cells, triggering the insertion of aquaporins (water channels) into their membranes.
Definition
Aquaporin
Aquaporins are specialized proteins that facilitate water movement across cell membranes.
Think of aquaporins as water faucets that open under ADH influence, allowing water to flow out of the collecting ducts and back into the bloodstream.
Aquaporins Allow Efficient Water Reabsorption
- With ADH: Aquaporins increase water permeability, allowing the body to reabsorb water efficiently.
- Without ADH: Aquaporins are absent, and the collecting ducts remain impermeable to water, resulting in dilute urine.
