16.6 - The Role Of The Nephron Is Osmoregulation Flashcards
What is osmoregulation and how does the kidney maintain it?
Osmoregulation is the maintenance of the water potential of plasma and tissue fluid. The kidney regulates osmoregulation through the nephron, which carries out several stages:
- Formation of glomerular filtrate by ultrafiltration
- Reabsorption of glucose and water by the proximal convoluted tubule
- Maintenance of a sodium ion gradient in the medulla by the loop of Henle
- Reabsorption of water by the distal convoluted tubule and collecting ducts
What are the main structures of the nephron involved in osmoregulation?
The nephron consists of:
- Renal (Bowman’s) capsule: Surrounds the glomerulus and collects filtrate
- Glomerulus: A network of capillaries involved in filtration
- Proximal convoluted tubule: Reabsorbs glucose, amino acids, and water
- Loop of Henle: Maintains a sodium ion gradient for water reabsorption
- Distal convoluted tubule: Adjusts ion and pH levels
- Collecting duct: Final site of water reabsorption
How is glomerular filtrate formed?
Blood enters the kidney via the renal artery, which branches into arterioles. Each arteriole enters a renal capsule as an afferent arteriole and forms the glomerulus. Due to the smaller diameter of the efferent arteriole, hydrostatic pressure builds, forcing water, glucose, and mineral ions out into the renal capsule, forming filtrate.
What resists the movement of filtrate out of the glomerulus?
- Connective tissue and epithelial cells of the capillary
- Epithelial cells of the renal capsule
- Hydrostatic pressure in the renal capsule
- Low water potential of blood in the glomerulus
—> Despite this resistance, filtration occurs due to structural modifications.
What structural modifications reduce resistance to filtration?
Podocytes in the renal capsule have spaces between them, allowing filtrate to pass beneath and between branches.
Endothelium of the glomerular capillaries has 100 nm spaces, enabling fluid passage.
These modifications allow filtrate to pass into the renal capsule.
What is present and absent in the glomerular filtrate?
Contains: Water, glucose, mineral ions, urea
Does not contain: Blood cells, plasma proteins (too large to pass through)
How does the proximal convoluted tubule reabsorb substances?
85% of filtrate is reabsorbed
Epithelial adaptations:
Microvilli + microvilli → Large surface area
Infoldings at the basal end → Large surface area for transport of ions+water into capillary
Many mitochondria → ATP for active transport
Describe the steps of reabsorption in the proximal convoluted tubule.
- Sodium ions are actively transported out of tubule cells into blood capillaries.
- This lowers sodium concentration inside tubule cells.
- Sodium ions diffuse into tubule cells from the glomerular filtrate in the lumen via facilitated diffusion through carrier proteins.
- Carrier proteins co-transport glucose, amino acids, or chloride ions.
- These substances diffuse into the blood, allowing their full reabsorption
How is water reabsorbed in the proximal convoluted tubule?
180 dm³ of water enters nephrons daily, but only 1 dm³ is excreted as urine.
85% of water reabsorption occurs in the proximal tubule.
The remaining water is reabsorbed in the collecting duct via the loop of Henle’s function.
What is the role of the loop of Henle in osmoregulation?
The loop of Henle creates a sodium ion gradient that enables water reabsorption in the collecting duct, concentrating urine.
What are the two regions of the loop of Henle?
Descending limb: Narrow, thin-walled, highly permeable to water.
Ascending limb: Wide, thick-walled, impermeable to water.
How does the loop of Henle create a sodium ion gradient?
- Sodium ions are actively transported out of the ascending limb.
- This lowers the water potential in the medulla’s interstitial space.
- Water leaves the descending limb by osmosis into blood capillaries.
- At the base, sodium ions diffuse out, and are actively pumped up the ascending limb.
- This maintains a decreasing water potential gradient deeper into the medulla.
How does water leave the collecting duct?
- The collecting duct is permeable to water.
- As filtrate moves down, water osmoses out into surrounding capillaries.
- This occurs along the entire length of the collecting duct due to the counter-current multiplier system.
What is the counter-current multiplier system in the loop of Henle?
- Filtrate in the collecting duct encounters progressively lower water potential in the interstitial fluid.
- This maintains a constant gradient, drawing water out continuously.
- Ensures maximum water reabsorption—around 80% of water is reabsorbed this way.
How does ADH regulate water loss?
- ADH controls aquaporin channels in the collecting duct.
- More ADH → More aquaporins → More water reabsorbed, less urine.
- Less ADH → Fewer aquaporins → More water lost, more urine
What is the function of the distal convoluted tubule?
- Adjusts water and salt levels in response to body needs.
- Controls blood pH by selectively reabsorbing ions.
- Its permeability is controlled by hormones.
