Lecture 9 - Control of plasma osmolarity Flashcards
Homeostatic osmolarity
280- 310 mOsm/Kg
approx 300 mOsm/kg
Increase in plasma osmolarity
Water intake is less than water excretion
How is water balanced if osmolarity increases?
- Remove water from the urine (ultrafiltrate) and reabsorb it into the ECF without the solute
Juxtamedullary nephrons
Responsible for making concentrated urine
Long loop of Henle
10% of nephrons
Vasa recta
The ordered structure of capillaries off of the efferent arteriole that is parallel to the Loop of Henle
MAINTAINS the osmotic gradient
Why does the juxtamedullary nephron have a long loop of Henle?
Creates a vertical osmotic gradient
Urea
Acts as an effective osmole in the kidney as hydrophilic so does not readily permeate lipid bilayers
In the body, it can move more freely as there are urea transporters that facilitate urea diffusion across membranes so is not an effective osmole
Contributes 600 mOsm/kg in medullary interstitium
Medullary counter current multiplication
- At the corticomedullary border, the interstitium is isotonic (300mOsm/kg)
- In the ascending limb, Na+ can be transferred to the interstitium via the Na+/Cl- cotransporter without the movement of water.
- Therefore, the filtrate becomes hyposomotic
- Because the interstitium is now hyperosmotic, water from the descending limb diffuses out leaving the Na+ behind.
- The filtrate in the descending limb is now hyperosmotic.
- The hyperosmotic fluid passes through the loop of Henle to the ascending limb which creates a cycle and maintains the Na+ concentration gradient.
- The hypososmotic filtrate in the ascending limb is transferred to the DCT
- The osmolarity in the interstitium reaches the max 1200 mOsm/kg
What causes medullary counter current multiplication
- Active NcCl transport in the thick ascending limb with no water reabsorption
- Recycling of urea
- Vasa recta
Urea in the proximal tubule
- Na+/urea cotransporter on apical surface - Sodium dependent urea transporter
- Urea diffuses into the capillary via passive urea transport
- Not a lot of urea reabsorbed compared to water therefore in the collecting duct the urea concentration is high
Urea recycling
- There are urea channels on the apical surface of the medullary collecting ducts allowing urea to diffuse out of the tubule into the medulla
- In the thin ascending limb there are also urea channels that secrete urea INTO the tubule
- The urea is then transferred to the collecting duct which causes a cycle and allows a high conc of urea in the medullary interstitium.
- The high concentration in the medulla in the collecting duct, allows a high concentration of urea to be excreted as less will be reabsorbed into the interstitium.
Cortical collecting duct and urea
Impermeable to urea
ADH
Increases the expression of aquaporins and urea channels.
More water and urea is reabsorbed into the interstitium so excretion of urea decreases and recycling increases.
Vasa recta maintenance of osmolarity gradient
- Low blood flow - decrease hydrostatic pressure in the vasa recta so oncotic pressure greater and water retained.
- Blood flow of the vasa recta occurs in the opposite direction to filtrate flow so water is the descending limb diffuses out into the vasa recta making the loop of Henle more concentrated
- No active transport
- The vasa recta loops down parallel to the loop of Henle so at the to near the cortex, it equalises with the lower concentration interstitium before entering the vein
Descending vasa recta
- Isosmotic blood in the vasa recta enters the hyperosmotic medulla
- The Na+, Cl- and urea diffuse into the lumen with slow flow so its equilibrates at each stratified level
- Osmolarity in the blood increases as it reaches tip of loop
