Session 5: Control of Plasma Osmolarity Flashcards
What is the normal osmolality of ECF?
280-310 mOsm/kg
Roughly around 300 mOsm/kg
What is the osmolality of an isotonic fluid?
Roughly 300 mOsm/kg
What is the range of urine osmolality?
50-1200 mOsm/kg
What is the major cation of of the ECF?
Na+
What is usually the problem affecting plasma osmolality?
Water balance and not electrolytes.
Water is not isotonic but hypotonic.
What happens if you simply add water to plasma?
Water is hypotonic, this means that adding water causes the plasma to become hyposmotic. This leads to water wanting to move out of the ECF in the kidney and more diluted urine will be excreted.
Which class of nephron majorly allows the movement of water and can affect osmolality?
Juxtamedullary nephrons.
The osmolality of the protein-free blood plasma is determined by electrolytes. It is roughly 300 mOsmoles. The osmolality of the total volume of the blood plasma is 301 mOsmoles.
What is the force of that 1 mOsm called?
This is due to the pull of proteins which is called colloid osmotic pressure or oncotic pressure.
Briefly explain the concentration gradient of the nephron.
There is a vertical concentration gradient in the nephron where the osmotic gradient gradually increase as you go deeper into the kidney (cortex to medulla).
Why is the vertical concentration gradient of the nephron important?
Because it allows ADH to easily control the concentration of urine.
The collecting duct use the gradient along with hormone ADH to produce urine in varying concentrations.
What solute helps in the urine concentration mechanism?
Urea
What helps to maintain the osmotic gradient?
Vasa recta
What is the osmolality at the corticomedullary border?
Isotonic aka 300 mOsm/kg
Explain the thick ascending limb of the loop of Henle’s role in the medullary gradient.
The thick ascending limb is largely impermeable to water. This means that no water will move in this part of the kidney tubule.
It’s purpose is to remove solute without water. This leads to a hyposmotic filtrate and an increase in the osmolality of the interstitium. There are no aquaporins in this region of the tubule.
The channel responsible for the movement of ions in the thick ascending limb is NaKCC. This leads to ions moving into ICF to increase the osmolality. These ions then move into the interstitium.
Explain the action of loop diuretics like furosemide in regards to the medullary gradient.
Blocks the NaKCC transporter. This leads to no increase in osmolality of the interstitium and leaves the interstitium to be isosmotic. This means that a lot of dilute urine will be excreted.
Briefly explain what happens in the ascending limb.
Active transport of Na+ and Cl- from lumen to interstitium by NaKCC.
The ascending limb is impermeable to water.
NaCl leaves and water remains. Filtrate osmolality decreases.
Fluid entering the DCT will be hyposmotic compared to plasma.
Briefly explain what happens in the descending limb.
Highly permeable to water due to aquaporin channels which are always open.
The descending limb is impermeable to Na+ so Na+ will remain in the descending limb and osmolality of filtrate will increase.
What is the maximum osmolality of the tip of the loop of henle?
1200 mOsm/kg
What is the maximum difference in concentration gradient that can be achieved between the interstitium and the lumen?
200 mOsm/kg
Explain the counter current and its role in achieving the medullary gradient.
Active salt pump in the ascending limb establishes a 200 mOsm gradient at each horizontal level. The fluid flows forwards into the DCT a mass of 200 mOsm fluid exits into the DCT and a new mass of 300 mOsm fluid enters from the proximal tubule.
The ascending limb will the transport out more NaCl and the descending limb passive fluxes of water reestablish the 200 mOsm gradient. However this time the osmolality has increased but not the gradient.
The same thing happens where fluid flows forward several frames and the 200 mOsm gradient must be established once again.
The final vertical osmotic gradient is established and maintained by the ongoing countercurrent multiplication of the long loops of Henle.
Explain urea as an effective osmole
It is only an effective osmole in the kidneys where in the proximal tubule it can be taken up by a sodium-dependent urea transport on the apical membrane to go from the lumen into the tubular cells and then into the interstitium or the capillary.
The urea adds to the osmotic gradient.
Explain urea recycling in the nephron.
Urea is reabsorbed in the medullary collecting duct. Cortical CD cells are impermeable to urea so this only happens in the medullary part.
Urea then moves into the interstitium and diffuse back into loop of Henle.
Under the influence of ADH fractional excretion of urea decreases and urea is increasingly recycled instead.
The concentration gradient produced by the loop of henle act as a counter current multiplier. What maintains the concentration gradient?
Vasa recta
Explain the properties of the vasa recta that allows it to maintain the concentration gradient.
Capillaries are leaky because we need to supply the medullary kidney. But too high blood flow would quickly was out the concentration gradient.
This means that in the vasa recta there is:
Low flow to maintain the medullary hypertonicity.
The cells are endothelium with no active transport where movement in or out depends on passive diffusion.
Where can the vasa recta be found?
Only in the juxtamedullary nephrons running parallel to the loop of henle.
Describe the flow of the vasa recta.
Low rate of flow and its direction is also opposite to the flow of the tubular fluid.
Explain the movement of solutes and solvent in the descending vasa recta.
Runs along the ascending loop of henle.
This is highly impermeable to water so mainly solutes like Na+, Cl- and K+ will move from the thick ascending limb to the descending vasa recta as the concentration gradient in the interstitium is higher than the plasma. The slow flow allows Na+, Cl- and urea to diffuse into the vasa recta so it equilibrates at each stratified level.
This leads to the plasma of the descending vasa recta becoming increasingly hyperosmotic.
Explain the movement of solutes and solvents in the ascending vasa recta.
Runs alongside the descending limb of loop of Henle.
The descending limb is largely impermeable to Na+ and highly permeable to water.
This leads to a lot of water moving via aquaporin channels into the vasa recta as the plasma of the vasa recta has at this point been hyperosmotic as well compared to the interstitium.
What does a decrease in osmolality do to ADH?
Decrease ADH secretion
