Controlling Osmolarity Flashcards
What defines osmolarity?
What drives osmotic movement?
The total concentration of solute in a solution (that can’t cross cell membranes)
Water will move across semi-permeable membranes according to the osmotic gradient: from areas of low –> high osmolarity
Name 2 semi-permeable membranes
- Cell membranes
2. Capillary walls
What generates oncotic pressure?
Large molecules that accumulate in the capillaries, as capillary walls don’t allow passage of large molecules, i.e; proteins or cells
Name 3 ways you can uncontrollably lose water, one way you can lose water controllably and 2 methods that will increase the water in your body.
Uncontrollably lose:
- Evaporation from the lungs
- skin and sweating
- lose it from the gut
Controllably lose:
The kidney can variably lose water (but can’t replace it)
Thirst
Increasing water: thirst and ADH (more on that later)
When would you require diuresis and anti-diuresis?
Diuresis: diluting urine to lose excess water, used when the fluid is hypotonic
Anti-diuresis: when fluid is hypertonic need to retain more fluid to balance the solute: solution ratio and keep the fluid at the right osmolarity. Stimulates more water resorption and thirst
Which parts of the nephron are diluting segments?
The LOH, DCT and the CT
How does the LOH dilute the urine?
Variable sodium and water resorption in the descending segment. The ascending segment is impermeable to water and ions are pumped out. This creates hypotonic filtrate and a hypertonic medullary interstitium
How do the DCT and the CT concentrate urine?
What is the typical osmolarity in the DCT vs the medullary interstitium surrounding the LOH?
When you need to concentrate urine, the DCT and CT would have more ADH/aquaporins so water can be resorbed, and water will leave the CT as it passes through the hypertonic medullary interstitium
The osmolarity is typically 300 milli osmoles/L whereas in the medullary interstitium is typically 1200 milli osmoles/L
Which hormone makes walls more and less permeable to water in the DCT and CT?
Why isn’t this permanent?
ADH/Vasopressin: stimulates aquaporins which are holes in the membrane walls that allow water to leave the filtrate and follow the osmotic gradient
Aquaporins aren’t permanent due to quick membrane turnover.
How do the DCT and CT dilute urine?
Their walls become less permeable to water leaving the filtrate
What stimulates ADH?
Osmoreceptors in the hypothalamus are sensitive to their own size: if they shrink this signals a need for anti-diuresis as the plasma has become too hypertonic, so they fire AP’s (if they swell, they don’t). This signals thirst and ADH
What is counter-current multiplication?
The ascending LOH is impermeable to water and actively pumps out ions due to the Na-K-Cl cotransporter and Na-K-ATPase - creating a hypertonic medullary interstitium: generating a local gradient of 100-200 milli osmoles. Since the descending LOH is permeable to water, water can leave the filtrate due to this osmotic gradient
Is counter-current multiplication enough to keep an extreme hypertonic medullary interstitium?
Urea is also recovered into the interstitial fluid in the DCT to further concentrate the medulla
Explain how the blood supply fuels the kidneys without interrupting the process?
How much blood supply does the kidney require at rest, why?
Countercurrent multiplication requires a lot of ATP, so kidney needs 25% of cardiac output at rest
The blood vessels are arranged in a U shaped vasa recta from the cortex - down into the medulla and back again
What determines the volume of the ECF and circulating fluid?
As long as the patient can drink, the ECF volume/circulating fluid is determined by the amount of solute.
