Urine concentration and dilution Flashcards
Where does ADH act on?
the distal tubule and collecting duct
- makes the tubules more permeable to water
- secreted by the pituitary gland when the body fluid osmolarity is increased
Where is most of the water reabsorbed in the absence of ADH?
- water is absorbed in equal parts with solutes in the proximal tubule
- water is reabsorbed by osmosis in the descending loop of Henle
- the interstitial fluid in the renal medulla = very hypertonic, so can reabsorb a lot of water –> tubular fluid becomes more concentrated
What it’s osmolarity of the tubular fluid leaving the early distal tubular segment, with or without ADH?
ADH does not act on the thick segment of the ascending loop of Henle. This portion is impermeable to water but does have avid reabsorption of Na+, Cl- and K+.
- therefore, the fluid entering the early distal tubule is always dilute
What’s the mechanism of making dilute urine?
reabsorbing solutes from the distal segments of the tubular system while reducing water reabsorption
What’s the counter current multiplier mechanism?
Reabsorption of large amounts of solute with very minimal water reabsorption to maintain a hyper-osmolar medullary interstitum to allow to water diffusion when ADH is present.
1. thick segment of the ascending loop of Henle: large amount of Na+, Cl-, and K+ reabsorption
2. Active transport of ions in the collecting duct
3. diffusion of large amounts of urea in the collecting duct
4. minimal diffusion of water in the collecting duct
What’s the role of urea in maintaining hyperosmolar renal medulla?
urea recirculates –> about 50% is reabsorbed in the descending limb of loop of Henle, then some more is transported out of the medullary collecting duct, and secreted into the ascending limb of the loop of Henle
What’s the countercurrent exchange in the vasa recta?
the vein is “U” shaped so as blood descends into the medullar, solutes diffuses in and water diffuse out but as the blood ascending again, solutes are reabsorbed and water comes back in, thus maintaining the hyperosmolarity of the interstitum
What are some disorders that can impair the kidney’s ability to concentrate urine?
Urine concentration is due to ADH.
1. lack of ADH. Inability to produce or secrete ADH - central diabetes insipidus
2. inability to respond to ADH at the kidneys - nephrogenic diabetes insipidus.
- could be due to medullary washout from increased renal blood flow
- failure of the countercurrent mechanism to form hyperosmolar renal medullary interstitium
- drugs impairing solute reabsorption in the loop of Henle
- drugs impairing the response of distal nephron segments to ADH
What are the 2 mechanisms involved in regulating the concentration of Na+ and the extracellular fluid osmolarity?
- osmoreceptor-ADH feed back
- thirst mechanism
Describe the osmoreceptor-ADH feedback mechanism.
- increased extracellular osmolarity (increased Na+)
- stimulates the osmoreceptors in the anterior hypothalamus
- action potential sent to the posterior pituitary gland
- ADH released from the secretory granules
- ADH transported in blood to the kidneys
- acts on the late distal tubules and collecting ducts
- increases water permeability/ reabsorption of water
- dilutes the extracellular fluid
What other reflex mechanisms can stimulate ADH release?
- arterial baroreceptor reflex
- cardiopulmonary reflex
vagus and glossopharyngeal nerve
What are the 3 most important stimulus for thirst?
- increased extracellular fluid osmolarity
- decreased extracellular fluid volume and arterial blood pressure
- Angiotensin II
Why can’t angiotensin II and aldosterone regulate extracellular osmolarity?
- angiotensin II and aldosterone lead to reabsorption of both Na+ and water – so there is an increased quantity of Na+ but the concentration remains the same
- If ADH feedback mechanism is still intact, any change in osmolarity will be countered by ADH secretion
