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Flashcards in Renal 7 Deck (9)
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What is countercurrent exchange? How would it be different if it was a regular tube?

Exchange between vasarecta and interstitum
- Maintains osmotic gradient in the medulla
- U shaped design is very important here

Descending loop of henle:
- When you're at the same level… the deeper you get into the medulla, its hypoosmotic to the interstitum
- Plasma osmolality is less than interstitium
- Solutes (enters) move from interstitium to plasma
- H20 (leaves) moves from plasma to interstitium
○ Both contribute to concentrating plasma.

Ascending loop of henle.
= Ascending blood is hyperosmotic
- Osmolality plasma
- Solutes (leaving) move from plasma to interstitium
- Water (comes in) moves from int
No permeablility to vesssels or ions

OVERALL: he osmolality is the same as it was when it came in.
Allows you to supply blood to deeper sections (especially long loop ones) without washing out the gradient)
This process prevents washout of the osmotic tradient.

How would it be different if it was a regular tube?
Cortical medullary jucntions to apex of medulla.
If it was a tube, blood would be hyperosmotic at the end, washout the gradient! Blood would equibrilate just like the U, but there is no mechanism to reestablish normal plasma osmolality, solute would be drawn out and prevent maintenance of the gradient in the medulla.
Since it’s a U, it prevents the washout and


How is urine regulated?

Urine [ ] regulation
- Determined in distal portion (distal tuble and collecting duct)
- Modulate amount of water reabsorption with AVP (arginine vasopressin)
- Acts in collecting duct on principle cells, and depending on the amount present, it will influence population of of aquaporin2 (AQP2)
- When AVP is absent, the the AQP won't be present on apical membrane
- AQP 3 4 on basolateral, but since water can't get in via AQP 2, in AVP absent scenario, there wont be water reabsorption
- When AVP is present, AQP2 expressed apically,w ater comes in and then completes reabsorption through 3 and 4 (always present on basolateral side0



What is AVP? where is it released/

Posterior Pituitary Gland
- High plasma osmolality (osmorecpeotrs detected)
○ Increased plasma osmolality, increase AVP secretion. Don't want it to become more concentrated, so you take in as much water as possible.
- Detected by osmoreceptors (change in firing rate)
- Supraoptic or paraventricular nucleus (produce and secrete AVP themselves)

- Low BP: baroreceptors
- Low blood volume: baroreceptors
○ Secretion of AVP in response to the decrease in BP or volume

- Detected by baroreceptors (stretch/pressure receptors like aortic arch, carotid sinus, wall of right atrium)
§ Send a signal to vasomotor center in medulla oblongata, that have projections to supraoptic or paraventricular nuclei to regulate AVP

Depending on blood volume state you can change how much you respond to changing osmolality
- Lower blood volume/lower blood pressure
1. AVP is secreted at a lower volume (lower threshold, 270 osmolality instead of 280 you have secretion
2. AVP secreted will increase more rapidly for given change to osmolality (270-275 the increase in AVP scretion is greater than in the 275)
- So increases threshold and sensitivity to changes in osmolality

AVP (vasopressin or antidiuretic hormone)
AVP retains water.


How does AVP alter our ability to reabsorb water?

- Released from posterior pituitary
- Travel through blood to collecting duct
- Act on V2 receptors
Activate G protein couples receptor (GPCR) increase andenylyl cylclase to increase CAMP and PKA activation
PKA activates nuclear elements, AQP2 channels
1. Make more of the aquapores (AQP2)
2. Increase insertion of pores into apical membrane

Now you can reabsorb water


How does AVP influence reabsorption along the nephron?

Along the nephron,
Beginning portion: same amount of water reabsorption regardless of AVP presence

Loop of Henele: no more high water permeability, no more water reabsorption here (Ascending loop and DCT)

Collecting duct: permeability depends on AVP
- Permeability of water is high with AVP present.

AVP present: more h20 reabsorption, concentrated urine
AVP absent: the opposite.

Increase in osmolality as you move through loop of henele
When we have tubular fluid leaving loop of henle towards distal portions, the osmolality is the same. Both AVP present and absent states will get the same fluid delivered to the collecting duct ( in the fluid that gets to the distal portions (regulation does not occur here)

Classic tuble: osmolality goes up slightly with AVP present. It will stay similar until collecting duct where a lot of water is reabsorbed. Leads to concentrated urine.
Without AVP: not the same water reabsorption (since water is stuck inside the tubule, you have progressive dilution of tubular fluid)

Lots of water reabsorption influences how concentrated inner portion of the medulla is which changes driving force of water (down limb) and sodium in ascending limb.


What happens when you have high/low water intake?

Water restricted vs high water intake.
- Loss lots of water
- Decreased BP and blood volume and plasma osmolality
- Two simuli that promote AVP release.
- AVP acts on collecting duct to make sure you are reabsorbing lots of water.

Can't increaes blood volume through this mechanism. They prevent water loss but they can't take water in.
To increase blood volume, you need to DRINK WATER.

High water intake: decreased osmolality, increased blood volume/pressure.
AVP release will be minimal, leading to reduced H20 reabsorption in collecting duct
- You pee lots !

Alcohol and caffeine inhibit avp release :)


How is plasma osmolality restored?

Restoring normal plasma osmolality
Low blood volume/pressure stimulates hypovolemic thirst response
High plasma osmolality stimulates osmotic thirst response.

Increased h20 adds more water to system to reduce plasma osmolality and icnrase blood volume


How does the body respond to low blood pressure?

Juxtaglomerular apparatus: interface for macula densa (ascending thick limb and distal tubule boundary) with afferent and efferent glomerulus

- Responds to low blood pressure
- Secretion of renin (because of reduction in blood pressure)
- Incrases circulatin levels of antiogensin 2 plasma levels


What stimulates renin secretion?

Macula densa: physically interafcing with afferent and efferent glomerulus

Justaglomerulus apparatus (JGA)
- Responds to changes in flow
- Regulates GFR in response to changes in flow
- Increased flow (increase glom filtration, tubular feedback leads to vasoconstriction of afferent arteriol to lower BP)
- Regulation of blood pressure mediated by renin and angiotensin-aldosterone system
- Still involves all the same cells, but now a decrease in blood pressure will lead to renin secretion.

What leads to renin release?
- Reduced GFR: deteced by reduced sodium in macula densa
- Reduced blood pressure: detected by atrial stretch receptors, carotid and aortic baroreceptors, leads to increased activation of sympathetic neurons that innervating afferent arterioles.
- Reduced blood pressure detected directly by afferent arterioles.

All leads to renin secretion ^^