Renal 6 Flashcards
Normal function requires ECF Osmolarity =
300
mOsm
H2O and Na+ input via diet:
– Too much
– Not enough.
H2O and Na+ loss: (4)
– Insensible
– Sweat
– Feces
– Urine - To a large extent, kidneys balance the
books by adjusting water reabsorption and
excretion.
Mechanisms to eliminate excess water by excreting a
dilute urine;
Mechanisms to conserve water by excreting a
concentrated urine;
Renal feedback mechanisms that control the
extracellular fluid sodium concentration and osmolarity
Thirst and salt appetite mechanisms that determine the
intakes of (2), which also help to control
extracellular fluid volume, osmolarity, and sodium
concentration
water and salt
Concentration and Dilution of the Urine Accomplished independently of major changes in solute excretion: – Maximal urine concen- tration: – Minimal urine concen- tration:
1200 mOsm (specific gravity ~ 1.030).
50 mOsm (specific gravity ~ 1.003).
Filtrate is — in proximal tubule.
isosmotic
Filtrate is isosmotic in proximal tubule. Becomes hyperosmotic as passes through tDL (2)
– Water reabsorption
– No solute reabsorption.
Becomes hyposmotic as passes
through TAL and early distal tubule (2)
– Solute reabsorption
– No water reabsorption.
Osmolarity of fluid will vary as pass
through distal tubule and collecting
duct (2)
– Stays hyposmotic in absence of
ADH (dilute urine)
– Dilute Urine (as low as 50 mOsm/L)
Urine Formed With ADH is Concentrated (4)
ADH increases H2O permeability of distal tubule and collecting duct. Large volume of H2O diffuses into interstitium. Enters capillaries of Vasa Recta and removed. Creates concentrated urine (as high as 1200 mOsm/L)
Obligatory Urine Volume
The maximal concentration ability of the
kidney dictates how much urine volume
must be excreted each day to rid the body of
metabolic waste products and ions that are
ingested.
Adult must excrete — mosmol daily
(solutes ingested and [mainly] produced by
metabolism).
600
(600 mosmol/day)/(1200 mosmol/L) = 0.5
L/day.
(600 mosmol/day)/(1200 mosmol/L) = 0.5
L/day.
– Adds to
other sources of H2O loss (skin,
lungs, GI).
Renal disease impairs — ability:
concentrating
Requirements for Excreting a Concentrated
Urine (2)
- High levels of ADH
2. Hyperosmotic medullary interstitial fluid
Hyperosmotic medullary interstitial fluid (4)
Surrounds collecting duct
Sets gradient for water reabsorption
Requires Counter Current Multiplier Mechanism
Function of Juxtamedullary nephrons
Function of Juxtamedullary nephrons (3)
- Long Loop of Henle
- Vasa Recta
- Slow flow rate
High Interstitial fluid Osmolarity a result of: (4)
- Active transport of Na+ and other ions by ascending limb of LOH
- Active transport of ions from collecting duct into interstitium
- Facilitated diffusion of Urea by Inner medullary collecting ducts
- Movement of only small amounts of water into medullary
interstitium
Continuous delivery of NaCl from proximal convoluted tubule into
loop of Henle
Concentration of filtrate by H2O reabsorption by
Descending Limb of Henle
Continuous reabsorption of solute into interstitium by
thick ascending limb
Na+-K+ pumps in CD pump Na+ into interstitium to
↑ osmolarity (to ≈1,200 mOsm/l) => formation of interstitial osmolar gradient
In Absence of ADH (2)
Reabsorb solute
Little to no water reabsorption
n Presence of ADH (3)
Distal tubule and collecting ducts become highly permeable to water and reabsorb much water into cortical interstitium Medullary collecting duct cells reabsorb water but overall amounts much lower (keeps medullary interstitium from being diluted). Reabsorbed water carried away by Vasa Recta
Role of Urea in Concentrating Urine
Waste product of protein metabolism; produced continuously by
liver.
urea:
Normally excrete –% of filtered load
20-50
urea:
Non—, but constitutes an osmotic load that must be —
(mw = 60 g/mol; plasma level 5 mM).
toxic
excreted
urea:
Contributes —% of medullary interstitial osmolarity (500-600
mOsm/L)
40-50
Reabsorbed passively by — collecting duct cells
medullary
Secreted into descending limb and thin ascending limb of Loop of
Henle - Requires: ADH (2)
Concentrates urea in filtrate (water reabsorption in cortical and
medullary collecting ducts)
Activates carriers (UT-A1; UT-A3) for the facilitated diffusion of
urea by medullary collecting duct cells
Countercurrent Multiplier Mechanism
Depends on anatomical relationships of (3)
loop of Henle, vasa recta, & collecting ducts.
~25% of nephrons are —, with long loops of Henle that extend into renal medulla parallel to vasa recta & collecting ducts.
juxtamedullary
Filtrate in descending & ascending limbs of loop of Henle flows in opposite directions of blood flow in
Vasa Recta
Clears Reabsorbed water so does not dilute
medullary interstitial fluid
Blood flow only –%
of total but sufficient
for O2 supply and
CO2 removal
2
Tubule Segment: Proximal Tubule
% H2O Reabsorbed
Filtrate Osmolarity
70
Isosmotic
Tubule Segment: Descending LOH
% H2O Reabsorbed
Filtrate Osmolarity
20
Hyperosmotic
Tubule Segment Ascending limb of LOH
% H2O Reabsorbed
Filtrate Osmolarity
0
hyposmotic
Tubule Segment: Late DT and CCD
% H2O Reabsorbed
Filtrate Osmolarity
variable (adh)
variable
Tubule Segment: MCD
% H2O Reabsorbed
Filtrate Osmolarity
variable (ADH)
variable
