Kidney Function II Flashcards
What is “clearance”
The volume of plasma that is cleared of a substance in given time
Renal Clearance
= u v /p
u= concentration in urine
v= volume of urine / min
p= concentration in plasma
where all the values are calculated at the same point in time
Inulin clearance rate-
It can be used to measure the GFR because:
it is freely filtered
it is not absorbed
Is not metabolised
easily measured
Good for experimental because it is administered intravenously
Creatinine examples
used clinically to estimate the GFR-
it is an estimate because it is slightly secreted- so value calculated would be a slight overestimate
Use of PAH
PAH is filtered, completely secreted, not reabsorbed
This means that all the plasm that enters the kidney per unit time is cleared of PAH
Rate of excretion = u x v
Since all the plasma that enters the kidney per unit time is cleared of PAH it is assumed that:
PAH clearance = renal plasma flow
Effective Renal Plasma flow ~ 600 ml/min
NB Effective because some is directed to perineal fat region
Renal Plasma FLow ——> Renal Blood Flow
Blood flow = Plasma Flow/ ( 1- haemocrit)
whole blood contains 45% cells
blood contains 55% plasma
Renal blood flow = 600/0.55 =1100 ml/min
The importance is stressed as this is 20% of cardiac output
Blood flow = Plasma flow / ( 1- haemocrit)
Osmalilty
mOSm/kg
the higher the solution osmolality the lower the water concentration
Sodium Balance
The main osmotically active solute in the plasma
Sodium freely filtered at renal corpuscle
Na concentration in plasma = 140 mol/l
Amount filtered = [Na] x GFR
Where is Sodium reabsorbed
Thin ascending limb (passive)
Proximal Tubule (LOTS OF REABSORPTION)
Thick ascending (LOTS OF REABSORPTION)
distal tubule
Collecting duct
Na transport Pathways (PT)
The Na+/K+ pump ensures that the [Na] intraceullarly is low which allows for the
Na+/H+ exchanger and Na+/nutrient symporter to transport Na+ inside
Due to the fact that Na+ and Cl- move together and due to the electrochemical gradient, Cl- moves down in-between the epithelium cells by diffusion
Na transport Pathways (Thick ascending limb)
The Na+/K+ pump ensures that the [Na] intraceullarly is low which allows for the
Na+/K+/2Cl- contransporter
to transport Na+ inside
Since there’s a buildup of K+ inside the cell, K+ can diffuse out of the epithelial cell by diffusion down its concentration gradient. This will encourage the Na+ to move inbetween the cells to IF due to there being a greater positive charge in the lumen
Na transport Pathways (distal tubule)
The Na+/K+ pump ensures that the [Na] intraceullarly is low which allows for the
Na+/Cl- co transporter
The filtrate in the lumen is negatively charged so Cl- will move inbetween the cells
Na transport Pathways (collecting duct)
The Na+/K+ pump ensures that the [Na] intracellulgvarly is low which allows for the
Na+ facilitated diffusion
The filtrate in the lumen is negatively charged so Cl- will move inbetween the cells
Water Reabsorption
Osmosis
Sodium Reabsorption
Tubule permeability- variable since different segments will express different water channels
and different segments will have tight junctions between the cells decreasing the movement of water between the cells
Proximal Tubule example
Na+ moves into the tubular epithelial cells from the tubular lumen by the symporters or the H+/Na+ anti porters. Na+ moves into the IF through Na+/K+ ATPase
The osmolarity decreases in the tubular lumen whilst increases in the IF
The osmolarity in the tubular lumen is less than the osmolarity in the IF
Water moves by osmosis through the epithelial cells to the IF or by osmosis in between the cells
BULK FLOW - due to the higher hydrostatic pressure in the iF, water will move into the plasma of the capillaries
Key–> volume of filtrate reduced and osmolarity is not changed
How to produce concentrated urine
Separate the water reabsorption with the Na+
Create a renal medulla IF with high osmolarity to drive the water reabsorption- this is done in the loop of Henle
Separating the Na+ and water reabsorption
Flow is said to be countercurrent- the flow in the limb is in the opposite direction
Since Henle’s loop reabsorbs more salt than water, it is implied that Na+ and water reabsorption is separated
In the ascending limb, walls are impeccable to water
Descending limb- express AQPI water channels so water an be reabsorbed. NaCl is also being secreted- moving into the filtrate passively
Blood Supply?
would wash away the osmolarity gradient but the vasa recta mirrors the shape of the loop
therefore gradient not washed away
Urea Recycling by the kidney
1) Urea is fully filtered at the Renal Corpuscle
2) Passive reabsorption in they the PT
3) Secreted back in the LOH
4) Reabsorbed at the CD via UT-A1 and UT-A3
ADH action
causes the fusion of AQP2 with the CSM facing the filtrate
This means that the water can move from the filtrate into the epithelial cell via aquaporins and then into IF by AQP3 and 4
