Reabsorption Flashcards
180L/day are filtered through the glomerulus into the renal tubule, however only 1-2L/day are excreted as urine, what happens to the remainder of the filtrate
More than >99% of the plasma entering the kidney return to the systemic regulation though the mechanism of reabsorption
What is responsible for reabsorption
The peritubular capillaries
How does reabsorption occur
The pressure in the peritubular capillaries is very low because the hydrostatic pressure overcoming the frictional resistance of the efferent arteriolar
The Oncotic pressure is high, due to the peritubualr capillaries having a higher concentration of plasma proteins compared to filtrate
As a consequences , of the high osmotic pressure and low pressure of peritubualr capillaries, it favour reabsorption of filtrate from the lumen to the peritubular capillaries
What is reabsorbed at the proximal tubule
NaCl 65-75%
Water 65-75%
Amino acids -100%
Glucose -100%
Urea -50%
K+ -100%
Ca2+ -100%
(percentages in normal physiology)
What is reabsorbed at the distal tubule
NaCl 5-20%
Water 5-20%
What is the major site of NaCl and water reabsorption
proximal tubule 65- 75%
How are many substances (excluding Na+ and H2O) reabsorbed
By a carrier mediated transport system
What prevents an excess of reabsorption of substances of filtrate
limited by a
number of carriers
as the carried mediated transport system have a maximum transport capacity (Tm)
What is the purpose of the Tm
Tm is set at normal level of plasma concentrations, so if TM is saturated, then the excess substates remains in the urine and is excreted, so plasma regulations is maintained
Define renal threshold
The plasma concentration at which saturation of TM occurs
Explain the titration curve for glucose
Glucose is freely filtred, so whatever its plasma concentration it will be filtered
The plasma glucose that is reabsorbed is up to 10mmoles/L (= renal threshold for glucose)
If the glucose filtered is above 10mmoles/L, 10mmole/L will be reabsorbed and the remainder will remain in filtrates and be excreted
Why is the Tm for glucose higher than the plasma concentration of 5mmoles/L
Ensures that all the valuable nutrient is normally reabsorbed
Tm is set above any possible level of non diabetic glucose concentration
Why is the Tm for amino acids so high
So that amino acids are not excreted into urine, as some proteins does get through (0.5%) but want to make sure all is reabsorbed in normal physiology
Why is the TM value close to the plasma value for most other substances
So automatically excrete excess
How is Na+ reabsorbed from the filtrate
Active transpot
not bt Tm as couldn’t cope with the demand
How as a gradient established for Na+ across the tubule wall
There is a high sodium concentration in the intercostal fluid due to the active transport, so leaves a low sodium concentration the tubular cells
The high sodium concentration n the tubule lumen compared to the tubule cell is so passively diffuses in
So the active transport sodium drives the gradient on the other side
Where is the sodium pumps located
On the basolateral surface, where there is a high density of mitochondria
How is the proximal tubules permeable to Na+
Has a high permeability to Na+ ions than most other membrane in the body due to the enormous surface are offered by the microvilli and large number of Na+ ion channel, so facilitates the diffusion of Na+
What is the additional benefit of active transport sodium reabsorption
Due to it being a charged particle, it is key to the reabsorption of other components of the filtrates as creates a concentration gradient
Na+ is also used for carriers mediated transport system
What components of the filtrate are reabsorbed via the initiation of sodium concentration gradient
Negative ions such as Cl- diffuse passively
This creates an osmotic force and draws H2O out of the tubules
The removal of H2O concentrates, all the substances left creating concentration gradients
Causing permeable solutes K+ Ca2+ and urea to be reabsorbed by diffusion
What does the rate of reabsorption of non actively permeable solutes (K+, Ca2+, urea) depend upon
The amount of H2O removal (which determines the extent of the concentration gradient)
The permeability of the membrane to any particular solute
How permeable is the membrane tubule to urea and how much urea is reabsorbed
Only moderately permeable so only about 50% is reabsorbed
What substances are impermeable to the tubular membrane
Insulin
mannitol
How is Na+ used for a carrier mediated transport system
As substances such as glucose, amino acids share the same carrier molecule as Na+ symport
What is the benefit it Na+ used a carrier mediate transport system
The active drive of Na+ is strong enough to co-transport glucose against its concentration gradient
(the transport of glucose needs energy which is provided indirectly by Na+ pump)
Drugs are usually lipid soluble, how are they not reabsorbed, when the concentration gradient is set up to the active transport of sodium
As the liver metabolises drugs to make them non polar and reduce there permeability, preventing reabsorption occurring in the kidney, so will remain in tubule and be excreted
What stimulates Na+ reabsorption at the distal tubule
Aldosterone
