Reabsorption and secretion Flashcards
Why is peritubular capillary pressure low?
Hydrostatic pressure overcoming frictional resistance in efferent arterioles
Why is oncotic pressure high in peritubular capillaries?
Loss of 20% plasma fluid in glomerular filtration concentrates plasma proteins
Why does only reabsorption occur at the peritubular capillaries?
Oncotic pressure is larger than normal
Hydrostatic pressure is less
Balance of starlings forces entirevly in favour of reabsorption
How much water, glucose sodium and urea are reabsorbed within the tubule?
Particularly what area of the tubule?
Particularly the proximal convoluted tubule
99% water
100% glucose
99.5% Na+
50% urea
How are many substances like glucose and amino acids reabsorbed?
Give some other examples of substances reabsorbed in this way
Carrier mediated transport systems
Organic acids, sulphate and phosphate are other examples
What is the maximum transport capacity?
Carriers have a maximum transport capacity Tm which is due to saturation of the carriers.
If Tm is exceeded, then the excess substrate enters the urine
How does a transporter protein work?
Opened to one side of the membrane
High affinity for solute and solute binds
Conformational change of protein causes it to open to otherside of the cellular membrane.
Conformational change also causes reduced affinity for binding substrate so the solute is now released
No energy required just follows chemical gradient
Why do we need carrier proteins?
Enables larger molecules such as glucose to cross the membrane.
Capacity is limited by the numver of carriers
What is the renal threshold?
Plasma threshold at which saturation occurs
Plasma [substrate] at which Tm is reached
Plasma concentration has become high enough that all the carriers have been used up and the limit of how much solute can be transported has been reached
How much glucose can be filtered?
Glucose is freely filtered, so whatever its [plasma] that will be filtered
How much glucose can be reabsorbed?
In man for plamsa glucose up to 10mmol/l, all will be reabsorbed
Beyond this level of plasma [glucose], it appears in the urine = Renal plasma threshold for glucose
e.g. if plasma [glucose] = 15mmol/l, 15 will be filtered, 10 reabsorbed and 5 secreted
What is normal plasma [glucose]?
Around 5mmol
Enterocyte concentration must be >5mmol for GLUT-2 to open
How do we know that the kidneys do not regulate [glucose]?
Normal [glucose] of 5 mmoles/l, so Tm is set way above any possible level of (non-diabetic) [glucose].
Ensures that all this valuable nutrient is normally reabsorbed
What does regulate glucose?
Insulin
Counter-regulatory hormones
The appearance of glucose in urine of diabetic patients is due to what?
Who should be followed up?
Glycosuria is due to a failure of insulin, NOT, the kidney
Any patient with glucose in their urine should be followed up
How is the reabsorption of amino acids simular to glucose?
Tm is also set high that urinary excretion does not occur, regulated by insulin and counter-regulatory hormones
What substances are regulated by the kidney?
Sulphate
Phosphate ions
How does the kidney regulate some substances?
Tm mechanism
Tm is set at a level whereby the normal [plasma] causes saturation
Any increase above the normal level will be excreted, therefore achieving its plasma regulation.
(also subject to PTH regulation for phosphate, PTH decreases reabsorption)
How much Na+ ions are reabsorbed every day?
Na+ ions are the most abundant in the ECF, a ver large amount is filtered every day
180l/day x 142mmoles/l = 25560 moles/day,
- 5% is reabsorbed
- This is about 1.5kg
Where does Na+ reabsorption occur?
65-75% of Na+ ion reabsorption occurs in the proximal tubule
How is Na+ reabsorbed?
Not by Tm mechanism but by active transport, which established a gradient for Na+ accross the tubule wall
Na+ K+ ATPase on basolateral surface
Decreases [Na+] in the epithelial cells, increasing the gradient for Na+ ions to move into the cells PASSIVELY across the luminal membrane
How does the transport of Na+ ions eventually effect the transport of water in the proximal tubule?
Negative ions such as Cl- diffuse passively across the proximal tubular membrane down the electrical gradient established and maintained by the active transport of Na+
The active transport of Na+ out of the tubule followed by Cl- creates an OSMOTIC FORCE, drwaing H2O out of the tubules.
H2O removed by osmosis from the tubule fluid CONCENTRATES all the substances left in the tubule creating outgoing concentration gradients
How does Na+ reabsorption cause the reabsortion of K+, Ca2+ and urea?
Na+ gradient creates electrical gradient
Anions like chlorine follow this gradient
Water will follow this movement of ions
This increases lumin concentration fo other substances move
In total what does the active transport of Na+ drive?
Reabsorption of:
- Anions like chlorine
- Water
- Potassium
- Calcium
- Urea
