Flashcards in Reabsorption and secretion Deck (46):
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
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]?
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?
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?
How does the kidney regulate some substances?
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,
99.5% 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?
-Anions like chlorine
What does the rate of non-actively reabsorbed solutes depend on?
1. The amount of H2O removed, which will determine the extent of the concentration gradient.
2. The permeability of the membrane to any particular solute
How permeable is the tubule membrane to urea?
Only MODERATELY permeable to urea so that only about 50% is reabsorbed and the remainder stays in the tubule.
Give an example of some substances to which the tubular membrane is impermeable
Inulin and mannitol
What will happen if the active transport of Na+ is disrupted?
Active transport of Na+ establishes the gradients down which other ions, H2O and solutes pass passively.
Importance of active transport of Na+ also for carrier mediated transport systems.
What substances depend on Na+ carrier mediated transport systems?
Glucose, amino acids etc
Share the sma ecarrier molecule as Na+ (symport)
High [Na+] in the tubule facillitates and low [Na+] inhibits glucsoe transport
Give an example of something that would decrease Na+ transport
Decreased Blood flow
How is Na+ reabsorption linked to acid/base balance?
Na+ reabsorption linked to bicarbonate ion reabsorption
What Na+ glucose symport causes the reabsorption of glucose from the tubule?
Why is this important?
Sodium Glucose Linked Transporter
Pulls glucose into the cell against its concentration gradient
Even at low urine glucose it means that all of it is taken up and no glucoise is wasted in the urine
What is tubular secretion?
Secretory mechanisms transport substances FROM the peritubular capillaries INTO the tubule lumen and
Therefore provide a second route into the tubule
Why do we need secretion?
Important for substances that are protein-bound, since filtration at glomerulus is very restricted.
Also for potentially harmful substances, means can be eliminated more rapidly
How are substances secreted?
Tm-limited carrier-mediated secretory mechanisms known for a large number of endogenous as well as exogenous substaces such as drugs
Carrier mechanisms are not very specific
Give some examples of how carrier mechanisms are not very specific
e.g. organic acid mechanism, which secreted lactic and uric acid can also be used for substances such as penicillin, aspirin and PAH (para-amino-hippuric acid)
Similarly, organic base mechanism for choline, creatinine etc can be used for morphine and atropine
Where are most of these substances secreted?
Why is K+ so important?
K+ is the major cation in the cells of the body and the maintenance of K+ balance is essential for life
What is the normal ECF [K+]?
What occurs during hyperkalaemia?
If potassium increases to 5.5mmoles/l = hyperkalaemia
Decreased resting membrane potential of excitable cells and eventually ventricular fibrillation and death
What occurs during hypokalaemia?
If [K+] cardiac arrhythmias and eventually death.
How do the kidneys handle K+?
Renal handling of K+ is complex.
K+ filtered at the glomerulus is reabsorbed primarily at the proximal tubule
Changes in K+ EXCRETION are due to changes in its SECRETION in the distal parts of the tubule.
Any increase in renal tubule cell [K+] due to increased ingestion will -> K+ secretion, while any decrease in intracellular [K+] leads to reduced secretion
What hormone regulates K+ secretion?
The adrenal cortical hormone aldosterone
How does aldosterone regulate K+?
An increase in [K+] in ECF bathing the aldosterone secreting cells stimulates aldosterone release which circulates to the kidneys to stimulate increase in renal tubule cell K+ secretion.
What effect does aldosterone have on Na+?
Stimulates Na+ reabsorption at the distal tubule.
Different reflex pathway than K+