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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

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?


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?

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,
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?

Reabsorption of:
-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?

Proximal tubule


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+]?

around 4mmoles/l


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+


Describe H+ secretion

H+ ions are actively secreted from the tubule cells (not the peritubular capillaries) into the lumen