Reabsorption and Secretion Flashcards
How are many substances e.g. glucose and amino acids reabsorbed in the kidneys?
Carrier mediated transport systems
What happens when the carriers become saturated?
They reach their maximum transport capacity (Tm)
What happens when Tm is exceeded?
Excess substrate enters the urine
What is the renal threshold?
Plasma threshold at which saturation occurs
What is the renal threshold for glucose?
Plasma conc. > 10mmol/l
What happens to the glucose when the renal threshold is exceeded?
It will appear in the urine
Do the kidneys regulate glucose concentration?
No
Is glycosuria in diabetic patients a sign of kidney failure?
No it is a sign of insulin failure
is the amino acids Tm high or low?
It is set so high that urinary excretion does not occur (amino acids are too important to be excreted)
How is Tm set for substances such as sulphate and phosphate ions?
The Tm is set so that normal plasma conc. causes saturation allowing the kidneys to regulate the level
How are sodium ions reabsorbed?
Active transport: creates a gradient across the tubule wall
Where are the majority of sodium ions reabsorbed?
Proximal tubule
Where are the active sodium pumps located?
Basolateral surfaces (they have a high density of mitochondria)
Why does the brush border of the proximal tubule cells have a higher permeability to sodium ions than other membranes?
The microvilli and large number of sodium ion channels
How are chlorine ions reabsorbed?
They diffuse passively across the membrane down the electrical gradient established and maintained by the active transport of sodium ions
What happens to the water in the tubules?
The active transport of sodium out of the tubule creates an osmotic force which draws water out of the tubule
What happens to all the other substrates in the tubules after the water and sodium have been reabsorbed?
The tubule fluid is more concentrated and the substances now all have outgoing conc. gradients
What does the rate of reabsorption of non-actively reabsorbed solutes depend on?
Amount of water removed and permeability of the membrane to the particular solute
How much urea is reabsorbed and why?
Approx. 50% as the tubule membrane is only moderately permeable to urea
Name two substances for which the tubular membrane is impermeable?
Inulin and mannitol
What effect does the sodium concentration on glucose transport?
High sodium conc. facilitates glucose transport
Low sodium conc. inhibits glucose transport
What is the function of the secretory mechanisms?
Transporting substances from the peritubular capillaries into the tubule lumen and providing a 2nd route into the tubule
Why are the secretory mechanisms important?
Important for substances that are protein-bound and for rapid elimination of potentially harmful substances
How can the carrier mechanism be used for drugs?
The carrier mechanisms are not specific and so can be used for both the organic mechanism and certain drugs
Where are the substances secreted?
The proximal tubule
Why is the handling of potassium ions so important?
Potassium is the major cation in cells and is essential for life
What is the normal ECF [potassium]?
4 mmol/l
What happens if the potassium conc. > 5.5mmol/l?
Hyperkalaemia: causes ventricular fibrillation and death eventually
What happens if the potassium conc < 3.5mmol/l?
Hypokalaemia: causes cardiac arrhythmias and death eventually
Where is the majority of potassium rebsorbed?
Proximal tubule
How is the amount of potassium excreted controlled?
Changes in the secretion of potassium in the distal parts of the tubules
Which adrenal cortical hormone regulates potassium secretion?
Aldosterone
Which part of the kidneys is effected by aldosterone?
Distal tubule
Which cells in the kidneys secrete hydrogen ions?
Tubule cells
