Reabsorption and Secretion Flashcards
What is Tm?
Maximum transport capacity
What happens when Tm is exceeded by carrier transport systems?
The excess substrate enters the urine
What is the renal threshold for glucose?
10mmoles/l
What occurs when plasma glucose concentration exceeds the renal threshold for glucose?
10mmoles/l will be reabsorbed and the remainder will be excreted although all is filtered
When is glucosuria common?
During insulin failure i.e uncontrolled diabetes
What percentage of Na+ ions are reabsorbed?
99.5%
Where does the majority of Sodium Ion reabsorption occur?
the proximal tubule
How are sodium ions reabsorbed?
Via active transport rather than a Tm Mechanism
Where are sodium pumps located?
basolateral surfaces
Why does the brush border of the proximal tubule have a higher permeability to Na+ ions than other membranes?
Large surface area from microvilli
Large number of Na+ ion channels
How do negative ions diffuse across the membrane?
Via passive transport down the electrical gradient maintained by sodium active transport
How is an osmotic force created in the tubules of the kidney?
Active transport of Na out of the tubule followed by Cl ion passive transport created an osmotic force which draws H2O out of the tubules into ECF
How are substances within the tubule concentrated ?
Since H2O is removed from tubule fluid, the left over substances within the tubule are more concentrated.
This creates an outgoing concentration gradient
What is the rate of absorption dependent on?
Amount of water removed which determines extent of concentration gradient
AND
The permeability of the membrane to any particular solute
How much urea is absorbed?
~50%
What is tubular secretion?
Secretory mechanisms transport substances from peritubular capillaries into tubule lumen and provide a second route into the tubule
Why is tubular secretion important for protein bound substances?
Because their filtration at the glomerulus is limited
What is the normal concentration of potassium in ECF?
4mmoles/l
What occurs if potassium concentration exceeds 5.5 mmol/l?
HYPERKALAEMIA which decreases resting membrane potentials of excitable cells = VF= Death
What occurs if potassium drops below 3.5mmol/l ?
HYPOKALAEMIA = increase in resting membrane potential = hyperpolarisation of cells = cardiac arrhythmia’s and cells death
Where is potassium filtered?
Within glomerulus, primarily at proximal tubule
When are changes in potassium observed?
When changes in secretion at the distal parts of the tubule are witnessed
What occurs when there is an increase in renal tubule cell potassium concentration?
Potassium secretion ensues and intracellular potassium concentration decreases which then leads to a decrease in secretion
What is potassium secretion regulated by?
Aldosterone
When is aldosterone released?
When there is an increase in [K+] in ECF
What is the affect of aldosterone on secretion?
It stimulates renal tubule cell [K+] secretion
If Drug A’s clearance is greater than inulin clearance, then what would happen to Drug A?
Net secretion of drug A
