Reabsorption and Secretion Flashcards
Where does reabsorption primarily occur?
Peritubular capillaries
What are many substances reabsorbed by?
Carrier mediated transport systems
Carriers have a maximum transport capacity, what happens if this is excedded?
Excess substrates go into urine
Describe briefly how carrier mediated transporters allow for reabsorption.
There is a binding site for the transported solute and when it binds, there is conformational change.
The outer side closes, meaning the substrate is momentarily trapped in the membrane. The inner side opens, allowing the substrate to pass into the cell
Carrier protein enable larger molecules to cross the membrane.
Give an example.
Glucose
Renal threshold?
Plasma threshold at which saturation occurs
If plasma glucose concentration is increased, what happens to reabsorption?
Reabsorbed until plasma threshold is reached and then rest is excreted in urine
If there is glucose in the urine, what is this due to the failure of?
Insulin
->not the kidneys, do not regulate glucose levels
There are some examples in which the kidneys do regulate the concentration in the plasma. Give some examples.
Sulphate and phosphate ions
Which type of ion is the most abundant in the ECF?
Sodium ions
Where does the majority of sodium reabsorption take place?
Proximal tubule
What % of sodium is reabsorped?
99.5%
Through which mechanism is sodium reabsorbed?
Active transport
What allows the uptake of sodium into the cell?
The sodium-potassium pump, sodium leaving the cell and potassium entering establishes a gradient for sodium. Low sodium in cells facilitates the uptake of sodium
Reabsorption of which ion is key to the reabsorption of other filtrate components?
Sodium
How does water get into the cell through the lipid membrane if it isn’t lipid soluble?
Through aquaporins
What is the normal counter ion for sodium?
Chloride
How do chloride ions get into the cell?
Diffuse passively down the electrical gradient established and maintained by the active transport of sodium.
What happens as a result of the active transport of sodium followed by the chloride ions?
An osmotic force is created which draws out water from the tubules
->by water being taken out, this concentrates the substances left in the tubule creating outgoing concentration gradients
What does the rate of reabsorption of non-actively reabsorbed solutes depend on?
-Amount of water removed, as determines extent of concentration gradient
-Permeability of the membrane to the particular solute
Is the tubule membrane permeable to urea?
Moderately- only 50% is reabsorbed and the rest stays in the tubule
Name some substances in which the tubular membrane is impermeable.
Inulin and mannitol
Active transport of which ion establishes the gradients in which other ions, water and solutes can pass passively?
Sodium
Anything that decreases active transport has what effect on renal function?
Disrupts renal function
What is tubular secretion?
Secretory mechanism used to transport substances from the peritubular capillaries into the tubule lumen and therefore provide a second route into the tubule
Why is tubule secretion important?
Potentially harmful substances can be removed more quickly
Important for protein-bound substances
What level of potassium could lead to hyperkalaemia?
> 5.5mmoles/L
-> 4mmoles/L is normal
What level of potassium could lead to hypokalaemia?
<3.5,,oles/L
What happens as a result of hyperkalaemia?
Decrease in resting membrane potentials, ventricular fibrillation and death
What happens as a result of hypokalaemia?
Increase in resting membrane potential, cardiac arrhythmias and death
->therefore, potassium balance is essential for life
RECAP- what part of the kidney does the filtering?
Glomerulus
What hormone has a role in the regulation of potassium secretion?
Aldosterone
RECAP- where is aldosterone released from?
Zona glomerulosa of the adrenal cortex
