Transport of Ions and Water (B2: W4) Flashcards
Define the different movements that occure in the tubule
- Filtration - movement of water or solute from the capillaries into the tubular space
- Secretion - things moving into the tubule
- Usually from peritubular capillaries
- Or ammonium from epithelial cells
- Reabsorption - from tubule to peritubular space
- Excretion - what gets out in the urine
What are the routes by which molecules pass in and out of the tubule?
- Transcellular - moves through two membranes
- Apical membrane (luminal)
- Laminal membrane
- Paracellular - between tight junctions
What kind of transporters are used in the tubule?
- Active transport
- Primary - use ATP
- Secondary (coupled) - use concentration gradient
- Symport
- Antiport
- Passive transport
- Simple diffusion
- Uniport facilitated diffusion
How is sodium moved out of the tubular lumen in the PCT?
Glu/Amino acid symporter
H antiporter
How is sodium moved out of the tubular lumen in the thick ascending limb of loop of Henle?
K symporter
Cl symporter
Loop diuretics affect this - block transport
How is sodium moved out of the tubular lumen in the DCT?
Cl symporter
How is sodium moved out of tubular lumen in collecting duct?
Ion channel
Aldosterone enhances conductance
How is sodium moved into peritubular space?
Na/K ATPase
What perentage of sodium is excreted from the original filtered load of 100%?
Fractional excretion = 0.5%
67% of Na is reabsorbed in PCT
How is chloride moved up its electrochemical gradient from the tubular lumen?
- In PCT: antiporter stored energy
- In ascending loop: K+ symporter
- In DCT: Na+ symporter
How is a concentration gradient for passive Cl reabsorption created?
- Na is reabsorbed into the body
- Water follows
- Causes a negative potetial increase
- Concentration of luminal Cl increases
- Now it wants to diffuse out
How does the concentration of sodium change along the proximal tubule?
Na concentration doesn’t change
Water goes with it
Concentration stays the same as a result
How does the concentration of chloride change along the proximal tubule?
Cl increases slightly and then plateaus
As water and sodium leave, chloride gets concentrated
Enough to support passive reabsorption of Cl
How does the concentration of inulin change along the proximal tubule?
Concentration of inulin continues to increase
It is stuck in the tubule and cannot be reabsorbed into body
As water is removed, it becomes more concentrated
What is the significance of potassium homeostasis?
Need a balance between what is consumed and what is excreted
- Keep a relatively low extracellular concentration
- Important to get it into cells from extracellular space
How do hydrogen and potassium behave in the case of metabolic acidosis?
- pH outside of the cells decreases
- H+ is high
- H+ moves into cells
- Shifts K+ out of cells
How do hydrogen and potassium behave in the case of metabolic alkalosis?
- pH outside of the cells increases
- Decrease in H+
- H+ moves out of cell
- K+ moves into cell
How do hydrogen and potassium behave in the case of hypokalemia?
- K+ concentration outside of cells decreases
- K+ moves out of cell
- H+ moves into cell
- Extracellular pH increases
How do hydrogen and potassium behave in the case of hyperkalemia?
- Extracellular K+ increases
- K+ moves into cell
- H+ moves out of cell
- Extracellular pH decreases
What direction is potassium shifted in response to consuming a meal?
Insulin works to shift K into cells
Indirectly stimulates Na-H antiporter
What effect do ß agonists have on potassium concentration?
ß agonists shift K into cells
Directly stimulate Na-K ATPase pump
What effect does aldosterone have on potassium concentration?
Moves K into cells
Stimulates pump
What effect does exercise have on potassium concentration?
Moves K out of cells
How does most potassium leave the tubular lumen?
Through intercellular channels
How is potassium handled in the distal tubule and collecting ducts?
Potassium is secreted by principal cells
How much potassium is excreted from the initial filtered load of 100%?
Fractional excretion = 10-20%
- Potassium concentration outside of cells is really low
- Need to excrete a greater percentage
- Collecting duct is sensitive to aldosterone
- Causes secretion
How does aldosterone regulate potassium secretion?
- Aldosterone increases Na conductance
- Na moves out of collecting duct
- Causes depolarization
- Potassium moves in as sodium rushes out
What are the two mechanisms for release of aldosterone from the adrenal gland?
- Blood pressure pathway that we know (angiotensin II)
- An increase in potassium directly stimulates aldosterone release from the adrenal gland
How does a blocked aldosterone system affect the plasma concentration of potassium?
Blocked aldosterone: increase in potassium intake increases plasma concentration of potassium
- As serum potassium concentration goes up, aldosterone goes up
- The amount of potassium consumed does not normally affect potassium concentration
How does potassium consumption affect potassium excretion?
The more potassium consumed, the more excreted
Which are the potassium losing diuretics versus the potassium sparing diuretics?
- Potassium-losing
- Furosemide
- Thiazide
- Potassium-sparing
- Amiloride
Why is furosemide a potassium-losing diuretic?
Blocks transporter in DCT
- Na and water are not reabsorbed, excess remains in tubule
- Flow is increased
- Membrane is depolarized
- Enhances K secretion
NaCl excretion increases, K excretion increases
Thiazide works the same way
Why is amiloride a potassium-sparing diuretic?
Works on aldosterone sensitive conductance
- Blocks K conductance
- Membrane hyperpolarizes
- No driving force for K to be secreted
Flow increases, NaCl excretion increases, K is spared
