Obligatory Reabsorption and Secretion in the PCT Flashcards
What are the functions of the proximal tubule functions?
- Recovery of many ions, sugars, amino acids, peptides and is achieved by Na-dependent in the PCT.
- Reabsorption of Cl, bicarbonate, organic ions and carboxylates and remaining glucose and secretion of waste and cations occurs in PST.
- New bicarbonate formed at the expense of H+ secretion
- 65% of water reabsorbed.
Describe the movement of urea, lipid soluble substances, phosphate/sulphate and proteins/peptides.
Urea - Simple diffusion via urea transporters and paracellular pathways.
Lipid soluble - Simple diffusion,
Phosphate/sulphate - Sodium linked transport,
Proteins/peptides - Reabsorbed and degraded
What is reabsorbed at the PCT and describe the mechanism in how it is absorbed
Sodium - Sodium Potassium- ATPase (generates electrochemical gradient for passive solute diffusion, osmosis and secondary active transport)
Virtually all nutrients - Cotransport with Na,
Cations (K+, Ca+ Mg+) - Passive transport driven by electrochemical gradient
Anions - Passive for Cl_ and active for bicarbonate
Water - Osmosis
Urea and lipid soluble solutes - Passive diffusion
Small proteins - Endocytosed by tubule cells
What is the difference between Cations and Anions
Cations - Positively charged,
Anions - Negatively charged.
What is reabsorbed in the descending limb and what is the mechanism?
Water via osmosis
What is reabsorbed in Ascending limb and by what mechanisms?
Sodium, Potassium and Chloride ions - Active transport by the sodium, potassium chloride cotransporter (in thick) and paracellular.
Calcium and Magnesium - Passive transport via paracellular route driven by electrochemical gradient.
What are the two pathways molecules and ions can cross the proximal tubule epithelium?
Transcellular (through cell body),
Paracellular (through leaky ‘tight’ junctions between cell bodies
What is reabsorbed at the distal convoluted tubule and by what mechanism?
Sodium - Primary active transport but requires aldosterone.
Calcium - Parathyroid mediated primary active transport.
Chloride - Diffusion as it follows the electrochemical gradient produced by reabsorption of sodium.
Water - Osmosis which is dependant on ADH increasing the porosity of epithelium.
What is reabsorbed at the collecting duct and via what mechanism
Na + , H + , K + , HCO 3 - , and Cl - (aldosterone mediated primary active transport of Na which creates conditions for passive transport of other ions.)
- Water (Osmosis dependant on ADH)
- Urea (Facilitated diffusion in response to concentration gradient in deep medulla region.
Summerise the different forces which allow for the movement of ions
1) Electrochemical gradient due to active transport of 3Na out and 3K in.
2) Na gradient across apical membrane.
3) Na gradient sets up transepithelial potential of -3mV (tubule lumen negative which causes paracellular efflux of cations)
4) Osmotic gradient caused by pumping Na out of cell
5) Solvent drag (water moving paracellular drags solutes with it)
6) Chemical concentration of solutes left behind when water leaves the tubule.
What is the transport maximum?
Limit as to how much can be moved. It can be measure in mg/min or mmol/min (rate)
What is the threshold?
It is the point at which the amount filtered is close to the Tmax. Above the threshold, substance appears in urine.
What is clearance?
The virtual volume of plasma which is totally cleared of the substance per unit time.
What are the three process which modify composition of urine?
A) Glomerular Filtration,
B) Tubular reabsorption,
C) Tubular secretion
To use the clearance equation to measure GFR the substance cleared from the plasma must be what?
- Freely filtered at the glomerulus,
- Neither secreted or reabsorbed,
- Not metabolised,
- Not toxic
