Past Papers · SAQ
Renal Tubular Physiology — Tubular Reabsorption
2015A Q04
Exam questionDescribe the mechanisms by which water and electrolytes are reabsorbed across the renal tubules.
CICMWrecks answer
Master answer
This is a list of all the mechanisms by which the renal tubules reabsorb water and electrolytes.
Most substances involve multiple mechanisms, the answer is structured to describe the different transport “mechanisms” and hence the examples are not extensive.
| Mechanism | Energy Expenditure | Electrochemical gradient | Example | ||||
|---|---|---|---|---|---|---|---|
| Diffusion | Passive diffusion | Molecule crosses a membrane to which it is permeable by diffusion | No | With | Most electrolytes and molecules at different levels of kidney (along with other mechanisms) – H+, NH4+, HCO3-, Urea |
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| Facilitated diffusion | Molecule crosses a membrane via a channel, without energy expenditure | No | With | Amino acid movement from tubules to blood | |||
| Active transport | Primary | Primary active transport | Molecule crosses a membrane via a channel, with energy expenditure (ATP) | Yes | Against | 3Na+ /2K+ ATPase pump – Na from cell into interstitium, in exhange for K | |
| Secondary | Symport | Molecule crosses membrane against its electrochemical gradient, with the energy being provided by the transport of another molecule (Co-transported) | Not directly | May be Passive or Active based on gradient of 2nd | sodium and an amino acid | ||
| Antiport | Molecule crosses membrane against its electrochemical gradient, with the energy being provided by the transport of another molecule (Anti-transported) | Not directly | May be Passive or Active based on gradient of 2nd | Na+ / H+ antiporter in proximal convoluted tubule | |||
| Ligand-gated ion channel | Binding of a ligand causes conformational change in membrane channel, allowing movement of ion across membrane | No | With | ENaC channel – Control resorption of Na Located in collecting duct |
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| Exocytosis | Substance packed in vesicle, moves to cell membrane, two membranes merge -> substance exits cell | Yes | Usually against | Exocytosis of Water by Aquaporin2 channel (regulated by vasopressin) | |||
| Endocytosis | Cell membrane extends to engulf a substance or object, which is then contained in the cell within a vesicle | Yes | Can be with, against or refer to a macroscopic object | Reabsorption of filtered proteins by receptor-mediated endocytosis in the proximal tubule | |||
| Reabsorption | Multi-mechanism:
Starling forces, diffusion, and active transport. |
Depending on mechanism | Depending on mechanism | Resorption of glucose, water | |||
| Osmosis | Spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides
(Diffusion of water across semi-permeable membrane) |
No | Yes | Water | |||
| Solvent Drag | solutes in the ultrafiltrate that are transported back from the renal tubule by the flow of water rather than specifically by ion pumps or other membrane transport proteins.
Occurs in paracellular pathways between tubular cells |
Usually No | NA | Renal Na and Cl resorption, Renal urea handling |
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JC 2019
Past papers
Exam appearances
| Exam | Exact exam wording | Candidate success |
|---|---|---|
| 2015A Q04 | Describe the mechanisms by which water and electrolytes are reabsorbed across the renal tubules. | 33% |