Canonical Question
Electrolytes
Master answer
Potassium
- Predominany intracellular cation
- Total Stores: Approx 3200mmol (50mmol/kg)
- Key Functions:
- Main determinant of ICF osmolality and tonicity
- Responsible for RMP of excitable cells via Goldmann-Hodgkin-Katz due to ↑↑gK relative to other species
- Role in action potential → repolarisation phase
- Secretion of insulin and multiple other KATP dependant processes
- Regulation of IC processes (protein/glycogen synthesis)
- Involved in Na+/K+ ATPase in cell membranes
Potassium balance / Determinants of Serum Potassium
- Intake
- Oral – very variable
- Approx. 50-200mmol/day
- Distribution
- Exchangable Pool
- ICF (90%): [K+] 150 mmol/L (1° ICF cation)
- ECF (2%): [K+] 3.5-5 mmol/L
- Non-exchangable Pool
- Bone (8%)
- Exchangable Pool
- Transcellular balance
| Factor | Mechanism | Effect on Serum K+ |
|---|---|---|
| Insulin | Causes intracellular shift of K+ | ↓ |
| β2 adrenergic agonism | Causes intracellular shift of K+ | ↓ |
| Aldosterone | Upregulate Na/K ATPase → Intracellular shift of K+ | ↓ |
| pH | Increased H+ (decreased pH) causes extracellular shift of K+ | ↑ |
| Plasma osmolarity | Hyperosmolar plasma initiall causes osmosis of water out of cells increasing intracellular osmolarity and [K+] → causes extracellular shift of K+ | ↑ |
| Skeletal muscle activity | Causes K+ leakage into serum | ↑ |
| Cell death | Causes K+ leakage into serum | ↑ |
- Elimination
- Faecal – 8mmol/day
- Renal – 92mmol/day (See next heading for details)
- Urinary K+ excretion = [K+ filtered by glomerulus] + [K+ secreted by CCD/LDCT] – [K+ reabsorbed by renal tubules]
- Glomerular filtration: Freely filtered = 756mmol/day (180L x 4.2mmol/L)
- PCT: 65% reabsorption
- LoH: 25~30% reabsorption
- DCT/Collecting Ducts – variable
- Determined by Aldosterone, Plasma [K+], Tubular Flow rate
- Secreted by principal cells
- Reabsorbed by intercalating cells
Renal K+ regulation:
- Renal K+ regulation occurs MAINLY via control of K+ secretion at the distal nephron (CCD and LDCT) via the following factors:
- Circulating factors:
- Plasma [K+ ]
- ↑ [K+ ] causes ↑ K+ secretion by → (i) Directly stimulating Na+ /K+ ATPase in principal cells, and (ii) Directly stimulating Aldosterone release from adrenal cortex
- Aldosterone
- ↑ aldosterone causes ↑ K+ secretion by → ↑ production of Na+ /K+ ATPase, K+ channels and ENaC Na+ channels in the principal cells
- Plasma pH
- Alkalosis via ↓ plasma [H+ ] causes ↑ K+ secretion by → stimulating Na+ /K+ ATPase in principal cells
- Plasma [K+ ]
- Luminal factors:
- Flow of tubular fluid in DCT and CCD
- ↑ tubular fluid flow causes ↑ K+ secretion by → maintaining ↓ luminal [K+ ] (Ie. continuously washing it away) → permits passive diffusion of K+ ↓ its [ ] gradient into tubular lumen
- ↑ Na+ delivery rate to DCT and CCD → ↑ K+ secretion
- -ve lumen potential difference → ↑ K+ secretion
- Flow of tubular fluid in DCT and CCD
- Circulating factors:
JC / Gladwin / Sakurai 2020
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2013A Q12 | Describe the physiological role, distribution and regulation of potassium (K+) | historical_member | — |
| 2022B Q14 | Describe the physiological role, distribution and regulation of potassium ions (K+ ). | historical_member | — |