Page 24

LSS Urinary System 

Alexandra Burke-Smith

Potassium mainly comes from diet. By eating meat, you effectively are eating cells, thus eating IC space which contains lots of potassium. This leads to increased plasma potassium, which must be taken up into tissues o This tissue uptake is driven by insulin (+ aldosterone and adrenaline) Tissue uptake can be seen as the immediate response to dietary K+, which involves the Na/K pump transporting potassium out of the plasma into the cell. o Potassium then leaks out of the cell via potassium channels, into the EC space surrounding cells

Potassium handling by the kidneys  ~70% of potassium reabsorption occurs in the PCT, with ~30% of the filtered load reaching to loop of Henle  Further reabsorption occurs in the loop of Henle, with ~10% remaining to reach the DCT  Potassium secretion then occurs in the principle cells of cortical collecting duct, and this varies from 1-80% of the initial filtered load (prior to reabsorption). Secretion is stimulated by: o Increased plasma [K+] o Increased aldosterone o Increased tubular flow rate o Increased plasma pH  On a cellular level, potassium secretion is driven by the basolateral Na/K pump, and the apical/luminal K+ channel o The cell membrane potential also drives the secretion. o Aldosterone stimulates the action of both the basolateral Na/K pump, and the apical/luminal K+ channel o An increase in tubular flow rate also increases K secretion. The rate increase is detected by cilia on principal cells, linked to the PDK1 enzyme which activation increases the intracellular calcium.  This increases stimulates the activity of the luminal K+ channel, causing more potassium to be excreted Potassium imbalances  Hypokalemia  Low plasma potassium levels – one of the most common electrolyte imbalances (common in 20% hospitalised patients)  Caused by: o Diuretics o Surreptitious vomiting (intentional in secret, e.g. in eating disorders) o Diarrhoea o Genetics - Gitelman’s syndrome; mutation in the Na/Cl transporter in the distal nephron  Hyperkalemia  High plasma potassium levels – present in 1-10% hospitalised patients  Seen in: o K+ sparing diuretics o ACE inhibitors o Elderly

24

Alex's Urinary  

Alex's Urinary

Advertisement