Describe how the kidney handles sodium. (50 marks) What factors influence urinary sodium excretion (50 marks)

[Click here to toggle visibility of the answers]

College Answer

Candidates were essentially expected to describe the fate of sodium as t passes through the
kidney from filtration at the glomerulus to ending up in the urine. Essentially, most Na+ is
reabsorbed at the proximal tubule (65%), reabsorption being down an electrochemical
gradient (inside cell negative and low Na+ concentration) which is maintained by active
Na+/K+ ATPase activity at the basolateral membrane. Then Loop of Henle reabsorbs a
further 15% of filtered Na+. At descending limb – no Na+ reabsorption. At ascending limb,
thick segment – active process as per proximal tubule, but mostly coupled to K+ and Cl- and
paracellular diffusion through tight junctions. Finally the distal convoluted tubule and
collecting ducts, a further almost 20% reabsorbed – leaving < 1%
Factors influencing loss are
- aldosterone = stimulates Na+ reabsorption at the collecting tubules
- intra renal factors such as interstitial pressure which is lowered during
hypovolaemia and reduced renal perfusion, thus promoting Na+ reabsorption
- sympathetic nervous system – influences interstitial pressure and increases renin
- angiotensin II – stimulates reabsorption at proximal tubule
- Atrial Naturetic Peptide/Factor – inhibits Na+ reabsorption
- Others – dopamine, cortisol, insulin => increase Na+ reabsorption, but minor

Reference – Textbook of medical Physiology, Guyton, Chp 28