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Urinary excretion questions

Why is there a rapid rise in the volume of urine excreted with water only ingestion? Why are salts not increased?

Account for the reduced diuresis with saline ingestion compared to water only.

What would you expect the excretion patterns to be after the separate administration intravenously of 1 litre of isotonic NaCl solution, 1 litre of isotonic glucose solution and a concentrated solution of 50g of urea?
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Plasma osmolality (or osmolarity) decreases with pure water, i.e. more water than solutes, so it becomes more dilute. This is detected by osmoreceptors in the hypothalamus sending neurosecretory signals (action potentials) to the posterior lobe of the pituitary gland stimulating secretions of ADH (a.k.a. vasopressin). This acts on the kidneys (collecting duct and d.c.t.) to increase permeability to water (by inserting more aquaporins in luminal cell membrane of cells) hence less water is reabsorbed. Also the increase in blood volume is detected with increased distention of the atria stimulating the secretion of ANP (atrial natriuretic peptide) which increases sodium and water excretion. Salts are not increased because of ANP, the dilution effect of water on the plasma and because capillaries are (for this question) freely permeable to ions.

Saline would be the opposite to the above description.

I don't think isotonic solutions (of anything) would have any effect on diuresis. I'm not sure about urea, technically it doesn't dissociate in water, so it might have a low osmolality.

Hope that helps.

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