Inherited disorders like cystinuria, Homocystinuria.

Clinical significance: 1. Fasting or inadequate protein in take, in excessive tissue protein catabolism, with liberation of Intra-cellular fluid components, results in an increase in urinary K and a change in Na:K ratio (in fasting, there is lack of NaCl intake). 2. Mineralo corticoid, aldosterance, increases the reabsorption of Na and excretion of K. In primary aldosteronism (Conn’s syndrome) the excretion of K will increase. 3. Effect of acidification of urine: K enters the urine from the plasma by glomerular filtration and also by active tubular secretion, the latter mechanism apparently competing withh that for urine acidification. The amount of K in urine is thus influenced by the requirement of urine acidification. K excretion will be increased if acidification is low e.g. in alkaline ash diet, and in alkalosis. K excretion will be dicreased if acidification is high eg in highly acid diet, and in acidosis. 4. Abnormal loss, through extrarenal channels of Na e.g in excessive sweating, diarrhoea etc, or of K (in diarrhoea) results in decreased urinary excretion of these elements.

V. Calcium and Magnesium

Ordinarily the greater proportions of Ca and Mg are excreted in the faces. Generally, the urinary excretion of Ca lies between 0.1gm (100mg) and 0.3gm (300mg) and of Mg between 0.1gm (100mg) and 0.2gm (200mg) per day. Absorption depends on reaction of the intestinal contents – increased alkalinity of intestinal contents causes precipitation of increased amounts of Ca and Mg phosphates which are lost in the faeces, with less being absorbed in blood and excreted in urine. Reverse occurs with increased acidity of intestinal contents.

Influence of Diet

Urinary excretion of both Ca and Mg normally is primarily dependant on the nature of the diet and there is wide variation. Under usual dietary conditions, in normal healthy subjects, urinary excretion accounts for about 15-30% of the total Ca and about 20-50% of the total Mg elimination, the remainder being excreted in faeces. At low or moderate levels of Ca intake (0.1-0.5gm daily) about 30-50% is eliminated in the urine, whereas at high levels of intake (1.0gm), about 10-25% is eliminated. The renal threshold for excretion of Ca lies between 6.5-8.0mg/100ml of plasma, little being eliminated at lower plasma Ca concentration.