SODIUM

Sodium: serum and urinary. 

Serum sodium

All the movements of sodium produce the movement of a variable amount of water. The volume of fluid in the extracellular compartment is directly dependent on the total amount of sodium in the body. The plasma sodium concentration is identical to that of the interstitial fluid. 

In its movements, to achieve an electrical balance, Na+ is followed by anions and primarily by Cl- and HCO3-. Being an alkaline metal and in its movement entraining the bicarbonate anion, Na+ intervenes in maintaining the acid-base balance. Also, Na+ intervenes in the neuromuscular excitability and in the dynamics of the polarization and depolarization phenomena of the cell membrane, opposing the effects of potassium. 

The mechanisms by which the body maintains constant sodium levels in the plasma and extracellular sector are represented by: renal blood flow carbonic anhydrase activity, renin-angiotensin-aldosterone system, ADH, vasopressin, other steroid hormones whose plasma concentration is controlled by he anterior pituitary. 

Biological reference range: premature babies: 132-140 mEq/L; newborns: 133-142 mEq/L; children (1-16): 136-145 mEq/L; adults: 136-145 mEq/L. 

Urinary sodium

The main way to eliminate sodium is through the kidneys. Sodium excretion by the kidneys is influenced by changes in glomerular filtration rate, serum sodium concentration, adrenal cortex activity, the amount of non-resorbable solvates in the filtrate and the volume of extracellular fluid. 

Biological reference range: children: 41-115 mEq/24h; adults: 40-220 mEq/24h.