Sodium in biology mEq/liter (135 - 145 mmol/L). A serum sodium level of less than 135 mEq/L qualifies as hyponatremia, which is considered severe when the serum sodium level is below 125 mEq/L. The renin–angiotensin system and the atrial natriuretic peptide indirectly regulate the amount of signal transduction in the human central nervous system, which depends on sodium ion motion across the nerve cell membrane, in all nerves. Sodium is thus important in neuron function and osmoregulation between cells and the extracellular fluid; the distribution of sodium ions are mediated in all animals by sodium–potassium pumps, which are active transporter solute pumps, pumping
Serum-separating tube particles to cause blood to clot quickly. The blood sample may then be centrifuged, allowing the clear serum to be removed for testing. These tubes should be used with care when measuring drug or hormone levels because the drug or hormone may diffuse from the serum into the gel, causing a reduction in measured level. The gel in SST II tubes (which appears slightly less opaque) is supposed to have less effect on drug levels in serum. Serum-separating tube Serum-separating tubes, also known as serum separator tubes or SSTs, are used in medical clinical chemistry tests requiring blood serum. SSTs