Plasma is the liquid component that accounts for about 55% of the volume of blood and in which are suspended the 45% taken up by cellular elements. When the two components are separated by centrifuge, using a test tube sample of blood treated to prevent clotting, the plasma separates as a yellowish layer. Plasma is different from serum — the fluid which is extruded when blood is allowed to clot — because some plasma constituents take part in clot formation. Plasma is about 95% water, with dissolved or suspended substances ranging from simple chemicals to complex protein molecules and fat particles. Its osmolarity (close to 300 milliosmoles/litre) and pH (close to 7.4) are closely controlled; likewise the main anions (chloride, bicarbonate) and cations (sodium, potassium, calcium, magnesium) and the major proteins (albumin and globulins). Nutrients (glucose, fatty acids, amino acids) and the nitrogenous waste product urea, vary more in their concentration, along with meals and metabolic activity. Other substances in much smaller concentration are equally important, such as the hormones, factors required for coagulation, and trace elements. Plasma is continually exchanging its constituents, apart from the large proteins, with all tissue fluids across capillary walls; it is also continually ‘sampled’ and its composition corrected by the kidneys, which partially filter it off from the blood at a rate of 2-3 times the whole 2-3 litres of circulating plasma every hour, process it, and restore it to the circulation — except for about 1% which forms the urine.
— Stuart Judge
See blood; body fluids; kidneys.
