Plasma

Plasma is a straw-coloured liquid that is mostly water - typically 80-92% by volume - with everything else dissolved or suspended in it. Dissolved in that water are the plasma proteins, inorganic constituents (about 0.9%, mainly the chlorides, carbonates and phosphates of sodium, potassium, calcium and magnesium), organic constituents (about 0.1%, including urea, amino acids, glucose, fats and vitamins), and dissolved respiratory gases. Four plasma proteins, all synthesised in the liver, do most of the work: albumin maintains the blood's osmotic pressure (keeping fluid from leaking excessively out of the vessels); globulin helps transport ions, hormones and lipids and supports immune function; and prothrombin and fibrinogen work together in blood clotting. Plasma composition is not fixed - it shifts constantly with digestion and metabolism. Immediately after a meal, blood in the hepatic portal vein (which drains the intestine) carries a sharp spike of glucose on its way to the liver for storage, a spike that fades as the glucose is absorbed into tissues. Similarly, if a person eats more protein than the body can store as amino acids, the liver breaks the surplus down and produces urea, so blood leaving the liver in the hepatic vein carries a higher urea load than blood arriving in the hepatic portal vein or hepatic artery. This liver traffic uses two separate inflow routes and one outflo …