Blood — Plasma and Composition

An adult human carries roughly five to five-and-a-half litres of blood, and this single fluid tissue is what connects every other tissue of the body to every other. Blood qualifies as a connective tissue for exactly the same reason bone or cartilage does — a comparatively small population of cells sits within a much larger volume of extracellular matrix that those cells (or their relatives) maintain — except that in blood this matrix, the plasma, is a true liquid rather than a solid or semi-solid, which is what lets blood flow continuously through a closed system of vessels rather than staying fixed in place the way a bone or a cartilage does.

Roughly fifty-five percent of blood's total volume is plasma, a pale straw-coloured fluid that is itself roughly ninety to ninety-two percent water. Dissolved in that water is a wide mixture of substances the body is either transporting or has produced. The most abundant dissolved solids are the plasma proteins, of which three kinds matter most for this chapter: albumins, the smallest and most numerous plasma proteins, are chiefly responsible for maintaining the blood's colloidal osmotic pressure — the pressure that pulls water back into the capillaries from the surrounding tissue fluid, so that fluid does not simply leak out of the circulation and accumulate in the tissues; globulins include the immunoglobulins (antibodies) that form part of the immune defence, as well as proteins that carry hormones, fats and metal ions through the blood; and fibrinogen is the soluble precursor of fibrin, the insoluble protein thread that forms the physical mesh of a blood clot, as described later in this chapter. All three groups of plasma protein are synthesised chiefly by the liver.

Beyond the proteins, plasma also carries the small dissolved molecules the body's cells need or must get rid of: nutrients absorbed from digestion, such as glucose, amino acids and lipids, being carried from the intestine to every tissue that needs them; hormones, being carried from the endocrine gland that secretes them to the distant target tissue they act on; nitrogenous excretory products, chiefly urea, being carried from the tissues where they are produced to the kidney for removal; and the respiratory gases themselves — a small amount of oxygen and, much more significantly, carbon dioxide, most of which travels dissolved in plasma as bicarbonate ion rather than bound to haemoglobin. If whole blood is allowed to clot and the clot (containing the fibrin mesh and the trapped cells) is then removed, the pale yellow fluid that remains is called serum — serum is, in effect, plasma with the fibrinogen and other clotting factors already used up, which is why serum, unlike plasma, cannot itself clot again.

The remaining forty-five percent of blood's volume is made up of the formed elements — red blood cells, white blood cells and platelets — suspended within this plasma. Because these formed elements are visibly cellular or cell-derived structures, each specialised for a single distinct role (gas transport, defence, or clotting respectively), they are treated as a separate topic in the section that follows.