The main components of blood are plasma a pale yellow liquid in which are dissolved mineral salts, and nutrients such as glucose and amino acid, and vitamins. Waste products such as urea (which causes the slightly yellow colour) and some carbon dioxide are also dissolved in the plasma. Suspended in the plasma are numerous proteins which can be divided into two major types, albumens and globulins. One of the globulins is fibrinogen which is necessary in the clotting process. The albumens perform a range of functions, including stabilising the osmotic pressure of the blood, and buffering the plasma against pH changes. Plasma, Serum and Lymph.

The composition of the plasma is:

Water 90% + 10% dissolved solids made up of the following:-

Plasma proteins: Albumins, Globulins, prothrombin, fibrinogen and enzymes

Mineral ions Na⁺,  K⁺, Ca²⁺, Mg²⁺, H₂PO₄^-, HPO₄^2-, PO₄^3-, Cl^-, HCO₃^- and SO₄^2-

Also present in varying amounts are: Dissolved products of digestion, Dissolved excretory products, Vitamins and Hormones.

Serum is the liquid remaining after coagulation of the blood and therefore lacks fibrinogen and prothrombin.

Lymph is a filtered solution of blood and it contains most of the plasma contents but tends to have lower levels of plasma proteins present.

The cells found in the blood include erythrocytes (red corpuscles) and leucocytes, the white cells. The leucocytes can be divided into a wide range of types but for simplicity we can divide them into phagocytes which engulf and digest foreign materials, lymphocytes which produce antibodies and antitoxins, and the T helper cells which activate the immune system.

In terms of the numbers of cells we expect to find approximately 5,000,000 red cells mm^-3, 7,000 leucocytes mm^-3 and 250,000 platelets. The platelets or thrombocytes are fragments of large bone marrow cells called megakaryocytes which are released into the blood stream and function in the clotting process. Because it is composed of a number of distinct cell types or tissues the blood is considered as an organ even though it is liquid! There is about 4.5 litres of blood in an average adult.