Blood is a living tissue the greater part of which is fluid plasma, which is almost colourless and in which float cells of two kinds, red and white blood corpuscles. Red blood corpuscles are slightly biconcave little discs, 0-0087 mm. in diameter in man, produced by the red marrow in bones and possibly by the spleen. When first produced they each possess a nucleus which persists in all vertebrates except mammals. Their dark-red colour when seen in masses is due to the presence of a pigment, haemoglobin, which is an iron compound. In the presence of oxygen, e.g. at a respiratory surface, a scarlet compound is formed called oxyhaemoglobin, which yields up its oxygen if the corpuscles are in plasma where the oxygen concentration is low, e.g. in capillaries in the tissues. This is the means whereby oxygen is transported in the blood. The fact that it is in chemical combination enables much more oxygen to be carried than if it were merely in physical solution. We therefore term haemoglobin a respiratory pigment.
The white blood corpuscles or leucocytes are far less numerous than the red, but are of the highest importance. Structurally they are the least specialized of all the cells in the body, strongly resembling Amoeba in appearance and in the way in which they carry out their activities. Their main function is to engulf andon bacteria which gain entrance into the body, and for this reason are also referred to as phagocytes. The remaining functions of blood are carried out by the plasma.
Most of the materials carried in the blood and water pass through the walls of capillaries into the spaces between the cells of the tissues. The red corpuscles are too large to do so ; the fluid therefore in these spaces is almost colourless lymph. The white corpuscles can pass through tiny gaps in the capillary walls, thereby enabling the corpuscles to come to the aid of tissues being attacked by bacteria. Lymph has therefore all the properties of blood, except that its oxygen is carried in physical solution and not combined as oxyhemoglobin in red corpuscles, and it acts as a ’middleman ’between blood capillaries and the actual cells of a tissue, passing useful materials to the cells from the capillaries and useless materials from the cells to the capillaries. Fluid collects in these spaces more rapidly than it can pass back into the blood capillaries. This is removed by tiny vessels with extremely thin walls called lymph vessels, which drain away the excess and pour it into the internal jugular veins. The lacteal vessels of the villi of the small intestine are a special set which drain away digested lipides from the gut.