The exchange of gases between the capillaries and the actual tissue cells is the converse of what takes place in the lungs ; oxygen leaves the blood and passes into the tissues ; carbon di oxide leaves the tissues and passes into the blood. As there is not known to be any mechanism other than diffusion for effecting this transference, it follows that the oxygen pressure in the tissues must be lower than that in the venous blood, and the carbon dioxide pressure higher.
The pressure of oxygen in the tissues is regulated by (I) the rate at which oxygen is being used, and (2) the rate at which it is being supplied. The former depends upon the activity of the organ, the latter on nature of the blood supply, i.e., the quan tity of blood which circulates through the organ, the surface which it presents in the capillaries, and the pressure of oxygen which exists in the capillary blood. All these factors are subject to considerable variations. The mean oxygen pressure in the capillary blood being perhaps the least inconstant, and being in the region of 40 mm. of mercury. The variations in oxygen con sumption in various organs are very great. In skeletal muscle the oxygen used per gram of muscle per minute varies from 0.003 to 0.08 cu.cm. The mechanism by which the quantity of blood to organs is regulated is discussed in VASCULAR SYSTEM.
The variations in the surface which the blood presents have been investigated recently by Krogh; in the resting organ rela tively few of the capillaries are open ; as the activity of the organ increases the number of open capillaries increases also, so that the surface of blood from which diffusion can take place is very much increased. Information on the subject of what happens to
the oxygen when it arrives in the cell is still rather obscure, but much work has been carried out since the World War and the following statements may perhaps be made : (I) As a model the following reaction may be considered : In the presence of a ferrous salt (A) and hydrogen peroxide (E) the oxygen of the air will oxidize butyric into aceto-acetic acid.
(2) In potatoes in the presence of some substance A, which plays the same role as a ferrous salt and which is precipitable by alcohol, a lecithin-like substance is turned into a peroxide B which in air will oxidize, guiacum turning it blue. The lecithin-like sub stance is not precipitated by alcohol. The substance A is called a peroxidase, the substance forming B is called an oxygenase, or auto-oxidizabie substance. The associate oxygenase and peroxide are together called an oxidase system. This type of system seems to exist in many vegetable cells.
(3) A material glutothione was discovered by Hopkins in 1921 in most animal and some other tissues. This material is prob ably capable of forming hydrogen peroxide and so playing a role similar to an oxygenase. Certain material contents of the animal cell known as "thermostable tissue residues" which are only oxidized very slowly in air, become oxidized rapidly if a little glutothione be added.
(4) Peroxidases are present in most animal tissues; of these one called cytochrome is known in some detail. It contains iron and is nearly related to the haemoglobin of blood.