The "tissue" tension of oxygen and its relation to hematocrit and erythropoiesis.

A N ADEQUATE SUPPLY OF OXYGEN to the individual cell in a multicellular organism is dependent on a circulatory system by means of which oxygen, reversibly bound to various pigments, is transported from an uptake organ to the capillaries of the tissues. From these capillaries, oxygen diffuses to the cell via the capillary wall, the extracellular water, and the cellular membranes. An adequate supply of oxygen is thus dependent on a multitude of systems; the most obvious of these are: the ambient oxygen pressure, the effectiveness of the uptake organ, the blood flow through the consuming organ, the number and size of the capillaries per cubic unit, the concentration and chemical properties of the transport pigment and the oxygen consumption by the cells. This complex of Systems 5 integrated by a number of humoral and nervous mechanisms which enable the organism to adjust toward optimal working conditions and to counteract disturbances in one system by changing the function of others. Under physiologic conditions the oxygen supply to an organ can be adjusted through changes in the rate of blood perfusion and in the oxygencarrying capacity. These two factors cannot be changed independently however, since the flow is dependent upon the blood viscosity which varies with changes in the hematocrit value. From determinations of the viscosity hematocrit relationship, it is possible to calculate the correlation between flow and hematocrit. According to Castle and Jandl,3 this correlation is linear, based on in vitro determinations of the blood viscosity, Pirofsky.12 This is in good agreement with in vivo flow studies by Murphy et al.’#{176} who studied the flow through the kidney in normovolemic dogs and found a linear dependency of the flow to the hematocrit. In studies on cardiac output, T. Q. Richardson and Guyton,’Ti Murray et al.,” and R. Replogle’4 also found a linear dependency in normovolemic dogs. Provided unchanged oxygen saturation of the hemoglobin at various hematocrit levels, the oxygen flow can be expressed as blood flow times hematocrit