The problem of diffusion of gases between blood and tissues is much more
complicated in peripheral capillaries than in the lungs. Instead of an alveolar space
where a uniform 02 tension down the length of a capillary may be assumed, there
will be gradients of P02 in the tissues surrounding the capillary, not only in a direction
at right angles to the axis of flow, but also along the length of the capillary.
These gradients will probably be considerable, since modem measurements of the
P02 of cells indicate levels often far below the venous P02. It appears that equilibration
is never achieved even in the full length of the capillary. In these circumstances
the existence of "mixing" due to the peculiarities of bolus flow may be of great
importance.