ABSTRACT
The movement of oxygen from the peripheral capillaries to the mitochondria is the final link in the oxygen cascade. In muscle cells, the diffusion of oxygen may be facilitated by the presence of myoglobin, and it is possible that convection also occurs in the cytoplasm of some cells. There is good evidence that the PO2 in the immediate vicinity of the mitochondria of many cells is very low, of the order of 1 mmHg. Many investigators believe that much of the pressure drop from the capillary to the mitochondria occurs very close to the capillary wall because of the limited surface area available for diffusion. This leads to the conclusion that the diffusion distance from the capillary wall to the mitochondria is relatively unimportant as a barrier to oxygen transport. This diffusion distance is decreased at high altitude, mainly because of the reduction in diameter of the muscle fibers. There is also an increase in myoglobin concentration and mitochondrial density at moderate altitudes. At extreme altitudes, mitochondrial volume in human skeletal muscle is decreased. Increases in the concentration of oxidative enzymes are seen at moderate altitudes, as is the case following training at sea level. The reverse occurs at extreme altitudes,
The diffusion of oxygen from the peripheral capillaries to the mitochondria, and its subsequent utilization by these organelles, constitutes the final link of the oxygen cascade which begins with the inspiration of air. Despite its critical importance, many uncertainties remain concerning the changes that occur in peripheral tissues both in acclimatized lowlanders and in the adaptation of high altitude natives. An obvious reason for this paucity of knowledge is the difficulty of studying peripheral tissues in intact humans. Much of our information necessarily comes from measurements on experimental animals exposed to low barometric pressures, though some additional studies have been made on tissue biopsies in humans.
