Oxygen supply to contracting skeletal muscle at the microcirculatory level: diffusion vs. convection

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Abstract
An adequate supply of oxygen is essential for the normal function of all cells. Because skeletal muscle cells have the ability to vary their oxygen demand by over an order of magnitude on going from rest to vigorous contraction, it is important that mechanisms be in place to ensure that the supply of oxygen is maintained at sufficient levels. Microcirculation plays a critical role in this process, as the terminal branches of this intricate network of blood vessels determine the distribution of perfusion, as well as the structural framework for diffusion. The oxygen supply depends on proper functioning of both the convective and diffusive components of the transport system. Convection is responsible for the long-range, rapid transport of oxygen by bulk flow of the blood and diffusion is the efficient mechanism for transport over the short distances between capillaries and muscle cells. Convective transport is dominated by the movement of red blood cells, as virtually all the oxygen at normal haematocrit is carried inside them, reversibly bound to haemoglobin. Over the years, specialized techniques, many of them video-based, have been developed for use in intravital microscopy to measure the parameters needed to quantify convection and diffusion in both capillaries and the larger microvessels, arterioles and venules. Most of our knowledge of oxygen transport in the microcirculation of muscle pertains to the resting condition, because one must be able to visualize the structures of interest, such as microvessels and muscle cells, and the large tissue movements that occur during contraction preclude measurements during that time. In resting muscle it has been found that the arterioles are the primary site of the diffusion of oxygen from the circulation, where the oxygen is utilized by nearby muscle cells or diffuses directly to nearby venules or capillaries. Diffusive interactions among neighbouring capillaries have also been observed. In contracting muscles, microvessels observed immediately following the period of stimulation exhibit enhancements of both convective (increased flow of red blood cells) and diffusive (increased perfused capillary surface area) transport. The use of computational models in the interpretation of experimental studies is leading to an increased understanding of the processes that underlie the oxygen transport system in skeletal muscle.