....increases the speed of transfer of oxygen and fuel to tissue.
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Oxygen is known to play a key role in cellular energetics. Both oxidation and other forms of energy production depend on a continuous supply of oxygen to the cells. In mammals, oxygen is extracted from the atmospheric air in the lungs, and carried by the bloodstream through the circulation to the tissue, where it is utilized mainly within the mitochondria. Behind this simple picture lie many questions concerning physical mechanisms of transport in different parts of the pathway. Is oxygen transported in blood mainly by pure convection, and what are the roles of diffusion and chemical kinetics? How important are the resistances to transport provided by various membranes (red blood cell, endothelial cell, parenchymal cell) along the pathway? Does oxygen cross these membranes by pure diffusion, or is the diffusion facilitated by a carrier? What are the mechanisms of transport inside the cells? Does active transport play any role in oxygen delivery? What is the main site of oxygen exchange between the blood and tissue: arterioles, capillaries, or venules? Are these sites different for different physiological conditions and for different tissues? Currently, we do not have definitive and complete answers to these important questions. A clear understanding of the physical mechanisms of oxygen transport throughout the pathway is a prerequisite to understanding the regulation of blood flow.
Krogh102 laid the foundation of the theory of oxygen transport to tissue. He proposed that oxygen is transported in the tissue by passive diffusion driven by gradients of oxygen tension (PO2). He then formulated a simple geometrical model of the elementary tissue unit supplied by a single capillary. This geometrical model is commonly referred to as the Krogh tissue cylinder or simply Krogh’s model. Together with his colleague, the mathematician Erlang, Krogh formulated a differential equation governing oxygen diffusion and uptake in the tissue cylinder. The solution to this equation expresses oxygen tension in the tissue as a function of spatial position within the tissue cylinder. This simple equation, known as the Krogh or Krogh-Erlang equation, has been the basis of most physiological estimates for the last 70 years.
The major subsequent advances in qualitative and quantitative understanding of oxygen transport to tissue have come from studies of hemoglobin-oxygen kinetics, the role of hemoglobin and myoglobin in facilitating oxygen diffusion, and the role of morphologic and hemodynamic heterogeneities.
This review focuses on theories of oxygen transport to tissue. It is intended to be comprehensive in that it systematically covers the pathway of oxygen molecules to and from the red blood cell, through the plasma, endothelial cell, other elements of the vascular wall, and through the extra- and intracellular space to the mitochondria. Mitochondrial oxygen transport is not considered here since, as an important area of biochemistry (oxidative phosphorylation), it is described in numerous textbooks and surveys. Only a few references are made to experimental studies and to physiological aspects of oxygen transport. The reader is referred to a recent monograph by Weibel186 and to several surveys that discuss the role of oxygen in regulation of blood flow in skeletal muscle,29,50 heart muscle,36 and brain.1
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