In all organs, an increase in metabolic rate is associated with increased blood flow and extraction of oxygen to meet the metabolic needs of the tissues. In addition, a reduction in oxygen within the blood is associated with dilation of the arterioles and increased blood flow, assuming neural reflexes to hypoxia are not activated. The local regulation of the microvasculature in response to the metabolic needs of tissues involves many different types of cellular mechanisms, one of which is linked to oxygen availability.
Oxygen is not stored in appreciable amounts in tissues, and the oxygen concentration will fall to nearly zero in about one minute if blood flow is stopped in any organ. An increase in metabolic rate would decrease the tissue oxygen concentration and possibly directly signal vascular muscle to relax by limiting the production of ATP for the contraction of smooth muscle cells. Figure 16.7 shows examples of the changes in oxygen partial pressure (tension) around arterioles (periarteriolar space), in the capillary bed, and around large venules during skeletal muscle contractions. At rest, venular blood oxygen tension is usually higher than in the capillary bed, possibly because venules acquire oxygen that diffuses out of nearby arterioles. Although both periarteriolar and capillary bed tissue oxygen tensions decrease at the onset of contractions, both are restored as ar-teriolar dilation occurs. The oxygen tension in venular blood rapidly and dramatically decreases at the onset of skeletal muscle contractions and demonstrates little recovery despite increased blood flow. The sustained decline in venular blood oxygen tension probably reflects increased extraction of oxygen from the blood. It is apparent from Figure 16.7 that the oxygen tension of venular blood in skeletal muscle is not a trustworthy indicator of the oxygen status of the capillary bed at rest or during contractions.
In contrast to venules, many arterioles have a normal-to-slightly increased periarteriolar oxygen tension during skeletal muscle contractions because the increased delivery of oxygen through elevated blood flow offsets the increased use of oxygen by tissues immediately around the arteriole. Therefore, as long as blood flow is allowed to increase substantially, it is unlikely that oxygen availability at the arteriolar wall is a major factor in the sustained vasodilation that occurs during increased metabolism.
Recent studies indicate that vascular smooth muscle cells are not particularly responsive to a broad range of oxygen tensions. Only unusually low or high oxygen tensions seem to be associated with direct changes in vascular smooth muscle force. However, either oxygen depletion from an organ's cells or an increased metabolic rate does cause the release of adenine nucleotides, free adenosine, Krebs cycle intermediates, and, in hypoxic conditions, lactic acid. There is a large potential source of various molecules, most of which cause vasodilation at physiological concentrations, to influence the regulation of blood flow.
An increase in hydrogen ion concentration, resulting from accumulation of carbonic acid (formed from CO2 and water) or acidic metabolites (such as lactic acid), causes vasodilation. However, usually only transient increases in venous blood and interstitial tissue acidity occur if blood flow through an organ with increased metabolism is allowed to increase appropriately.
Arteriolar dilation and tissue oxygen tensions during skeletal muscle contractions.
The decrease in arteriolar, capillary bed, and venous oxygen tensions at the start of contractions reflects increased oxygen use, which is not replenished by increased blood flow until the arterioles dilate. As arteriolar dilation occurs, arteriolar wall and capillary bed oxygen tensions are substantially restored, but venous blood has a low oxygen tension. During recovery, oxygen tensions transiently increase above resting values because blood flow remains temporarily elevated as oxygen use is rapidly lowered to normal. (Modified from Lash JM, Bohlen HG. Perivascular and tissue Po2 in contracting rat spinotrapezius muscle. Am J Physiol 1987;252:H1192-H1202.)
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