Cerebral Blood Oxygen Tension and Hemoglobin Oxygen Saturation at Rest

The brain is a highly oxidative organ. It consumes approximately 20 percent of the body's oxygen and 25 percent of the body's glucose supply. Compared to its small fraction of total body weight (only 2%), a disproportionately large percentage of oxygen and glucose is consumed in the brain. The supply of oxygen to the brain depends on the vas culature, and the brain receives approximately 15 percent of the cardiac output. By reversibly binding oxygen to hemoglobin located in erythrocytes, oxygen transport capacity within the vasculature is significantly increased compared to the amount of oxygen physically dissolved in plasma. Blood oxygen tension (PO2), hemoglobin oxygen saturation (SO2), and blood flow of brain microvessels on the one hand, and tissue oxygen tension on the other hand are the basic parameters determining oxygen delivery to tissue. Under resting conditions, a tight but nonlinear relationship exists for brain interstitial oxygen tension and regional cerebral blood flow. The difference in oxygen partial pressure between capillary blood and the tissue determines the PO2 gradient and, hence, the oxygen flux from microvasculature to nerve and glial cells.

In systemic arterial blood, Po2 values of 80 to 90 mmHg result in an oxygen saturation of hemoglobin of 95 percent. Studies in barbiturate-anesthetized rats have shown that following brain passage Po2 values are reduced to approximately 40 mmHg, leading to an oxygen saturation of hemoglobin of around 60 percent within the blood taken from the sagittal sinus, the venous outflow system of the brain. Blood oxygen tension decreases significantly from pial arteries to intracerebral arterioles, resulting in a comparably low Po2 of approximately 60 mmHg in precapillary resistance vessels. Blood oxygen saturation within the capillary bed drops from approximately 80 percent (Po2 60 mmHg) to 60 percent (Po2 40 mmHg), and the major part of oxygen supply to brain tissue occurs within capillaries.

However, Po2 on the venous side of the capillary tree further decreases in small venules. Surprisingly, oxygen saturation of approximately 55 percent in postcapillary venules is significantly lower than in the blood of larger intracerebral veins and the superior sagittal sinus [1]. This higher Po2 in larger venules and veins compared to postcapillary venules can be explained by the well-known fact that arterial O2 is lost to countercurrent venous exchange before reaching the tissue. In general it can be concluded that the bulk of oxygen delivery to brain tissue occurs at the level of the microcirculation, and not only capillaries but also small arterioles and venules significantly contribute to oxygen supply to brain tissue during normoxia.

Microvascular oxygenation strongly depends on erythrocyte flux within the vasculature. Whereas all capillaries in the brain are perfused by plasma, erythrocyte perfusion under resting conditions is characterized by a pronounced heterogeneity, suggesting that many capillaries are not maximally used. Rapid fluctuations and spatial heterogeneity of density and velocity of red blood cells occur within the capillary network. During global stimulation of cerebral blood flow following systemic hypercapnia, mean blood cell flux and velocity increase, capillary perfusion with red blood cells becomes more homogenous, and the number of poorly blood-cell perfused capillaries decreases. In addition there is evidence for a dilation of capillaries. "Capillary recruitment" within brain microcirculation exists only in the sense of a dynamic adjustment of red blood cell perfusion in continuously plasma-perfused capillaries, but not as opening or closing of capillaries [2].

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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