Cerebral microcirculatory changes during exposure to hypoxia.

Changes in the red blood cell content of the cat cerebral cortical vessels were monitored using reflecting light during a decrease in PIO2. The following observations were made: 1) the capillary bed red blood cell content increased during hypoxia prior to the arteriole response at an arterial oxygen tension of 52.0 mmHg; 2) at arterial oxygen tensions below 52 mmHg the red blood cell content increased in all vessels simultaneously; 3) as the PIO2 decreased, the increase in red blood cell content occurred earlier in all vessels. Capillaries, therefore, can respond to hypoxia independently of the arteriole in cat cerebral cortical tissue.