Near-IR spectroscopy was used to quantify blood content and oxygenation dynamics in abdominal organs and skeletal muscle of 18 anesthetized rabbits during hypoxic hypoxia. Liver, kidney, and hindlimb muscle were exposed surgically. Laser diode pulses transmitted across the tissues were detected by means of a photomultiplier. The amount and redox level of tissue hemoglobin were estimated from the near-IR signals and monitored during 5- min-long hypoxic challenges and subsequent recovery periods. In the kidney, exposure to 10% FiO$-2$/ resulted in rapid and symmetrical changes in oxygenated and reduced hemoglobin with 50% of the variations occurring within 1 min and a plateau after 3 min. Total hemoglobin did not change and hemoglobin oxygenation returned to baseline within 1 min of hypoxia cessation. Exposure to 6% FiO$-2$/ doubled the decrease in oxygenated hemoglobin and induced a sustained vasoconstriction which decreased total hemoglobin content 2 min after initiation of hypoxia. Comparable patterns were observed in the liver and skeletal muscle with the following exceptions: local vasoconstriction was generally not observed at 6% FiO$-2$/, return to baseline oxygen availability was much slower in skeletal muscle than in the other organs.
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