ABSTRACT
The highest circulating oxygen levels in the human body occur in freshly oxygenated blood leaving the heart, while at the other extreme, the level of oxygen drops to zero in the anaerobic interior of the large intestine. Oxygen levels also uctuate among organs and in different locations within organs, such as the brain (Figure 15.1) (Phelps, 2000; Kubicki et al., 2007), and especially in mitochondria due to their oxygen-scavenging ability. There are numerous mechanisms to explain why oxygen levels are so variable in different regions of the body, and it is clear that in many cases O2 concentration is carefully and purposely regulated. Recent data suggest oxygen avoidance is widely practiced by many organisms as a key mechanism to protect their highly unsaturated membranes against oxidative damage (see Section III). Oxygen avoidance or limitation is an effective mechanism because it chokes off the supply of O2 needed as substrate for membrane peroxidation.
