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The odor of blood is a complex mixture of volatiles. There are hundreds of volatiles in blood odor, and the substances at very low concentrations are often the most potent. Mammalian predators use the odor of blood to home in on wounded prey, whereas prey species show avoidance behavior and increased vigilance towards blood odors. The volatile trans-4,5-epoxy-(E)-2-decenal, present in mammalian blood, is as strong an attractant to four mammalian predator species as the full blood mixture.

Origin and Production

trans-4,5-Epoxy-(E)-2-decenal is generated by lipid peroxidation, a physiological process present in all mammals. It is very likely found in the blood of all mammals. It can also form when fats that contain linoleic acid are baked. 13-Hydroperoxy-9,11-octadecadienoic acid and 9-hydroperoxy-10,12-octadecadienoic acid are intermediates in that process. It also forms in cooked beef that sits too long in the refrigerator, contributing to a stale smell. It is also an important part of the smell of raw and cooked mutton. Rubbing blood over skin gives a metallic smell similar to the smell of iron on skin, based on the same scent molecules. Blood also contains iron atoms.

Chemistry

trans-4,5-Epoxy-(E)-2-decenal has the formula C₁₀H₁₆O₂ and a molar mass of 168.236 g/mol. The compound is an oxygenated α,β-unsaturated aldehyde, and its IUPAC name is (E)-3-[(2S,3S)-3-pentyloxiran-2-yl]prop-2-enal. The related trans-2-decenal differs from it only in lacking the epoxy group. Humans can smell it at a concentration of 1.5 pg/L in air, 15 ng/L in water and 1.3 μg/L in oil. Humans are more sensitive to its smell than mice. Humans are also more than one order of magnitude more sensitive to the (-)-enantiomer (S,S).

Behavioral Effects

In a two-compartment test arena, mice spent significantly more time in the near-odorless compartment than with either trans-4,5-epoxy-(E)-2-decenal or real blood, and their responses to the two did not differ significantly. In human participants, the compound triggered postural avoidance, raised physical arousal and improved the detection of emotional visual stimuli. Ethical restrictions on pathogen transfer prevented a direct comparison with blood in humans.

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