|Controversy in Existance in Humans|
Reasons for Doubt
There is ongoing debate over how many effects of the diving response humans demonstrate in comparison to diving mammals, and whether the same mechanisms are at work. This is based upon the well-supported observation that the response in humans is less prompt and exhibited to a lesser degree (Gooden, 1994; Ross and Steptoe, 1980).
Factors such as body position, previous activity, depth of immersion, and varying lung volumes present conflicting arguments for the existance of a diving reflex in humans. Since the is markedly weaker, there is a greater influence by other factors. Peripheral vasoconstriction has also been shown to be insufficient to adequately reduce oxygen transfer as would be necessary for a diving mammal. There is conflicting evidence in the existence of all aspects of the response except bradycardia, which may not even be related to oxygen conservation (Manley, 1990). Even within this similarity between divining mammals and humans, there is still a major difference: the effect of temperature. Temperature effect upon bradycardia is only seen in humans (Speck and Bruce, 1978).
In Defense of Humans
Others argue that it is not valid to generalize that the diving response has become vestigial in humans (Gooden, 1992). The logic supporting this belief is grounded in several studies which show humans having similar effects, if less dramatic, to diving mammals and birds (Sarnaik and Vohra, 1986; Rosengren et al., 2002; Gooden, 1994). In direct opposition to the claim that peripheral vasoconstriction is not strong enough to have an effect on oxygen conservation, some research claims that limb blood flow is still reduced by 30-50% (Fagius and Sundlöf, 1986).
There is also a marked variety in research protocols involving the diving response in humans, with a lack of uniformity in methods and interpretation of results. Attempts to isolate factors have not necessarily excluded all variables, as we are now just discovering the impact of certain conditions. This confuses the data and makes a definite conclusion hard to reach (Manley, 1990).
Table 2 adapted from (Manley, 1990). A compilaton of previous research regarding the various influcences upon the diving reflex in humans and the conflicting results.
Most researchers can agree that in humans an unusual circulatory state is triggered by breath holding, and intensified by immersion of face in water (Manley, 1990). Some have pointed out that the main difference between diving species and humans may come not from the response, but rather from pre-existing physiological adaptions. Previously available oxygen within the body is stored more economically in diving mammals due to their anatomy. Human musculature has only 12% of total oxygen storage, while aquatic mammals can store 25-30% of available oxygen in muscles (Ramirez et al., 2007).
The question is, given these physiological differences, is the initial reflex to dive exposure the same in diving mammals and humans? The generally accepted consensus is that the human response is qualitatively similar but quantitatively not as profound as that found in diving mammals and birds(Gooden, 1994; Ramirez et al., 2007; Butler, 1982)