Firstly, bradycardia is very apparent in humans where the diving response may be quicker than in some marine mammals, and sometimes to equally astounding levels. In untrained divers a heart rate reduction of up to 30% can be attained whilst trained divers can experience slowing of up to and in some cases even over 50% similarly to semi-aquatic mammals. Umberto Pelizzari records drops to 8 beats/min while there have been records of drops as low as 6 beats/min. Results generated from an experiment by Sara M. Hiebert and Elliot Burch, (Simulated human diving and heart rate: making the most of the diving response as a laboratory exercise [online] Available at
SIMULATED HUMAN DIVING AND HEART RATE: MAKING THE MOST OF THE DIVING RESPONSE AS A LABORATORY EXERCISE -- Hiebert and Burch 27 (3): 130 -- Advances in Physiology Education [Cited 11 July 2007]), conducted by American college students, with the aim to "demonstrate that the bradycardia associated with the diving response is a robust effect that can easily be measured" is a good example of a laboratory experiment used to demonstrate one aspect of the diving reflex in human freedivers. While this is a somewhat simple experiment for measuring bradycardia in humans, and does not show fully to what extent bradycardia affects human freedivers, it adequately demonstrates the presence of a dropped heart rate due the diving response triggered by facial immersion. When held in context with other broad results like those mentioned, it clearly shows the strong presence of bradycardia in freedivers, especially since these are completely untrained participants with little or no experience in freediving. The students simulated dives by submerging the face in 15°C water in order to trigger the diving response and measured the heart rate of the participant for thirty seconds. The results (see fig one) show that for both a small group (left hand graph) and a larger pooled sample (right hand side) there is a significant drop in heart rate from an average 76bpm to 56bpm in a simulated dive compared to a drop of only 3bpm maximum when breathing in air. It also shows that diving bradycardia becomes more pronounced with time.
Secondly, peripheral vasoconstriction in human freedivers is not as developed as in marine mammals and tends to only occur greatly at deep depths or in trained freedivers, although some vasoconstriction occurs even in simple facial immersion. Associated with the blood shunt is a great increase in arterial blood pressure which in many cases leads normal blood pressure of 120/80mmHg to increase to 280/200mmHg with some systolic peaks higher than 300mmHg. This dramatic hypertensive response reflects extreme peripheral vasoconstriction in the human circulatory system and is often used as a measurement in experiments. For example, an experiment by Johan P. A. Andersson et al, Cardiovascular and respiratory responses to apneas with and without face immersion in exercising humans 5 [online] Available at
Cardiovascular and respiratory responses to apneas with and without face immersion in exercising humans -- Andersson et al. 96 (3): 1005 -- Journal of Applied Physiology [Cited 11 July 2007], looking into cardiovascular and respiratory responses to apneas in exercising humans carried out on ten active freedivers measured a 35% increase in mean arterial blood pressure (see MAP fig. 2), with a average 40% reduction in skin blood flow (see SkBF fig. 2) when participants held a 40 second apnea with full facial immersion (AFI) . The participants were asked to perform dynamic leg exercises for 50 minutes whilst conducting apneas of 40 seconds, with and without facial immersion, with a five minute break in-between. The results also show a drop in heart rate (see HR fig.2) providing further support for bradycardia in humans.
Thirdly, blood shift, much like peripheral vasoconstriction, only generally occurs at depths, where pressure causes the lung size to decrease beyond that of normal residual volume. It is often an adaptation that is best observed in a single participant, rather than used as the basis of an experiment, since there are not that many people who freedive to such depths and are available for experiments. A study on world class freediver Francisco Pipin Ferreras during his World Record Breaking breathhold dive to 127.5 metres in Florida, 1994, showed that beyond 100m, where the 8.2 litres of air in his lungs were compressed to less than 0.25 litres, blood plasma had began to, and soon after, filled his lungs. On returning to the surface, the blood plasma began to return to the circulatory system from 80m onwards. (Free diving [online] Available at
UKDivers.net - Choosing the right equipment[Cited 11 July 2007]). This occurrence is widely acknowledged by every world-class freediver descending to such depths. Peripheral vasoconstriction is directly linked to blood plasma filling the lungs, since the increased arterial pressure around the lungs, caused by redirection of blood from other areas of the body, forces blood plasma into the lungs.
Furthermore, the Bohr effect is directly related to haemoglobin, and not specific to marine mammals, humans or any animal for that matter. It is difficult to conduct an experiment on this natural occurrence, and somewhat pointless since the Bohr effect has almost equal effect on all mammals. As mentioned before it helps during freedives, where the concentration of carbon dioxide in the blood is high, by facilitating the transport of oxygen around the body.
However, the Bohr effect seems to have no effect on the cardiovascular responses which actually reduce the uptake of oxygen in the lungs during a breath-hold. Not much is known about this response, but studies such as one by Lindholm and Linnarson on pulmonary gas exchange during apnoea in exercising men show that, during progressive breath-holds, the uptake of oxygen from the lungs decreased 74%. This decrease is 20% higher than could be accounted for by the fall in arterial oxygen saturation alone, suggesting that cardiovascular responses contribute greatly to reducing pulmonary oxygen uptake during apnea, which help to prolong a dive by preserving the body’s main oxygen store. This is also apparent in marine mammals but seeing as the lungs are not the primary dive store of oxygen for marine mammals, the decrease in pulmonary uptake is less pronounced and less important than in humans.
Finally, spleenic contraction occurs, like in all marine mammals, to increase the number of oxygen carrying red blood cells in the circulatory system. However this often does not kick in until fifteen minutes after diving has begun, unlike the Northern Elephant Seal which is measured to have a decrease spleen volume up to 16% that of the predive volume within 3 minutes of a dive (Effects of forced diving on the spleen and hepatic sinus in northern elephant seal pups [online] Available at
http://www.pnas.org/cgi/reprint/98/16/9413.pdf[Cited 11 July 2007]) and is only obviously pronounced in trained divers. Spleenic contractions have been observed in the Korean Ama divers, with ultrasonic measurements of the spleens of ten women freedivers taken before and after repeated dives to approximately six meters. The results were compared with three Japanese male divers who were not frequent breath-hold divers. In the Ama divers, spleen volume was reduced by 19.5 plus or minus 8.7% with haemoglobin concentration increasing 9.5 plus or minus 5.9%whilst in the Japanese divers spleen volume remained unaffected by continued dives with haemoglobin concentrations increasing only 3.0 plus or minus 0.6%. (Spleenic contraction during breath-hold diving in the Korean Ama [online] Available at
Splenic contraction during breath-hold diving in the Korean ama -- Hurford et al. 69 (3): 932 -- Journal of Applied Physiology [Cited 11 July 2007]). The results of the Ama divers back up the theory that spleen contractions are part of the diving reflex in humans as well as marine mammals although this response is far more pronounced in trained divers than non-trained divers.
