The mammalian diving reflex: Physiology Practical Report
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The mammalian diving reflex is a mechanism where respiration is optimised to permit a mammal to remain underwater for as long as possible. It is hypothesised that it’s ordained by cold water touching the face, apnea, and the position of the body relative to a horizontal plane It is strongly exhibited in aquatic mammals (dolphins, seals, or otters) but is weaker in other mammals such as humans, or other diving birds, like penguins. The reflex is triggered when the face becomes in contact with cold water.
The diving reflex causes three changes to a living organism’s body:
• Bradycardia: the heart rate slows down to under 60bpm which reduces the need for oxygen from the bloodstream, allowing other organs to use it instead.
• Peripheral vasoconstriction: under extreme high pressure, capillaries in the periphery begin closing off allowing blood to only circulate to the essential organs in the body core– most importantly the heart and brain. This is done to conserve the uptake of oxygen and keep a steady body temperature. Human muscles tend to only hold 12% of the body’s overall oxygen storage, so muscles usually suffer from cramping at this stage. Aquatic mammals’ muscles contain up to 30% of the body’s total oxygen storage so activity is not affected when capillaries stop supplying blood.
• Blood shift: this arises when water pressure becomes very high. Organ and circulatory walls permit plasma or water to be absorbed into the chest cavity to keep a constant pressure and protect the organs from getting crushed. At this point, blood plasma fills up the alveoli in lungs, and when the mammal returns to an environment of lower pressure, the plasma is reabsorbed.
The aim of the experiment was to observe heart rate variability in healthy volunteers.
The hypothesis was that heart rate, systolic and diastolic pressure are not affected by cold or warm water on the face or by holding breath.
1. The participant sat in a comfortable position and allowed the heart rate to come to its normal resting rate.
2. A blood pressure monitor was attached to the subject’s arm adjacent to the heart position, to maintain an accurate reading. The subject did not move or talk for 5 minutes.
3. An initial resting reading was taken for heart rate and blood pressure. This reading was used as a control to compare to the other variables.
4. The subject was given 5 minutes to rest, to avoid the results from getting affected.
5. The subject held their breath for 5 seconds, allowing internal conditions to adjust, and then the blood pressure monitor was switched on while the subject was still holding their breath until after the reading was taken.
6. The subject was given 5 minutes to rest, to avoid the results from getting affected by the previous conditions.
7. The subject’s whole face was submerged into a bowl of room temperature water (25 degrees Celsius) and given a few seconds to adjust, another reading was taken as “the warm water” measurement.
8. The subject was given 5 minutes to rest, to avoid the results from getting affected by the previous conditions.
9. The subject’s whole face was submerged into a bowl of cold water (<10 degrees Celsius) and given a few seconds to adjust, another reading was taken as “the cold water” measurement.
The program IBM SPSS was used to compare the average readings for the heart rate, diastolic pressure and systolic pressure under all four conditions. Results were collected for 23 participants, however 1 case was excluded when working out the mean, as it was assumed to be an anomaly.
A one way ANOVA test was carried out on the raw results to determine whether there was a statistically significant difference between the means for each variable – heart rate, diastolic pressure, and systolic pressure respectively.
ANOVA and Post Hoc for Heart Rate under all four conditions
The above table shows the output of the ANOVA analysis for heart rate. The significance level is 0.000 (p=0.000) which is below 0.05, hence there is a statistically significant difference in the mean heart rate under the four varying conditions (rest, holding breath, warm water, cold water). The null hypothesis is rejected. However, to find out under which spicifc conditions were different, a multiple comparisons table containing the results of a post hoc test was used.
A Tukey post hoc test revealed that the mean heart rate was statistically significantly lower under cold water conditions (+63/-12 bpm – p=0.000) and warm water conditions (67+/-9 bpm p=0.022) compared to resting heart rate (78+/-10bpm). Holding breath rate (71+/-9 p=.336) was not statistically significantly different to resting rate.There was no statistically significant difference in heart rate between warm water and cold water conditions ( p=0.528 ).
ANOVA and Post Hoc for Diastolic blood pressure under all four conditions
The above table shows the output of the ANOVA analysis for diastolic blood pressure. The significance level is 0.000 (p=0.000) which is below 0.05, hence there is a statistically significant difference in the mean diastolic blood pressure under the four varying conditions (rest, holding breath, warm water, cold water). The null hypothesis is rejected. However, to find out under which spicifc condition differed, a Tukey Post hoc test was carried out.
The Tukey post hoc results revealed that the diastolic blood pressure was statistically significantly higher under warm conditions (89+/-12 mm/Hg, p=0.001) and under cold conditions (94 +/-11 mm/Hg, p=0.000) compared to resting pressure (77+/-8 mm/Hg). There was no statistically significant change between restting pressure and pressure while holding breath (82 +/-9 mm/Hg). There was no statistically significant difference in diastolic pressure between cold water and warm water (p=0.475).
ANOVA and Post Hoc for Systolic Blood Pressure under all four conditions
The above table shows the output of the ANOVA analysis for systolic blood pressure. The significance level is 0.000 (p=0.000) which is below 0.05, hence there is a statistically significant difference in the mean systolic blood pressure under the four varying conditions (rest, holding breath, warm water, cold water). The null hypothesis is rejected. However, to find out under which spicifc condition differed, a Tukey Post hoc test was carried out.
The Tukey post hoc results revealed that the systolic blood pressure was statistically significantly higher under warm conditions (138+/-15 mm/Hg, p=0.000) and under cold conditions (141 +/-17 mm/Hg, p=0.000) compared to resting pressure (122+/-10 mm/Hg). There was no statistically significant change between restting pressure and pressure while holding breath (121 +/-8 mm/Hg). There was no statistically significant difference in systolic pressure between cold water and warm water (p=0.832).
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The null hypothesis was rejected because the ANOVA tests showed there was a statistically significant difference in heart rate, systolic and diastolic pressure under the conditions.
It was initially assumed that water touching the face and apnea trigger the diving reflex, however after analysation of the above results, it seemed that water alone was the main trigger in this experiment. Due to low p-values in the Post Hoc test for heart rate, it was determined that, whether cold or warm, water triggers the diving reflex as the heart rate did statistically significantly change under cold (+63/-12 bpm – p=0.000 –because p is <0.001 statistically it is highly significant and has a 9.99% chance of being significantly different) and warm conditions (67+/-9 bpm p=0.022). However, there was no statistically significant change during apnea (71+/-9 bpm p=.336). Furthermore, systolic and diastolic blood pressure were also statistically significantly higher than resting systolic and diastolic pressure, however, apnea wasn’t statistically significant for either pressure. These results prove that submersion of the face in water was the main trigger for the response of bradycardia in the diving reflex.
Theoretically, only cold water should have triggered the diving reflex, however in the above experiment both warm and cold water produced a an equally similar response, that was not statistically significantly different for heart rate (p=0.528 ); diastolic pressure (p=0.475); and systolic pressure (p=0.832). From the above experiment, it seemed that bradycardia and peripheral vasoconstriction are not dependant on water temperature to be triggered.
The physiological and neurological response once the face becomes in contact with water.
Skin mechanoreseptors transmit information through the large trigeminal nerves (fifth cranial nerve) to the second order neurons of the trigeminal nucleus which are then projected to the thalamus and ultimately the somatosensory cortex. A graphical representation is found below.
The diagram below shows physiological and neurological path of the diving reflex from detection of wetness to the physiological response.
The autonomic nervous system is automatic and not under conscious control. Complementary action by the sympathetic system speeds up the peripheral vasoconstriction mechanism and the parasympathetic system slows down heart rate.6
Following facial submersion blood pressure increased statistically significantly in the experiment showing that as shown above. This is due to vasoconstriction which redirects blood to brain and heart. Diving animals’ diving reflex differs to humans. For example, weddel seals exhale just before diving as their body contains 87ml of oxygen per kilogram – 33% of it is in their muscles bonded with myoglobin and the rest is in the blood. This is why seals could stay under water for longer than humans, as in humans only 20ml kg-1 is available. When seals dive, blood volume is 21% of body mass (in humans, it’s 7%), and their haematocrit (volume percentage of red blood cells) is 60% compared to humans (46% in men and 42% in women) as they have large spleens. Their alveoli collapse under the depth of 60 meters (as apposed to humans’ 290 meters), air passes into the re-inforced non absoptive regions.7
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Many of the results contain a high standard deviation; this could be due to participants having different lung capacities and were able to hold their breath for a shorter period of time than average so did not stay under water for long enough for the diving reflex to take full effect. It could also be because some participants started the blood pressure measuring equipment much earlier or much later after submersion under water than the rest of the subject pool – hence the big fluctuation in heart rate and blood pressure. Starting the blood pressure monitor too quickly would produce a higher heart rate than expected due to initial physiological shock as water touches the face. Furthermore, results would have been skewed because subjects may not have been fully relaxed. To avoid the results from being inacurate, extreme anomolies were ommitted – when systolic pressure was above 200 and heart rate above 100. Even after ommitting some results, not all subjects had as low a heart rate as theoretically expected during bradycardia (60bpm). This is due to the experiment being limited because it was not safe to expect participants to stay under water long enough to ensure that the diving reflex was fully triggered.
In conclusion, this experiment showed, through empirical evidence and statistical analysis, the significant effects of the diving reflex; it proved that once water is detected by receptors in the face the diving reflex takes effect. However, there was no significant difference in the response depending on the temperature of the water, Lawrence Folinsbee (1974) showed that cold water causes the heart rate to decrease much lower than in warm water, which wasn’t detected here. 8 Furthermore, this experiment successfully showed the effect the diving reflex has on the blood pressure.
Here's a link to the mammalian diving reflex report that you could use as a template. https://www.dropbox.com/s/9yk9rph71g0yjhk/physiology%20osman%20elmais.docx?dl=0
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