The Effect of External Temperature on Heart Rate and Blood Pressure
CU Boulder, Fall 2007
Temperature affects a large amount of the processes that occur in the human body. Most enzymes within the body have optimum temperatures, which can increase the rate of cellular respiration in the body, and thus increase the rate at which blood must be pumped to the respiring tissues. We tested the effects of external temperature on heart rate and blood pressure in female college students in freshmen through junior years. Because many enzymes involved in cellular respiration have higher optimum temperatures, we hypothesized that heart rate would be directly proportional to temperature and blood pressure would be inversely proportional to temperature; i.e. when temperature increased, heart rate would increase, and blood pressure would decrease, and when external temperature decreased, heart rate would decrease and blood pressure would increase.
To test this, four females were used as subjects; their blood pressures and heart rates were taken twice at room temperature (26.0 ¡C) to have a standard to which the other data could be compared. Next, the females were required to stand in a freezer of temperature 5.5 ¡C for 5 minutes. Their blood pressures and heart rates were again measured and recorded. There was then a 5 minute period in which the subjectsÕ heart rates and blood pressures were allowed to return to normal. The female subjects were then exposed to a hot sauna of temperature 60.1 ¡C for 5 minutes. Their blood pressures and heart rates were measured and recorded. The prediction was that in the cold environment, heart rate would be lower than at room temperature and blood pressure would be higher than at room temperature; likewise, when temperature increased it was predicted that heart rate would be higher than at room temperature and blood pressure would be lower than at room temperature.
Our results indicated that the heart rate was lower at 5.5 ¡C than at 26.9 ¡C, and was higher at 60.1 ¡C than at 26.9 ¡C (room temperature.) However, the results indicated that blood pressure was higher at 5.5 ¡C than at room temperature, and blood pressure was also higher at 60.1 ¡C than at room temperature. We ran six t-tests to determine the significance of each of the different sets of data:
á The p value from the t-test of hot temp heart rate vs. room temp heart rate was 0.028455044
á The p value from the t-test of cold temp heart rate vs. room temp heart rate was 0.082872375
á The p value from the t-test of cold temp systolic BP vs. room temp systolic BP was 0.001913779
á The p value from the t-test of hot temp systolic BP vs. room temp BP was 0.147196817
á The p value from the t-test of cold temp diastolic BP vs. room temp diastolic BP was 0.000981792
á The p value from the t-test of hot temp diastolic BP vs. room temp diastolic BP was 0.401123416
This illustrates that the decrease in heart rate in cold temperature was not significant and the increase in heart rate in hot temperature was significant. The increase in systolic blood pressure in cold temperatures was significant, and the increase in systolic blood pressure in hot temperature was not significant. Lastly, the increase in diastolic blood pressure in cold temperature was very significant, and the increase in diastolic blood pressure in hot temperature was not significant.
The results are only half consistent with the predictions based on the hypothesis because the heart rate significantly increased in the hot temperature, and the increase in both systolic and diastolic blood pressures was significant, but the decrease in heart rate was insignificant in colder temperatures, and the blood pressure did not decrease as predicted. A study done by Landsteiner and Hayes (1943) showed that when blood temperature was from 26.5 ¡C to 43.0 ¡C, the heart rate (pulse) fluctuated from 63 to 165 and had an nearly linear resemblance in relation to blood temperature (256). In this case, the temperature of the blood was measured and not the temperature of the environment. In the future, we would use a more accurate device for measuring blood pressure and heart rate than a portable machine, and we would also allow more time for the subjects to acclimate to the cold temperature and hot temperatures so that their internal temperature would decrease and increase more respectively.