Final answer:
A rectal thermometer should remain in place for approximately 2 to 3 minutes to obtain an accurate temperature reading. This is different from the retention time of food in the human intestine, which is 4 hours for the small intestine and 24 to 72 hours for the entire intestine.
Step-by-step explanation:
A rectal thermometer is typically used to take a highly accurate temperature reading by inserting it into the rectum. For a proper and accurate reading, the rectal thermometer should generally stay in place for about 2 to 3 minutes. This method is often used for infants and young children, as well as in certain medical examinations for adults. As for the experiment mentioned, it relates to understanding the growth of E. coli in varying oxygen environments within the human intestine, rather than to the use of thermometers.
It is noteworthy that the retention time of food in the human small intestine is quite different from the recommended duration for a rectal thermometer to remain in place. The small intestine's approximate retention time is around four hours for food, whereas the entire length of the intestine, which averages 7.5 meters in length, retains food for about 24 to 72 hours depending on various factors.