Final answer:
Running at top speed increases a mammal's metabolic rate, which in turn increases the demand for oxygen, causing the heart to increase blood flow and more oxygen to be absorbed into the bloodstream more rapidly.
Step-by-step explanation:
The question pertains to how a mammal's body responds to increased physical activity, specifically running at top speed, and its effect on oxygen absorption. When running at top speed, a mammal's metabolic demands increase, thus increasing the need for oxygen. The correct answer to why oxygen would enter the blood of a mammal faster if the mammal were running at top speed is: a) Running increases blood flow, allowing more oxygen to enter the bloodstream. Increased physical activity raises the heart rate and blood flow, enhancing the delivery of oxygen to the tissues. Increased cardiac output leads to a higher rate of blood circulation through the lungs, thereby more oxygen is taken up by the blood and transported to the muscles that are engaged in the activity.
When exercising, the body also experiences physiological changes, such as the production of more BPG by erythrocytes which enhances the dissociation of oxygen from hemoglobin, allowing for more efficient oxygen delivery to the working muscles. Additionally, when the body is exercising, the partial pressure of oxygen in the muscles decreases, which increases the oxygen gradient between the blood and the muscle tissues, thus facilitating a faster diffusion of oxygen into the muscles.