Final answer:
Walter Cannon, an American physiologist at Harvard Medical School, was the first person to carry out well-planned experiments relating to body functions, particularly the body's reactions to stress.
Step-by-step explanation:
The first person to carry out well-designed and careful experiments relating to body functions was Walter Cannon, an American physiologist at Harvard Medical School. His work, in the early part of the 20th century, was pivotal in identifying the body's physiological reactions to stress. Cannon's approach paved the way for a deeper understanding of how the body functions under stress, influencing subsequent research in physiology and medicine.