Final answer:
The earliest physiological response to breathing 100% oxygen is an increased heart rate.
Step-by-step explanation:
The earliest physiological response to breathing 100% oxygen is an increased heart rate. When a person starts breathing pure oxygen, the body's oxygen levels rise quickly, stimulating the respiratory centers in the brain to increase the heart rate in order to pump more oxygenated blood to the tissues. When monitoring the earliest physiologic response to breathing 100% oxygen, the typical initial response is an increase in heart rate. This is known as the "hyperoxic response," where breathing high concentrations of oxygen can lead to a transient increase in heart rate. It's important to note that individual responses can vary, and other factors may influence the overall cardiovascular response to high oxygen concentrations.