Final answer:
The primary stimulus for breathing is an increase in carbon dioxide levels in the blood, or hypercapnia, which is more effective than a decrease in oxygen levels, or hypoxemia, for stimulating respiration. Central and peripheral chemoreceptors detect these changes, leading to an increased rate and depth of breathing through the activation of the respiratory centers in the brain.
Step-by-step explanation:
Stimulus for Breathing
The normal stimulus to breathe is an increase in carbon dioxide (CO2) in the blood, also known as hypercapnia, or a decrease of oxygen (O2) in the blood, also known as hypoxemia. Among these two, the response to elevated levels of carbon dioxide is more effective for stimulating respiration. This is because the respiratory rate and depth are principally controlled by the respiratory centers of the brain, which respond robustly to changes in blood CO2 levels due to its role in regulating blood pH.
Central chemoreceptors located in the brain detect increased CO2 levels and stimulate the medulla oblongata and pons to increase the depth and rate of breathing. Similarly, peripheral chemoreceptors located in the aortic arch and carotid arteries sense changes in O2 levels, but this mechanism is not as sensitive as the response to CO2. An increase in stimuli, such as elevated CO2, triggers a stronger ventilatory response to maintain homeostasis by removing excess carbon dioxide and regulating blood pH.
Physical activity can also affect breathing through hyperpnea, which is an increase in the rate and depth of ventilation. This occurs through psychological stimuli, motor neuron activation of muscles, and activation of proprioceptors due to exercise. However, the chemical stimuli in relation to CO2 and O2 in the blood remain the primary factors regulating the respiratory system under regular conditions.