Now that we have a foundation of buffers and the control centers for respiration, we are ready to discuss respiratory regulation of blood gases and pH.

The body has a built-in mechanism to regulate CO₂ , O₂ , and H⁺ levels in the blood. Chemoreceptors send input to the inspiratory center to increase respirations if CO₂ and/or H⁺ are high or PO₂ is low.

Increased CO2, increased H⁺ (i.e. lower pH), or decreased O₂ would lead to an increase in rate and depth of breathing.

Can you die by holding your breath?

If the PO₂ drops below a normal level of 100 mm Hg, chemoreceptors are stimulated. You are consciously holding your breath through the influence of the motor cortex. If your PO₂ drops below a certain level, fainting (syncope) follows, and the brainstem will take over the work of breathing while you are unconscious.

A low PCO₂ (below 40 mmHg) does not signal chemoreceptors and will not stimulate the inspiratory center. (The inspiratory center responds to high CO₂ , not low CO₂ ). So, if you hyperventilate, blowing off CO₂ and increasing O₂ the inspiratory center slows and you can hold your breath longer.

Swimmers were once encouraged to hyperventilate before swimming to be able to hold their breath longer.

Why is it dangerous to hyperventilate and then swim underwater?

This is dangerous because O₂ levels may fall to a dangerously low condition before the chemoreceptor reflex is activated, causing fainting (syncope). Passing out under water is not a good idea as drowning may occur!

Because CO₂ easily converts to an acid, the respiratory system can help control blood pH by either speeding up breathing to get rid of CO₂ or slowing down breathing to retain CO₂.

A condition of too much alkali (too high a pH, too low a [H⁺]) in the blood is called alkalosis.

Hyperventilation, or excessive ventilation is a mechanism used by the body to lower the blood [H⁺]. Hyperventilation during your favorite exam may also lead to alkalosis. (You knew exams were bad for your health!)

A condition of too much acid (too low a pH, too high a [H+]) in the blood is called acidosis.

Hypoventilation retains more CO₂. While the body may use this mechanism in conditions of alkalosis to decrease the pH, if it lasts more than a few seconds it usually results from pathology. Patients with chronic obstructive pulmonary diseases such as emphysema have difficulty exhaling and often build up CO₂ leading to acidosis.