55. Chemical control of respiration. Acidosis, alkalosis

Chemoreceptors

Chemoreceptors are sensitive to changes in the level of certain compounds. The most important chemoreceptors are those which sense changes in pCO2, pO2 and H+. There are chemoreceptors both centrally and peripherally in the body.

When the chemoreceptors are activated, they will stimulate the respiratory centre, which will stimulate ventilation.

Central chemoreceptors:

Central chemoreceptors are located in the medulla oblongata. They’re sensitive to changes in pH ([H+]) and in pCO2. The central chemoreceptors are activated if:

• pH decreases (acidosis)
• pCO2 increases (hypercapnia)

Peripheral chemoreceptors:

Peripheral chemoreceptors are located in the carotid and aortic bodies. They’re sensitive to changes in pO2. The peripheral chemoreceptors are activated if pO2 decreases below 60 mmHg (hypoxia).

Acid-base disorders

Acid-base disorders like acidosis and alkalosis were described in detail in topic 21.

Respiratory acidosis:

Respiratory acidosis occurs when there is hypoventilation, i.e. breathing is not sufficient to get rid of enough CO2. This causes pCO2 to accumulate, which causes acidosis. This can occur in diseases like:

• Chronic obstructive pulmonary disease
• Muscle weakness involving the diaphragm

The body will compensate for the respiratory acidosis by decreasing renal HCO3– excretion.

Respiratory alkalosis:

Respiratory alkalosis occurs when there is hyperventilation, i.e. breathing is excessive compared to the body’s needs. This causes pCO2 to decrease as more CO2 is exhaled. This causes alkalosis.

Respiratory alkalosis can occur when a person voluntarily hyperventilates, or if a person is in high altitude and is not acclimatized yet.

The body will compensate by increasing renal HCO3– excretion.