Effects of Blood PCo2 and pH on Ventilation

Chemoreceptor input to the brain stem modifies the rate and depth of breathing so that, under normal conditions, arterial PCo2, pH, and Po2 remain relatively constant. If hypoventilation (inadequate ventilation) occurs, PCo2 quickly rises and pH falls. The fall in pH is due to the fact that carbon dioxide can combine

Carotid sinus

Common carotid artery

Carotid sinus

Common carotid artery

—Sensory nerve fibers

(in glossopharyngeal nerve)

Carotid body

Sensory nerve fibers (in vagus nerve)

Aortic bodies

Heart

Aorta

—Sensory nerve fibers

(in glossopharyngeal nerve)

Carotid body

Sensory nerve fibers (in vagus nerve)

Aortic bodies

Aorta

Heart

■ Figure 16.26 Sensory input from the aortic and carotid bodies.

The peripheral chemoreceptors (aortic and carotid bodies) regulate the brain stem respiratory centers by means of sensory nerve stimulation.

with water to form carbonic acid, which, as a weak acid, can release H+ into the solution. This is shown in these equations:

The oxygen content of the blood decreases much more slowly because of the large "reservoir" of oxygen attached to hemoglobin. During hyperventilation, conversely, blood PCo2 quickly falls and pH rises because of the excessive elimination of carbonic acid. The oxygen content of blood, on the other hand, is not significantly increased by hyperventilation (hemoglobin in arterial blood is 97% saturated with oxygen during normal ventilation).

Chemoreceptors

Aortic and carotid bodies

Chemoreceptors

Medulla oblongata

Chemoreceptors

Aortic and carotid bodies

Chemoreceptors

Medulla oblongata

-Spinal cord

Motoneurons to respiratory muscles

■ Figure 16.27 The regulation of ventilation by the central nervous system. The feedback effects of pulmonary stretch receptors and "irritant" receptors on the control of breathing are not shown in this flowchart.

For the reasons just stated, the blood PCo2 and pH are more immediately affected by changes in ventilation than is the oxygen content. Indeed, changes in PCO2 provide a sensitive index of ventilation, as shown in figure 16.28. In view of these facts, it is not surprising that changes in PCO2 provide the most potent stimulus for the reflex control of ventilation. Ventilation, in other words, is adjusted to maintain a constant Pœ2; proper oxygenation of the blood occurs naturally as a side product of this reflex control.

The rate and depth of ventilation are normally adjusted to maintain an arterial PCO2 of 40 mmHg. Hypoventilation causes a rise in PCO2—a condition called hypercapnia. Hyperventilation, conversely, results in hypocapnia. Chemoreceptor regulation of breath figure 16.29.

tion of breathing in response to changes in PCO2 is illustrated in

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

Get My Free Ebook


Post a comment