Control of respiration

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Control of ventilation refers to the physiological mechanisms involved in the control of physiologic ventilation. Gas exchange primarily controls the rate of respiration.

The most important function of breathing is gas exchange (of oxygen and carbon dioxide). Thus the control of respiration is centered primarily on how well this is achieved by the lungs.

There are four main centers in the brain to regulate the respiration:

  1. Inspiratory center
  2. Expiratory center
  3. Pneumotaxic center
  4. Apneustic center

The first two centers are present on the medulla oblongata whereas the last two centers on the pons region of brain.

Ventilatory Pattern[edit]

The pattern of motor stimuli during breathing can be divided into inspiratory and expiratory phases. Inspiration shows a sudden, ramped increase in motor discharge to the inspiratory muscles (including pharyngeal dilator muscles). Before the end of inspiration, there is a decline in motor discharge. Exhalation is usually silent, except at high minute ventilation rates.

The mechanism of generation of the ventilatory pattern is not completely understood, but involves the integration of neural signals by respiratory control centers in the medulla and pons. The nuclei known to be involved are divided into regions known as the following:

There is further integration in the anterior horn cells of the spinal cord.[citation needed]

Control of ventilatory pattern[edit]

Ventilation is normally controlled by the autonomic nervous system, with only limited voluntary override. An exception to this is Ondine's curse, where autonomic control is lost.

Determinants of ventilatory rate[edit]

Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of carbon dioxide as determined by metabolic rate. Blood levels of oxygen become important in hypoxia. These levels are sensed by chemoreceptors in the medulla oblongata for pH, and the carotid and aortic bodies for oxygen and carbon dioxide. Afferent neurons from the carotid bodies and aortic bodies are via the glossopharyngeal nerve (CN IX) and the vagus nerve (CN X), respectively.

Levels of CO2 rise in the blood when the metabolic use of O2 is increased beyond the capacity of the lungs to expel CO2. CO2 is stored largely in the blood as bicarbonate (HCO3-) ions, by conversion first to carbonic acid (H2CO3), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H+ and HCO3-. Build-up of CO2 therefore causes an equivalent build-up of the disassociated hydrogen ion, which, by definition, decreases the pH of the blood.

During moderate exercise, ventilation increases in proportion to metabolic production of carbon dioxide. During strenuous exercise, ventilation increases more than needed to compensate for carbon dioxide production. Increased glycolysis facilitates release of protons from ATP and metabolites lower pH and thus increase breathing.

Mechanical stimulation of the lungs can trigger certain reflexes as discovered in animal studies. In humans, these seem to be more important in neonates and ventilated patients, but of little relevance in health. The tone of respiratory muscle is believed to be modulated by muscle spindles via a reflex arc involving the spinal cord.

Drugs can greatly influence the control of respiration. Opioids and anaesthetic drugs tend to depress ventilation, especially with regards to carbon dioxide response. Stimulants such as amphetamines can cause hyperventilation.

Pregnancy tends to increase ventilation (lowering plasma carbon dioxide tension below normal values). This is due to increased progesterone levels and results in enhanced gas exchange in the placenta.

Ventilation is temporarily modified by voluntary acts and complex reflexes such as sneezing, straining, burping, coughing and vomiting.

Feedback control[edit]

Receptors play important roles in the regulation of respiration; central and peripheral chemoreceptors, and mechanoreceptors.

In addition to involuntary control of respiration by the respiratory center, respiration can be affected by conditions such as emotional state, via input from the limbic system, or temperature, via the hypothalamus. Voluntary control of respiration is provided via the cerebral cortex, although chemoreceptor reflex is capable of overriding conscious control.

References[edit]

  1. ^ Coates EL, Li A, Nattie EE. Widespread sites of brain stem ventilatory chemoreceptors. J Appl Physiol. 75(1):5-14, 1984.
  2. ^ Cordovez JM, Clausen C, Moore LC, Solomon, IC. A mathematical model of pH(i) regulation in central CO2 chemoreception. Adv Exp Med Biol. 605:306-11, 2008.

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