Control of Ventilation

The process of ventilation, regulates and controls the oxygen entering the body, is tightly monitored and controlled in the brainstem (specifically the medulla) and relies on a delicate sensory-effector system to keep dissolved gases in the blood in narrow physiologic range. There are three basic elements of the respiratory control system that allow for such accuracy:

  1. Sensors: monitor specific information about dissolved gases and other factors and are responsible for relaying this information to the medulla.
  2. Central Controller: located in the medulla it receives sensory information and relays a bodily response to correct any abnormality detected.
  3. Effectors: receive information from the brain and “effect” a change in some physiologic way to correct for the abnormalities detected.

Many common parts of the body are cleverly used in this sensory-effector system to keep dissolved gases in a narrow physiologic range. Figure 4 illustrates these areas:

Figure 4.  Schematic representation of factors that affect medullary control of pulmonary ventilation.

As shown in Figure 4, the medial portion of the medulla is the central controller that regulates the normal respiratory cycle. This portion of the brain receives input from various “sensors” to either increase or decrease respiration (the amount of breaths), and the duration of each breath (how much air we take into the lungs). The overall goal of the central control mechanism is to maintain the correction oxygenation state of the blood to maximize oxygenation of body tissues.

Different control mechanisms for ventilation predominate during rest and during exercise. The main controllers at rest are the central and peripheral chemoreceptors:

  • Central chemoreceptor is located near the ventral surface of the medulla and responds to changes in the chemical composition of the blood and extracellular fluid around it. The composition of the cerebrospinal fluid (CSF) has the greatest influence on the central chemoreceptor. It is separated from the central chemoreceptor by the blood-brain barrier. This barrier is relatively impermeable to hydrogen (H+) and bicarbonate ions (HCO3), but CO2 diffuses across it easily. As PCO2 rises in the blood, it diffuses into the CSF, causing a drop in pH (i.e. making the CSF more acidic). This results in an increase in ventilation.
  • Peripheral chemoreceptors located in the aortic arch and at the bifurcation of the carotid arteries (Fig 5) actively monitor actively monitor the PO2, PCO2, hydrogen ion and potassium concentrations in arterial blood and send signals to the brain accordingly.

For example, increased ventilation is the body’s response to high CO2 levels in the blood: chemoreceptors sense high PCO2 and send a signal to the medulla to increase ventilation and exhale the excess CO2 in order to return PCO2 values back to resting state.

 Figure 5. The aortic arch and bifurcation of the carotid arteries contain the “peripheral chemoreceptors” that are sensitive to reduce PO2, increased PCO2, H+ and K+ concentrations.

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