Involuntary respiration is any form of respiratory control that is not under direct, conscious control. Breathing is required to sustain life, so involuntary respiration allows it to happen when voluntary respiration is not possible, such as during sleep. Involuntary respiration also has metabolic functions that work even when a person is conscious.

The Respiratory Centers

Involuntary respiration is controlled by the respiratory centers of the upper brainstem (sometimes termed the lower brain, along with the cerebellum). This region of the brain controls many involuntary and metabolic functions besides the respiratory system, including certain aspects of cardiovascular function and involuntary muscle movements (in the cerebellum).

Anatomy of the brainstem: The brainstem, which includes the pons and medulla.

The respiratory centers contain chemoreceptors that detect pH levels in the blood and send signals to the respiratory centers of the brain to adjust the ventilation rate to change acidity by increasing or decreasing the removal of carbon dioxide (since carbon dioxide is linked to higher levels of hydrogen ions in blood).

There are also peripheral chemoreceptors in other blood vessels that perform this function as well, which include the aortic and carotid bodies.

The Medulla

The medulla oblongata is the primary respiratory control center. Its main function is to send signals to the muscles that control respiration to cause breathing to occur. There are two regions in the medulla that control respiration:

• The ventral respiratory group stimulates expiratory movements.
• The dorsal respiratory group stimulates inspiratory movements.

The medulla also controls the reflexes for nonrespiratory air movements, such as coughing and sneezing reflexes, as well as other reflexes, like swallowing and vomiting.

The Pons

The pons is the other respiratory center and is located underneath the medulla. Its main function is to control the rate or speed of involuntary respiration. It has two main functional regions that perform this role:

• The apneustic center sends signals for inspiration for long and deep breaths. It controls the intensity of breathing and is inhibited by the stretch receptors of the pulmonary muscles at maximum depth of inspiration, or by signals from the pnuemotaxic center. It increases tidal volume.
• The pnuemotaxic center sends signals to inhibit inspiration that allows it to finely control the respiratory rate. Its signals limit the activity of the phrenic nerve and inhibits the signals of the apneustic center. It decreases tidal volume.

The apneustic and pnuemotaxic centers work against each other together to control the respiratory rate.

Chemoreceptors

A chemoreceptor, also known as chemosensor, is a sensory receptor that transduces a chemical signal into an action potential. The action potential is sent along nerve pathways to parts of the brain, which are the integrating centers for this type of feedback. There are many types of chemoreceptors in the body, but only a few of them are involved in respiration.

The respiratory chemoreceptors work by sensing the pH of their environment through the concentration of hydrogen ions. Because most carbon dioxide is converted to carbonic acid (and bicarbonate ) in the bloodstream, chemoreceptors are able to use blood pH as a way to measure the carbon dioxide levels of the bloodstream.

The main chemoreceptors involved in respiratory feedback are:

1. Central chemoreceptors: These are located on the ventrolateral surface of medulla oblongata and detect changes in the pH of spinal fluid. They can be desensitized over time from chronic hypoxia (oxygen deficiency) and increased carbon dioxide.
2. Peripheral chemoreceptors: These include the aortic body, which detects changes in blood oxygen and carbon dioxide, but not pH, and the carotid body which detects all three. They do not desensitize, and have less of an impact on the respiratory rate compared to the central chemoreceptors.

Chemoreceptor Negative Feedback

Negative feedback responses have three main components: the sensor, the integrating sensor, and the effector. For the respiratory rate, the chemoreceptors are the sensors for blood pH, the medulla and pons form the integrating center, and the respiratory muscles are the effector.

Consider a case in which a person is hyperventilating from an anxiety attack. Their increased ventilation rate will remove too much carbon dioxide from their body. Without that carbon dioxide, there will be less carbonic acid in blood, so the concentration of hydrogen ions decreases and the pH of the blood rises, causing alkalosis.

In response, the chemoreceptors detect this change, and send a signal to the medulla, which signals the respiratory muscles to decrease the ventilation rate so carbon dioxide levels and pH can return to normal levels.

There are several other examples in which chemoreceptor feedback applies. A person with severe diarrhea loses a lot of bicarbonate in the intestinal tract, which decreases bicarbonate levels in the plasma. As bicarbonate levels decrease while hydrogen ion concentrations stays the same, blood pH will decrease (as bicarbonate is a buffer) and become more acidic.

In cases of acidosis, feedback will increase ventilation to remove more carbon dioxide to reduce the hydrogen ion concentration. Conversely, vomiting removes hydrogen ions from the body (as the stomach contents are acidic), which will cause decreased ventilation to correct alkalosis.

Chemoreceptor feedback also adjusts for oxygen levels to prevent hypoxia, though only the peripheral chemoreceptors sense oxygen levels. In cases where oxygen intake is too low, feedback increases ventilation to increase oxygen intake.

A more detailed example would be that if a person breathes through a long tube (such as a snorkeling mask) and has increased amounts of dead space, feedback will increase ventilation.

Respiratory feedback: The chemoreceptors are the sensors for blood pH, the medulla and pons form the integrating center, and the respiratory muscles are the effector.