Although the respiratory muscles can be controlled voluntarily, normal breathing is a rhythmic, involuntary act that continues even when a person is unconscious.

Breathing is controlled by a poorly defined group of neurons in the brain stem called the respiratory center. The components of this center are widely scattered throughout the pons and medulla oblongata. However, two areas of the respiratory center are of special interest. They are the rhythmicity area of the medulla and the pneumotaxic area of the pons. The medullary rhythmicity area includes two groups of neurons that extend the length of the medulla. They are called the dorsal respiratory group and the ventral respiratory group. The dorsal respiratory group is responsible for the basic rhythm of breathing. The neurons of this group emit bursts of impulses that signal the diaphragm and other inspiratory muscles to contract.

The ventral respiratory group is quiescent during normal breathing. However, when more forceful breathing is necessary, the neurons in this group generate impulses that increase inspiratory movements.

Other neurons of the group activate the muscles associated with forceful expiration. The neurons in the pneumotaxic area transmit impulses to the dorsal respiratory group continuously and regulate the duration of inspiratory bursts originating from the dorsal group. In this way, the pneumotaxic neurons control the rate of breathing. More specifically, when the pneumotaxic signals are strong, the inspiratory bursts have shorter duration's, and the rate of breathing is increased; when the pneumotaxic signals are weak, the inspiratory bursts have longer duration's, and the rate or breathing is decreased.

Factors Affecting Breathing
In addition to the controls exerted by the respiratory center, the breathing rate and depth are influenced by a variety of other factors. These include the presence of certain chemicals in the body fluids, the degree to which the lung tissues are stretched, and person's emotional state. For example, there are chemosensistive areas within the respiratory center. These areas are located in the ventral portion of the medulla oblongata near the origins of the vagus nerves. They are very sensitive to changes in the blood concentrations of carbon dioxide and hydrogen ions. Thus, if the concentration of carbon dioxide or hydrogen ion rises, the chemosensitive areas signal the respiratory center, and the rate and depth of breathing are increased. Then, as a result of increase breathing rate, more carbon dioxide is lost in exhaled air, the blood concentrations of these substances are reduced, and the breathing rate decreases.

Low blood oxygen seems to have little direct effect on the chemosensitive areas associated with the respiratory center. Instead, changes in the blood oxygen concentration are sensed by chemoreceptors in specialized structures called the carotid and aortic bodies, which are located in walls of certain large arteries (the carotid arteries and the aorta) in the neck and thorax. When these receptors are stimulated, impulses are transmitted to the respiratory center, and the breathing rate is increased. However, this mechanism is usually not triggered until the blood oxygen concentration reaches a very low level; thus, oxygen seems to play only a minor role in the control of normal respiration.

An inflation reflex helps to regulate the depth of breathing. This reflex occurs when stretch receptors in the visceral pleura, bronchioles, and alveoli are stimulated as result of lung tissues being overstretched. The sensory impulses of this reflex travel via the vagus nerves to the pneumotaxic area of the respiratory center and cause the duration of inspiratory movements to shorten. This action prevents over inflation of the lungs during forceful breathing.

The normal breathing patter may also be altered if a person is emotionally upset. Fear, for example, typically cases and increased breathing rate, as does pain. In addition, because the respiratory muscles are voluntary, breathing can be altered consciously. In fact. breathing ca be stopped altogether if a person desires.

If breathing is stopped, the blood concentrations of carbon dioxide and hydrogen ions begin to rise, and the concentration of oxygen falls. These changes stimulate the respiratory center, and soon the need to inhale overpowers the desire to hold the breath. On the other hand, a person can increase the breath holding time by breathing rapidly and deeply in advance. This action, called hyperventilation, causes a lowering of the blood carbon dioxide concentration, and following hyperventilation it takes longer than usual for the carbon dioxide concentration to reach the level needed to produce an overwhelming effect on the respiratory center. (Note: Hyperventilation should never be used to help in holding the breath while swimming, because the person who has hyperventilated may lose consciousness under water and drown. )