Mechanism which controls respiration

It achieves this by sending constant inhibitory impulses to the inspiratory centre in the medulla to limit the period of inspiration. The lungs contain stretch receptors or baroreceptors which also appear to influence respiration. When the lungs expand during inspiration, stretch receptors in the lung walls are activated and act via the vagus nerve to inhibit the inspiratory centre in the medulla oblongata and allow reflex expiration to occur Bourke, These receptors are particularly important in animals and in young babies who have a poorly organised brainstem Stocks, but their role in adults remains uncertain, especially during quiet respiration.

Marieb suggests that this mechanism is probably protective rather than regulatory. Other receptors in the lungs are sensitive to irritants such as gases, debris, inhaled foreign bodies and excess mucus. When they are activated, these receptors influence the respiratory centre via the vagus nerve so that coughing can occur to clear the irritant. The higher centres of the brain are the areas where we understand and manipulate information and experience thoughts, feelings and emotions.

These centres can also influence respiration. Respiratory rate and depth alter when the centres of the limbic system involved with emotions such as pain, anger or excitement are activated, though this effect is involuntary and outside our control.

Centres in the hypothalamus are activated and influence both the rate and the depth of respiration via the pons Martini and Bartholomew, and the medullary inspiration centre. Respiration can be increased or decreased via this pathway. Examples of this mechanism in action include gasping with fear or cold, a rise in respiratory rate when the body temperature is high, and breath-holding during times of anger. From the cerebral cortex we can also voluntarily change our respiratory pattern by sending signals direct to the muscles of inspiration and bypassing the medullary centres Marieb, The cortex is the area of the brain where we interpret and manipulate information and when we need, for example, to swim a length under water, sing or simply chat to friends, we can consciously control our breathing pattern.

Many of us, in our younger days, tried to hold our breath until we collapsed, but it is impossible to alter our breathing beyond certain limits because the other respiratory control mechanisms ultimately override the influence of the higher centres.

Perhaps the most important influence on respiratory rate and depth are chemicals. Specialised receptors chemoreceptors respond to chemical changes in the blood and cerebrospinal fluid CSF.

Peripheral chemoreceptors in the aortic arch and the carotid bodies respond to changes in the oxygen O2 , carbon dioxide CO2 and acidity pH levels in arterial blood. It is these chemoreceptors that are ultimately responsible for the homeostasis of O2 and CO2 levels in the blood. They ensure that there is adequate oxygen circulating for the needs of cells throughout the body and that the waste products of cellular metabolism, carried as CO2, can be deposited in the lungs. Arterial pressures of O2 and, especially, CO2 are maintained within narrow limits despite large changes in consumption and production.

Normally it is a small rise in arterial CO2 that triggers these chemoreceptors and results in a negative feedback homoeostatic response to reduce these levels. As a result, the CSF rapidly becomes more acidic and its pH falls. This increased acidity stimulates the central chemoreceptors, which act directly upon the medullary and pontine centres to increase both the rate and depth of respiration by increasing the strength and duration of neuronal impulses from the inspiratory centre to the muscles of inspiration.

The result is that the excess CO2 is blown out of the lungs. When arterial CO2 levels are abnormally low hypocapnia , respirations become shallow and slow hypoventilation and periods of apnoea may occur as the stimulus to breathe is absent.

People can also control their breathing when they wish, for example during speech, singing, or voluntary breath holding. Sensory organs in the brain and in the aorta and carotid arteries monitor the blood and sense oxygen and carbon dioxide levels. Normally, an increased concentration of carbon dioxide is the strongest stimulus to breathe more deeply and more frequently. Conversely, when the carbon dioxide concentration in the blood is low, the brain decreases the frequency and depth of breaths.

During breathing at rest, the average adult inhales and exhales about 15 times a minute. See also Overview of the Respiratory System Overview of the Respiratory System To sustain life, the body must produce sufficient energy.

Energy is produced by burning molecules in food, which is done by the process of oxidation whereby food molecules are combined with The lungs have no skeletal muscles of their own. The work of breathing is done by the diaphragm, the muscles between the ribs intercostal muscles , the muscles in the neck, and the abdominal muscles.

The diaphragm, a dome-shaped sheet of muscle that separates the chest cavity from the abdomen, is the most important muscle used for breathing in called inhalation or inspiration.

The diaphragm is attached to the base of the sternum, the lower parts of the rib cage, and the spine. As the diaphragm contracts, it increases the length and diameter of the chest cavity and thus expands the lungs. The intercostal muscles help move the rib cage and thus assist in breathing. The process of breathing out called exhalation or expiration is usually passive when a person is not exercising. The elasticity of the lungs and chest wall, which are actively stretched during inhalation, causes them to return to their resting shape and to expel air out of the lungs when inspiratory muscles are relaxed.

The Pons The pons is the other respiratory center and is located underneath the medulla. 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. Key Points The ventral respiratory group controls voluntary forced exhalation and acts to increase the force of inspiration. The dorsal respiratory group nucleus tractus solitarius controls mostly inspiratory movements and their timing. Ventilatory rate minute volume is tightly controlled and determined primarily by blood levels of carbon dioxide as determined by metabolic rate.

Chemoreceptors can detect changes in blood pH that require changes in involuntary respiration to correct. The apneustic stimulating and pnuemotaxic limiting centers of the pons work together to control rate of breathing.