Process of breathing out
Ventilation brings oxygen into the lungs for gas exchange and removes carbon dioxide.Since the amount of oxygen we need varies depending on activity ,there must be a mechanism which controls the breathing process.
The rhythmic sequence of and is brought about by alternating series of signals from the inspiratory and expiratory cells in the center of the (medulla).During the phase in which the inspiratory nerve cells are activated, the expiratory nerve cells do not discharge, and conversely.Complex circuits and networks of are responsible for this mechanism. Signals from located in or near the have a stabilizing influence on this mechanism.At rest regular nerve impulses are sent to the diaphragm and the intercostals muscles ,causing rythymic contraction and relaxation of these muscles .This causes the volume and pressure changes in the thorax which cause breathing.
Mechanism of control of breathing
Breathing is clearly an involuntary process(unless in cases like forced breathing), and like many involuntary processes (such as heart rate, coughing and sneezing) it is controlled by a region of the brain called the medulla. It is located at the top of the spinal cord. The medulla and its nerves are part of the autonomic nervous system (i.e. involuntary). The region of the medulla that controls breathing is called the respiratory centre. It receives inputs from various receptors around the body and sends output through two nerves to the muscles around the lungs.Impulses are sent along the phrenic nerve to the diaphragm and along intercostals nerves to the intercostal muscles.Unlike the heart, the muscles that
cause breathing cannot contract on their own, but need nerve impulses from the brain for each breath
Ventilation is controlled by impulses form-
Chemoreceptors – in the medulla, aorta and carotid arteries.
Stretch receptors –in the wall of the bronchi.
Nervous control of breathing
Biological Sciences 1 and 2 by D.J Taylor,N.P.O Green and G.W.Stout.)
During inspiaration the lungs get inflated.The inflation causes the stretch receptors (or proprioceptors) located in the bronchial tree to get stimulated.More and more nerve impulses are sent via the vagus nerve to the expiratory centre.The vagus nerve connects the bronchial tree to the respiratory centre in the brain.Inspiration is temporarily inhibited.The external intercostals muscles therefore relax,elastic recoil of the lung tissues occurs and expiration takes place.After this has occurred the bronchial tree is no longer stimulated .Therefore the expiratory centre becomes inactive and inspiration can begin again.The whole cycle is repeated rhythmically throughout the life of the organism.
The depth and rate of breathing also needs to be regulated,so that oxygen supply is matched to the body’s changing demands.The respiratory centre detects carbon dioxide concentration in the blood.If carbon dioxide rises,it sets off deeper and more rapid breathing .If it falls abnormally low,breaths become shallower and less frequent.When carbon dioxide level increases chemoreceptors are stimulated and they send nerve impulses via nerve fibres to the respiratory centre in the medulla..The control centre in the medulla gets stimulated and sends out impulses via the external intercostals and phrenic nerves to the intercostals muscles and diaphragm.This increases the rate and depth of breathing.This reduces the level of carbon dioxide in the blood and brings in more oxygen.When the level of carbon dioxide falls the chemoreceptors are no longer stimulated and the breathing rate returns to normal.Carbon dioxide can become harmful if allowed to build up in the body.It dissolves to form acid which can denature enzymes and proteins.An increase of 0.25 % carbon dioxide concentration in the air can double the ventilation rate.
The control of breathing by chemoreceptors is an example of negative feedback mechanism.Negative feedback occurs when any change in the normal level is detected,corrected and returned to normal level.eg any change in the carbon dioxide level in the blood is detected ,corrected and returned to normal level.
Conclusion
The mechanism of control of ventilation is extremely important for the survival of the organism.As mentioned earlier,this process is involuntary and takes place automatically.However there are certain cases where the rate and depth of breathing are under voluntary control as shown by the ability to hold one’s breath.
Voluntary control is used during forced breathing,speech,singing,sneezing and coughing.When such control is being exerted,impulses originating in the cerebral hemispheres pass to the breathing centre which then carries out the appropriate action.Hyperventilation is forced deep breathing.Breathing in and out deeply reduces the carbon dioxide level in the blood.Increase in carbon dioxide concentration triggers breathing.A low level can suppress the stimulus to breathe.Divers hyperventilate before going underwater,so that the urge to breathe is reduced.This allows them to stay underwater for a longer period of time.Hyperventilation on the other hand could also be dangerous. Hyperventilation during panic or fear could reduce the urge to breathe which may result in fainting as the brain is deprived of oxygen.Once the conscious control is
lost ,breathing resumes.In cases when a person has stopped breathing,it is essential to restart ventilation before permanent brain damage occurs due to lack of oxygen.Mouth to mouth resuscitation increases the carbon dioxide concentration in the patient’s blood and hence stimulates their medulla to start the breathing process.Hospitals have resuscitators which also serve the same purpose.