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Describe how the mechanism of ventilation is controlled. Describe the roles of the phrenic nerves and medulla in generating a basic breathing rhythm.

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Introduction

Describe how the mechanism of ventilation is controlled.Describe the roles of the phrenic nerves and medulla in generating a basic breathing rhythm. Introduction The tidal flow of air into and out of the lungs is caused by differences in pressure between the thorax and the atmosphere.Air always moves from an area of higher pressure to an area of lower pressure.A ventilation mechanism ensures that a constant supply of fresh air is available to provide enough oxygen for gas exchange and to remove carbon dioxide. There are two processes of breathing- inhalation and exhalation Inhalation Air moves from high to low pressure.Hence the pressure in the lungs is low.The diaphragm muscle contracts and flattens.The external intercostals muscles contract and pull the rib cage out and up.The volume in the lung increases.This reduces the pressure in the lungs and air moves from the atmosphere(high pressure) to the lungs(low pressure) Process of breathing in Exhalation In this process the pressure in the lungs must be higher than the pressure in the atmosphere so that air move in an opposite direction to that in inspiration.The ...read more.

Middle

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. ...read more.

Conclusion

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. ?? ?? ?? ?? 1 ...read more.

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