Regulation of Respiration

Prof. K. Sivapalan

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Introduction

• Respiration is altered by several factors.

• The usage of oxygen and production of carbon dioxide and acids vary depending on the metabolic needs.

• There is a good reserve of respiratory function- rate, depth and respiratory epithelium.

• The respiratory system is controlled to meet the needs.

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Median Sagital Section of Brain

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• All respiratory muscles are striated muscles under voluntary control.

• The nerves driving the respiratory muscles arise from the ventral horn of the spinal cord.

• The anterior horn cell is the final pathway.

• The voluntary efforts are controlled by cerebral cortex which activates the AHC.

• Autonomic control is effected by centers in the brain stem which also influences the AHC

Control of Respiratory Muscles

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Respiratory Centre

• Respiratory center is located bilaterally in Medulla and Pons.

• The dorsal group of cells are mainly expiratory and the ventral cells are bothe expiratory and inspiratory.

• Cells in pons inhibit medullary inspiratory cells.

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Rhythmic Activity of the Centers

• The inspiratory center, left alone on its own, discharges for about 2 seconds which results in inspiration.

• It then stops for about 3 seconds during which the recoil of the lungs results in expiration.

• The influence from the pons, pneumotaxic center, can reduce the inspiration to 0.5 second and absence can result in arrest in inspiration

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The Hering-BreuerInflation Reflex

• Afferents in the vagus inhibit inflation

• The afferents originate from the stretch receptors in the bronchi and broncheols.

• In human, they seem to be activated when the inflation is 1500 ml.

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Chemical Control of Respiration

• Respiration is modified changes in partial pressures of oxygen and carbon dioxide and pH of the blood.

• The receptors responsible are collectively called chemo receptors.

• The receptors located in Medulla are central chemo receptors and those in aorta and carotid arteries are peripheral chemo receptors

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Central Chemo-receptors• Cells in ventral Medulla respond

to changes in Hydrogen ions in CSF which is proportional to PCO2.

• Blood pH has no effect as H+ is not permeable through Blood brain barrier.

• The central chemo receptors activate respiratory centre when CO2 increases and inhibit when it decreases.

• The effect is potent immediately but after one day the HCO3

- diffuses into CSF and reduces the effect. [Renal compensation]

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Peripheral Chemo-receptors

• Most receptors- Carotid bodies near the bifurcation – glossopharingeal nerve.

• Some receptors in the arch of the aorta- vagus nerve

• A few receptors in abdominal and thoracic vessels.

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• Each body receives its own blood supply through an artery directly from the arterial trunk.

• The blood flow in each 2-mg carotid body is about 0.04 mL/min, or 2000 mL/100 g of tissue/min compared with a blood flow 54 mL or 420 mL per 100 g/min in the brain and kidneys.

• The blood flow is well above the need of the bodies: exposed to arterial blood.

• Type I cells have catecholamines which are released to stimulate the afferents.

• They increase discharge in response to reduction in PO2, increase in PCO2 and reduction of Hydrogen ion.

Receptors

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• There is normal discharge at the PO2 of 100 mm Hg.

• It decreases as partial pressure increases and increases when partial pressure decreases.

• The maximal change is between PO2 of 60-30 mmHg. The response is not affected by duration of hypoxia.

• Denervation of carotid bodies results abolishes response to changes in – Oxygen tension- totally

– pH- great extent

– Carbon dioxide by 30%

• Central for carbondioxide and peripheral for oxygen and hydrogen.

Response to Oxygen

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Factors that Affect Respiration

• Inherant activity of respiratory center and reflex inhibition by stretch.

• Changes in oxygen, carbon dioxide and hydrogen.

• Voluntary control of respiration, higher centers- pain and emotion.

• Motor cortical activity, proprioceptors: in exercise

• Deglutition center, vomiting centre , straining, hiccup, yawning.

• Changes in REM sleep, sleep apnoea.

• Baro-receptor – mild inhibition.

• Irritants in respiratory passage- cough and sneeze.

• Cerebral oedema- compression of vessels

• Anesthetics and narcotics.2013