Class: B.Sc. HONOURS ZOOLOGY Paper: Animal Physiology: LSS (Theory Class) Teacher’s name: Meenakshi Rana Date: 25th April, 2020 Time: 12:30 – 2:30 PM

Regulation of the Respiratory Centers cont….

2. Regulation of

Certain chemical stimuli modulate how quickly and how deeply we breathe. The functions to maintain proper levels of CO2 and O2 and is very responsive to changes in the levels of these gases in body fluids. The respiratory centers are influenced by stimuli received from sensory neurons that are responsive to chemicals, called .

A. (nerves of the central nervous system), located in or near the , monitor the chemistry of cerebrospinal fluid (CSF). They are sensitive to changes in the pH of CSF and participate in a negative feedback system . An increase in arterial

PCO2 causes a similar rise in PCO2 of CSF of brain as CO 2 diffuses down its gradient from the + blood vessels into the CSF. Increase in PCO2 causes a fall in pH. The fall in pH (increased H concentration) occurs because CO2 can combine with H2O to form H2CO3 (carbonic acid, weak – + acid), which dissociates into HCO 3 and H . In response to the low pH, the central chemoreceptors cause inspiratory area to become highly active, and the rate and depth of increase. Rapid and deep breathing, called hyperventilation, allows the + of more O2 and exhalation of more CO2 until PCO2 and H are lowered to normal.

Thus, the central chemoreceptor do not monitor CO2 itself, however, they are sensitive + to changes in CO2 induced by H concentration in the CSF that bathes them.

B. Peripheral chemoreceptors (nerves of the peripheral nervous system), located in aortic bodies in the wall of the aortic arch and in carotid bodies in the walls of the carotid arteries, monitor the chemistry of the blood. These receptors are especially important for detecting changes in O2 in the blood, although they also respond to a lesser extent to changes in CO2 and

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+ H concentrations. When O2 concentration in the arterial blood falls below normal, these receptors become strongly stimulated and the response is same as by central chemoreceptors. + An increase in either Co2 or H concentration also excites the chemoreceptors. However, the effects of both these factors in the central chemoreceptors itself are so much more powerful than their effects mediated through the peripheral chemoreceptors.

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C. Baroreceptors or stretch receptors located in the walls of bronchi and bronchioles are activated when the expand to their physical limit. When these receptors become stretched during overinflation of the lungs, nerve impulses are sent along the vagus (X) nerves to the inspiratory and apneustic areas. In response, the inspiratory area is inhibited directly, and the apneustic area is inhibited from activating the inspiratory area. As a result, exhalation begins. As air leaves the lungs during exhalation, the lungs deflate and the stretch receptors are no longer stimulated. Thus, the inspiratory and apneustic areas are no longer inhibited, and a new inhalation begins. This reflex, referred to as the inflation (Hering–Breuer) reflex, is mainly a protective mechanism for preventing excessive inflation of the lungs rather than a key component in the normal regulation of respiration.

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