For Pneumograph: Central chemoreceptors are primarily sensitive to changes in the pH in the blood, (resulting from changes in the levels of carbon dioxide) and they are located on the medulla oblongata near to the medullar respiratory groups of the respiratory center (1. Pneumotaxic center - various nuclei of the pons and 2. Apneustic center -nucleus of the pons). The peripheral chemoreceptors that detect changes in the levels of oxygen and carbon dioxide are located in the arterial aortic bodies and the carotid bodies. Information from the peripheral chemoreceptors is conveyed along nerves to the respiratory groups of the respiratory center. There are four respiratory groups, two in the medulla and two in the pons. From the respiratory center, the muscles of respiration, in particular the diaphragm, are activated to cause air to move in and out of the lungs. Hyperventilation: When a healthy person takes several deep and fast breaths (hyperventilation), PCO2 in the lungs and blood falls. As a result there is an increase in diffusion of CO2 to the alveolar air from dissolved state in blood. As the conc. of H2CO3 is in equilibrium with that of dissolved CO2 in blood, hyperventilation increases the [HCO3]/ [H2CO3] ratio by falling of CO2 conc. Thus there is a rise of blood pH (greater than pH 7.4). The breathing centre detects this change and stops or reduces stimulation and work of the respiratory muscles (called apnea). (Blood leaving the lungs is normally fully saturated with oxygen, so hyperventilation of normal air cannot increase the amount of oxygen available.) Breath-holding: The person naturally holds their breath until the CO2 level reaches the initially pre- set value. Breath holding accumulates more carbon dioxide and hence decrease in blood pH. One obvious hypothesis to explain the breakpoint is that once PaO2 (partial arterial pressure)falls below or PaCO2 rises above a certain threshold partial pressure, or rate of change of partial pressure reaches a threshold, then chemoreceptor of breathing centre senses this pH (lower than pH 7.4) and this stimulation causes an involuntary breath called breaking point. The breathing centre then intensifies breathing. This over breathing is going to continue until extra CO2 is removed and the pre-set value is reached again. Interpretation: A normal respiratory movement of chest was recorded followed by a period of breath holding. After the breaking point, the normal respiratory movements were taken. Then the subject was asked to hyperventilate for sometime which was followed by breath holding. It was observed that after voluntary hyperventilation the breath can be hold for a longer period of time than the time of the first breath hold without prior hyperventilation. This is because hyperventilation causes excess washout of CO2 (decrease PCO2 in lungs) and hence diffusion of CO2 from blood to alveoli. As a result blood pH increases which causes the respiratory centres to reduce or stop stimulation and work of the respiratory muscles. This cessation of breathing is added to the breath holding time when the later was done just after hyperventilation. .