 Dear students, in this topic we shall discuss the respiratory responses to hypercapnia. Hypercapnia is a condition of abnormally elevated levels of carbon dioxide in the blood. Animals respond to hypercapnia by increasing the rate of ventilation. This response happens almost immediately after increasing carbon dioxide level and is maintained for long periods till the compensatory mechanisms take control. Dear students, several receptors mediate response to hypercapnia. These receptors send messages to the medullary respiratory center. These receptors include chemoreceptors of aortic and carotid bodies and mechanoreceptors which are found in the lungs. The increase in the concentration of carbon dioxide in blood means that it will result in fall of pH level of the blood. The changes in pH level of blood are responded by the central hydrogen ion receptors. These receptors also increase in the rate of ventilation. Now, the increase in the pH level of the body does not last long. Therefore, the compensatory mechanisms start at this level. If the ventilation rate is controlled at the pH level, the carotid level becomes normal. In such a way, the concentration of bicarbonates and their levels are increased by increasing. When the bicarbonate level is increased in blood and CSF, the pH falls and the breathing rate reaches the normal level. The increased bicarbonates and the pH level are taken at the normal level even if the carbon dioxide levels are still maintained at a higher level. The correction of cerebrospinal fluid pH is very important in the return of ventilation rate to normal.