 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 ventilation rate. Now, the addition of pH to the body does not tolerate this much. Therefore, compensatory mechanisms start at this level. The ventilation rate of pH level is controlled at the end of the car normal level. In this way, the concentration of bicarbonates in the blood and their levels are increased. When the bicarbonate level is increased in the blood and CSF, the pH falls and the breathing rate is reached at the normal level. The increased bicarbonate and pH level is reached 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.