Carbon dioxide is the waste product of the respiratory system. It is a colorless gas formed in the tissues during metabolism and carried in the blood to lungs where it is exhaled. An increase in the concentration of carbon dioxide in the blood stimulates respiration. Bohr’s effect – it states that if the alveolar CO2 levels are low than the oxygen molecules will not dissociate from the haemoglobin molecules to the optimal level. Thus it is responsible for transport of oxygen to tissues. Carbon dioxide regulates the pH levels of the blood. It is a dilator of the smooth muscles. Smooth muscles surround the hollow spaces of the body for e.g. bronchial tubes, arteries, bowel etc.