Carbon monoxide slows down the blood's capacity to carry oxygen to body tissues including vital organs like the heart and brain. When inhaled, it combines with the oxygen carrying haemoglobin of the blood to form carboxyhemoglobin (COHb) causing that haemoglobin to be no longer available for transporting oxygen.
1 Additional Answer
Carbon Monoxide goes into the blood stream once breathed in and mixes with haemoglobin. The blood becomes unable to carry oxygen. The lack of oxygen causes the bodys tissue to die leading to death in a few hours.