What will happen if carbon monoxide combines with hemoglobin?

Our red blood cells (RBC's) are basically little bags of a substance called hemoglobin. In fact, RBC's are the only cells in our body without nuclei; they lose them when they mature so that more hemoglobin can be packed into the cell. Hemoglobin (heme) molecules are little oxygen carriers. When you breathe, oxygen from the air enters your lungs. Each molecule has four "seats" that can be filled with oxygen. They can pick up oxygen in your lungs and deliver it all over your body. Every cell needs a supply of oxygen.

Carbon monoxide (CO) is very dangerous because it sticks to your hemoglobin better than oxygen does. It "hogs the seats" so that oxygen can't get a ride. And those CO molecules keep riding around, never giving their seats up to the oxygen. This means there's no way to get oxygen to your brain, heart, or other cells and those cells start to die. The chemical reactions that stop happening when there's no oxygen are the ones that make ATP, the form of energy that all of our cells use. Then, parts of your body do not receive the essential oxygen, and effectively suffocate.

heme + O2 ↔ (hem-O₂)

(Reversible)

heme+ CO → (hem-CO)

(Irreversible)

We can't see, smell, or taste CO, so it's very dangerous. Things that burn fuel (furnaces, cars, barbeque grills, etc.) produce CO, and that's why they have to be ventilated to the outside.