Each hemoglobin molecule can carry up to **4 oxygen molecules**. Hemoglobin is a protein that is found in red blood cells. It is responsible for transporting oxygen from the lungs to the rest of the body. Each hemoglobin molecule has four heme groups, which are each capable of binding to one oxygen molecule. When hemoglobin binds to oxygen, it changes shape, which allows it to release the oxygen into the tissues.

The amount of oxygen that hemoglobin can carry depends on the concentration of oxygen in the blood. When the concentration of oxygen is high, hemoglobin is fully saturated with oxygen. When the concentration of oxygen is low, hemoglobin is only partially saturated with oxygen.

Hemoglobin is also important for transporting carbon dioxide from the tissues to the lungs. Carbon dioxide is a waste product that is produced by the cells during metabolism. Hemoglobin binds to carbon dioxide and carries it to the lungs, where it is exhaled.

Hemoglobin is a vital protein that plays a critical role in oxygen and carbon dioxide transport. It is essential for life and without it, humans would not be able to survive.