Hemoglobin is a protein molecule in red blood cells that binds oxygen to the red blood cell and enables the blood to deliver that oxygen to all the cells in the body. Hemoglobin accounts for up to 3% of teh dry weight of red blood cells. It can bind 4 oxygen molecules in the pulmonary artery. In the pulmonary vein, the oxygen has been delivered, and the hemoglobin picks up the carbon dioxide, which is a waste product created by the process of cellular respiration. Each hemoglobin protein can bind up to 4 carbon dioxide molecules, which will be delivered to the lungs and exhaled, which rids the body of the carbon dioxide. Blood is normally a dark red while contained within the enclosed circulatory system. When a person gets a cut, blood oozes from the capillaries and turns a bright ruby red, courtesy of the hemoglobin molecule binding with oxygen molecules in the air.
I would like to add a few thoughts. The heart can be divided into a left side and a right side. The right side of the heart has a very low oxygen content because the oxygen contained on the hemoglobin molecules has been delivered to the cells of the body. In human circulation all blood is returned to the right atrium of the heart after it has circulated throughout the body. The left side of the heart has a very high oxygen content because when blood leaves the right ventricle it travels to the lungs via the pulmonary arteries. In the lungs the blood picks up oxygen and gets rid of carbon dioxide. When the blood leaves the lungs it travels to the left atrium and then the left ventricle before passing this oxygen rich blood to the rest of the body.