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Humans breathe oxygen, which is distributed from the lungs through the bloodstream to tissues in the body. This oxygen is carried principally by hemoglobin, a protein that is a primary component of red blood cells. Oxyhemoglobin is the form of hemoglobin that carries oxygen. It also causes the blood to be bright red.
When the oxyhemoglobin reaches the tissues of the body, the oxygen is released into the cells. The then depleted hemoglobin is known as deoxyhemoglobin, and causes the blood to appear purple. The blood returns to the lung, where the carbon dioxide diffuses out, to be exhaled. Oxygen then diffuses into the blood and binds to hemoglobin. Oxyhemoglobin can also be produced by cellular respiration.
Whether or not hemoglobin binds oxygen depends on the partial pressure of the gases. For instance, in the aveoli of the lungs — the areas where gas is exchanged — most of the gas is composed of oxygen. Hemoglobin thus binds oxygen and is in the oxyhemoglobin form. Alternatively, in the tissues, most of the gas is comprised of carbon dioxide. The oxyhemoglobin cannot hold on to its oxygen under these conditions, and the oxygen is released into the tissues.
The name 'hemoglobin' comes from heme and globin. The oxyhemoglobin structure involves four chains of polypeptides, chains of amino acids shorter than proteins, each folded into a globular structure called a globin. Each of these globins contains a heme group within it. This is a planar nitrogen structure containing iron that reversibly binds oxygen. Oxyhemoglobin can have up to four oxygen molecules bound to it, with one molecule binding per heme.
The hemoglobin of unborn babies has a much higher affinity for oxygen than that of the mother. Known as fetal hemoglobin, this protein obtains oxygen from the mother’s blood. After the baby is born, it gradually acquires the adult type of hemoglobin.
Each red blood cell has about 280 million molecules of hemoglobin. Thus, blood can transport a tremendous amount of oxygen. Other compounds can also bind to hemoglobin. Carbon monoxide binds more tightly than oxygen. One can be exposed to this compound from automobile exhaust and cigarette smoke. Cyanide is another compound that can bind hemoglobin.
Given the importance of hemoglobin in physiology, errors in its production can have dire consequences for people. A change in one amino acid in the hemoglobin molecule causes the red blood cells to develop in the shape of sickle in some circumstances, resulting in a severe disease known as sickle cell anemia. This disease is recessive, meaning that a person has to inherit a defective gene from both parents in order to develop this disorder.