Hemoglobin

Hemoglobin, iron-protein compound in red blood cells that gives blood its red color and transports oxygen, carbon dioxide, and nitric oxide. Hemoglobin is present in all but the least complex of animals. It carries oxygen from the lungs or gills, where blood is oxygenated, to body cells. When saturated with oxygen, hemoglobin is called oxyhemoglobin. After releasing oxygen to the body tissues, hemoglobin reverses its function and picks up carbon dioxide, the waste product of cellular respiration, for transport to the lungs, where it is expired. When saturated with carbon dioxide, hemoglobin is known as carboxyhemoglobin.

In 1996 scientists discovered that, in addition to oxygen and carbon dioxide, hemoglobin takes up and releases a third gas, nitric oxide. Nitric oxide plays an important role in regulating blood pressure by relaxing the blood vessel walls, thus increasing blood flow. Hemoglobin controls the expansion and contraction of blood vessels, and thus blood pressure, by regulating the amount of nitric oxide to which the vessels are exposed.

The hemoglobin molecule is a combination of two molecules (heme and globin) that play other roles in the body. Research published in 2007 indicates that by itself the iron-containing protein heme functions as a hormone. Inside cells, heme molecules bind with special receptor proteins. The heme-protein combination works in the nucleus of the cell to control specific genes coded in DNA that help regulate the sleep-wake cycle, weight gain, how food is metabolized, and how fat is stored. Understanding the role of heme could lead to better treatments for obesity, diabetes, and cancer.

Globins are a family of complex proteins that carry oxygen. Heme and globin are also found in myoglobin, a protein that stores oxygen in muscle tissues, and in neuroglobin, a protein that occurs in brain and nerve tissue. Cytoglobin is a related protein found in neurons and connective tissue.

Hemoglobin is contained entirely in the red blood cells, amounting to perhaps 35 percent of their weight. To combine properly with oxygen, red blood cells must contain adequate hemoglobin. Hemoglobin, in turn, is dependent on iron for its formation. A deficiency of hemoglobin caused by a lack of iron in the body leads to anemia.

Hemoglobin carries more than 20 times its volume of oxygen. Some chemicals, such as carbon monoxide, combine so firmly with hemoglobin that it can no longer combine with oxygen and asphyxiation results.

After a life of perhaps 120 days, red blood cells are destroyed in the spleen, or in the course of circulation, their hemoglobin is broken into its constituents, including iron, which enters new blood cells formed in the bone marrow.

When blood vessels rupture, as in an injury, the red cells are released and escape into tissue, where they are broken down. The hemoglobin is converted into bile pigments, the color of which is responsible for the appearance of bruises.

Alterations in the structure of hemoglobin can lead to life-threatening illnesses. The most important of these conditions is sickle-cell anemia, which involves a hereditary change in one of the amino acids that make up hemoglobin. The thalassemias are a group of hereditary diseases of similar origin.

Increased numbers of hemoglobin-containing red blood cells can occur as a physiological adaptation to life at high altitudes where oxygen levels are lower. Some Native American peoples who live in the Andes mountains produce higher levels of hemoglobin in their blood than do lowland populations. Athletes may train at high altitudes to raise the hemoglobin levels in their blood as a way to improve performance at lower altitudes with extra oxygen in their bodies. This effect can also be achieved by blood transfusions of extra red blood cells, a practice known as blood-doping that is widely banned for athletic competition. Some drugs can also raise the number of red blood cells. See also Performance-Enhancing Drugs.

Studies of marine mammals such as seals, whales, and dolphins indicate that higher concentrations of hemoglobin in their brains, along with raised levels of neuroglobin and cytoglobin, allow them to make long dives despite low oxygen levels in their blood. Some animals have evolved forms of hemoglobin that are better at taking up and releasing oxygen than the hemoglobin in human red blood cells. Crocodiles are able to remain underwater for up to an hour by releasing oxygen from their hemoglobin more efficiently than other animals do. Emperor penguins also appear to have extremely efficient hemoglobin that allows them to make long, deep dives. Some birds that migrate at extremely high altitudes have hemoglobin that is better at binding oxygen.