Hemoglobin, the essential component of red blood cells (erythrocytes), transports oxygen (
) through the bloodstream from the lungs to all the tissues of the body. Hemoglobin also carries carbon dioxide (
) back to the lungs to complete the process of respiration. Vertebrate hemoglobin is a nearly spherical protein molecule consisting of an array of four globin polypeptide chains, each containing a heme group, which is the oxygen-binding site. Its molecular weight is approximately 64,500 daltons. The oxygen uptake of hemoglobin exhibits cooperativity, such that each
successively increases the affinity of the molecule for adding another
, up to four. The saturation
is a measure of the fractional occupancy of the oxygen-binding sites. It increases as a sigmoid-shaped function of the partial pressure of
in its immediate environment. In the alveoli of the lungs,
is approximately 100 torr.
Myoglobin, which is located in muscle cells, serves as a reserve supply of oxygen for muscle functioning. This molecule consists of a single globin unit (MW ≈ 16,700 Da) with just one oxygen binding site. In contrast to hemoglobin, myoglobin will absorb or release an
molecule at a much lower partial pressure. The skeletal muscles of aquatic mammals, such as whales and dolphins, are particularly rich in myoglobin, which allows them to be submerged for long periods of time.
A phenomenon known as the Bohr effect was discovered in 1904 by Danish physiologist Christian Bohr (father of physicist Niels Bohr). This constitutes a reduction in the oxygen affinity of hemoglobin as blood acidity increases, with pH decreasing from its normal value of 7.4, usually as a consequence of an increase of
concentration in the blood. Myoglobin does not exhibit the Bohr effect.
This Demonstration plots the dependence of oxygen saturation
and pH. The average number of bound
molecules is shown in the top illustrations. These are highly schematic, with the
molecules being greatly magnified.