Equilibrium plays a role in oxygen trasport through the human body

Humans need oxygen to function. Oxygen must be transported from outside the body to all the organs and tissues in the body. Oxygen is carried through the body by a protein called hemoglobin which binds oxygen in the lungs and carries it to the tissues. In the tissues, the hemoglobin releases the oxygen and returns to the lungs to repeat the cycle.

Hemoglobin contains four heme groups. Each heme group contains a central iron to which the oxygen is bound. Hence, each hemoglobin molecule is capable of carrying four oxygen molecules. A graphical representation of hemoglobin shows the overall structure of the molecule in a "cartoon" form (i.e. few bonds ar shown explicitly, alpha helices are shown as ribbon-like corkscrews). In this representation, the iron molecules are grey balls, and bonds between the non-hydrogen atoms of heme are drawn in red.

Graphical Representation of hemoglobin: Two complete hemoglobin molecules are shown in this picture. Try to identify the four heme groups and the four iron atoms in each. (One may be difficult to find)

A detailed structure for heme: The top structure shows all the bonds and atoms in the heme molecule. The bottom structure is a more common representation carbons represented by the junction of two bonds, hydrogens implied (not shown, it is assumed that each carbon has sufficient hydrgens attached to complete its valence) and hetero (non-carbon or hydrogen atoms) shown explicitly.

Whether each hemoglobin molecule carries its complete set of four oxygen atoms depends on the partial pressure of oxygen. In the air the partial pressure of oxygen is approximately 150 torr. Once inside the lungs, the air is mixes with water vapor and other gases (which remain from prior breaths) and the partial pressure is reduced to about 100 torr. The saturation of hemoglobin as a function of partial oxygen pressures is shown below:

In the lungs (partial pressure of oxygen=100 torr), hemoglobin is >95% saturated. Virtually all of the hemoglobin leaves the lungs carrying four oxygen molecules. At higher elevations, the partial pressure of oxygen is closer to 70 torr, reducing the saturation to about 90%. In order to compensate for the reduced pressure the body must synthesize more hemoglobin.

Le Châtlier's principle and hemoglobin saturation

Each oxygen molecule becomes bound to hemoglobin in a step-wise manner. Each addition can be represented by a reaction:

Using this set of equations, explain why hemoglobin binds oxygen in the lungs, and releases oxygen in the tissues. [Answer]

Explain why the synthesis of additional hemoglobin molecules (when the partial pressure of oxygen is lowered) will increase the amount of oxygen that is delivered to the tissues. [Answer]

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