What is the name of the hormone at D?

 

In response to inadequate oxygen transport, the kidneys and liver increase the synthesis and secretion of the glycoprotein hormone erythropoietin (EPO). This hormone is released into the systemic circulation to act as a chemical messenger. It serves as the primary stimulus for the acceleration of red blood cell production.

A. 6: Hemoglobin lacks the structural capacity to bind 6 oxygen molecules simultaneously. The protein is a tetramer, meaning it consists of only 4 polypeptide subunits. Each subunit is limited to the binding of a single heme group and its associated iron ion.

B. 2: This number represents only half of the potential carrying capacity of a fully saturated hemoglobin molecule. In the high partial pressure of oxygen found in pulmonary capillaries, hemoglobin typically binds more than two molecules. This would reflect a low oxygen saturation level of 50%.

C. 3: Binding 3 molecules would result in 75% oxygen saturation, which occurs as blood unloads oxygen to resting tissues. However, it does not represent the maximum theoretical or physiological limit of the transport protein. The molecular structure allows for one additional binding site to be filled.

D. 4: Each hemoglobin molecule is a tetramer composed of 4 globin chains, each containing a central heme group with a ferrous iron atom. Each iron atom can reversibly bind 1 molecule of O2. Therefore, a single hemoglobin molecule can carry a maximum of 4 oxygen molecules.

E. 5: Human hemoglobin does not possess a fifth binding site or heme group to accommodate an extra oxygen molecule. The quaternary structure is strictly limited to 4 subunits. Any value above 4 is biologically and chemically impossible for a standard adult hemoglobin A molecule.