All of these gases can be bound by hemoglobin. Hemoglobin transports oxygen from the lungs to the necessary tissue, and carbon dioxide from the tissue to the lungs. Hemoglobin has a much higher affinity for carbon monoxide than oxygen, which is why it is so dangerous to inhale carbon monoxide.

High levels of carbon dioxide (CO₂), low pH, and high temperatures all act to decrease oxygen's affinity toward human hemoglobin. Think of working muscle, which produces hot, acidic, high CO₂ conditions in the blood; in this environment, it is important for hemoglobin to release transported oxygen to provide an aerobic environment to the muscle.

A decrease in pH is generally caused by an increase in carbon dioxide in the blood, and will cause hemoglobin to have a lower affinity to oxygen. This makes sense, because we want to easily release oxygen to tissues with high levels of carbon dioxide, and quickly bind and remove the carbon dioxide from the cells to the lungs for expiration from the body.

Increased temperature will also decrease the affinity of hemoglobin for oxygen.

The oxygen dissociation curve is a graph of hemoglobin saturation versus oxygen partial pressure. As oxygen partial pressure increases, hemoglobin will generally be more saturated. The oxygen partial pressure will affect the saturation percentage of hemoglobin in the blood, but will not shift the curve itself. All other options will shift the oxygen dissociation curve to the right, lowering hemoglobin's affinity for oxygen.

Hemoglobin displays the unique property of cooperativity, meaning that once one molecule of oxygen has bound to the hemoglobin molecule, adding the three next molecules is easier. This gives the hemoglobin binding curve the typical sigmoidal oxygen dissociation curve.

Cooperativity helps hemoglobin bind four oxygen molecules in the lungs and maintain the bonds until it enters a region of very low oxygen partial pressure. The most energy is required to remove the first oxygen; the next three are easier because the cooperativity has been lessened. As hemoglobin travels, it is able to selectively distribute oxygen the areas of the body that need it most because of this property.

Hemoglobin's affinity for oxygen can vary based upon the environment. A right shift, which lowers hemoglobin's affinity for oxygen, occurs when there is a need for oxygen to be released to surrounding tissue. This occurs during exercise, increased temperatures, increased 2,3-bisphosphglycerate, increased carbon dioxide, and decreased pH.

Hyperventilation will increase oxygen and decrease carbon dioxide, which will effectively cause a left shift. Left shifts occur under circumstances opposite from the right shift. A decrease in temperature, 2,3-bisphosphoglycerate, or carbon dioxide will cause a left shift.

Several factors are capable for shifting the oxygen dissociation curve. A decrease in pH or an increase in carbon dioxide partial pressure indicates a need to remove carbon dioxide wastes from the system. Hemoglobin affinity for oxygen will decrease under these conditions in order to accommodate the carbon dioxide for removal. Decreased carbon dioxide partial pressure will increase the affinity for oxygen rather than decrease it.

Increased temperature and increased 2,3-bisphosphoglycerate will also decrease the hemoglobin affinity for oxygen.

A decrease in binding affinity shifts the oxygen dissociation curve to the right, while an increase shifts the curve to the left.

The basic idea of cooperativity is that oxygen will bind with lower affinity once an oxygen atom is removed. Once you remove the first oxygen atom, the remaining ones are more likely to come off to supply tissue. This change is instigated by conformational changes in hemoglobin structure when an oxygen is removed.

Most adaptations of the fetal circulatory system are designed to bypass the lungs and liver, which develop especially slowly. The ductus venosus shuttles blood directly from the umbilical vein to the inferior vena cava, avoiding the liver entirely. The foramen ovale is a right-to-left shunt between the atria that sends blood away from the right ventricle; the ductus arteriosus shunts the remaining blood from the right ventricle to the aorta to bypass the lungs. Finally, fetal hemoglobin has a high affinity for oxygen because it must compete with maternal blood in the placenta.

Fetal myoglobin does not have a low affinity for oxygen.

Carbon dioxide (CO₂) is nonpolar, and thus can dissolve well only in nonpolar solvents. Since blood is an aqueous (and polar) solvent, CO₂ needs to be converted to a polar form via blood enzymes to allow it to be dissolved directly in water.