As blood passes from the systemic arteries into the capillaries,
it is exposed to lower PO2; oxygen is released from Hb, leaving the
blood and entering the tissue. The PO2 in the systemic capillaries
varies from tissue to tissue. The oxyhemoglobin dissociation curve
is very steep between PO2 = 60-20 mm Hg. Therefore, within this
steep range, a small decrease in PO2 in the tissues will result in
great unloading of oxygen to the tissues.