How much oxygen is present in blood?
Oxygen saturation (medicine)
Oxygen saturation is the fraction of oxygen-saturated hemoglobin relative to total hemoglobin (unsaturated + saturated) in the blood. The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal blood oxygen levels in humans are considered 95–100 percent. If the level is below 90 percent, it is considered low resulting in hypoxemia. Blood oxygen levels below 80 percent may compromise organ function, such as the brain and heart, and should be promptly addressed. Continued low oxygen levels may lead to respiratory or cardiac arrest. Oxygen therapy may be used to assist in raising blood oxygen levels. Oxygenation occurs when oxygen molecules () enter the tissues of the body. For example, blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygenation is commonly used to refer to medical oxygen saturation.
Oxygen–hemoglobin dissociation curve
Carbon dioxide affects the curve in two ways. First, CO accumulation causes carbamino compounds to be generated through chemical interactions, which bind to hemoglobin forming carbaminohemoglobin . CO is considered an Allosteric regulation as the inhibition happens not at the binding site of hemoglobin. Second, it influences intracellular pH due to formation of bicarbonate ion. Formation of carbaminohemoglobin stabilizes T state hemoglobin by formation of ion pairs. Only about 5–10% of the total CO content of blood is transported as carbamino compounds, whereas (80–90%) is transported as bicarbonate ions and a small amount is dissolved in the plasma.The formation of a bicarbonate ion will release a proton into the plasma, increasing acidity which also shifts the curve to the right as discussed above; low CO levels in the blood stream results in a high pH, and thus provides more optimal binding conditions for hemoglobin and O. This is a physiologically favored mechanism, since hemoglobin will drop off more oxygen as the concentration of carbon dioxide increases dramatically where tissue respiration is happening rapidly and oxygen is in need. 
2,3-Bisphosphoglycerate or 2,3-BPG (formerly named 2,3-diphosphoglycerate or 2,3-DPG) is an organophosphate formed in erythrocytes during glycolysis and is the conjugate base of 2,3-bisphosphoglyceric acid. The production of 2,3-BPG is likely an important adaptive mechanism, because the production increases for several conditions in the presence of diminished peripheral tissue O availability, such as hypoxaemia, chronic lung disease, anaemia, and congestive heart failure, among others. High levels of 2,3-BPG shift the curve to the right (as in childhood), while low levels of 2,3-BPG cause a leftward shift, seen in states such as septic shock, and hypophosphataemia. In the absence of 2,3-BPG, hemoglobin's affinity for oxygen increases. 2,3-BPG acts as a heteroallosteric effector of hemoglobin, lowering hemoglobin's affinity for oxygen by binding preferentially to deoxyhemoglobin. An increased concentration of BPG in red blood cells favours formation of the T, low-affinity state of hemoglobin and so the oxygen-binding curve will shift to the right.