Oxygen-hemoglobin disassociation curve

Oxygen–hemoglobin dissociation curve



Also called the oxyhemoglobin dissociation curve or oxygen dissociation curve.

A curve that plots the proportion of hemoglobin in its saturated form on the vertical axis against the prevailing oxygen tension on the horizontal axis.

The oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2).

It is determined by the hemoglobin affinity for oxygen.

It indicates how hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.

Hemoglobin (Hb) is the primary vehicle for transporting oxygen in the blood.

Oxygen is also carried dissolved in the blood’s plasma, but to a much lesser degree than in hemoglobin.

Oxygen bound to the hemoglobin in RBCs is released into the blood’s plasma and absorbed into the tissues.

Each hemoglobin molecule has the capacity to carry four oxygen molecules, and these molecules of oxygen bind to the iron of the heme group hemoglobin in a reversible way.

The binding of the first oxygen molecule induces a conformational change of hemoglobin that increases the affinity for the remaining three oxygen molecules.

The capacity of hemoglobin that is filled by oxygen at any time is called the oxygen saturation.

Expressed as a percentage, the oxygen saturation is the ratio of the amount of oxygen bound to the hemoglobin, to the oxygen-carrying capacity of the hemoglobin.

The oxygen-carrying capacity of hemoglobin is determined by the type of hemoglobin present in the blood.

The amount of oxygen bound to the hemoglobin at any is in large part, due to the partial pressure of oxygen to which the hemoglobin is exposed.

At the alveolar–capillary interface of the lung, the partial pressure of oxygen is typically high, and oxygen binds readily to hemoglobin that is present.

As the blood circulates around the body, the partial pressure of oxygen is less and hemoglobin releases the oxygen into the tissue because the hemoglobin cannot maintain its full bound capacity of oxygen in the presence of lower oxygen partial pressures.

Hemoglobin saturation curve is described as sigmoid, resulting from the interaction of bound oxygen molecules with incoming molecules.

The binding of the first oxygen molecule is facilitates the binding of the second, third and fourth.

There is an induced conformational change in the structure of the hemoglobin molecule by the binding of an oxygen molecule.

This conformational change allows hemoglobin’s affinity for oxygen to increase as successive molecules of oxygen bind.

The curve levels out as the hemoglobin becomes saturated with oxygen.

The standard dissociation curve is relatively flat at pressures above about 60 mmHg, meaning that the oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure.

To provide more oxygen to tissues would require blood transfusions to increase the hemoglobin concentration or supplemental oxygen that would increase the oxygen dissolved in plasma.

Oxygen is unloaded to peripheral tissue as the hemoglobin’s affinity diminishes.

The partial pressure of oxygen in the blood at which the hemoglobin is 50% saturated, typically about 26.6 mmHg for a healthy person, is known as the P50.

P50 is a conventional measure of hemoglobin affinity for oxygen.

The P50 changes in the presence of disease or other conditions that change the hemoglobin oxygen affinity and, consequently, shift the curve to the right or left.

An increased P50 indicates a rightward shift of the standard curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation, indicating a decreased affinity.

A lower P50 indicates a leftward shift and a higher affinity.

The plateau area of the oxyhemoglobin dissociation curve is the range that exists at the pulmonary capillaries.

The steep part of the oxyhemoglobin dissociation curve is the range that exists at the systemic capillaries.

A rightward shift in the hemoglobin oxygen disassociation curve indicates that the hemoglobin has a decreased affinity for oxygen.

With a rightward shift in the hemoglobin oxygen disassociation curve makes it more difficult for hemoglobin to bind to oxygen, but it makes it easier for the hemoglobin to release oxygen bound to it.

This rightward shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, such as during exercise, or hemorrhagic shock.

A right shift shows decreased affinity, as would appear with an increase in either body temperature, hydrogen ions, 2,3-bisphosphoglycerate (2,3-BPG) concentration or carbon dioxide concentration.

A leftward shift indicates that the hemoglobin has an increased affinity for oxygen so that hemoglobin binds oxygen more easily, but unloads it more reluctantly.

Left shift: has a higher O2 affinity

Right shift: has a lower O2 affinity.

Fetal hemoglobin has higher O2 affinity than adult hemoglobin; primarily due to much-reduced affinity to 2,3-bisphosphoglycerate .

Factors that move the oxygen dissociation curve to the right are those physiological states where tissues need more oxygen: exercise as muscles have a higher metabolic rate, need more oxygen, produce more carbon dioxide and lactic acid, and their temperature rises.

A decrease in pH shifts the standard curve to the right, while an increase shifts it to the left.

The Bohr effect leads to the T state of stabilized deoxyhemoglobin, with a lowered affinity of oxygen, than in the R state.

Carbon dioxide accumulation causes carbamino compounds to be generated and binding to hemoglobin forming carbaminohemoglobin, that stabilizes T state hemoglobin by formation of ion pairs.

Carbon dioxide accumulation influences intracellular pH due to formation of bicarbonate ion.

The formation of a bicarbonate ion decreases acidity, which also shifts the curve to the right.

Low CO2 levels in the blood stream results in a high pH, and thus provides more optimal binding conditions for hemoglobin and O2.

Hemoglobin drops 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.

2,3-BPG production increases for several conditions in the presence of diminished peripheral tissue O2 availability, such as hypoxemia, chronic lung disease, anemia, and congestive heart failure.

High levels of 2,3-BPG shift the curve to the right, 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.

Hemoglobin binds with carbon monoxide 200–250 times more readily than with oxygen.

Carbon monoxide is a highly successful competitor and displaces oxygen even at minuscule partial pressures.

HbCO + O2 almost irreversibly displaces the oxygen molecules forming carboxyhemoglobin

Carbon monoxide binding to the iron center of hemoglobin is much stronger than that of oxygen, and the binding site remains blocked for the remainder of the life cycle of that affected red blood cell.


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