Blood is a ‘watery body fluid’. In other words, it is water that contains a whole range of substances.
It is contained in a complex network called the vascular system and is pumped around the body by the heart.
Blood has two main functions:
- Provides defense against disease.
- It transports compounds, ions, and some elements to and from other tissues and cells.
Oxygen is one of the substances transported with the help of red blood cells. Red blood cells contain a pigment called hemoglobin , each molecule that binds to four oxygen molecules.
Oxygen molecules are transported to individual cells in body tissue where they are released. The binding of oxygen is a reversible reaction.
At high oxygen concentrations, oxyhemoglobin is formed, but at low oxygen concentrations, oxyhemoglobin dissociates into hemoglobin and oxygen.
At relatively low oxygen concentrations, there is uncombined hemoglobin in the blood and little or no oxyhemoglobin, for example, in body tissue.
At relatively high oxygen concentrations, there is little or no uncombined hemoglobin in the blood, it is in the form of oxyhemoglobin, such as in the lungs.
To transport carbon dioxide to the respiratory tissues of the lungs, hemoglobin can bind to carbon dioxide, but in smaller amounts.
The presence of carbon dioxide aids in the release of oxygen from hemoglobin, which is known as the Bohr effect.
When determining the amount of oxygen in the blood, the first important point to establish is the partial pressure. This term is easy to understand when used with a mixture of gases, such as that of an alveolus.
A number of substances bind to hemoglobin and alter the relative affinity of hemoglobin for oxygen. In particular, carbon dioxide, H + and 2,3-bisphosphoglycerate, bind to the protein portion of hemoglobin.
When this term is used with blood, it refers to the gas with which the blood is in equilibrium.
In a healthy person, there is a long time for the balance between the gas in an alveolus and the blood flowing through a pulmonary capillary.
Even at the peak of exercise, the blood spends enough time in a pulmonary capillary to reach equilibrium with the gas in the alveolus (except in exceptional athletes with powerful hearts).
Therefore, in healthy people, the partial pressures of oxygen and carbon dioxide in the systemic arterial blood are the same as in the alveoli.
This breaks down in the case of certain respiratory disorders, such as those that cause fluid to build up in the alveoli.
But while only partial pressures determine the direction a gas will diffuse, additional factors determine the amount of a gas in the blood.
Oxygen in the blood
Oxygen is found in two forms in the blood:
Oxygen in solution
The oxygen in the solution is easy to calculate because it is directly proportional to the partial pressure.
Therefore, to determine the amount of oxygen in the solution, just multiply the partial pressure by the solubility.
Since the partial pressure of oxygen at sea level in normal systemic arterial blood is 100 mm Hg, the amount in solution is equal to 3 ml O2 / liter of blood.
Obviously, hemoglobin accounts for almost all the oxygen in the blood. This fact has many physiological ramifications.
Oxygen in the hemoglobin molecule
To determine the amount of oxygen bound to hemoglobin, it is first necessary to determine the percent saturation of hemoglobin.
This is done using an oxygen dissociation curve for hemoglobin. This curve is determined experimentally by balancing hemoglobin with various partial pressures of oxygen.
The main function of hemoglobin, the supply of oxygen from the lungs to the cells of the tissues, depends on the variable affinity that hemoglobin has for oxygen.
This affinity depends mainly on the local partial pressure of oxygen, but the pH, the partial pressure of carbon dioxide and the concentration of organic phosphates are also significant.
Local conditions in the lungs such as relatively high local oxygen partial pressure, low carbon dioxide partial pressure among other factors are associated with high affinity, so hemoglobin easily binds oxygen here; The product of this union is oxyhemoglobin.
On the contrary, in the microvasculature of the tissues, local conditions such as: relatively low local partial pressure of oxygen, high partial pressure of carbon dioxide among other factors, are associated with low affinity of hemoglobin for oxygen and oxyhemoglobin dissociates easily, releasing oxygen to tissue cells.
Pulse oximetry has become the standard of care in operating rooms, intensive care units.
Before pulse oximetry was available, doctors relied on invasive procedures, such as arterial puncture for blood gas analysis, to identify the presence of hypoxemia.
Unlike arterial blood gas analysis, pulse oximetry enables non-invasive and continuous monitoring of arterial blood oxygen saturation.