Importance of hemoglobin
Oxygen is transported in the blood through two distinct mechanisms: its dissolution in the plasma and its binding to the hemoglobin contained in red blood cells or erythrocytes.
Since oxygen is scarcely soluble in aqueous solutions, the survival of the human organism depends on the presence of adequate quantities of hemoglobin. In fact, in a healthy individual more than 98% of the oxygen present in a given volume of blood is bound to hemoglobin and transported by erythrocytes.
Link between hemoglobin and oxygen
The binding of oxygen to hemoglobin is reversible and dependent on the partial pressure of this gas (PO2): in the pulmonary capillaries, where plasma PO2 increases due to the diffusion of oxygen from the alveoli, hemoglobin binds to oxygen; in the periphery, where oxygen is used in cellular metabolism and plasma PO2 drops, hemoglobin transfers oxygen to the tissues.
But what is PO2?
Partial Oxygen Pressure
The partial pressure of a gas such as oxygen, inside a limited space (lungs) containing a mixture of gases (atmospheric air), is defined as the pressure that this gas would have if it occupied alone the space under consideration.
To simplify the concept, let's imagine partial pressure as the amount of oxygen: the higher the partial pressure of oxygen, the higher its concentration. This is a very important aspect if we consider that a gas tends to diffuse from a point with a higher concentration (higher partial pressure) to a point with a lower concentration (lower partial pressure).
This law governs the exchange of gases in the lungs and tissues.
In fact, at the pulmonary level, where the air of the alveoli is in close contact with the very thin walls of the blood capillaries, the oxygen molecules pass into the blood because the partial pressure of oxygen in the alveolar air is higher than the PO2 of the blood.
Data on the hand, the PO2 of the venous blood that reaches the pomone in rest conditions is approximately equal to 40mmHg, while at sea level the alveolar PO2 is equal to approximately 100 mmHg; consequently the oxygen diffuses according to its own concentration gradient (partial pressure) from the alveoli towards the capillaries. Conceptually, the passage will stop when the PO2 in the arterial blood leaving the lungs has equaled the atmospheric one in the alveoli (100 mmHg ).
As arterial blood reaches the tissue capillaries, the concentration gradient reverses.In fact, in a cell at rest the intracellular PO2 is on average 40mmHg; Since, as we have seen, the blood at the arterial end of the capillary has a PO2 of 100 mmHg, oxygen diffuses from the plasma to the cells. Diffusion stops when the venous capillary blood reaches the same partial pressure of oxygen as the blood. intracellular environment, ie 40 mmHg (in resting conditions). During physical exertion the concentration of oxygen in the cellular environment decreases and with it the partial pressure of the gas (even up to 20 mmHg); consequently the release of oxygen from the plasma occurs more rapidly and consistently.
As we have seen, the adequate intake of oxygen by the blood flowing in the pulmonary capillaries strictly depends on the partial pressure of the air packed into the alveolar sacs; we have also seen how here the alveolar PO2 is normally (at sea level) equal to 100 mmHg; if this value is excessively reduced, the diffusion of oxygen from the air to the blood is insufficient and a dangerous condition known as hypoxia arises.
Hypoxia: Little Oxygen in the Blood
The partial pressure of the alveolar air can drop at high altitudes (because the atmospheric pressure is reduced) or when pulmonary ventilation is inadequate (as happens in the presence of lung diseases, such as chronic obstructive bronchitis, asthma, fibrotic lung diseases , pulmonary edema and emphysema).
The same situation arises when the wall of the alveoli thickens or the area of their surface is reduced. The speed of diffusion of oxygen from the air to the blood is in fact directly proportional to the area of the alveolar surface available and inversely proportional to the thickness. of the alveolar membrane.
Emphysema, a degenerative lung disease mainly caused by cigarette smoke, destroys the alveoli reducing the surface area available for gas exchange; in pulmonary fibrosis, on the other hand, the deposition of scar tissue increases the thickness of the alveolar membrane. In both cases, the diffusion of oxygen through the alveolar walls is much slower than normal.
Hypoxia can also result from a reduced concentration of hemoglobin in the arterial blood. Pathologies that decrease the amount of hemoglobin in red blood cells or their number negatively affect the ability of the blood to carry oxygen. In extreme cases, such as in subjects who have lost significant amounts of blood, the hemoglobin concentration may be insufficient to meet the oxygen requirements of the cells; in these cases, the only solution to save the patient's life is blood transfusion.
Hemoglobin Dissociation Curve
The physical relationship between plasma PO2 and the amount of oxygen linked to hemoglobin has been studied in vitro and is represented by the characteristic hemoglobin dissociation curve.
Observing the curve shown in the figure, it can be seen that at a PO2 equal to 100 mmHg (value normally recorded in the alveolar area) 98% of hemoglobin is bound to oxygen.
Note that at values higher than 100 mmHg the percentage of hemoglobin saturation does not increase further, as evidenced by the flattening of the curve; for the same reason, as long as the alveolar PO2 remains above 60 mmHg, the hemoglobin is saturated for more than 90%, therefore it maintains an almost normal capacity to transport oxygen in the blood. To learn more, see the article dedicated to hemoglobin and the Bohr effect.
All the factors listed in the article can be evaluated through simple blood tests, such as red blood cell count, hemoglobin dosage and blood oxygen saturation (percentage of oxygen saturated hemoglobin compared to the total amount of hemoglobin present in the blood).
