The uptake of Nitrous Oxide (N2O), Carbon Monoxide (CO), and O2 in blood is influenced by their partial pressures and the time red blood cells spend in the capillary. For gases that are perfusion-limited (N2O and O2), their partial pressures equalize with alveolar pressure before leaving the capillary. On the other hand, the partial pressure of CO, which is diffusion-limited, does not reach equilibrium with alveolar pressure. In rare conditions, O2 uptake can be diffusion-limited.

Diffusion limitation for O2 and CO2 may occur when red blood cells spend less than 0.25 seconds in the capillary bed. This can be observed in highly fit athletes during intense exercise and in healthy individuals exercising at high altitudes.

Oxygen is transported in blood through two forms: dissolved O2 and O2 bound to Hgb (hemoglobin). The dissolved form is clinically measured as the partial pressure of arterial oxygen (PaO2) in an arterial blood gas sample. Under normal conditions, the contribution of dissolved O2 to O2 transport is minimal due to its low percentage. However, in cases of severe hypoxemia, dissolved O2 becomes significant. The primary transport mechanism of O2 is its binding to Hgb, forming oxyhemoglobin within red blood cells. When O2 is not bound to Hgb, it is referred to as deoxyhemoglobin or reduced Hgb. The ability of blood to carry O2 is significantly enhanced by approximately 65 times through its binding to Hgb.

Hgb serves as the major transport molecule for O2. It is a protein consisting of two major components: four nonprotein heme groups, each containing iron in the reduced ferric (Fe+++) form, which is where O2 binds, and a globin portion comprising four polypeptide chains. Normal adults possess HgbA, which consists of two α-globin chains and two β-globin chains, while infants younger than 6 months primarily have fetal Hgb (HgbF), consisting of two α chains and two γ chains. The structural difference in HgbF increases its affinity for O2 and aids in O2 transport across the placenta. Furthermore, HgbF is not inhibited by 2,3-diphosphoglycerate (2,3DPG), a glycolysis product, further enhancing O2 uptake.

The binding of O2 to Hgb affects Hgb's light absorption capabilities. This effect is responsible for the color change observed between oxygenated arterial blood (bright red) and deoxygenated venous blood (dark red-bluish). The binding and dissociation of O2 with Hgb occur rapidly in milliseconds, facilitating O2 transport as red blood cells spend a short duration of 0.75 seconds in the capillaries. With approximately 280 million Hgb molecules per red blood cell, O2 transport is efficiently facilitated. Myoglobin, a protein similar in structure and function to Hgb, consists of only one subunit of the Hgb molecule. It assists in the transfer of O2 from blood to muscle cells and plays a critical role in O2 storage, particularly in conditions of oxygen deprivation.
End of 0 .25 .50 .75 Time in capillary (sec) •Fig. 24.3 Uptake of Nitrous Oxide (N2O), Carbon Monoxide (CO), and O2in Blood in Relation to Their Partial Pressures and the Transit Time of the Red Blood Cell in the Capillary. Forgasesthatareperfusionlimited(N2OandO2),theirpartialpressureshaveequilibratedwithalveolarpressurebeforeexitingthecapillary.Incontrast,thepartialpressureofCO,agasthatisdiffusionlimited,doesnotreachequilibriumwithalveolarpressure.Inrareconditions,O2uptakecanbecomediffusionlimited.
Diffusion limitation for O2 and CO2 would occur if red blood cells spent less than 0.25 seconds in the capillary bed. This is occasionally the case in very fit athletes during vigorous exercise and in healthy subjects who exercise at high altitude.
Oxygen is carried in blood in two forms: dissolved O2 and O2 bound to Hgb. The dissolved form is measured clinically in an arterial blood gas sample as the partial pressure of arterial oxygen (PaO2). Only a small percentage of O2 in blood is in the dissolved form, and its contribution to O2 transport under normal conditions is almost negligible. However, dissolved O2 can become a significant factor in conditions of severe hypoxemia. Binding of O2 to Hgb to form oxyhemoglobin within red blood cells is the primary transport mechanism of O2. Hgb not bound to O2 is referred to as deoxyhemoglobin or reduced Hgb. The O2-carrying capacity of blood is enhanced about 65 times by its ability to bind to Hgb.
Hgb is the major transport molecule for O2. The Hgb molecule is a protein with two major components: four nonprotein heme groups, each containing iron in the reduced ferric (Fe+++) form, which is the site of O2 binding, and a globin portion consisting of four polypeptide chains. Normal adults have two α-globin chains and two β-globin chains (HgbA), whereas children younger than 6 months of age have predominantly fetal Hgb (HgbF), which consists of two α chains and two γ chains. This difference in the structure of HgbF increases its affinity for O2 and aids in the transport of O2 across the placenta. In addition, HgbF is not inhibited by 2,3-diphosphoglycerate (2,3DPG), a product of glycolysis; thus O2 uptake is further enhanced.
Binding of O2 to Hgb alters the ability of Hgb to absorb light. This effect of O2 on Hgb is responsible for the change in color between oxygenated arterial blood (bright red) and deoxygenated venous blood (dark red-bluish). Binding and dissociation of O2 with Hgb occur in milliseconds, thus facilitating O2 transport because red blood cells spend only 0.75 seconds in the capillaries. There are approximately 280 million Hgb molecules per red blood cell, which provides an efficient mechanism to transport O2. Myoglobin, a protein similar in structure and function to Hgb, has only one subunit of the Hgb molecule. It aids in the transfer of O2 from blood to muscle cells and in the storage of O2, which is especially critical in O2-deprived conditions.