Oxygen is essential for the cells. Carbon dioxide is a waste product produced in the cells and it must be expelled from cells and body.

Lungs serve to exchange these two gases with blood.

Exchange of Respiratory Gases in Lungs

In lungs, exchange of respiratory gases takes place between the alveoli and blood.

Exchange of gases occurs through bulk flow diffusion.

Exchange of respiratory gases takes place through respiratory membrane.

It is formed by epithelium of respiratory unit and endothelium of pulmonary capillary.

Epithelium of respiratory unit is a very thin layer.

Since capillaries are in close contact with this membrane, the alveolar air is in close proximity with capillary blood.

This facilitates the gaseous exchange between air and blood.

Layers of Respiratory Membrane

Different layers of respiratory membrane from inside out are:

Average thickness of the respiratory membrane is 0.6 μm.

Total surface area of the respiratory membrane in both lungs is about 70 sq m.

Average diameter of pulmonary capillary is only 8 μm, which means that the RBCs with a diameter of about 7.4 μm squeeze through the capillaries.

Therefore, the membrane of RBCs is in close contact with capillary wall.

This facilitates quick exchange of oxygen and carbon dioxide between the blood and alveoli.

Diffusing Capacity for Oxygen and Carbon Dioxide

Thus, diffusing capacity for carbon dioxide is about 20 times more than that of oxygen.

Factors Affecting Diffusing Capacity

1. Pressure gradient

   Diffusing capacity is directly proportional to the pressure gradient.

   Pressure gradient is the difference between partial pressure of a gas in alveoli and pulmonary capillary blood.

   It is the major factor which accelerates diffusing capacity.

2. Solubility of gas in fluid medium

   Diffusing capacity is directly proportional to solubility of the gas.

   If the solubility of a gas is more in the fluid medium, large number of molecules dissolve in it and diffuse easily.

3. Total surface area of respiratory membrane

   Diffusing capacity is directly proportional to surface area of respiratory membrane.

   Surface area of respiratory membrane in each lung is about 70 sq m.

   If the total surface area of respiratory membrane decreases, diffusing capacity for the gases is decreased.

4. Molecular weight of the gas

   Diffusing capacity is inversely proportional to molecular weight of the gas.

   If the molecular weight is more, the density is more and rate of diffusion is less.

5. Thickness of respiratory membrane

   Diffusing capacity is inversely proportional to the thickness of respiratory membrane.

   More the thickness of respiratory membrane less is the diffusion.

   It is because the distance through which the diffusion takes place is long.

Entrance of Oxygen from Atmospheric Air into Alveoli

Partial pressure of oxygen in the atmospheric air is 159 mm Hg and, in the alveoli, it is 104 mm Hg.

Because of the pressure gradient of 55 mm Hg, oxygen easily enters from atmospheric air into the alveoli.

Diffusion of Oxygen from Alveoli into Blood

When the blood is flowing through the pulmonary capillary, RBC is exposed to oxygen only for 0.75 sec at rest and only for 0.25 sec during severe exercise.

So, diffusion of oxygen must be quicker and effective.

Fortunately, this is possible because of pressure gradient.

Partial pressure of oxygen in pulmonary capillary is 40 mm Hg and in the alveoli, it is 104 mm Hg.

Thus, pressure gradient of 64 mm Hg facilitates diffusion of oxygen from alveoli into the blood.

Partial Pressure and Content of Oxygen and Carbon Dioxide

Diffusion of Oxygen from Blood into Tissues

Partial pressure of oxygen in arterial end of systemic capillary is 95 mm Hg.

Average oxygen tension in tissues is 40 mm Hg.

It is because of continuous metabolic activity and constant utilization of oxygen.

Thus, a pressure gradient of about 55 mm Hg exists between capillary blood and the tissues, so that oxygen can easily diffuse into the tissues.

Oxygen content in arterial blood is 19 mL%, and in the venous blood, it is 14 mL%.

Thus, the diffusion of oxygen from blood to tissues is 5 mL/100 mL of blood.

Diffusion of Carbon Dioxide from Tissues into Blood

Due to continuous metabolic activity, carbon dioxide is produced constantly in the cells of the tissues.

So, the partial pressure of carbon dioxide is high in the cells and is about 46 mm Hg.

Partial pressure of carbon dioxide in arterial blood is 40 mm Hg.

Pressure gradient of 6 mm Hg is responsible for diffusion of carbon dioxide from tissues to blood.

Carbon dioxide content in arterial blood is 48 mL%, and in the venous blood, it is 52 mL%.

So, diffusion of carbon dioxide from tissues to the blood is 4 mL/100 mL of blood.

Calculation

Normal Values of Respiratory Exchange Ratio

Normal value of respiratory exchange ratio depends upon the type of food substance that is metabolized.

However, when a balanced diet containing average quantity of proteins, carbohydrates and fat is utilized, the R is about 0.825.

In steady conditions, respiratory exchange ratio is equal to respiratory quotient.

Respiratory Exchange Ratio for Different Food Substances

Definition

Normal Value

For about 1 hour after meals, the respiratory quotient is 1.0.

It is because usually, immediately after taking meals, only carbohydrates are utilized by the tissues.

During the metabolism of carbohydrates, one molecule of carbon dioxide is produced for every molecule of oxygen consumed by the tissues.

Respiratory quotient is 1.0, which is equal to respiratory exchange ratio.

After utilization of all the carbohydrates available, body starts utilizing fats.

Now the respiratory quotient becomes 0.7.

When the proteins are metabolized, it becomes 0.8.

During exercise, the respiratory quotient increases.

Blood transports the respiratory gases.

Oxygen, which is essential for cells of the body, is transported from alveoli to lungs tissues.

Carbon dioxide, the waste product produced in cells, is transported from tissues to alveoli of lungs.

Transport of Oxygen

Oxygen is transported from alveoli to tissue by blood in two forms:

1. As simple physical solution.
2. In combination with hemoglobin.

Transport of Oxygen as Simple Solution

Oxygen dissolves in water of plasma and is transported in this physical form.

Amount of oxygen transported in this form is very negligible.

It is only 0.3 mL/100 mL of plasma.

It is about 3% of total oxygen in blood.

Transport of Oxygen in Combination with Hemoglobin

Oxygen combines with hemoglobin in blood and is transported as oxyhemoglobin.

Transport of this form is important, because maximum amount (97%) of oxygen is transported by this method.

Oxygen combines with hemoglobin only as a physical combination.

It is only oxygenation and not oxidation.

This type of combination of oxygen with hemoglobin has some advantages.

Oxygen can be readily released from hemoglobin when it is needed.

Oxygen combines with the iron in heme part of hemoglobin.

Oxygen Carrying Capacity of Blood

Oxygen carrying capacity of blood is the amount of oxygen transported by blood.

One gram of hemoglobin carries 1.34 mL of oxygen.

It is called oxygen carrying capacity of hemoglobin.

Normal hemoglobin content in blood is 15 g%.

So, the blood with 15 g% of hemoglobin should carry 20.1 mL of oxygen, i.e. 20.1 mL of oxygen in 100 mL of blood.

But, the blood with 15 g% of hemoglobin carries only 19 mL% of oxygen, i.e. 19 mL of oxygen is carried by 100 mL of blood.

Oxygen carrying capacity of blood is only 19 mL% because the hemoglobin is not fully saturated with oxygen.

It is saturated only for about 95%.

Normally, hemoglobin is saturated with oxygen only up to 95%.

Saturation of hemoglobin with oxygen depends upon the partial pressure of oxygen.

When partial pressure of oxygen is more, hemoglobin accepts oxygen and when partial pressure of oxygen is less, hemoglobin releases oxygen.

Normal Oxygen-Hemoglobin Dissociation Curve

Under normal conditions, oxygen-hemoglobin dissociation curve is ‘S’ shaped or sigmoid shaped.

Upper part of the curve indicates acceptance of oxygen by hemoglobin depending upon the partial pressure of oxygen.

Lower part of the curve indicates dissociation of oxygen from hemoglobin.

P₅₀

When partial pressure of oxygen is 25 to 27 mm Hg, the hemoglobin is saturated to about 50%.

That is, the blood contains 50% of oxygen.

At 40 mm Hg of partial pressure of oxygen, the saturation is 75%.

It becomes 95% when the partial pressure of oxygen is 100 mm Hg.

Factors Affecting Oxygen-Hemoglobin Dissociation Curve

Oxygen-hemoglobin dissociation curve is shifted to left or right by following conditions:

Shift to Left

Shift to left indicates acceptance (association) of oxygen by hemoglobin.

1. In fetal blood, because fetal hemoglobin has got more affinity for oxygen than adult hemoglobin.
2. Decrease in hydrogen ion concentration and increase in pH (alkalinity).

Shift to Right

Shift to right indicates dissociation of oxygen from hemoglobin.

1. Decrease in partial pressure of oxygen.
2. Increase in partial pressure of carbon dioxide (Bohr effect).
3. Increase in hydrogen ion concentration and decrease in pH (acidity).
4. Increased body temperature.
5. Excess of 2,3-diphosphoglycerate (DPG), which is a byproduct of carbohydrate metabolism in blood corpuscles.

Bohr Effect

The partial pressure of carbon dioxide is very high in tissues.

Because of this, carbon dioxide enters the blood.

Presence of carbon dioxide in blood decreases the affinity of hemoglobin for oxygen, so that oxygen is released for the tissues.

Oxygen dissociation curve is shifted to right.

Carbon dioxide is transported in blood from tissues to the alveoli.

Partial pressure and content of carbon dioxide in arterial blood and venous blood are given below.

Carbon dioxide is transported in the blood in four ways:

1. As dissolved form — 7%
2. As carbonic acid — negligible
3. As bicarbonate — 63%
4. As carbamino compounds — 30%

I. Transport of Carbon Dioxide as Dissolved Form

Carbon dioxide diffuses into blood and dissolves in the fluid of plasma forming a simple solution.

Only about 3 mL/100 mL of plasma of carbon dioxide is transported as dissolved state.

It is about 7% of carbon dioxide in the blood.

II. Transport of Carbon Dioxide as Carbonic Acid

Part of dissolved carbon dioxide in plasma combines with the water to form carbonic acid.

Transport of carbon dioxide in this form is negligible.

III. Transport of Carbon Dioxide as Bicarbonate

About 63% of carbon dioxide is transported as bicarbonate.

From plasma, carbon dioxide enters the RBCs.

In RBCs, carbon dioxide combines with water to form carbonic acid.

This reaction inside RBCs is rapid due to the presence of an enzyme called carbonic anhydrase.

This enzyme accelerates the reaction.

Carbonic anhydrase is present only inside the RBCs and not in the plasma.

This is why the carbonic acid formation is at least 200 to 300 times more in the RBCs than in plasma.

Chloride Shift or Hamburger Phenomenon

Chloride shift occurs when carbon dioxide enters the blood from tissues.

In plasma, plenty of sodium chloride is present.

It dissociates into sodium and chloride ions.

When the negatively charged bicarbonate ions move out of RBC into the plasma, the negatively charged chloride ions move into the RBC in order to maintain the electrolyte equilibrium (ionic balance).

Reverse Chloride Shift

This occurs in lungs.

When the blood reaches the alveoli, sodium bicarbonate in the plasma dissociates into the sodium and bicarbonate ions.

Bicarbonate ion moves into the RBC.

It makes chloride ion to move out of the RBC into the plasma, where it combines with sodium and forms sodium chloride.

Bicarbonate ion inside the RBC combines with hydrogen ion, forms carbonic acid, which dissociates into water and carbon dioxide.

Carbon dioxide is then expelled into alveoli.

IV. Transport of Carbon Dioxide as Carbamino Compounds

About 30% of carbon dioxide is transported as carbamino compounds.

Carbon dioxide is transported in blood in combination with hemoglobin and plasma proteins.

Carbon dioxide combines with hemoglobin to form carbaminohemoglobin.

It combines with plasma proteins to form carbamino proteins.

Carbaminohemoglobin and carbamino proteins are together called carbamino compounds.

Carbon dioxide combines with proteins or hemoglobin with a loose bond, so that carbon dioxide is easily released into alveoli where the partial pressure of carbon dioxide is low.

Thus, the combination of carbon dioxide with proteins and hemoglobin is a reversible one.

Amount of carbon dioxide transported in combination with plasma proteins is very less compared to the amount transported in combination with hemoglobin.

It is because the quantity of proteins in plasma is only half of the quantity of hemoglobin.

Normal Carbon Dioxide Dissociation Curve

Normal carbon dioxide dissociation curve shows that carbon dioxide content in the blood is 48 mL% when the partial pressure of carbon dioxide is 40 mm Hg.

It becomes 52 mL% when the partial pressure of carbon dioxide is 46 mm Hg.

Carbon dioxide content becomes 70 mL% when the partial pressure is about 100 mm Hg.

Haldane Effect

Significance of Haldane Effect

1. Release of carbon dioxide from blood into alveoli of lungs.
2. Uptake of oxygen by the blood.