Exchange of Respiratory Gases Physiology Notes

Exchange of Respiratory Gases

Exchange Of Respiratory Gases In Lungs

• In the lungs, the exchange of respiratory gases takes place between the alveoli and the blood. The exchange of gases occurs through bulk flow diffusion.
• The respiratory unit is the structure through which the exchange of gases between blood and alveoli takes place.

Respiratory Membrane:

• The respiratory membrane is the membranous structure through which the exchange of respiratory gases takes place.
• It is formed by the epithelium of the respiratory membrane and the endothelium of the pulmonary capillary.
• The epithelium of the respiratory unit is a very thin layer.

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• As the capillaries are in close contact with this membrane, the alveolar air is in close proximity to capillary blood.
• This facilitates the gaseous exchange between air and blood. The respiratory membrane is formed by different layers of structures belonging to the alveoli and capillaries.
• The different layers of the respiratory membrane are as follows from within and outside:

From Alveolar Portion:

1. A monomolecular layer of surfactant, which spreads over the surface of the fluid lining of alveoli
2. A thin layer of fluid that lines the alveoli
3. The alveolar epithelial layer, which is composed of thin epithelial cells resting on a basement membrane

In between Alveolar Portion and Capillary Portion: An interstitial space

From Capillary Portion

1. Basement membrane of capillary
2. Capillary endothelial cells.
3. In spite of having many layers, the respiratory membrane is very thin.
4. The average thickness of the respiratory membrane is about 0.1 p.
5. The total surface area of the respiratory membrane in both lungs is about 70 sq. meters.
6. The average diameter of pulmonary capillaries is only 8 p, which means that the red blood cells with a diameter of 7.4 p actually squeeze through the capillaries.
7. Therefore, the membrane of red blood cells is in close contact with the capillary wall.
8. This facilitates the quick exchange of oxygen and carbon dioxide between the blood and alveoli.

Diffusing Capacity:

• The diffusing capacity is defined as the volume of gas that diffuses through the respiratory membrane each minute for a pressure gradient of 1 mm Hg.

Diffusing Capacity for Oxygen and Carbon Dioxide:

1. Diffusing capacity for oxygen is 21 mL/minute/1 mm Hg.
2. Diffusing capacity for carbon dioxide is 400 mL/minute/ 1 mm Hg.
3. Thus, the diffusing capacity for carbon dioxide is about 20 times more than that of oxygen.

Factors Affecting Diffusing Capacity: The diffusing capacity is affected by various factors:

Pressure gradient: Diffusing capacity is directly proportional to the pressure gradient.

1. A pressure gradient is the difference between the partial pressure of a gas in the alveoli and pulmonary capillary blood (see below). It is the major factor that affects the diffusing capacity.
2. Solubility of gas in a fluid medium
3. Diffusing capacity is directly proportional to the solubility of the gas.
4. If the solubility of a gas is more in the fluid medium, a large number of molecules dissolve in it and diffuse easily.

Total surface area of respiratory membrane:

• Diffusing capacity is directly proportional to the surface area of the respiratory membrane.
• The surface area of the respiratory membrane in each lung is about 70 sq. m.
• If the total surface area of the respiratory membrane decreases, the diffusing capacity for the gases is decreased.
• The diffusing capacity is decreased in emphysema in which many of the alveoli are collapsed because of heavy smoking or oxidant gases.

Molecular weight of the gas:

• Diffusing capacity is inversely proportional to a molecular weight of the gas.
• If the molecular weight is more, the density is more and the rate of diffusion is less.

Thickness of respiratory membrane:

Diffusion is inversely proportional to the thickness of the respiratory membrane.

• The more the thickness of the respiratory membrane less is the diffusion.
• It is because the distance through which the diffusion takes place is long.
• In conditions like fibrosis and edema, the diffusion rate is reduced, because the thickness of the respiratory membrane is increased.

Relation between Diffusing Capacity and Factors Affecting it:

The relation between the diffusing capacity and the factors affecting it is expressed by the following formula:

DC – diffusing capacity

Pg = pressure gradient

S = solubility of gas

A = surface area of the respiratory membrane

Mw = molecular weight

D = thickness of the respiratory membrane.

Diffusion Coefficient And Fick’S Law Of Diffusion

Diffusion Coefficient:

• The diffusion coefficient is defined as a constant (a factor of proportionality), which is the measure of a substance diffusing through the concentration gradient.
• It is also known as the diffusion constant. It is related to the size and shape of the molecules of the substance.

Fick’s Law of Diffusion:

Diffusion is well described by Fick’s law of diffusion.

• According to this law, the amount of a substance crossing a given area is directly proportional to the area available for diffusion, the concentration gradient, and a constant known as diffusion coefficient. Thus,

The amount diffused = Area × Concentration gradient x Diffusion coefficient

The formula of Fick’s law:

where,

J = amount of substance diffused

D = diffusion coefficient

A = area through which diffusion occurs

dc/dx = concentration gradient

• The negative sign in the formula indicates that diffusion occurs from a region of higher concentration to a region of lower concentration.
• The diffusion coefficient reduces when the molecular size of the diffusing substance is increased.
• It increases when the size is decreased, i.e. the smaller molecules diffuse rapidly than the larger ones.

Diffusion Of Oxygen:

Entrance of Oxygen from Atmospheric Air into the Alveoli:

• The 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 the 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 the diffusion of oxygen must be quicker and more effective.
• Fortunately, this is possible because of the pressure gradient.
• The partial pressure of oxygen in the pulmonary capillary is 40 mm Hg and in the alveoli, it is 104 mm Hg.
• The pressure gradient is 64 mm Hg. It facilitates the diffusion of oxygen from alveoli into the blood.

Diffusion Of Carbon Dioxide:

Diffusion of Carbon Dioxide from Blood into Alveoli

• The partial pressure of carbon dioxide in alveoli is 40 mm Hg whereas in the blood it is 46 mm Hg.
• The pressure gradient of 6 mm is responsible for the diffusion of carbon dioxide from the blood into the alveoli.

Exit of Carbon Dioxide from the Alveoli into the Atmospheric Air:

In the atmospheric air, the partial pressure of carbon dioxide is very insignificant and is only about 0.3 mm Hg whereas, in the alveoli, it is 40 mm Hg. So, carbon dioxide enters and passes to the atmosphere from the alveoli easily.

Exchange Of Gases At Tissue Level

Diffusion Of Oxygen From Blood Into The Tissues:

1. The partial pressure of oxygen in the venous end of the pulmonary capillary is 104 mm Hg.
2. However, the partial pressure of oxygen in the arterial end of the systemic capillary is only 95 mm Hg.
3. It may be because of the physiological shunt in the lungs. Due to the venous admixture in the shunt, 2% of blood reaches the heart without being oxygenated.
4. The average oxygen tension in the tissues is 40 mm Hg. It is because of continuous metabolic activity and constant utilization of oxygen.
5. 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.
6. The oxygen content in arterial blood is 19 mL% and, in venous blood, it is 14 mL%. Thus, the diffusion of oxygen from the blood to the tissues is 5 mL/100 mL of blood.

Diffusion Of Carbon Dioxide From Tissues Into The Blood:

• Due to the 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. The partial pressure of carbon epoxide in arterial blood is 40 mm Hg.
• The pressure gradient of 6 mm Hg is responsible for the diffusion of carbon dioxide from tissues to the blood and the carbon dioxide content in arterial blood is 48 mL%. And, in the venous blood, it is 52 mL%.
• So, the diffusion of carbon dioxide from tissues to the blood is 4 mL/100 mL of blood.

Respiratory Exchange Ratio

Respiratory Exchange Ratio Definition: The respiratory exchange ratio (R) is the ratio between the net output of carbon dioxide from the tissues to the simultaneous net uptake of oxygen by the tissues.

Normal Values:

The value of R depends upon the type of food substance that is metabolized.

• When a person utilizes only carbohydrates for metabolism, R is 1.0. That means during carbohydrate metabolism, the amount of carbon dioxide produced in the tissue is equal to the amount of oxygen consumed.
• If only fat is used for metabolism, the R is 0.7. When fat is utilized, oxygen reacts with fats and a large portion of oxygen combines with hydrogen ions to form water instead of carbon dioxide.
• So, the carbon dioxide output is less than the oxygen consumed. And, the R is less.
• If only protein is utilized, R is 0.803.
• However, when a balanced diet containing average quantity of proteins, carbohydrates, and lipids is utilized, the R is about 0.825.
• In steady conditions, the respiratory exchange ratio is equal to the respiratory quotient.

Respiratory Quotient

Respiratory Quotient Definition: The respiratory quotient is the molar ratio of carbon dioxide production to oxygen consumption. It is used to determine the utilization of different foodstuffs.

Normal Value:

For about one hour after meals, the respiratory quotient is 1.0. It is because usually, immediately after taking meals, only the 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.
• The respiratory quotient is 1.0, which is equal to the respiratory exchange ratio.
• After the utilization of all the carbohydrates available, the body starts utilizing fats. Now the respiratory quotient becomes 0.7. When proteins are metabolized, it becomes 0.8.
• During exercise, the respiratory quotient increases.