• For diffusion to occur there must be a source of energy. This is provided by the kinetic motion of the molecules themselves

Net diffusion - is the movement of particles (such as gas molecules) from an area where they are highly concentrated to an area where they are less concentrated.

for example, oxygen in the lungs moves from the air sacs (where it’s in high concentration) into the blood (where it’s in lower concentration). This flow continues until the concentration of particles is balanced across both areas.

Diffusion of Gases Between the Gas Phase in the Alveoli and the Dissolved Phase in the Pulmonary Blood.

The partial pressure of each gas in the alveolar respiratory gas mixture tends to force molecules of that gas into solution in the blood of the alveolar capillaries.

Conversely, the molecules of the same gas that are already dissolved in the blood are bouncing randomly in the fluid of the blood, and some of these bouncing molecules escape back into the alveoli.

If the partial pressure is greater in the gas phase in the alveoli, as is normally true for oxygen, then more molecules will diffuse into the blood than in the other direction. Alternatively, if the partial pressure of the gas is greater in the dissolved state in the blood, which is normally true for carbon dioxide, then net diffusion will occur toward the gas phase in the alveoli.

Diffusion of Gases Through Fluids—Pressure Difference Causes Net Diffusion

factors affect the rate of gas diffusion in a fluid.

• the solubility of the gas in the fluid (Higher solubility of a gas means more molecules are available to diffuse for a given partial pressure difference)

• the cross-sectional area of the fluid (A larger area for diffusion allows more molecules to pass through simultaneously.)

• the distance through which the gas must diffuse (Greater distances slow down the diffusion process, as molecules take longer to travel.)

• the molecular weight of the gas (Faster-moving gas molecules (lower molecular weight) increase the rate of diffusion, as their kinetic energy enhances movement.)

• the temperature of the fluid

Diffusion of Gases Through Tissues

The gases that are of respiratory importance are all highly soluble in lipids and, consequently, are highly soluble in cell membranes. Because of this, the major limitation to the movement of gases in tissues is the rate at which the gases can diffuse through the tissue water instead of through the cell membranes

Compositions of Alveolar Air and Atmospheric Air Are Different

Alveolar air does not have the same concentrations of gases as atmospheric air by any means

• the alveolar air is only partially replaced by atmospheric air with each breath.

• oxygen is constantly being absorbed into the pulmonary blood from the alveolar air.

• carbon dioxide is constantly diffusing from the pulmonary blood into the alveoli

• dry atmospheric air that enters the respiratory passages is humidified even before it reaches the alveoli.

Humidification of the Air in the Respiratory Passages

shows that atmospheric air is composed almost entirely of nitrogen and oxygen; it normally contains almost no carbon dioxide and little water vapor. However, as soon as the atmospheric air enters the respiratory passages, it is exposed to the fluids that cover the respiratory surfaces. Even before the air enters the alveoli, it becomes (for all practical purposes) totally humidified. (tungod sa sinuses base sa gi ingon ni ma’am rochie)

Rate at Which Alveolar Air Is Renewed by Atmospheric Air

only 350 milliliters of new air is brought into the alveoli with each normal inspiration, and this same amount of old alveolar air is expired

the volume of alveolar air replaced by new atmospheric air with each breath is only one seventh of the total, so multiple breaths are required to exchange most of the alveolar air.

Importance of the Slow Replacement of Alveolar Air

importante kay para dili sudden ang change sa gas concentration sa blood para mas ma control and saturations sa tissue if need pa taas or baba og dapat dili mokalit rag taas or baba

Oxygen Concentration and Partial Pressure in the Alveoli

The more rapidly oxygen is absorbed, the lower its concentration in the alveoli becomes; conversely, the more rapidly new oxygen is breathed into the alveoli from the atmosphere, the higher its concentration becomes. Therefore, oxygen concentration in the alveoli, as well as its partial pressure, is controlled by

• the rate of absorption of oxygen into the blood

• the rate of entry of new oxygen into the lungs by the ventilatory process.

CO2 Concentration and Partial Pressure in the Alveoli

Carbon dioxide is continually being formed in the body and then carried in the blood to the alveoli; it is continually being removed

Expired Air Is a Combination of Dead Space Air and Alveolar Air

The overall composition of expired air is determined by

• (1) the amount of the expired air that is dead space air and

• (2) the amount that is alveolar air

Diffusion of Gases Through the Respiratory Membrane

flow of blood in the alveolar wall has been described as a “sheet” of flowing blood. Thus, it is obvious that the alveolar gases are in very close proximity to the blood of the pulmonary capillaries. Further, gas exchange between the alveolar air and the pulmonary blood occurs through the membranes of all the terminal portions of the lungs, not merely in the alveoli themselves. All these membranes are collectively known as the respiratory membrane, also called the pulmonary membrane.

Factors That Affect the Rate of Gas Diffusion Through the Respiratory Membrane

• the thickness of the membrane,

  • Because the rate of diffusion through the membrane is inversely proportional to the thickness of the membrane, any factor that increases the thickness to more than two to three times normal can interfere significantly with normal respiratory exchange of gases.

• the surface area of the membrane

  • in emphysema, many of the alveoli coalesce, with dissolution of many alveolar walls. Therefore, the new alveolar chambers are much larger than the original alveoli, but the total surface area of the respiratory membrane is often decreased as much as fivefold because of loss of the alveolar walls.

• the diffusion coefficient of the gas in the substance of the membrane

  • carbon dioxide diffuses about 20 times as rapidly as oxygen. Oxygen diffuses about twice as rapidly as nitrogen.

• the partial pressure difference of the gas between the two sides of the membrane.

  • The partial pressure represents a measure of the total number of molecules of a particular gas striking a unit area of the alveolar surface of the membrane in unit time, and the pressure of the gas in the blood represents the number of molecules that attempt to escape from the blood in the opposite direction. Therefore, the difference between these two pressures is a measure of the net tendency for the gas molecules to move through the membrane.

Increased Oxygen Diffusing Capacity During Exercise.