Alveolar Gas Exchanges
While other parts of the respiratory system move air and conduct it into and out of the air passages, the alveoli carry on the vital process of exchanging gases between the air and the blood.

Alveoli
The alveoli are microscopic air sacs clustered at the distal ends of the finest respiratory tubes, the alveolar ducts. Each alveolus consists of a tiny space surrounded by a thin wall, which separates it from adjacent alveoli.

Respiratory Membrane

The wall of an alveolus consists of an inner lining of simple squamous epithelium and a dense network of capillaries, which are also lined with simple squamous epithelial cells. Thin fused basement membranes separate the layers of these flattened cells, and between them there are elastic collagenous fibers, which help to support the wall. There are at least two thicknesses of epithelial cells and a layer of fused basement membranes between the air in an alveolus and the blood in a capillary. These layers make up the respiratory membrane, which is of vital importance because it is through this membrane that gas exchanges occur between the blood and alveolar air.

Diffusion Through The Respiratory

Membrane

Gas molecules diffuse from regions where they are in higher concentration toward

regions where they are in lower

concentration. Similarly, gases move from

regions of higher pressure toward regions of lower

pressure, and the pressure of gas

determines the rate at which it will diffuse from one region to

another.

Measured by volume, ordinary air is about 78

percent nitrogen, 21 percent oxygen, and 0.04 percent carbon dioxide.

Air also contains small amounts of other gases that have little or no

physiological importance.

In a mixture of gases, such as air, each gas is

responsible for a portion of the total weight of pressure produced by

the mixture. The amount of pressure each gas creates is called the

partial pressure, and it is directly related to the concentration of the

gas in the mixture. For example, because air is 21 percent oxygen,

this gas is responsible for 21 percent of the atmospheric pressure.

Since 21 percent of 760 mm Hg is equal to 160 mm Hg, it is said that

the partial pressure of oxygen, symbolized Po₂, in atmospheric air is 160 mm Hg. Similarly, the partial

pressure of carbon dioxide (Pco₂) in air can be calculated as 0.3 mm Hg.

When a mixture of gases dissolves in the blood,

each gas exerts its own partial pressure in proportion to its

dissolved concentration. furthermore, each gas will diffuse between

the blood and its surroundings, and this movement will tend to

equalize its partial pressures in the two regions.

For example, the Pco₂ in capillary blood is 45 mm

of Hg, but the Pco₂ in alveolar air is 40 mm of Hg. As a consequence of the

difference between these partial pressures, carbon dioxide diffuses

from the blood, where its pressure is higher, through the respiratory

membrane and into the alveolar air. When the blood leaves the lungs,

its Pco₂ is

40 mm Hg, which is about the same as the Pco₂ of the alveolar air.

Similarly, the Po₂ of capillary blood is 40 mm

Hg, but that of alveolar air is 104 mm Hg. Thus, oxygen diffuses from

the alveolar air into the blood, and the blood leaves the lungs with

a Po₂ of 104

mm Hg.

Transport Of Gases

The transport of oxygen and carbon dioxide between the lungs and body

cells is a function of the blood. As these gases enter the blood,

they dissolve in the liquid portion (plasma). They combine chemically

with various blood component, and most are carried in combination

with other atoms and molecules.

Oxygen Transport

Almost all of the oxygen (over 98 percent) carried in the blood is

combined with the iron-containing compound, hemoglobin, that occurs within

the red blood cells. The remainder of the oxygen is dissolved in the

blood plasma.

In the lungs, where the Po₂ is relatively high, oxygen

dissolves in the blood and combines rapidly with the iron atoms of

hemoglobin. The result of this chemical reaction is a new substance

called oxyhemoglobin.

The chemical bonds that form between the oxygen

and hemoglobin molecules are relatively unstable, and as the

Po₂

decreases, oxygen is released from oxyhemoglobin molecules. This

happens in tissues where the cells have used oxygen in their

respiratory processes, and the free oxygen diffuses from the blood

into nearby cells.

The amount of oxygen released from oxyhemoglobin

is affected by several other factors, including the blood

concentration of carbon dioxide, the blood pH, and the blood

temperature. Thus, as the concentration of

carbon dioxide increase, as the blood becomes more acidic, or as the

blood temperature increases, more oxygen is released.

Due to these factors, more oxygen is released to the skeletal

muscles during periods of physical

exercise, because the increased muscular activity accompanied by an

increase use of oxygen causes an increase

in the carbon dioxide concentration, a

decrease in the pH, and a rise in the

temperature. At the same time, less active

cells receive relatively smaller amounts of oxygen.

Carbon Dioxide Transport

Blood flowing through the capillaries of the body tissues gains

carbon dioxide because the tissues have a relatively high

Pco₂. This

carbon dioxide is transported to the lungs in one of three forms: as

carbon dioxide dissolved in the blood, as part of a compound formed

by bonding to hemoglobin, or as part of a bicarbonate ion.

The amount of carbon dioxide that dissolves in the

blood is determined by its partial pressure. The higher the Pco

₂ of the

tissues, the more carbon dioxide will go into solution. However, only

about 7 percent of the carbon dioxide is transported in this

form.

Unlike oxygen, which combines with the iron atoms

of hemoglobin molecules, carbon dioxide bonds with the amino groups

(-NH₂) of

these molecules. Consequently, oxygen and carbon dioxide do not

compete for bonding sites, and both gases can be transported by a

hemoglobin molecule at the same time.

When carbon dioxide combines with hemoglobin, a

loosely bound compound called carbaminohemoglobin is formed.

This substance decomposes readily in regions where the Pco

₂ is low

and, thus, releases its carbon dioxide. Although this method of

transporting carbon dioxide is theoretically quite effective,

carbaminohemoglobin forms relatively slowly. It is believed that only

about 23 percent of the total carbon dioxide in the blood is carried

this way.

The most important carbon dioxide transport

mechanism involves the formation of bicarbonate ions (HCO₃^-). Carbon dioxide reacts with

water to form carbonic acid (H₂CO₃)

CO ₂ + H₂O ---> H₂CO₃

Although this reaction occurs slowly in the blood

plasma, much of the carbon dioxide diffuses into the red blood cells,

and these cells contain an enzyme, called carbonic anhydrase, that speeds

the reaction between carbon dioxide and water.

The resulting carbonic acid then dissociates,

releasing hydrogen ions (H⁺) and bicarbonate ions

(HCO₃^-).

H₂CO₃ -----> H⁺ + HCO₃^-

Most of the hydrogen ions combine quickly with

hemoglobin molecules and, thus, are prevented from accumulating and

causing great change in the blood Ph, The bicarbonate ions tend to

diffuse out of red bllod cells and enter the blood plasma. It is

estimated that nearly 70 percent of the carbon dioxde transported in

the blood is carried in this form.

When the blood passes through the capillaries of

the lungs, it loses its dissolved carbon dioxide by diffusion into

the alveoli, This occurs in response to relatively low Pco

₂ of the

alveolar air. At the same time, hydrogen ions and bicarbonate ions in

the red blood cells recombine to form carbonic acid molecules, and

under the influence of carbonic anhydrase, the carbonic acid gives

rise to carbon dioxide and water,

H⁺ + HCO₃^- ----> CO₂ + H₂O

Carbaminohemoglobin also releases its carbon

dioxide, and as carbon dioxide continues to diffuse out of the blood,

and equillibrium is established between the Pco₂ of the blood and the

Pco₂ in the

alveolar air.