alveolar-capillary exchange


A – Introduction :

L & rsquo; pulmonary exchanger is the meeting of two circulations : Aerial (alveolar) and pulmonary capillary ; it represents the place of diffusion of oxygen from alveoli to blood, and carbon dioxide in the reverse direction, according to a pressure gradient between the two compartments. This transporting step follows alveolar ventilation in the breathing process

The hematosis is therefore defined by the set of physiological mechanisms that allow the enrichment of mixed venous oxygen.

The existence of pathology disrupting the broadcast will have negative consequences on the entire body such as tissue hypoxia and reduced cell metabolism.

B – by histological Reminder :

The alveolar-capillary membrane "MAC" is more 300 million alveoli and is characterized by a considerable area between 50 and 70 m² for a thickness less than 0.5 micrometer.

It is successively made by the following structures :

  • Surfactant
  • alveolar epithelium
  • Interstitium
  • capillary membrane
  • Plasma
  • Erythrocyte membrane

After crossing all these layer (by simple diffusion) oxygen must chemically react with hemoglobin to be fixed and transported to tissues, this is why we do not talk mailing but alveolar-capillary transfer.

C – distribution of physical law :

According to the Fick principle the conveying speed of a gas through a layer of tissue is :

  • proportional to the area of ​​the fabric
  • proportional to the difference on either gas concentration side of the fabric
  • inversely proportional to the thickness of the tissue layer
  • proportional to the solubility of the gas
  • inversely proportional to the square root of the molecular weight of the gas

We can then write :

Vx = K . s/e .ΔP

s : surface

e : thickness

K : physicochemical factor (solubility coefficient divided by the square root of the gas molecular weight)

D – Diffusion of oxygen across the alveolar-capillary membrane :

at PA02 averages 100 mmhg, the oxygen partial pressure at the entrance of the pulmonary capillary was 40 torrs : So what follows the oxygen pressure gracient.

Passing oxygen through the MAC

The membrane diffusing capacity depends on two factors in series, a physical factor of membrane conductance, a chemical factor or combination of blood with hemoglobin. All these phenomena led physiologists to use the term transfer instead of simple diffusion alveolar-capillary :

1/TL = 1 /Dm + 1/i. You

TL : lung transfer capacity

Dm : pulmonary diffusing capacity

i: speed combination with hemoglobin

You : pulmonary capillary blood volume

The time factor for the passage of oxygen through the alveolar-capillary membrane can be considered at two levels :

the transit time of which is that in which the blood through the capillary into contact with the socket (equal to 0.75 seconds, at 0.25 seconds during intense physical exercise)

– the balance of time defined by the time required for the oxygen partial pressures at the alveolus and capillary blood equilibrates (of the order of 0.25 sec)

So whether at rest or exercise, transit time is always enough for a balance can be between oxygen and the alveolar capillary blood.

E – C02 diffusion across the alveolar-capillary membrane :

The PC02 at the entrance of the pulmonary capillary is approximately 45mmHg, that in the alveolar air is 40 torrs : therefore the diffuse C02 capillary toward the alveolus.

Due to its high solubility (25 times greater than that of oxygen) C02 readily diffuses across the alveolar-capillary membrane despite the low concentration gradient between the two compartments.

F – Measuring the transfer capability of the MAC :

If we take the formula TL02 = V02 (PA02-Pc 02), PC02 the term which represents the capillary pressure of oxygen is very difficult to measure. then used carbon monoxide "CO" which not only follows the same path as oxygen, but has also a greater capacity to bind to hemoglobin (affinity 250 times greater than that of oxygen), despite negligible capillary pressures Pc CO = 0.

We then written : TL = V CO CO / PA CO

In young man 20 years of size 1.75 meters CO TL is approximately 35 ml/ mn/ KPa.

TL CO is disturbed by a set of diseases which :

  • To increase the thickness of the MAC as diffuse interstitial pneumonia
  • To decrease the MAC surface, example : infectious process, neoplastic or lung resection in the treatment of lung cancer
  • To induce a restriction of the pulmonary vascular bed (↓Vc) : embolism, emphysema
  • quantitative or qualitative abnormalities of hemoglobin (↑θ) :

anemia, sickle cell disease, thalassémie…

During the PR Aissaoui – Faculty of Constantine