Pulmonary System: Gas Exchange Process

Study Notes

The primary function of the pulmonary system is the exchange of gases between the environmental air and the blood.
Three steps are involved in this process:
- Ventilation (the movement of air into and out of the lungs)
- Diffusion (the movement of gases between air spaces in the lungs and the bloodstream)
- Perfusion (movement of blood into and out of the capillary beds of the lungs to the body organ and tissues)

Made up of a set of conducting airways (upper and lower) that deliver air to each section of the lung.
Upper conducting airways consist of nasopharynx and oropharynx, which help filter and moisturize the air that is inhaled.
Larynx connects upper and lower airways and prevents collapse during inspiration and expiration and swallowing.

There are continued branching of the conducting airways, known as generations, and at the end of the 16th division is the smallest of the conducting airways, known as the terminal bronchioles.
No gas exchange occurs in the conducting airways.
From the terminal bronchioles, there are continued divisions of respiratory bronchioles to the alveolar ducts.
Gas exchange is possible starting at the level of the respiratory bronchioles, but the primary gas exchange units of the lung are the alveoli.

The alveoli are the primary gas-exchange units of the lung where oxygen enters the blood and CO2 is removed.
There are two major types of epithelial cells that appear in the alveolus:
- Type I: provide structure (elastin)
- Type II: secrete surfactant, a lipoprotein that coats the inner surface of the alveoli.
The alveoli also contain alveolar macrophages, which ingest foreign material that reaches the alveolus and prepare it for removal through the lymphatics.

Facilitates gas exchange, delivers nutrients to the lungs, acts as a reservoir for the left ventricle, and serves as a filtering system that removes clots, air, debris from circulation.
Pulmonary circulation participates in gas exchange, and the entire cardiac output from the right ventricle goes into the lungs.
Pulmonary circulation has lower pressure and resistance than systemic circulation.

The shared alveolar and capillary walls compose the alveolo-capillary membrane (also referred to as the respiratory membrane).
Gas exchange occurs over this membrane.
Any disorder that thickens the membrane impairs gas exchange.

Four major factors that determine how rapidly a gas (O2 or CO2) will pass through the membrane:
- Thickness of the membrane
- Surface area of the membrane
- Diffusion coefficient of the gas
- Pressure difference between the two sides of the membrane

Ventilation is the mechanical movement of gas (air) into and out of the lung.
Ventilation is often misnamed or used interchangeably with the term "respiration", but in physiology, respiration refers to the diffusion of gases between an alveolus and the capillary which perfuses it.
Alveolar ventilation cannot be determined by observation of vent rate, pattern, or effort; an arterial blood gas analysis must be performed to measure PaCO2.

Three types of lung receptors send impulses from the lungs to the dorsal respiratory group (neurons located in the brain stem):
- Irritant receptors
- Stretch receptors
- J-receptors (juxtapulmonary capillary receptors)

Central chemoreceptors in the brain respond to changes in the hydrogen ion concentration of the CSF.
Peripheral chemoreceptors are located in aortic bodies, the aortic arch, and carotids, and are primarily sensitive to changes in pO2.

Alveolar ventilation (or distension) is made possible by surfactant, which lowers surface tension.
Surface tension refers to the tendency of water molecules to pull toward each other and to collapse a sphere.
Surfactant's primary function is to stabilize the alveoli at low lung volumes on expiration so that alveolar collapse does not occur.

Compliance is a measure of lung and chest wall distensibility.
Increased compliance: lungs or chest wall is abnormally easy to inflate and has lost some elastic recoil.
Decreased compliance: lungs are stiff or difficult to inflate.

Many of the pulmonary diseases that patients exhibit result in hypoxia and/or hypoxemia.
Hypoxia: reduced oxygenation of cells in tissues.
Hypoxemia: reduced oxygenation of arterial blood.
Causes of hypoxemia:
- Inadequate oxygenation of lungs due to extrinsic reasons (high altitudes, hypoventilation)
- Pulmonary disease (hypoventilation, abnormal V/Q)
- Shunting of blood
- Inadequate oxygen transport by blood to tissues (anemias, abnl hgb, tissue edema)
- Inadequate tissue capability of using oxygen (poisoning – cyanide; vitamin deficiencies)

Obstructive patterns: fundamental physiologic problem is increased resistance to airflow as a result of caliber reduction in conducting airways.
Restrictive patterns:
Examples of common obstructive disorders:
- Asthma
- Emphysema
- Chronic bronchitis