Pulmonary System Function

Pulmonary System Function

• 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, diffusion, and perfusion.

Structure of the Pulmonary System

• The pulmonary system is made up of conducting airways (upper and lower) that deliver air to each section of the lung.
• Upper conducting airways consist of the nasopharynx and oropharynx, which filter and moisturize the air that is inhaled.
• The larynx connects the upper and lower airways and has supporting cartilage that prevents collapse during inspiration, expiration, and swallowing.
• The lower conducting airways begin at the level of the trachea and branch into two main airways (or bronchi).
• There is continued branching of the conducting airways, known as generations, with the smallest being the terminal bronchioles.

Gas Exchange

• Gas exchange occurs in the alveoli, which are the primary gas-exchange units of the lung.
• There are two major types of epithelial cells in the alveoli: Type I, which provide structure (elastin), and Type II, which secrete surfactant.
• Alveolar macrophages ingest foreign material that reaches the alveolus and prepare it for removal through the lymphatics.

Pulmonary Circulation

• Pulmonary circulation 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, and debris from the circulation.

Alveolo-Capillary Membrane

• The alveolo-capillary membrane is composed of the shared alveolar and capillary walls.
• Gas exchange occurs over this membrane, and any disorder that thickens the membrane impairs gas exchange.

Factors Affecting Gas Exchange

• Four major factors 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

• Ventilation is the mechanical movement of gas (air) into and out of the lung.
• Alveolar ventilation cannot be determined by observation of vent rate, pattern, or effort; it requires an arterial blood gas analysis to measure PaCO2.
• Ventilation is controlled by the respiratory center in the lower brain stem, which transmits impulses to the respiratory muscles based on information from peripheral chemoreceptors and lung receptors.

Lung Receptors

• Three types of lung receptors send impulses from the lungs to the dorsal respiratory group (neurons located in the brain stem):
  • Irritant receptors, found on the epithelium of the conducting airways, sensitive to noxious stimuli
  • Stretch receptors, found in the lungs and airways, sensitive to changes in lung volume
  • J-receptors, found in the alveolar walls, sensitive to increases in interstitial fluid

Properties of Lung and Chest Wall

• Compliance is a measure of lung and chest wall distensibility, determined by alveolar surface tension and elastic recoil of the lung and chest wall.
• Increased compliance is seen in emphysema, resulting in chronic overinflation of the lungs.
• Decreased compliance is seen in pneumonia, edema, fibrosis, or adult respiratory distress syndrome (ARDS).

Hypoxia and Hypoxemia

• Hypoxia is reduced oxygenation of cells in tissues.
• Hypoxemia is reduced oxygenation of arterial blood.
• Causes of hypoxemia include:
  • 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, abnormal hemoglobin, tissue edema)
  • Inadequate tissue capability of using oxygen (poisoning, vitamin deficiencies)

Ventilation-Perfusion Ratio (V/Q)

• The ventilation-perfusion ratio, V/Q, is the ratio of airflow into the lungs divided by the pulmonary blood flow.
• An abnormal V/Q ratio is the most common cause of hypoxemia.