Pulmonary System: Structure, Function, and Control

Pulmonary System: Structure, Function, and Control

Basic Structures of the Pulmonary System

• Primary Components: The pulmonary system consists of two lungs, upper airways, lower airways, blood vessels, the chest wall, and the diaphragm.
• Upper Respiratory Tract: Responsible for filtering, humidifying, and warming inspired air.
  • Nasal Cavity: Contains nasal conchae.
  • Pharynx (Throat): Divided into three regions:
    • Nasopharynx: Located behind the nose.
    • Oropharynx: Located behind the mouth.
    • Laryngopharynx: Located above the larynx.
  • Larynx (Voice box): The voice box, which includes the epiglottis.
• Lower Respiratory Tract: Extends from the trachea to the alveoli.
  • Trachea ("windpipe"): Bifurcates at the carina.
  • Bronchi:
    • Primary bronchi
    • Secondary (lobar) bronchi
    • Tertiary (segmental) bronchi
  • Bronchioles: Smaller air passages branching from the bronchi.
  • Alveoli: Tiny air sacs, the primary site for gas exchange.
  • Alveolar duct
  • Alveolar sac
  • Capillary

Chest Cavity Anatomy

• Lungs:
  • Right Lobe: Composed of three lobes (upper, middle, lower) and has two fissures (horizontal and oblique).
  • Left Lobe: Composed of two lobes (upper, lower) and has one fissure (oblique).
• Mediastinum:
  • Separates the two lungs.
  • Contents: Houses vital organs including the heart, great vessels (aorta, superior and inferior vena cava, pulmonary artery, and pulmonary veins), trachea, esophagus, and thymus.
  • Function: Protects these vital organs.
• Pleura: A double-layered membrane encasing the lungs.
  • Visceral Pleura: Adhered directly to the lung surface, offering protection and support.
  • Parietal Pleura: Lines the inner surface of the chest wall, producing and absorbing pleural fluid.
• Pleural Space: The space between the visceral and parietal pleura.
  • Contains a lubricating pleural fluid that reduces friction between the lung and chest wall during breathing.
  • Maintains a negative pressure that is essential for lung inflation during inspiration.
• Diaphragm:
  • A dome-shaped muscle located inferior to the mediastinum.
  • It is the primary muscle responsible for breathing.

Mechanics of Breathing

• Boyle's Law: Explains the inverse relationship between volume and pressure, which drives air movement in and out of the lungs.
  • Inhalation: The diaphragm moves downwards
    ightarrow increasing the volume of the lungs
    ightarrow decreasing the pressure inside the lungs
    ightarrow air flows into the lungs.
  • Exhalation: The diaphragm moves upwards
    ightarrow decreasing the volume of the lungs
    ightarrow increasing the pressure inside the lungs
    ightarrow air is pushed out.
• Major Muscles of Breathing: Diaphragm.
• Accessory Muscles of Breathing: These muscles assist in more forceful inspiration or expiration.
  • External Intercostals: Primarily for inspiration.
  • Neck Muscles: Sternocleidomastoid and scalene muscles, primarily for inspiration.
  • Abdominal Muscles: Help push air out forcefully during expiration.
  • Back and Shoulder Muscles: Can also contribute.

Gas Exchange and Oxygenation

• Gas Exchange: The process of swapping gases (oxygen and carbon dioxide) between the lungs and the bloodstream.
  • Oxygen (O_2) is taken in during inhalation.
  • Carbon dioxide (CO_2) is expelled during exhalation.
  • This exchange primarily occurs in the alveoli.
  • Result: Oxygenation of the blood.
• Important Cells Involved in Gas Exchange:
  • Type I Alveolar Cells: Form the thin walls of the alveoli, facilitating the diffusion of O2 and CO2 in and out of the lungs.
  • Type II Alveolar Cells: Produce surfactant, a substance that reduces surface tension within the alveoli, preventing them from collapsing.
  • Red Blood Cells (RBCs): Pick up oxygen from the alveoli and transport it to tissues and organs throughout the body. RBCs contain hemoglobin, the protein responsible for binding to oxygen.
  • Endothelial Cells of the Capillaries: Line the blood vessels, regulating the passage of substances between the blood and the lungs.
• Oxygenation: Refers to the gas exchange process where oxygen binds to hemoglobin in red blood cells, which is then transported to tissues and organs.
• Ventilation: The mechanical process of moving air in and out of the lungs.
  • Inhalation: The chest cavity expands
    ightarrow lowering the pressure inside the lungs
    ightarrow air rushes in.
  • Exhalation: The chest cavity contracts
    ightarrow increasing the pressure inside the lungs
    ightarrow air moves out.

Pulmonary Circulation

• Distinction: Pulmonary circulation is distinct from systemic circulation and operates in a reversed manner concerning oxygenation.
  

• Control Center: Ventilation is primarily controlled by the brainstem, specifically the medulla oblongata and the pons.
• Brain Response to Chemical Signals: The brain monitors and responds to chemical signals in the blood, including levels of carbon dioxide (CO2), oxygen (O2), and pH.
• Brainstem Control Centers:
  • Pontine Respiratory Group: Located in the pons, it controls inspiration.
  • Apneustic Center: Moderates inspiration.
  • Dorsal Respiratory Group: Located in the medulla, it responds to chemoreceptors.
  • Ventral Respiratory Group: Located in the medulla, it sets the rhythm of inspiration and expiration.
  • Chemosensitive Center: Sensitive to changes in hydrogen ions (H^+), oxygen (O2), and carbon dioxide (CO2) levels.
  • The chemical reaction H2O + CO2
    ightleftharpoons H2CO3
    ightleftharpoons H^+ + HCO3^- illustrates the relationship between CO2 and pH.
• Chemoreceptors: Sensory receptors that detect changes in blood chemistry.
  • Central Chemoreceptors: Located in the brainstem.
  • Peripheral Chemoreceptors: Located in the carotid bodies and aorta bodies, they are sensitive to changes in P{O2}, P{CO2}, and pH.
  • Nerves involved include the Vagus nerve, Intercostal nerve, and Phrenic nerve (to the diaphragm).

Ventilation and Perfusion (V/Q Ratio)

• Ventilation (V): Represents the amount of air reaching the alveoli.
• Perfusion (Q): Represents the amount of blood reaching the alveoli via capillaries.
• V/Q Ratio: The ratio of ventilation to perfusion.
  • Normal V/Q Ratio: Approximately 0.8 on average.
• V/Q Mismatch (V/Q Defect): Occurs when there are imbalances in ventilation or perfusion.
  • Ventilation but No Perfusion (Dead Space): Occurs when there is air in the alveoli but no blood flow for gas exchange. The V/Q ratio approaches infinity (V/Q = ext{inf}).
  • Perfusion but No Ventilation (Shunt): Occurs when there is blood flow to the alveoli but no air for gas exchange. The V/Q ratio is zero (V/Q = 0).

Pulmonary Exam: Auscultation

• Purpose: Listening to lung sounds to assess respiratory health.
• Procedure: Listen through a full cycle of inspiration and expiration using a