Overview

1. Anatomy of the Respiratory System

1.1 Structural Overview and Associated Tissues

  • Primary Organs: Nose, nasal cavity, pharynx, larynx, trachea, bronchial tree, and lungs.

  • Vascular Components: Pulmonary circuit vessels (pulmonary trunk, pulmonary arteries, and pulmonary veins).

  • Thoracic Cage: Ribs, sternum, and thoracic vertebrae.

  • Respiratory Muscles:

    • Primary muscles: Diaphragm and intercostal muscles (external and internal).

    • Accessory muscles: Scalenes, sternocleidomastoid, and abdominal muscles (active during forced inhalation and exhalation).

1.2 Anatomical Division

  1. Upper Respiratory Tract (Nasal cavity through the larynx)

    • Nose and Nasal Cavity: Lined with pseudostratified ciliated columnar respiratory epithelium; highly vascularized capillary network warms incoming air; goblet cells secrete mucus to trap particulates.

    • Pharynx (Throat):

    • Nasopharynx: Posterior to nasal cavity; functions exclusively as an air conduit.

    • Oropharynx: Posterior to oral cavity; shared conduit for both air and food.

    • Laryngopharynx: Inferior portion; diverges anteriorly into the larynx and posteriorly into the esophagus.

    • Larynx ("Voice Box"): Composed of nine cartilages (including thyroid cartilage, cricoid cartilage, and epiglottis); houses vocal folds (true vocal cords) and regulates airflow into the lower tract.

  2. Lower Respiratory Tract (Trachea to the alveoli)

    • Trachea ("Windpipe"): Reinforced by C-shaped hyaline cartilage rings to maintain a patent lumen while allowing esophagus expansion; lined by ciliated mucosa operating the mucociliary escalator.

    • Bronchial Tree: Successive branching originating at the carina: primary (main) bronchi →\rightarrow secondary (lobar) bronchi →\rightarrow tertiary (segmental) bronchi →\rightarrow bronchioles.

    • Alveoli: Microscopic air sacs facilitating external gas exchange (continuous with the spongy tissue of the lungs):

    • Type I Alveolar Cells: Simple squamous epithelial cells forming the thin diffusion barrier.

    • Type II Alveolar Cells (Septal Cells): Secrete pulmonary surfactant to reduce alveolar surface tension and prevent atelectasis (alveolar collapse).

    • Alveolar Macrophages ("Dust Cells"): Phagocytose inhaled particulate matter and microorganisms.

    • Lungs: Paired organs enclosed within the pleural cavities:

    • Right Lung: Divided into three lobes (superior, middle, inferior) by horizontal and oblique fissures.

    • Left Lung: Divided into two lobes (superior, inferior) by an oblique fissure; contains the cardiac notch.

    • Parenchyma: Contains approximately 300 to 500 million alveoli with abundant elastic connective tissue stroma for passive recoil.

1.3 Functional Division

  1. Conducting Zone (External nares to terminal bronchioles) (where air is conducted from the outside to inside the lungs)

    • Transports, cleanses, humidifies, and warms incoming air to body temperature (37 ∘C37\,^{\circ}\text{C}).

    • Cartilage decreases progressively along the tract, while smooth muscle increases to enable autonomic regulation via bronchoconstriction (parasympathetic) and bronchodilation (sympathetic).

  2. Respiratory Zone (Respiratory bronchioles to alveoli) (where there is gas exchange)

    • Microscopic sites where external gas exchange occurs, including respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

    • Respiratory Membrane (Blood-Air Barrier): Formed by the alveolar epithelium, fused basement membranes, and capillary endothelial cells.

2. Basic Functions of the Respiratory System

2.1 Respiration

Provides metabolic cellular oxygenation and carbon dioxide waste removal via four integrated physiological stages:

  1. Pulmonary Ventilation: Mechanical movement of air into and out of the lungs (inspiration and expiration) driven by pressure gradients described by Boyle's Law.

  2. Pulmonary Gas Exchange (External Respiration): Passive diffusion of O2O_2 across the respiratory membrane into pulmonary capillaries, and CO2CO_2 from capillary blood into the alveoli.

  3. Gas Transport: Systemic circulatory transport of respiratory gases:

    • Oxygen transport: Predominantly bound to hemoglobin in erythrocytes (HbO2HbO_2), with a small portion dissolved in plasma.

    • Carbon dioxide transport: Converted into bicarbonate ions (HCO3−HCO_3^-) in plasma, bound to hemoglobin (HbCO2HbCO_2), and dissolved as gas in plasma.

  4. Tissue Gas Exchange (Internal Respiration): Passive diffusion of O2O_2 from systemic capillaries into peripheral cells, and CO2CO_2 from cellular metabolism into systemic blood.

2.2 Non-Respiratory Homeostatic Functions

  • Vocalization and Speech: Exhaled air vibrates vocal folds in the larynx; sound is resonated by paranasal sinuses and oral cavity, and articulated by the tongue, lips, and soft palate.

  • Olfaction: Olfactory receptor neurons in the superior nasal conchae and cribriform plate detect airborne chemicals.

  • Abdominal Pressure Regulation (Valsalva Maneuver): Glottis closure alongside thoracic and abdominal muscle contraction raises intra-abdominal pressure to assist in defecation, urination, vomiting, and childbirth.

  • Respiratory Pump: Cyclic changes between intrathoracic and intra-abdominal pressures during breathing facilitate venous return to the heart and enhance lymphatic drainage.

  • Acid-Base Regulation: Regulates systemic pH through ventilation depth and rate by shifting the carbonic acid-bicarbonate buffer equilibrium:   CO2+H2O⇌H2CO3⇌H++HCO3−CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-

    • Exhaling excess CO2CO_2 drives the reaction leftward, eliminating free H+H^+ to alleviate systemic acidosis.

  • Endocrine and Cardiovascular Regulation: Endothelial cells of pulmonary capillaries produce Angiotensin-Converting Enzyme (ACE), which converts angiotensin I into angiotensin II to regulate systemic vascular resistance and blood pressure.