The Respiratory System: Structure, Function, and Gas Exchange

The Respiratory System

Overview and Major Functions

The respiratory system is essential for supplying the body with oxygen (O2) required for cellular respiration and for removing carbon dioxide (CO2), a metabolic waste product. It also plays roles in olfaction (sense of smell) and speech production. The processes of respiration involve both the respiratory and circulatory systems, ensuring efficient gas exchange and transport throughout the body.

• Major Functions:

  • Supplying O2 for cellular respiration

  • Disposing of CO2

  • Olfaction and speech

• Four Processes of Respiration:

  1. Pulmonary ventilation: Movement of air into and out of the lungs (breathing)

  2. External respiration: Exchange of O2 and CO2 between lungs and blood

  3. Transport: Movement of O2 and CO2 in the blood

  4. Internal respiration: Exchange of O2 and CO2 between systemic blood vessels and tissues

Anatomy of the Respiratory System

Upper and Lower Respiratory Tract

The respiratory system is divided into the upper and lower respiratory tracts, each with specialized structures and functions.

• Upper Respiratory Tract: Nose, nasal passages, sinuses, pharynx

• Lower Respiratory Tract: Larynx, trachea, bronchi, bronchioles, lungs, alveoli

Upper Respiratory Tract: Structure and Function

The upper respiratory tract filters, warms, and humidifies incoming air, and provides a passageway for respiration. It also houses olfactory receptors and contributes to voice resonance.

• Filters large foreign material and microorganisms (trapped in mucus)

• Moistens and warms air

• Provides resonating chambers for voice

Lower Respiratory Tract: Structure and Function

The lower respiratory tract is responsible for gas exchange and includes the larynx, trachea, bronchi, bronchioles, and alveoli.

• Larynx: Maintains an open airway, routes air and food, and houses the vocal folds for sound production.

• Trachea: The windpipe, supported by C-shaped cartilage rings, transports air and is lined with ciliated epithelium to trap and remove debris.

• Bronchi and Bronchioles: Branching airways that transport, clean, warm, and humidify air. Bronchi contain cartilage; bronchioles do not.

• Alveoli: Tiny air sacs where gas exchange occurs.

Lungs and Bronchial Tree

The lungs occupy the thoracic cavity and are composed primarily of alveoli. The right lung has three lobes; the left lung has two lobes and a cardiac notch for the heart. The bronchial tree consists of branching bronchi and bronchioles, ending in alveolar sacs.

Mechanics of Breathing

Pulmonary Ventilation

Breathing consists of two phases: inspiration (air enters lungs) and expiration (air exits lungs). These processes depend on pressure changes in the thoracic cavity relative to atmospheric pressure.

• Inspiration: Diaphragm contracts and moves downward, external intercostals contract, thoracic volume increases, intrapulmonary pressure drops, and air flows in.

• Expiration: Diaphragm and intercostals relax, thoracic volume decreases, intrapulmonary pressure rises, and air flows out.

Pressure Relationships

• Atmospheric pressure (Patm): 760 mm Hg at sea level

• Negative respiratory pressure: Less than Patm

• Positive respiratory pressure: Greater than Patm

• Zero respiratory pressure: Equal to Patm

Respiratory Volumes and Capacities

• Tidal Volume (TV): Normal, quiet breathing (~500 mL)

• Inspiratory Reserve Volume (IRV): Additional air that can be inspired after TV

• Expiratory Reserve Volume (ERV): Additional air that can be expired after TV

• Residual Volume (RV): Air remaining after forced expiration

• Inspiratory Capacity (IC): Total air that can be inspired after normal expiration

• Functional Residual Capacity (FRC): Air remaining after normal expiration

• Vital Capacity (VC): Total exchangeable air

• Total Lung Capacity (TLC): Sum of all volumes

Dead Space

• Anatomical Dead Space: Air in passageways not involved in gas exchange (~150 mL)

• Alveolar Dead Space: Non-functional alveoli due to collapse or obstruction

• Total Dead Space: Sum of anatomical and alveolar dead space

Nonrespiratory Air Movements

• Include cough, sneeze, crying, laughing, hiccups, and yawns

• Mostly reflex actions, some voluntary

Gas Exchange and Transport

External and Internal Respiration

Gas exchange occurs by diffusion: O2 moves from alveoli to blood, CO2 moves from blood to alveoli (external respiration); O2 moves from blood to tissues, CO2 moves from tissues to blood (internal respiration).

Oxygen Transport

• 1.5% dissolved in plasma

• 98.5% bound to hemoglobin (Hb) in red blood cells (RBCs)

• Each Hb molecule binds up to 4 O2 molecules (as oxyhemoglobin, HbO2)

Carbon Dioxide Transport

• 7–10% dissolved in plasma

• 20% bound to globin of hemoglobin (carbaminohemoglobin)

• 70% transported as bicarbonate ions (HCO3–) in plasma

CO2 Transport and Exchange

CO2 combines with water to form carbonic acid (H2CO3), which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3–). This reaction is catalyzed by the enzyme carbonic anhydrase in RBCs.

Equation:

$\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

Influence of CO2 on Blood pH

The carbonic acid–bicarbonate buffer system helps maintain blood pH. Changes in respiratory rate and depth can adjust blood pH by altering CO2 levels:

• Slow, shallow breathing increases CO2 (acidosis)

• Rapid, deep breathing decreases CO2 (alkalosis)

Respiratory Disorders Caused by Microorganisms

Common Respiratory Infections

• Colds: Caused by over 100 different viruses; symptoms include coughing, runny nose, congestion, sneezing. Antibiotics are ineffective.

• Influenza: Caused by influenza virus; symptoms include sore throat, fever, cough, aches, chills, muscle pain. Can lead to pneumonia, especially in vulnerable populations. Annual vaccination is recommended.

Pneumonia

• Infection of the lungs caused by bacteria or viruses

• Alveoli fill with fluid, interfering with gas exchange

• Symptoms: fever, chills, shortness of breath, productive cough, chest pain

• Treatment depends on the cause

Tuberculosis

• Caused by Mycobacterium tuberculosis (bacteria)

• May be active (with symptoms) or dormant (reactivates later)

• Diagnosis: tuberculin skin test, chest X-ray

• Treatment: antibiotics