Chapter 21: The Respiratory System

Module 21.1 Overview of the Respiratory System

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It is divided into anatomical and functional zones, each with specific roles in respiration.

• Major Functions:

  • Gas exchange (O2 in, CO2 out)

  • Regulation of blood pH

  • Voice production

  • Olfaction (smell)

  • Protection from inhaled pathogens and debris

• Upper vs. Lower Respiratory Tracts:

  • Upper tract: Nose, nasal cavity, pharynx, larynx

  • Lower tract: Trachea, bronchi, bronchioles, alveoli, lungs

• Conducting vs. Respiratory Zones:

  • Conducting zone: Passages that carry air (nose to terminal bronchioles)

  • Respiratory zone: Sites of gas exchange (respiratory bronchioles, alveolar ducts, alveoli)

• Four Respiratory Processes:

  1. Pulmonary ventilation: Movement of air in and out of lungs

  2. Pulmonary gas exchange: Exchange of gases between alveoli and blood

  3. Gas transport: Movement of gases in blood

  4. Tissue gas exchange: Exchange of gases between blood and tissues

Module 21.2 Anatomy of the Respiratory System

The respiratory system consists of a series of structures that conduct air and facilitate gas exchange. Each structure has a specialized function.

• Pathway of Air During Inspiration: Nose → Nasal cavity → Pharynx → Larynx → Trachea → Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles → Alveoli

• Main Functions of Structures:

  • Nose: Filters, warms, and moistens air; detects odors

  • Pharynx: Passageway for air and food

  • Larynx: Voice production; routes air and food

  • Trachea: Conducts air to bronchi; lined with cilia and mucus

  • Lungs: Main organs of respiration; contain alveoli for gas exchange

• Bronchial Tree:

  • Primary bronchi: Enter each lung

  • Secondary bronchi: Supply each lobe

  • Tertiary bronchi: Supply bronchopulmonary segments

  • Bronchioles: Smallest airways, lead to alveoli

• Alveoli and Respiratory Membrane:

  • Alveoli: Tiny air sacs for gas exchange

  • Type I alveolar cells: Simple squamous cells for gas diffusion

  • Type II alveolar cells: Secrete surfactant to reduce surface tension

  • Alveolar macrophages: Remove debris and pathogens

• Lungs, Pleural Membranes, and Cavities:

  • Lobes: Right lung (3), left lung (2)

  • Segments: Subdivisions of lobes

  • Lobules: Smallest subdivisions

  • Pleural membranes: Parietal and visceral layers surround lungs

  • Pleural cavity: Space with lubricating fluid

Module 21.3 Pulmonary Ventilation

Pulmonary ventilation is the process of moving air into and out of the lungs, driven by pressure and volume changes.

• Pressure-Volume Relationship: Described by Boyle's Law: (at constant temperature, pressure and volume are inversely related)

• Inspiration and Expiration:

  • Inspiration: Diaphragm and external intercostals contract, increasing thoracic volume

  • Expiration: Usually passive; internal intercostals and abdominal muscles assist during forced expiration

• Pressure Changes:

  • Atmospheric pressure (Patm): Pressure of air outside body

  • Intrapulmonary pressure (Ppul): Pressure within alveoli

  • Intrapleural pressure (Pip): Pressure within pleural cavity (always less than Ppul)

• Factors Affecting Ventilation:

  • Airway resistance: Increased resistance decreases airflow

  • Pulmonary compliance: Ease of lung expansion; decreased in fibrosis

  • Alveolar surface tension: Surfactant reduces tension, preventing collapse

• Respiratory Volumes and Capacities:

  • Tidal volume (TV): Air moved per breath (~500 mL)

  • Inspiratory reserve volume (IRV): Extra air inhaled after normal inspiration

  • Expiratory reserve volume (ERV): Extra air exhaled after normal expiration

  • Residual volume (RV): Air remaining after maximal exhalation

  • Vital capacity (VC):

  • Total lung capacity (TLC):

• Minute Volume: Total air moved per minute:

Module 21.4 Gas Exchange

Gas exchange occurs in the lungs and tissues, governed by physical laws describing gas behavior.

• Dalton’s Law: Total pressure of a gas mixture equals the sum of partial pressures of each gas:

• Henry’s Law: Amount of gas dissolved in a liquid is proportional to its partial pressure and solubility

• Pulmonary Gas Exchange: O2 moves from alveoli to blood; CO2 moves from blood to alveoli

  • Affected by surface area, membrane thickness, and ventilation-perfusion matching

• Tissue Gas Exchange: O2 moves from blood to tissues; CO2 from tissues to blood

  • Affected by capillary surface area, diffusion distance, and tissue perfusion

Module 21.5 Gas Transport through the Blood

Oxygen and carbon dioxide are transported in the blood by different mechanisms, with hemoglobin playing a central role.

• Oxygen Transport:

  • Bound to hemoglobin (as oxyhemoglobin)

  • Dissolved in plasma

  • Hemoglobin Saturation: Full saturation = all heme sites bound to O2; partial = some sites bound

• Oxygen-Hemoglobin Dissociation Curve: Shows relationship between partial pressure of O2 and hemoglobin saturation

• Carbon Dioxide Transport:

  • Dissolved in plasma

  • Bound to hemoglobin (as carbaminohemoglobin)

  • As bicarbonate ion (HCO3-) via carbonic acid-bicarbonate buffer system

• Carbonic Acid-Bicarbonate Buffer System:

  • CO2 + H2O H2CO3 $\leftrightarrow$ H+ + HCO3-

• Effect of CO2 on pH: Increased CO2 lowers pH (more acidic); decreased CO2 raises pH (more basic)

• Hyperventilation vs. Hypoventilation:

  • Hyperventilation: Excessive breathing, decreases CO2, increases pH (alkalosis)

  • Hypoventilation: Reduced breathing, increases CO2, decreases pH (acidosis)

Module 21.7 Neural Control of Ventilation

Breathing is regulated by neural centers in the brainstem, responding to chemical and mechanical signals.

• Respiratory Pattern Generator (RPG): Located in the medulla oblongata; sets basic rhythm

• Ventral Respiratory Group (VRG): Controls forced breathing

• Dorsal Respiratory Group (DRG): Integrates sensory input, influences VRG

• Effect of CO2 and H+: Increased levels stimulate increased respiratory rate

• Receptors:

  • Central chemoreceptors: In medulla; respond to CO2/H+ in CSF

  • Peripheral chemoreceptors: In carotid and aortic bodies; respond to O2, CO2, and pH

  • Stretch receptors: In lungs; prevent overinflation

Module 21.8 Diseases of the Respiratory System

Respiratory diseases can be classified based on their effect on lung function.

• Restrictive Lung Disease: Decreased lung compliance; reduced expansion (e.g., pulmonary fibrosis)

• Obstructive Lung Disease: Increased airway resistance; difficulty exhaling (e.g., asthma, COPD)

• Chronic Obstructive Pulmonary Disease (COPD): Progressive airflow limitation, often due to smoking

• Asthma: Reversible airway inflammation and constriction

• Lung Cancer: Malignant growth in lung tissue, often related to smoking

Example: In asthma (an obstructive disease), airway smooth muscle constricts, increasing resistance and making exhalation difficult.