The Respiratory System

Major Functions of the Respiratory System

• Gas Exchange: Supplies the body with oxygen (O2) and removes carbon dioxide (CO2).

• Regulation of Blood pH: By controlling CO2 levels, the respiratory system helps maintain acid-base balance.

• Voice Production: The movement of air across the vocal cords produces sound.

• Olfaction: The nasal cavity contains olfactory receptors for the sense of smell.

• Protection: Filters, warms, and humidifies incoming air; defends against pathogens and particulates.

Four Respiratory Processes

• Pulmonary Ventilation: Movement of air into and out of the lungs (breathing).

• External Respiration: Gas exchange between air in alveoli and blood in pulmonary capillaries.

• Transport: Movement of O2 and CO2 in the blood between lungs and tissues.

• Internal Respiration: Gas exchange between systemic blood and body tissues.

Anatomy of the Respiratory System

Upper vs. Lower Respiratory Tracts

• Upper Respiratory Tract: Nose, nasal cavity, paranasal sinuses, pharynx.

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

Conducting vs. Respiratory Zones

• Conducting Zone: All respiratory passageways that carry air to the sites of gas exchange (nose to terminal bronchioles); functions in air transport, humidification, warming, and filtration.

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

Pathway of Air During Inspiration

1. Nasal cavity

2. Pharynx

3. Larynx

4. Trachea

5. Primary bronchi

6. Secondary (lobar) bronchi

7. Tertiary (segmental) bronchi

8. Bronchioles

9. Terminal bronchioles

10. Respiratory bronchioles

11. Alveolar ducts

12. Alveoli

Gross Anatomy and Function of Key Structures

• Nasal Cavities: Filter, warm, and moisten air; contain olfactory receptors.

• Paranasal Sinuses: Lighten skull, warm and moisten air, enhance resonance of voice.

• Pharynx: Passageway for air and food; divided into nasopharynx, oropharynx, laryngopharynx.

• Larynx: Maintains open airway, routes food and air, houses vocal cords.

• Trachea: Windpipe; conducts air to bronchi, lined with ciliated mucosa.

• Bronchi: Branch into lungs; primary, secondary, tertiary divisions.

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

• Pleural Membranes: Double-layered serous membranes (parietal and visceral pleura) surrounding lungs; reduce friction.

• Pulmonary Blood Vessels: Carry deoxygenated blood to lungs and oxygenated blood to heart.

• Thoracic and Pleural Cavities: House and protect lungs; pleural cavity contains pleural fluid.

• Diaphragm: Main muscle of inspiration; separates thoracic and abdominal cavities.

Microscopic Anatomy and Function

• Respiratory (Nasal) Mucosa: Pseudostratified ciliated columnar epithelium with goblet cells; traps particles and moves mucus.

• Tracheal Wall Layers: Mucosa, submucosa, hyaline cartilage, adventitia.

• Bronchi and Bronchioles: Bronchi have cartilage and cilia; bronchioles lack cartilage, have smooth muscle.

• Alveolar Cell Types:

  • Type I: Simple squamous cells for gas exchange.

  • Type II: Secrete surfactant to reduce surface tension.

  • Alveolar macrophages: Phagocytize debris and pathogens.

• Respiratory Membrane: Thin barrier (alveolar epithelium, fused basement membrane, capillary endothelium) for efficient gas exchange.

Changes in Epithelial and Connective Tissue

• Epithelium transitions from pseudostratified ciliated columnar (trachea/bronchi) to simple cuboidal (bronchioles) to simple squamous (alveoli).

• Cartilage decreases, smooth muscle increases as airways branch.

• These changes facilitate air conduction, filtration, and gas exchange.

Mechanics of Breathing

Definitions

• Pulmonary Ventilation: Movement of air into and out of lungs.

• Inspiration: Air flows into lungs.

• Expiration: Air flows out of lungs.

Muscles of Breathing

• Quiet Inspiration: Diaphragm, external intercostals.

• Forced Inspiration: Sternocleidomastoid, scalenes, pectoralis minor.

• Forced Expiration: Internal intercostals, abdominal muscles.

Pressures Involved in Breathing

• Atmospheric Pressure (Patm): Pressure exerted by air surrounding the body.

• Intrapulmonary Pressure (Ppul): Pressure within alveoli.

• Intrapleural Pressure (Pip): Pressure within pleural cavity; always negative relative to Ppul.

• Transpulmonary Pressure: Difference between Ppul and Pip; keeps lungs inflated.

Boyle’s Law and Breathing

• Boyle’s Law: The pressure of a gas varies inversely with its volume at constant temperature.

• During inspiration, thoracic volume increases, intrapulmonary pressure decreases, and air flows in.

• During expiration, thoracic volume decreases, intrapulmonary pressure increases, and air flows out.

Factors Affecting Pulmonary Ventilation

• Bronchiolar Smooth Muscle Contraction: Constriction increases resistance, reducing airflow.

• Lung and Thoracic Wall Compliance: High compliance (stretchiness) facilitates expansion; recoil aids expiration.

• Airway Resistance: Described by (Flow = Pressure difference / Resistance).

• Pulmonary Surfactant: Reduces alveolar surface tension, preventing collapse.

Forces Affecting Lung Collapse

• Collapsing Forces: Elastic recoil of lung tissue, surface tension of alveolar fluid.

• Opposing Forces: Surfactant, negative intrapleural pressure, chest wall expansion.

Respiratory Volumes and Capacities

• Anatomical Dead Space: Volume of air in conducting passages not involved in gas exchange (~150 mL).

Gas Laws and Gas Exchange

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

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

• Henry’s Law explains why O2 and CO2 move between alveoli and plasma based on partial pressure gradients and solubility.

External Respiration (Lungs)

• O2 diffuses from alveoli (high PO2) to blood (low PO2).

• CO2 diffuses from blood (high PCO2) to alveoli (low PCO2).

• At high altitude, lower PO2 reduces O2 diffusion into blood.

Ventilation-Perfusion Coupling

• Reduced alveolar ventilation leads to decreased pulmonary blood flow (vasoconstriction).

• Reduced pulmonary blood flow leads to decreased alveolar ventilation (bronchoconstriction).

Internal Respiration (Tissues)

• O2 diffuses from blood (high PO2) to tissues (low PO2).

• CO2 diffuses from tissues (high PCO2) to blood (low PCO2).

Oxygen and Carbon Dioxide Transport

• Oxygen Transport: 98.5% bound to hemoglobin (Hb), 1.5% dissolved in plasma.

• CO2 Transport: Dissolved in plasma, bound to Hb, or as bicarbonate ions (HCO3-).

• Factors Affecting O2 Loading/Unloading: Increased temperature, decreased pH, increased PCO2 shift the O2-Hb dissociation curve right (Bohr effect), enhancing O2 unloading to tissues.

• Increased PO2 increases Hb saturation; decreased PO2 decreases saturation.

Oxygen-Hemoglobin Saturation Curve

• Right Shift (Down and Right): Increased temperature, PCO2, 2,3-BPG, or decreased pH; promotes O2 unloading at tissues.

• Left Shift (Up and Left): Decreased temperature, PCO2, 2,3-BPG, or increased pH; promotes O2 loading in lungs.

Control of Respiration

• Brainstem Respiratory Centers: Medullary respiratory centers (ventral and dorsal groups) set rhythm; pontine centers modify rhythm.

• Chemical Stimuli: CO2 (most potent), O2, and pH detected by central and peripheral chemoreceptors.

• Neural Stimuli: Stretch receptors, irritant receptors, voluntary control, and emotional input.

Factors Affecting Respiratory Rate and Depth

• Arterial pH: Decreased pH (acidosis) increases rate/depth; increased pH (alkalosis) decreases rate/depth.

• PO2: Significant effect only when very low.

• PCO2: Small increases cause large increases in ventilation.

• Lung Reflexes: Hering-Breuer reflex prevents overinflation.

• Voluntary Control/Emotions: Can override automatic centers temporarily.

Respiratory Terminology

• Hyperventilation: Increased rate/depth of breathing, lowers CO2.

• Hypoventilation: Decreased rate/depth, raises CO2.

• Panting: Rapid, shallow breathing.

• Eupnea: Normal, quiet breathing.

• Hyperpnea: Increased ventilation in response to metabolic need (e.g., exercise).

• Apnea: Temporary cessation of breathing.

Homeostatic Responses and Disease

• High Altitude: Lower PO2 stimulates increased ventilation and erythropoiesis.

• Exercise: Increases ventilation to meet metabolic demand.

• Hyperventilation: Decreases CO2, may cause alkalosis.

Diseases Affecting Homeostasis

• Emphysema: Destruction of alveolar walls, loss of elasticity, impaired gas exchange.

• Asthma: Bronchoconstriction and inflammation increase airway resistance.

• Tuberculosis (TB): Infectious disease causing granuloma formation and reduced lung compliance.

• Chronic Bronchitis: Chronic inflammation and mucus production obstruct airways.

Additional info: This guide expands on the objectives by providing definitions, explanations, and examples for each major concept in respiratory anatomy and physiology. For more detail, refer to textbook figures (e.g., 22.24 for the O2-Hb curve, 22.27 for control of breathing).