The Respiratory System

Overview of the Respiratory System

The respiratory system is responsible for the exchange of gases between the body and the environment, providing oxygen to tissues and removing carbon dioxide. It consists of anatomical structures that facilitate air conduction, gas exchange, and protection against environmental hazards.

• Key Functions:

  • Extensive surface area for gas exchange between air and circulating blood

  • Transport of air to and from exchange surfaces

  • Protection from dehydration, temperature changes, and pathogens

  • Production of sounds for communication

  • Detection of olfactory stimuli

Anatomical Divisions of the Respiratory System

Upper Respiratory System

The upper respiratory tract filters, warms, and humidifies incoming air, protecting the delicate surfaces of the lower respiratory system.

• Main Structures:

  • Nose

  • Nasal cavity

  • Paranasal sinuses

  • Pharynx

Lower Respiratory System

The lower respiratory tract conducts air to and from the gas exchange surfaces of the lungs.

• Main Structures:

  • Larynx

  • Trachea

  • Bronchi

  • Bronchioles

  • Alveoli (site of gas exchange)

Note: Most of the respiratory system is conducting (no gas exchange); only alveoli are involved in gas exchange.

Respiratory Defense System

Protective Mechanisms

The respiratory defense system protects delicate gas exchange surfaces from debris and pathogens.

• Pseudostratified ciliated epithelium with mucous cells and lamina propria

• Mucus escalator: Secreted mucus traps particles; cilia sweep mucus and debris toward the pharynx, where it is swallowed and destroyed in the stomach

Structure of the Respiratory Epithelium

Cell Types and Functions

The respiratory tract is lined with specialized epithelial cells that vary by location.

• Ciliated columnar cells: Move mucus and trapped particles

• Mucous cells: Produce mucus for trapping debris

• Lamina propria: Connective tissue layer supporting the epithelium

• Basement membrane: Anchors epithelial cells

• Stem cells: Regenerate damaged epithelium

Phonation and Articulation

Sound Production in the Larynx

Speech production involves two processes: phonation and articulation.

• Phonation: Vibration of the vocal cords in the larynx produces sound waves. Pitch depends on the diameter, length, and tension of the vocal cords, controlled by voluntary muscles.

• Articulation: Modification of sounds by the tongue, teeth, and lips. Amplification and resonance occur in the pharynx, oral and nasal cavities, and paranasal sinuses.

Conducting Passageways in the Lower Respiratory Tract

Bronchi and Bronchioles

The lower respiratory tract includes conducting passageways that do not participate in gas exchange.

• Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles

• Bronchioles:

  • Walls lack cartilage; contain thick smooth muscle layers

  • Sympathetic stimulation causes bronchodilation (increased diameter and airflow)

  • Parasympathetic stimulation causes bronchoconstriction (decreased diameter and airflow)

  • Extreme bronchoconstriction can occur during allergic reactions (e.g., asthma)

Pulmonary Lobules and the Respiratory Membrane

Alveolar Structure and Function

Alveoli are the primary sites of gas exchange in the lungs.

• Each alveolus is supplied by an alveolar duct and attached to an alveolar sac

• Approximately 150 million alveoli per lung

• Surrounded by capillary networks and elastic fibers for expansion and recoil

Alveolar Cell Types

• Type I pneumocytes: Simple squamous epithelial cells for gas exchange

• Type II pneumocytes: Secrete surfactant to reduce surface tension and prevent alveolar collapse

• Alveolar macrophages: Patrol and remove debris and pathogens

Clinical Note: Without surfactant, alveoli would collapse due to high surface tension.

Respiration: External vs. Internal Respiration

Definitions and Comparisons

• External respiration: Exchange of gases between alveoli and blood

• Internal respiration: Exchange of gases between blood and tissues

Hypoxia: Low tissue oxygen levels; limits metabolic activity. Anoxia: Complete lack of oxygen; causes tissue damage (e.g., heart attack, stroke).

Ventilation and Pressure Changes

Mechanics of Breathing

Ventilation is the movement of air into and out of the lungs, driven by changes in thoracic volume and pressure.

• Inhalation: Thoracic volume increases, pressure falls, air flows in

• Exhalation: Thoracic volume decreases, pressure rises, air flows out

Equation:

Respiratory Muscles and Volumes

Muscles Involved in Breathing

• Quiet breathing:

  • Inhalation is active (diaphragm ~75%, external intercostals ~25%)

  • Exhalation is passive (relaxation of same muscles)

• Forced breathing: Accessory muscles are activated for forceful inhalation or exhalation

Respiratory Volumes and Capacities

• Tidal volume (VT): Air moved in/out per breath

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

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

• Residual volume: Air remaining after maximal exhalation

• Vital capacity:

• Inspiratory capacity:

• Functional residual capacity (FRC):

• Total lung capacity:

Ventilation: Respiratory Rate and Minute Volume

Key Definitions

• Respiratory rate: Number of breaths per minute (average 12-18/min)

• Respiratory minute volume (VE): Volume of air moved per minute

Equation: Where = breaths per minute, = tidal volume

Alveolar Ventilation

• Alveolar ventilation (VA): Amount of air reaching alveoli per minute

• Less than minute volume due to anatomic dead space () in conducting portions (~150 mL)

Equation:

Note: Slow, deep breaths ventilate alveoli more effectively than rapid, shallow breaths.

Gas Exchange and Transport

Partial Pressures and Gas Exchange

• External respiration: Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli

• Internal respiration: Oxygen diffuses from blood to tissues; carbon dioxide diffuses from tissues to blood

Oxygen Transport in Blood

• Hemoglobin (Hb): Protein with four subunits, each containing a heme group that binds oxygen

• 98.5% of oxygen is bound to hemoglobin; 1.5% is dissolved in plasma

• With bound oxygen, hemoglobin is called oxyhemoglobin (HbO2)

• Carbon monoxide can also bind to hemoglobin, preventing oxygen transport

Oxygen-Hemoglobin Saturation Curve

• Shows percentage of heme units bound to oxygen at different partial pressures

• S-shaped curve due to cooperative binding

• Blood entering systemic circuit: mm Hg, ~97% saturation

• Blood leaving peripheral tissues: mm Hg, ~75% saturation

• Active muscle tissue: drops to 15-20 mm Hg, saturation drops further

Factors Affecting Oxygen-Hemoglobin Binding

• pH (Bohr effect): Lower pH shifts curve right (more oxygen released); higher pH shifts curve left (less oxygen released)

• Temperature: Higher temperature shifts curve right (more oxygen released)

• 2,3-BPG: Increased levels shift curve right (more oxygen released)

Carbon Dioxide Transport

• CO2 is transported in three forms:

  • Dissolved in plasma (~7%)

  • Bound to hemoglobin (~23%)

  • As bicarbonate ions (HCO3-) (~70%)

Equation:

Regulation of Respiration

Neural and Chemical Control

• Baroreceptors in carotid and aortic sinuses monitor arterial blood pressure

• Drop in arterial pressure stimulates respiratory centers, increasing minute volume

• Increase in arterial pressure inhibits respiratory centers, decreasing minute volume

Impacts of Aging and Smoking on Lung Function

Aging and Respiratory Function

• Decreased lung compliance

• Decreased vital capacity and FEV1

• Diffusion capacity decreases

• Diaphragm muscles weaken

• Elastic tissue deteriorates

• Rib arthritis reduces compliance

• Some emphysema occurs

Lung Compliance

• Compliance: Distensibility of the lung

• Low compliance: Difficult inhalation, easy exhalation (e.g., fibrosis, pneumothorax)

• High compliance: Easy inhalation, difficult exhalation (e.g., emphysema)

Restrictive vs. Obstructive Lung Diseases

Smoking and Respiratory Function

• Emphysema worsens with smoking

• Respiratory function decreases with years of smoking

• Cigarette smoke contains carcinogens; 85-90% of lung cancer cases are from smoking

• Progression: Dysplasia (reversible) → Metaplasia (reversible) → Neoplasia/Anaplasia (irreversible)

Respiratory System and COVID-19

Clinical Note

• Some coronavirus patients experience "silent hypoxia" (low oxygen levels without symptoms)

• Research ongoing regarding mechanisms and detection

Summary Table: Key Respiratory Volumes and Capacities

Additional info: Some context and definitions were inferred and expanded for clarity and completeness.