Comprehensive Study Notes: The Respiratory System

The Respiratory System

Functions of the Respiratory System

The respiratory system is essential for gas exchange, pH regulation, voice production, and olfaction. It supports cellular metabolism and maintains homeostasis.

• Gas Exchange: Oxygen enters the blood for cellular respiration; carbon dioxide, a waste product, is removed.

• pH Regulation: Carbon dioxide (CO2) levels directly affect blood pH. The lungs help maintain acid-base balance by removing CO2.

• Voice Production: Air passing through the vocal cords produces sound.

• Olfaction: Smell receptors located in the nasal cavity detect odors.

Anatomy & Airflow Pathway

Air travels through a series of structures before reaching the alveoli, where gas exchange occurs.

• Airflow Pathway: Nose → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

Upper Respiratory Structures

• Nose/Nasal cavity: Filters (hairs), warms (blood vessels), and moistens (mucus) incoming air.

• Pharynx (throat): Divided into nasopharynx, oropharynx, and laryngopharynx.

Lower Respiratory Structures

• Larynx: Known as the "voice box"; contains vocal cords.

• Epiglottis: Covers the airway during swallowing to prevent aspiration.

• Trachea: Reinforced by C-shaped cartilage rings to keep the airway open.

• Bronchial Tree: Branches from primary to secondary to tertiary bronchi, then bronchioles and terminal bronchioles.

Alveoli

Alveoli are the primary sites of gas exchange, featuring specialized cells and a large surface area.

• Structure: Composed of simple squamous epithelium, surrounded by capillaries, providing a huge surface area.

• Cells:

  • Type I cells: Responsible for gas exchange (about 90% of alveolar cells).

  • Type II cells: Produce surfactant, a substance that reduces surface tension.

  • Dust cells: Alveolar macrophages that remove debris and pathogens.

• Surfactant: Reduces surface tension and prevents alveolar collapse (atelectasis). Premature infants may lack surfactant, leading to respiratory distress.

Mechanics of Breathing

Breathing involves pressure changes and muscle contractions, governed by physical laws.

• Pressure Relationships: Air moves from areas of high to low pressure. Boyle’s Law states that pressure and volume are inversely related: $P_1V_1 = P_2V_2$

• Pressure Gradients: Include atmospheric, intrapulmonary, and intrapleural pressures.

Inspiration (Inhalation)

• Main inspiratory muscles: diaphragm and external intercostals.

• Diaphragm contracts (moves downward), external intercostals contract, thoracic volume increases, pressure decreases, and air flows in.

Expiration (Exhalation)

• Diaphragm relaxes, thoracic volume decreases, pressure increases, and air flows out.

• Normally passive due to elastic recoil of lung tissue.

Physical Factors Affecting Gas Transport

• Airway resistance

• Alveolar surface tension

• Pulmonary compliance

Lung Volumes & Capacities

Lung volumes and capacities are important for assessing respiratory health and function.

• Tidal Volume (TV): Normal breath (~500 ml).

• Inspiratory Reserve Volume (IRV): Extra volume inhaled after a normal inspiration.

• Expiratory Reserve Volume (ERV): Extra volume exhaled after a normal expiration.

• Residual Volume (RV): Air remaining after maximal exhalation; prevents lung collapse.

• Vital Capacity: Total amount of air that can be moved in and out of the lungs.

• Forced Vital Capacity: Used to diagnose pulmonary diseases.

The Behavior of Gases

Gas exchange is governed by physical laws describing partial pressures and solubility.

• Dalton’s Law of Partial Pressures: The total pressure of a gas mixture is the sum of the partial pressures of individual gases.

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

Pulmonary (Alveolar) Gas Exchange (External Respiration)

Gas exchange between alveoli and blood occurs by diffusion, driven by partial pressure gradients.

• Oxygen: High concentration in alveoli diffuses into blood.

• Carbon dioxide: High concentration in blood diffuses into alveoli.

• Mechanism: Passive diffusion; no energy required.

• Factors Affecting Efficiency:

  • Surface area of respiratory membrane

  • Thickness of respiratory membrane

  • Ventilation-perfusion matching (coupling)

Tissue Gas Exchange (Internal Respiration)

Gas exchange between blood and tissues is influenced by surface area, distance, and tissue perfusion.

• Factors Affecting Efficiency:

  • Available surface area

  • Distance between capillaries and cells

  • Perfusion of tissue

Gas Transport in Blood

Oxygen and carbon dioxide are transported in the blood by different mechanisms.

Oxygen Transport

• ~98% bound to hemoglobin (Hb) in red blood cells.

• ~2% dissolved in plasma.

• Each hemoglobin molecule binds up to 4 O2 molecules.

• Bohr Effect: Increased CO2 and H+ lower hemoglobin's affinity for O2, promoting oxygen release.

• BPG (2,3-bisphosphoglycerate): Decreases hemoglobin's affinity for O2.

Carbon Dioxide Transport

• ~70% as bicarbonate (HCO3-)

• ~23% bound to hemoglobin

• ~7% dissolved in plasma

Neural Control of Breathing

Breathing is regulated by respiratory centers in the brainstem and chemoreceptors sensitive to CO2 and O2 levels.

• Three pairs of respiratory centers in the reticular formation of the medulla oblongata and pons:

  • VRG (Ventral Respiratory Group): Primary control center.

  • DRG (Dorsal Respiratory Group)

  • PRG (Pontine Respiratory Group)

• Central and Peripheral Chemoreceptors: Monitor CO2, O2, and pH.

• Main Stimulus: Increased CO2 leads to increased H+ and decreased pH, stimulating breathing.

• Secondary Stimulus: Low O2 (only in extreme cases).

Hypoxia vs Hypercapnia

• Hypoxia: Low oxygen levels in tissues.

• Hypercapnia: High CO2 levels in blood.

• CO2 is the main regulator of breathing.

pH Balance (Respiratory Role)

The respiratory system helps regulate blood pH by controlling CO2 removal.

• CO2 combines with H2O to form carbonic acid: $\mathrm{CO_2 + H_2O \rightarrow H_2CO_3}$

• More CO2 increases acidity (lowers pH).

• Lungs remove CO2 to raise pH.

Respiratory Diseases

Respiratory diseases are classified as infectious, restrictive, or obstructive, affecting lung function in different ways.

Infectious Respiratory Diseases

• COVID-19

• Influenza

Restrictive Lung Diseases

• Idiopathic pulmonary fibrosis

• Pneumoconiosis

• Neuromuscular diseases and chest wall deformities

Obstructive Lung Diseases

• COPD (Chronic Obstructive Pulmonary Disease):

  • Emphysema

  • Small airway disease

  • Chronic bronchitis

  • Asthma

  • Lung cancer

Summary Table: Lung Volumes and Capacities

Volume/Capacity	Definition	Typical Value
Tidal Volume (TV)	Amount of air inhaled or exhaled in a normal breath	~500 ml
Inspiratory Reserve Volume (IRV)	Extra volume inhaled after normal inspiration	~3000 ml
Expiratory Reserve Volume (ERV)	Extra volume exhaled after normal expiration	~1200 ml
Residual Volume (RV)	Air remaining after maximal exhalation	~1200 ml
Vital Capacity (VC)	Total amount of air that can be moved in and out of the lungs	~4700 ml

Example: Forced Vital Capacity (FVC) is measured during spirometry to assess lung function and diagnose diseases such as COPD and restrictive lung disorders.

Additional info: Typical values for lung volumes are inferred from standard physiology textbooks.