12. RESPIRATORY SYSTEM

RESPIRATORY SYSTEM

The respiratory tract forms the path through which air passes from the nose to the lungs. Exchange of gases during internal and external respiration is the major function of the respiratory system, along with filtering, warming, and humidifying inhaled air. The respiratory system includes the vocal cords for producing sound, lungs for controlling body pH levels, and olfactory bulbs for smelling.

EVENTS DURING RESPIRATION

1. Ventilation of lungs for inward and outward movement of air
2. Exchange of gases between blood and alveolar air
3. Excretion of water vapour
4. Supplying air to larynx for voice production

PARTS OF RESPIRATORY SYSTEM

The human respiratory system consists of: nose, pharynx (throat), larynx (voice box), trachea (windpipe), bronchi and bronchioles, lungs, and alveoli.

Functional Divisions

• Conducting Zone (Nose to Bronchioles): Transports air to alveoli, filters, humidifies, and maintains temperature.
• Respiratory Zone (Alveolar Duct to Alveoli): Site of O₂ and CO₂ exchange between blood capillaries and alveoli.

Anatomical Divisions

• Upper Respiratory Tract: Nose, pharynx, larynx
• Lower Respiratory Tract: Trachea, bronchi, bronchioles, alveolar duct, alveoli

NOSE

Functions of Nose

1. Respiration: Major pathway for inhaled air (middle meatus); nasal cycle every 30-180 minutes.
2. Air Conditioning: Warms air to 31-37°C and humidifies to 100%.
3. Defence: Mucociliary system traps 95% of particles; IgA antibodies; sneezing reflex.
4. Vocal Resonance and Speech: Provides sound quality; paranasal sinuses act as voice resonators.

PHARYNX

Parts of Pharynx

• Nasopharynx: Posterior to nasal cavity; extends from hard/soft palate to skull base.
• Oropharynx: Posterior to oral cavity; contains tonsils.
• Hypopharynx (Laryngopharynx): Inferior to oropharynx; superior to oesophagus (C3-C6 vertebrae).

Functions of Pharynx

1. Passageway for air and food
2. Taste (olfactory nerve endings)
3. Warming and humidifying inhaled air
4. Hearing (auditory tube to middle ear)
5. Protection (tonsils – immune defence)
6. Speech (resonator for sound from larynx)

LARYNX

Cartilages of Larynx

• Single Cartilages: Thyroid (largest, Adam’s apple), Cricoid (signet ring-like), Epiglottis (leaf-like)
• Paired Cartilages: Arytenoids (pyramid-shaped), Corniculates (small conical nodules), Cuneiforms (narrow, elongated)

Functions of Larynx

1. Phonation: Vocal cords vibrate to produce sound
2. Protection: Epiglottis prevents food/liquids from entering lungs
3. Respiration: Regulates airflow via glottis
4. Coughing and Sneezing: Reflex actions to clear airways
5. Speech Articulation: Provides basic sound source for speech

TRACHEA

The trachea (windpipe) is a 10-11 cm long pathway from the larynx to the 5th thoracic vertebra, where it splits into right and left bronchi. It is composed of 16-20 C-shaped rings of hyaline cartilage.

Functions of Trachea

1. Prevents airway kinking during head/neck movements
2. Acts as mucociliary escalator
3. Warms, humidifies, and filters inhaled air
4. Generates cough reflex
5. Transports air to and from lungs
6. Defends against microorganisms
7. Regulates temperature of inhaled air

BRONCHI AND BRONCHIOLES

The right primary bronchus is shorter and wider than the left. Secondary bronchi (3 right, 2 left) divide into tertiary (segmental) bronchi → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.

Functions of Bronchi and Bronchioles

1. Provide passageway for air
2. Regulate volume of air entering lungs (muscle contraction/relaxation)
3. Warm and humidify inhaled air
4. Filter air by removing particulate matter
5. Generate cough reflex

LUNGS

Lungs are two cone-shaped lobes in the thoracic cavity. The left lung has a cardiac notch and is smaller; the right lung is thicker, broader, and shorter. Each lung is enclosed in a pleural membrane (parietal and visceral pleura) with pleural fluid in between.

Parts of Lungs

• Apex: Round, extends to root of neck
• Base: Concave, semilunar, associated with diaphragm
• Costal Surface: Convex, associated with ribs and intercostal muscles
• Medial Surface: Concave, contains hilum (primary bronchus, pulmonary artery, pulmonary veins)

Functions of Lungs

1. Alter blood pH by altering partial pressure of CO₂
2. Filter small blood clots formed in veins
3. Filter gas micro-bubbles from venous blood
4. Alter blood concentration of some drugs
5. Convert angiotensin I to angiotensin II (ACE)
6. Form shock-absorbent protective layer for heart
7. Immunoglobulin-A in bronchial secretions protects against infections

ALVEOLI

Alveoli are hollow cavities in mammalian lungs (approx. 300 million per lung). Average diameter: 200-300 µm. They are the major sites of gaseous exchange with blood.

Cell Types in Alveoli

• Squamous (Type I) Alveolar Cells: 95% of surface area; thin, broad; rapid gaseous diffusion
• Cuboidal (Type II) Alveolar Cells: 5% of surface area; repair damaged cells; secrete pulmonary surfactant
• Alveolar Macrophages (Dust Cells): Most numerous; phagocytosis of debris, dust, bacteria

Functions of Alveoli

1. External Respiration: Gaseous exchange between alveoli and blood by diffusion
2. Protection Against Microbes: Lymphocytes, plasma cells, and macrophages
3. Surfactant Secretion (Type II cells): Lowers surface tension, prevents alveolar collapse

MECHANISM OF BREATHING

Inhalation (Inspiration)

1. Contraction of diaphragm (becomes flat) and external intercostals
2. Thoracic cavity increases in size (height, antero-posterior, lateral dimensions)
3. Intrapulmonary volume increases; gas pressure decreases
4. Partial vacuum sucks air into lungs until pressure equilibrium

Exhalation (Expiration)

1. Relaxation of inspiratory muscles; rib cage descends; lungs recoil
2. Thoracic and intrapulmonary volumes decrease
3. Intrapulmonary pressure increases above atmospheric pressure
4. Gases flow out until pressure equilibrium

RESPIRATORY VOLUMES AND CAPACITIES

Lung Volumes

• Tidal Volume (TV): 500 ml – air inspired/expired during normal respiration
• Inspiratory Reserve Volume (IRV): 2500-3000 ml – additional air inspired forcibly
• Expiratory Reserve Volume (ERV): 1000-1100 ml – additional air expired forcibly
• Residual Volume (RV): 1100-1200 ml – air remaining in lungs after forcible expiration

Lung Capacities

• Inspiratory Capacity (IC): TV + IRV ≈ 3500 ml
• Expiratory Capacity (EC): TV + ERV ≈ 1600 ml
• Functional Residual Capacity (FRC): ERV + RV ≈ 2300 ml
• Vital Capacity (VC): ERV + TV + IRV ≈ 4600 ml
• Total Lung Capacity (TLC): VC + RV ≈ 5800 ml