Breathing and Exchange of Gases (Pulmonary Physiology)

14.1 Respiratory Organs

Mechanisms of Breathing Across Animal Groups

• Lower Invertebrates (Sponges, Coelenterates, Flatworms): Exchange O₂ and CO₂ via simple diffusion over entire body surface
• Earthworms: Use moist cuticle for gas exchange
• Insects: Utilize network of tubes called tracheal tubes to transport atmospheric air within body NEET 1998
• Aquatic Arthropods and Molluscs: Respire through special vascularised structures called gills (branchial respiration)
• Vertebrates: Fishes use gills; amphibians, reptiles, birds, and mammals use vascularised bags called lungs (pulmonary respiration)
• Amphibians (e.g., Frogs): Can also respire through moist skin, known as cutaneous respiration NEET 1990

14.1.1 Human Respiratory System

Anatomical Pathway: External nostrils → nasal chamber → pharynx → larynx → trachea → primary/secondary/tertiary bronchi → terminal bronchioles → alveoli NEET 1994

Key Structures

• Pharynx: Common passage for both food and air
• Larynx: Cartilaginous box that helps in sound production (sound box)
• Epiglottis: Thin elastic cartilaginous flap that covers glottis during swallowing to prevent food entry into larynx
• Trachea: Straight tube extending up to mid-thoracic cavity. Divides into right and left primary bronchi at level of 5th thoracic vertebra
• Cartilaginous Rings: Incomplete rings support trachea, primary, secondary, tertiary bronchi, and initial bronchioles to prevent collapse Odisha NEET 2019
• Alveoli: Very thin, irregular-walled, vascularised bag-like structures at ends of terminal bronchioles Odisha NEET 2019. Do not collapse entirely after forceful expiration due to Residual Volume NEET 2017
• Lungs: Comprise branching network of bronchi, bronchioles, and alveoli. Covered by double-layered pleural membrane NEET 1996
• Pleural Fluid: Present between layers to reduce friction on lung surface. Outer pleural membrane contacts thoracic lining; inner membrane contacts lung surface

Functional Divisions

• Conducting Part (Nostrils to Terminal Bronchioles): Transports atmospheric air to alveoli, clears foreign particles, humidifies air, brings air to body temperature
• Exchange Part (Alveoli and their ducts): Actual site of diffusion of O₂ and CO₂ between blood and atmospheric air

Thoracic Chamber

Anatomically air-tight chamber formed:

• Dorsally: Vertebral column
• Ventrally: Sternum
• Laterally: Ribs
• Lower side: Dome-shaped diaphragm

Changes in thoracic volume are directly reflected in pulmonary (lung) volume.

Sequence of Respiration Steps

1. Pulmonary ventilation (breathing)
2. Diffusion of gases (O₂ and CO₂) across alveolar membrane
3. Transport of gases by blood
4. Diffusion of O₂ and CO₂ between blood and tissues
5. Utilisation of O₂ by cells for catabolic reactions and release of CO₂ NEET 2023 - Manipur

14.2 Mechanism of Breathing

Inspiration and Expiration

Inspiration Process

• Occurs when intra-pulmonary pressure is lower than atmospheric pressure (negative pressure in lungs) NEET 2020
• Initiated by contraction of diaphragm, which increases volume of thoracic chamber in antero-posterior axis Odisha NEET 2019
• Contraction of external inter-costal muscles lifts ribs and sternum, increasing thoracic volume in dorso-ventral axis
• Overall: ↑ thoracic volume → ↑ pulmonary volume → ↓ intra-pulmonary pressure → air moves into lungs NEET 2020, 2018

Expiration Process

• Takes place when intra-pulmonary pressure is higher than atmospheric pressure
• Triggered by relaxation of diaphragm and inter-costal muscles, returning diaphragm and sternum to normal positions
• Reduces thoracic/pulmonary volume → increases intra-pulmonary pressure → forces air out
• Abdominal Muscles: Help increase strength of both inspiration and expiration

Breathing Rate: Healthy human breathes 12-16 times/minute
Spirometer: Used for clinical assessment of pulmonary functions by estimating volume of air involved in breathing
Crucial Detail: Lungs do not collapse between breaths primarily because negative intrapleural pressure constantly pulls at lung walls NEET-II 2016

14.2.1 Respiratory Volumes and Capacities

Note: All volumes and capacities below have frequently appeared as direct match-the-following questions NEET 2024, 2024 Re, 2018

Respiratory Volume / Capacity	Abbreviation	Definition	Normal Value / Formula
Tidal Volume	TV	Volume of air inspired or expired during normal respiration	500 mL (6000-8000 mL/min)
Inspiratory Reserve Volume	IRV	Additional volume of air a person can inspire by forcible inspiration	2500 - 3000 mL
Expiratory Reserve Volume	ERV	Additional volume of air a person can expire by forcible expiration	1000 - 1100 mL
Residual Volume	RV	Volume of air remaining in lungs even after forcible expiration	1100 - 1200 mL
Inspiratory Capacity	IC	Total volume of air a person can inspire after normal expiration	TV + IRV
Expiratory Capacity	EC	Total volume of air a person can expire after normal inspiration NEET 2019	TV + ERV
Functional Residual Capacity	FRC	Volume of air that remains in lungs after normal expiration	ERV + RV
Vital Capacity	VC	Maximum air a person can breathe in after forced expiration (or breathe out after forced inspiration) NEET 2023, 2022 Re, 2009	ERV + TV + IRV
Total Lung Capacity	TLC	Total volume of air accommodated in lungs at end of forced inspiration NEET 2020 - Covid	RV + ERV + TV + IRV (or VC + RV)

14.3 Exchange of Gases

Diffusion Dynamics and Partial Pressures

• Alveoli are primary sites of gas exchange; exchange also occurs between blood and tissues
• Gas exchange strictly follows simple diffusion based on pressure/concentration gradients NEET 1998
• Factors affecting diffusion rate: Solubility of gases, thickness of membranes, and partial pressure gradients

Partial Pressures (in mm Hg) of Respiratory Gases

NEET 2021, NEET-II 2016

Gas	Atmospheric Air	Alveoli	Deoxygenated Blood	Oxygenated Blood	Tissues
O₂	159	104	40	95	40
CO₂	0.3	40	45	40	45

Solubility Exception: Solubility of CO₂ is 20-25 times higher than that of O₂. Thus, amount of CO₂ that can diffuse through membrane per unit pressure difference is much higher than O₂.

The Diffusion Membrane

• Total thickness is much less than a millimetre
• Composed of three major layers NEET 1997, 1990:
  1. Thin squamous epithelium of alveoli
  2. Endothelium of alveolar capillaries
  3. Basement substance in between them (basement membrane supporting squamous epithelium and basement membrane surrounding capillary endothelial cells)

14.4 Transport of Gases

Overview of Transport Mediums

O₂ Transport

• 97% transported by RBCs in blood NEET 2010
• 3% carried in dissolved state through plasma

CO₂ Transport

NEET 1995

• 70% carried as bicarbonate (HCO₃⁻) - represents bulk of CO₂ transport NEET 2014, 2011, 2006
• 20-25% transported by RBCs (as carbamino-haemoglobin)
• 7% carried in dissolved state through plasma

14.4.1 Transport of Oxygen

• O₂ binds reversibly with haemoglobin (Hb) to form oxyhaemoglobin
• Each haemoglobin molecule can carry a maximum of four molecules of O₂
• Binding is primarily related to partial pressure of O₂ (pO₂) NEET 2020

Oxygen Dissociation Curve

A sigmoid curve is obtained when percentage saturation of Hb with O₂ is plotted against pO₂.

Conditions Favourable for Oxyhaemoglobin FORMATION (in Alveoli)

NEET 2024, 2021

• High pO₂
• Low pCO₂
• Lesser H⁺ concentration (High pH)
• Lower temperature

Conditions Favourable for Oxyhaemoglobin DISSOCIATION (in Tissues)

NEET-I 2016

• Low pO₂
• High pCO₂
• High H⁺ concentration (Low pH)
• Higher temperature

Delivery: Every 100 ml of oxygenated blood can deliver around 5 ml of O₂ to tissues under normal physiological conditions

Crucial Detail: Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin NEET 2009

14.4.2 Transport of Carbon Dioxide

• CO₂ is carried by haemoglobin as carbamino-haemoglobin (20-25%)
• This binding is related to pCO₂, but pO₂ is a major interfering factor NEET 2024 Re
• In Tissues: High pCO₂ and low pO₂ → more binding of CO₂
• In Alveoli: Low pCO₂ and high pO₂ → dissociation of CO₂

Carbonic Anhydrase

An enzyme present in very high concentration in RBCs and minute quantities in plasma. It acts in both directions:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H

• At tissues (high pCO₂): CO₂ diffuses into blood and forms HCO₃⁻ and H⁺. Blood buffers play role in preventing blood from becoming highly acidic NEET 1995
• At alveoli (low pCO₂): Reaction reverses to form CO₂ and H₂O, releasing CO₂

Delivery: Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO₂ to alveoli.

14.5 Regulation of Respiration

Neural and Chemical Controls

Respiratory Rhythm Centre

Located in medulla region of brain; primarily responsible for respiration regulation NEET 1999, 1994

Pneumotaxic Centre

Located in pons region; moderates functions of rhythm centre. Neural signals from this centre can reduce duration of inspiration, altering respiratory rate NEET 2012

Chemosensitive Area

Situated adjacent to rhythm centre. Highly sensitive to CO₂ and hydrogen ions (H⁺). Increases in these trigger rhythm centre to eliminate them NEET 2015 Cancelled, 2004

Aortic Arch and Carotid Artery Receptors

Also recognize changes in CO₂ and H⁺ concentration and send corrective signals to rhythm centre.

Crucial Detail: The role of oxygen in regulation of respiratory rhythm is quite insignificant.

14.6 Disorders of Respiratory System

Pathological and Occupational Conditions

Asthma

Difficulty in breathing causing wheezing, exclusively due to inflammation of bronchi and bronchioles NEET 2019, 2018, NEET-I 2016

Emphysema

A chronic disorder characterized by damage of alveolar walls, which drastically decreases respiratory surface area. Major cause is cigarette smoking NEET 2018, NEET-I 2016, 2015

Occupational Respiratory Disorders

Industries involving grinding or stone-breaking (e.g., producing silica/dust) overwhelm body's defense mechanism. Long-term exposure leads to fibrosis (proliferation of fibrous tissues) and serious lung damage (e.g., Silicosis) NEET 2018, 2012. Workers must wear protective masks.