Chapter 2: Breathing and Exchange of Gases

Introduction

Concepts

• Reductionist approach studies life via physio-chemical methods; neither molecular nor organismic alone reveals full truth.
• Organisms use oxygen to break down glucose, fatty acids, amino acids for energy; carbon dioxide is released as waste.
• Breathing: intake of oxygen-rich air and removal of carbon dioxide-rich air from the body.
• Respiration: exchange of oxygen from environment with carbon dioxide produced in cells during food oxidation.
• Respiration includes breathing, gas diffusion, gas transport by blood, cellular oxygen use, and carbon dioxide release.

Memory Aids

• Breathing = air movement; Respiration = gas exchange + cellular use.
• Oxygen in, CO2 out = respiration cycle.

Quick Checkpoints

• What is the reductionist approach?
• Define breathing and respiration.
• What gases are exchanged during respiration?

Respiratory Organs

Concepts

• Breathing mechanisms vary by animal habitat and complexity.
• Animals use different organs for gas exchange:
• Whole body surface: sponges, flatworms.
• Moist skin: earthworms, frogs.
• Tracheae tubes: insects.
• Gills: fishes, molluscs.
• Lungs: amphibians, reptiles, birds, mammals.

Memory Aids

• Surface → Skin → Tracheae → Gills → Lungs (increasing complexity).
• Habitat dictates respiratory organ type.

Quick Checkpoints

• Which animals use skin for respiration?
• Name animals with gills.
• What respiratory organ do insects use?

Human Respiratory System

Concepts

• Air enters through external nostrils → nasal chamber → pharynx (common food/air passage) → larynx via glottis.
• Epiglottis covers glottis during swallowing to prevent food entry into larynx.
• Larynx (sound box) produces sound.
• Trachea divides into right and left primary bronchi at 5th thoracic vertebra.
• Bronchi branch into secondary, tertiary bronchi, then bronchioles ending in alveoli.
• Trachea and bronchi supported by incomplete cartilaginous rings.
• Alveoli: thin-walled, vascularized sacs for gas exchange; ~300 million in human lungs.
• Lungs enclosed by double-layered pleura with pleural cavity containing lubricating pleural fluid.
• Conducting part: nostrils to terminal bronchioles; transports, cleans, humidifies, and warms air.
• Respiratory part: alveoli and ducts; site of gas diffusion between blood and air.
• Lungs lie in airtight thoracic chamber formed by vertebral column, sternum, ribs, and diaphragm.
• Diaphragm movement changes thoracic volume affecting lung volume.
• Squamous alveolar cells (pneumocytes): type I for gas exchange, type II secrete surfactant (lecithin) to prevent alveolar collapse.
• Surfactant secretion develops late in pregnancy; absence causes lung collapse in newborns.
• Respiration steps: breathing, gas diffusion, blood transport, tissue gas exchange, cellular oxygen use, and CO2 release.

Memory Aids

• Epiglottis = food flap; Larynx = sound box.
• Trachea → Bronchi → Bronchioles → Alveoli = air path.
• Pneumocytes: I = exchange, II = surfactant.
• Thoracic cavity changes = lung volume changes.

Quick Checkpoints

• What prevents food from entering the larynx?
• Where does gas exchange occur in lungs?
• What is the role of surfactant?
• Describe the path of air from nostrils to alveoli.
• What forms the thoracic cavity?
• List the steps of respiration process.

Mechanism of Breathing

Process and Muscles Involved

• Breathing has two stages: inspiration (air intake) and expiration (air release).
• Air movement is driven by pressure gradients between lungs and atmosphere.
• Diaphragm and intercostal muscles (external and internal) between ribs create these pressure changes.

Memory Aids

• Inspiration = air in; Expiration = air out.
• Diaphragm flattens to inhale, relaxes to exhale.
• External intercostals lift ribs; internal intercostals aid expiration.

Quick Checkpoints

• What muscles cause thoracic volume changes?
• How is pressure gradient created for breathing?
• Define inspiration and expiration.

Inspiration

Mechanism and Air Pathway

• Occurs when intra-pulmonary pressure is less than atmospheric pressure (negative pressure).
• Diaphragm contracts, flattens, and lowers, increasing thoracic volume antero-posteriorly.
• External intercostals contract, lifting ribs and sternum, increasing thoracic volume dorso-ventrally.
• Increased thoracic volume raises pulmonary volume, lowering lung pressure, drawing air in.
• Abdominal muscles relax passively, allowing diaphragm movement.
• Air pathway: External nares → Nasal cavities → Internal nares → Pharynx → Glottis → Larynx → Trachea → Bronchi → Bronchioles → Alveolar ducts → Alveoli.

Memory Aids

• Diaphragm down = inhale.
• External intercostals lift ribs = more lung space.
• Air flows from high to low pressure.

Quick Checkpoints

• What causes intra-pulmonary pressure to drop?
• Which muscles contract during inspiration?
• Trace the air pathway during inspiration.

Expiration

Mechanism and Air Pathway

• Occurs when intra-pulmonary pressure exceeds atmospheric pressure.
• Diaphragm and intercostal muscles relax, reducing thoracic and pulmonary volume.
• Increased lung pressure forces air out.
• Air pathway: Alveoli → Alveolar ducts → Bronchioles → Bronchi → Trachea → Larynx → Glottis → Pharynx → Internal nares → Nasal cavities → External nares → Outside.
• Additional abdominal muscles can increase breathing strength.
• Normal breathing rate: 12-16 breaths/min; inspiration ~2 sec, expiration ~3 sec.
• Inspiration is active; expiration is passive.

Memory Aids

• Diaphragm up = exhale.
• Muscle relaxation causes air expulsion.
• Expiration pressure > atmospheric pressure.

Quick Checkpoints

• What triggers expiration?
• Which muscles relax during expiration?
• Describe the air pathway during expiration.

Respiratory Volumes and Capacities

Definitions and Clinical Use

• Respiratory volumes: air amounts inhaled, held, or exhaled under conditions.
• Pulmonary capacities: sums of respiratory volumes, used in diagnosis.
• Volumes and capacities are 20-25% less in women than men; higher in athletes than non-exercisers.
• Most volumes measured by spirometer except Functional Residual Capacity (FRC) and Residual Volume (RV).

Memory Aids

• TV = normal breath air.
• IRV = extra inhale air.
• ERV = extra exhale air.
• RV = air always in lungs.
• VC = TV + IRV + ERV (max air in/out).
• IC = TV + IRV (max inhale after normal out).
• EC = TV + ERV (max exhale after normal in).
• FRC = ERV + RV (air left after normal out).
• TLC = VC + RV (total lung air capacity).

Quick Checkpoints

• What is tidal volume?
• Define vital capacity.
• Which volumes form functional residual capacity?
• How is total lung capacity calculated?

Exchange of Gases

Gas Exchange Sites and Mechanism

• Alveoli are the primary sites for gas exchange.
• Gas exchange also occurs between blood and tissues by simple diffusion.
• Diffusion depends on pressure/concentration gradients.
• Solubility of gases and membrane thickness affect diffusion rate.
• Partial pressure (pO for oxygen, pCO for carbon dioxide) drives diffusion.

• Oxygen partial pressure is highest in the atmosphere and decreases through alveoli, blood, to tissues.
• Carbon dioxide partial pressure is low in atmosphere but higher in alveoli, blood, and tissues.
• CO gradient is opposite to O: from tissues to blood to alveoli.
• CO is 20-25 times more soluble than O, so diffuses more easily per unit pressure difference.
• Diffusion membrane consists of thin squamous epithelium of alveoli, endothelium of capillaries, and basement substance.
• Membrane thickness is less than a millimeter, favoring efficient gas diffusion.
• Arterial blood leaving alveoli has pO and pCO nearly equal to alveolar air (104 mmHg and 40 mmHg respectively).

Gas Exchange Process

• Air enters alveoli with high O and low CO.
• Deoxygenated blood arrives via pulmonary artery with low O and high CO.
• Oxygen diffuses from alveoli to blood; CO diffuses from blood to alveoli.
• Oxygenated blood returns to heart via pulmonary vein.
• Heart pumps oxygenated blood to body tissues through systemic arteries.
• Tissues use oxygen and release CO into blood.
• Deoxygenated blood carrying CO returns to heart via systemic veins.

Alveolar Structure and Gas Diffusion

• Alveolar wall is one cell thick for easy gas passage.
• Oxygen passes through alveolar wall, basement membrane, and capillary endothelium into red blood cells.
• Carbon dioxide moves from blood in capillaries into alveoli to be exhaled.
• Blood enters capillaries via pulmonary artery carrying CO-rich blood and leaves oxygenated.
• Process ensures oxygen supply and carbon dioxide removal.

Requirements for Efficient Gas Exchange

• Large surface area for gas exchange.
• Very short diffusion path between alveolar air and blood.
• Concentration gradients for oxygen and carbon dioxide.
• Presence of haemoglobin to increase O and CO carrying capacity.
• Thin, moist, and large respiratory membrane.

Memory Aids

• Partial pressure drives gas movement.
• CO diffuses faster due to higher solubility.
• Thin alveolar-capillary membrane aids diffusion.
• Haemoglobin boosts oxygen and carbon dioxide transport.
• Large surface area and short path ensure efficiency.

Quick Checkpoints

• What is the role of partial pressure in gas exchange?
• Why does CO diffuse faster than O?
• Which layers form the diffusion membrane?
• How does haemoglobin affect gas transport?
• What structural features optimize gas exchange in lungs?

Regulation of Respiration

Neural Control

• Breathing rate and depth controlled by neurons in medulla oblongata and pons varolii.
• Medulla oblongata has dorsal respiratory group (inspiration) and ventral respiratory group (inspiration or expiration).
• These groups form the respiratory rhythm centre.
• Pons varolii contains pneumotaxic centre that limits inspiration, controlling respiratory rate.

Chemical Control

• Chemoreceptors in carotid and aortic bodies monitor CO₂ and H⁺ levels.
• Changes in CO₂ and H⁺ send signals to respiratory rhythm centre for adjustment.
• Oxygen has minimal role in respiratory rhythm regulation.

Memory Aids

• "Dorsal = Drive In" (dorsal group causes inspiration).
• "Ventral = Versatile" (ventral group controls both inspiration and expiration).
• "Pneumo = Pause" (pneumotaxic centre limits inspiration).
• CO₂ and H⁺ are the main chemical triggers, not oxygen.

Quick Checkpoints

• Which brain regions control breathing rhythm?
• What roles do dorsal and ventral respiratory groups play?
• How does the pneumotaxic centre affect breathing?
• Where are chemoreceptors located for chemical regulation?
• Which chemicals primarily regulate respiration?
• Does oxygen significantly regulate respiratory rhythm?

Disorders of Respiratory System

Common Respiratory Disorders

• Asthma: Difficulty in breathing with wheezing and coughing due to excess mucus blocking bronchi and bronchioles.
• Emphysema: Chronic damage to alveolar walls reducing respiratory surface; major cause is cigarette smoking.
• Occupational Respiratory Disorders: Caused by long-term dust exposure (e.g., silica, asbestos) leading to lung inflammation and fibrosis; common in mining industries.
• Hypoxia: Condition of oxygen shortage in tissues.
• Asphyxia: Oxygen falls and carbon dioxide rises, paralyzing respiratory center, causing breathing to stop and death.
• Bronchitis: Inflammation of bronchi with excessive mucus causing cough and greenish-yellow sputum; caused by infection, smoking, and pollutants.
• COPD (Chronic Obstructive Pulmonary Disease): Includes chronic bronchitis, emphysema, and chronic asthma.

Memory Aids

• Asthma = Airway mucus blocks breathing.
• Emphysema = Elastic alveoli walls destroyed.
• Occupational disorders = Dust + long exposure = fibrosis.
• Hypoxia = Low oxygen in tissues.
• Asphyxia = Low O2 + high CO2 = respiratory failure.
• Bronchitis = Bronchi inflamed + mucus cough.
• COPD = Chronic lung obstruction diseases.

Quick Checkpoints

• What causes airway obstruction in asthma?
• How does emphysema reduce respiratory efficiency?
• Which industries are prone to occupational respiratory disorders?
• Define hypoxia and its effect on tissues.
• What happens to O2 and CO2 levels in asphyxia?
• Identify causes and symptoms of bronchitis.
• What diseases are included under COPD?

Respiratory System Structure

Pathway and Gas Exchange

• Air enters through trachea, kept open by cartilage rings.
• Trachea divides into left and right main bronchi, each leading to a lung.
• Bronchi branch into smaller bronchioles ending in alveoli (air sacs).
• Alveoli surrounded by capillaries where oxygen enters blood and carbon dioxide leaves.
• Deoxygenated blood arrives via pulmonary artery; oxygenated blood leaves via pulmonary veins.
• Alveolar pores allow air movement between alveoli for efficient gas exchange.

Memory Aids

• Trachea = Cartilage tube for air passage.
• Bronchi = Main airways to lungs.
• Bronchioles = Smaller branches ending in alveoli.
• Alveoli = Site of gas exchange with capillaries.
• Pulmonary artery = Carries deoxygenated blood to lungs.
• Pulmonary veins = Carry oxygenated blood to heart.
• Alveolar pores = Connect alveoli for air flow.

Quick Checkpoints

• Trace the path of air from trachea to alveoli.
• Where does gas exchange occur in the lungs?
• What is the role of alveolar pores?
• Which vessels carry oxygenated and deoxygenated blood?