Breathing and Exchange of Gases

Breathing and Respiratory Organs

Importance of Oxygen and Carbon Dioxide

• Living organisms require oxygen (O₂) for the oxidation of food substances such as glucose, amino acids and fatty acids.
• These reactions release energy needed for growth, repair and other life activities.
• Carbon dioxide (CO₂) is released as a waste product during these reactions.
• Therefore, O₂ must be continuously supplied to the cells and CO₂ must be continuously removed.

Breathing and Respiration

• Breathing is the physical process of taking O₂-rich air from the atmosphere and releasing CO₂-rich air from the body.
• Respiration is a broader process that includes breathing, diffusion of gases, transport of gases by blood, utilisation of oxygen by cells and release of CO₂.

Breathing is also called pulmonary ventilation.

Respiratory Organs in Different Animals

Different animals have evolved different respiratory organs depending on their habitat and body organisation.

• Lower Invertebrates:
  • Sponges, coelenterates, flatworms, etc., exchange gases by simple diffusion across their general body surface.
• Earthworms:
  • Respire through their moist cuticle, which allows diffusion of gases.
• Insects:
  • Possess a tracheal system consisting of tracheae and tracheoles (air-filled tubes) that directly deliver air to tissues.
• Aquatic Arthropods and Molluscs:
  • Respire through gills (branchial respiration).
• Terrestrial Forms:
  • Generally respire through lungs (pulmonary respiration).

Vertebrates:

• Fishes:
  • Use gills for respiration.
• Amphibians, Reptiles, Birds, and Mammals:
  • Respire through lungs.
• Frogs:
  • Can also respire through their moist skin (cutaneous respiration), especially in water and during hibernation.

Human Respiratory System

Structure of the Respiratory System

The human respiratory system consists of air passages + lungs.

• External Nostrils:
  • Located above the upper lips.
  • Lead to the nasal chamber through the nasal passage.
• Nasal Chamber:
  • Opens into the pharynx (a common passage for food and air).
• Pharynx:
  • The pharynx opens into the larynx through a slit-like opening called the glottis.
  • During swallowing, the glottis is covered by a cartilaginous flap called the epiglottis, preventing food from entering the airway.
• Larynx (Sound Box):
  • A cartilaginous structure responsible for sound production
  • Also called the sound box.
• Trachea:
  • A straight tube extending into the mid-thoracic cavity.
  • It divides into right and left primary bronchi at about the 5th thoracic vertebra.
  • The walls of trachea and bronchi are supported by incomplete cartilaginous rings that prevent collapse.
• Bronchi and Bronchioles:
  • Each primary bronchus divides into secondary and tertiary bronchi, which further divide into bronchioles.
  • Terminal bronchioles give rise to alveolar ducts ending in alveoli.
• Alveoli:
  • Thin-walled, highly vascularised, irregular sac-like structures for gas exchange.
  • Human lungs contain about 300 million alveoli.
  • Provides a large surface area for diffusion.

Lungs

• Location:
  • The lungs are located in the thoracic cavity on either side of the heart.
• Structure:
  • Each lung is enclosed by a double-layered pleura.
  • The outer pleural membrane lines the thoracic cavity.
  • Inner pleural membrane covers the lung surface..
• Pleural cavity:
  • The space between the two membranes is called the pleural cavity and contains pleural fluid.
  • This fluid reduces friction during breathing.

Respiratory Parts of the System

• A. Conducting Part:
  • Extends from the external nostrils to the terminal bronchioles.
  • Function: Transport of air, removal of foreign particles, humidification and temperature regulation of inhaled air.
• B. Exchange Part:
  • Consists of alveoli and alveolar ducts.
  • This is the site of actual diffusion of oxygen and carbon dioxide between blood and air.

Thoracic Chamber:

• Structure:
  • The lungs are enclosed in an air-tight thoracic chamber formed by the vertebral column (back), sternum (front), ribs (sides) and diaphragm (bottom).
• Importance:
  • Any change in the volume of the thoracic cavity directly affects lung volume and is essential for breathing movements.

Special Features of Alveoli:

• Alveoli are lined by squamous epithelial cells called pneumocytes.
• Type I pneumocytes are mainly involved in gaseous exchange.
• Type II pneumocytes secrete a phospholipid called surfactant, which reduces surface tension and prevents collapse of alveoli during respiration.
• Absence of surfactant in newborn babies can be fatal.

Steps in Respiration:

1. Breathing (Pulmonary Ventilation):
   • Inhalation of atmospheric air and exhalation of carbon dioxide-rich air.
2. Diffusion of gases:
   • Exchange of oxygen and carbon dioxide across the alveolar membrane..
3. Transport of gases:
   • Oxygen and carbon dioxide are transported by blood.
4. Diffusion between blood and tissues:
   • Exchange of gases between blood and body tissues.
5. Cellular Respiration:
   • Cells utilise oxygen for catabolic reactions and release carbon dioxide.

Mechanism of Breathing

Breathing Stages:

1. Inspiration: Drawing atmospheric air into the lungs.
2. Expiration: Releasing carbon dioxide-rich alveolar air out of the lungs.

How Air Moves In and Out:

• Pressure Gradient:
  • The movement of air into and out of the lungs occurs due to pressure differences between the lungs and the atmosphere.
• During inspiration, intra-pulmonary pressure becomes less than atmospheric pressure.
• During expiration, intra-pulmonary pressure becomes greater than atmospheric pressure
• These pressure gradients are generated by the coordinated action of respiratory muscles.

Muscles Involved in Breathing:

• Diaphragm: The diaphragm is a dome-shaped muscle forming the floor of the thoracic cavity.
  • During inspiration, it contracts, flattens and moves downward, increasing thoracic volume in the antero-posterior direction.
  • During expiration, it relaxes and returns to its dome shape.
• Intercostal Muscles: These muscles are present between the ribs.
  • External Intercostals:
    • Contract during inspiration.
    • Lift the ribs and sternum upward and outward, increasing thoracic volume side-to-side.
  • Internal Intercostals:
    • Relax during inspiration.
    • Contract during expiration to bring ribs back to their original position.
• Abdominal Muscles:
  • Play a passive role during normal breathing.
  • Help increase the force of inspiration and expiration during vigorous breathing (exercise, coughing, sneezing).

Process of Inspiration:

1. Diaphragm contracts and becomes flat.
2. External intercostal muscles contract, lifting ribs and sternum.
3. Thoracic cavity volume increases.
4. Pulmonary (lung) volume increases.
5. Intra-pulmonary pressure falls below atmospheric pressure.
6. Air moves from atmosphere into the lungs through the respiratory passage.

Pathway of air during inspiration:

External nostrils → Nasal cavity → Pharynx → Glottis → Larynx → Trachea → Bronchi → Bronchioles → Alveolar ducts → Alveoli

Inspiration is an active process because it involves muscle contraction.

Process of Expiration:

1. Diaphragm relaxes and arches upward.
2. Internal intercostal muscles contract; external intercostals relax.
3. Thoracic cavity volume decreases.
4. Pulmonary volume decreases.
5. Intra-pulmonary pressure rises above atmospheric pressure.
6. Air is expelled out of the lungs.

Pathway of air during expiration:

Alveoli → Alveolar ducts → Bronchioles → Bronchi → Trachea → Larynx → Glottis → Pharynx → Nasal cavity → External nostrils → Outside

Expiration is generally a passive process as it occurs due to relaxation of muscles.

“We can increase the force of inspiration and expiration using additional abdominal muscles.”

Breathing Rate:

• A healthy adult breathes 12–16 times per minute.
• Average inspiration lasts about 2 seconds and expiration about 3 seconds.

Measurement of Air Volume:

• Spirometer: Device used to measure respiratory volumes and capacities and to assess lung function.

Respiratory Volumes and Capacities

Respiratory volumes refer to the amount of air the lungs can receive, hold or expel under different conditions.

• Tidal Volume (TV):
  • Volume of air inspired or expired during normal breathing.
  • About 500 mL.
  • A healthy person breathes about 6000–8000 mL of air per minute.
• Inspiratory Reserve Volume (IRV):
  • Extra air that can be forcibly inhaled after a normal inspiration.
  • About 2500–3000 mL.
• Expiratory Reserve Volume (ERV):
  • Extra air that can be forcibly exhaled after a normal expiration.
  • About 1000–1100 mL.
• Residual Volume (RV):
  • Volume of air remaining in the lungs even after a forceful expiration.
  • About 1100–1200 mL.
  • Prevents lung collapse.

Pulmonary Capacities:

Pulmonary capacities are combinations of two or more respiratory volumes.

• Inspiratory Capacity (IC):
  • Total air that can be inhaled after a normal expiration.
  • IC = TV + IRV
  • About 3000–3500 mL.
• Expiratory Capacity (EC):
  • Total air that can be exhaled after a normal inspiration.
  • EC = TV + ERV
  • About 1500–1600 mL.
• Functional Residual Capacity (FRC):
  • Volume of air remaining in lungs after normal expiration.
  • FRC = ERV + RV
  • About 2100–2300 mL.
• Vital Capacity (VC):
  • Maximum volume of air that can be inhaled after forced expiration or exhaled after forced inspiration.
  • VC = TV + IRV + ERV
  • About 4000–4600 mL.
• Total Lung Capacity (TLC):
  • Total volume of air present in lungs after forced inspiration.
  • TLC = VC + RV or TLC = TV + IRV + ERV + RV

• Respiratory volumes and capacities are generally 20–25% lower in females than in males.
• They are higher in athletes compared to non-exercising individuals.

All volumes and capacities except RV and FRC can be measured using a spirometer.

Exchange of Gases

Exchange of gases is the process by which oxygen (O₂) and carbon dioxide (CO₂) are exchanged between the atmosphere, blood and body tissues.

Sites of Gas Exchange:

1. Alveoli: Primary sites for the exchange of gases in humans.
   • They are thin-walled, highly vascularised structures that provide a large surface area for diffusion.
2. Blood and Tissues: Exchange of gases also occurs between blood and body tissues during internal respiration.

How Gas Exchange Occurs

Gas exchange takes place by simple diffusion, mainly based on concentration or partial pressure gradients.

Important Factors Affecting Diffusion

1. Partial Pressure Gradient
   • Gases diffuse from regions of higher partial pressure to regions of lower partial pressure.
2. Solubility of Gases
   • Carbon dioxide is 20–25 times more soluble in blood than oxygen, so it diffuses more rapidly.
3. Thickness of Diffusion Membrane
   • Thinner membranes allow faster diffusion.
4. Surface Area
   • A large surface area of alveoli favours efficient gas exchange.

Partial Pressure:

• Definition: Partial pressure is the pressure contributed by an individual gas in a mixture.
  • Oxygen: represented as pO₂
  • Carbon Dioxide: represented as pCO₂
• Gradients: Partial Pressure Gradients in Gas Exchange
  • Oxygen: Diffuses from alveoli → blood → tissues
  • Carbon Dioxide: Diffuses from tissues → blood → alveoli

“A reverse gradient exists for CO₂ from tissues to blood and blood to alveoli.”

Approximate Partial Pressures (mm Hg)

• Atmospheric air: pO₂ ≈ 159
• Alveoli: pO₂ ≈ 104, pCO₂ ≈ 40
• Deoxygenated blood: pO₂ ≈ 40, pCO₂ ≈ 45
• Oxygenated blood: pO₂ ≈ 95, pCO₂ ≈ 40
• Tissues: pO₂ ≈ 40, pCO₂ ≈ 45

These gradients ensure continuous diffusion of O₂ and CO₂ in the required directions.

Diffusion (Respiratory) Membrane Layers:

1. Thin squamous epithelium of alveoli
2. Endothelium of alveolar capillaries
3. Basement membrane between them
   • Supports the epithelium and capillary endothelial cells.

• Thickness: The total thickness of the diffusion membrane is much less than one millimetre, making it ideal for rapid gas diffusion.

Conditions Favouring Efficient Gas Exchange

• Large surface area of alveoli
• Very short diffusion distance
• Thin, moist respiratory membrane
• Adequate partial pressure gradients
• Presence of respiratory pigment haemoglobin

As a result, oxygen easily diffuses from alveoli to tissues and carbon dioxide from tissues to alveoli.

Transport of Gases

Blood as Transport Medium: Blood serves as the medium for transport of oxygen and carbon dioxide.

• Oxygen (O2):
  • 97% by red blood cells (RBCs).
  • 3% dissolved in plasma.
• Carbon Dioxide (CO2):
  • 20-25% by RBCs.
  • 70% as bicarbonate.
  • 7% dissolved in plasma.

Transport of Oxygen:

Oxygen is transported in two forms:

1. As oxyhaemoglobin – about 97%
2. As dissolved oxygen in plasma – about 3%

Transport as Dissolved Oxygen: A small fraction of oxygen dissolves directly in plasma under normal temperature and pressure conditions.

Transport as Oxyhaemoglobin

• Haemoglobin:
  • Red pigment in RBCs.
  • Each haemoglobin molecule contains 4 haem groups with iron in ferrous (Fe²⁺) form
    • This allows it to bind 4 oxygen molecules.
    • Oxygen binding forms oxyhaemoglobin (HbO₂)
• Factors Affecting Oxygen Binding with Haemoglobin
  • Partial pressure of oxygen (pO₂)
  • Partial pressure of carbon dioxide (pCO₂)
  • Hydrogen ion concentration (H⁺)
  • Temperature

Oxygen-Haemoglobin Dissociation Curve

• The relationship between pO₂ and percentage saturation of haemoglobin with oxygen is shown by the oxygen-haemoglobin dissociation curve, which is sigmoid (S-shaped).
• In the Alveoli:
  • High pO₂, low pCO₂, low H+, low temperature.
  • Favour the association of oxygen with haemoglobin, forming oxyhaemoglobin.
• In the Tissues:
  • Low pO₂, high pCO₂, high H+, high temperature.
  • Favour dissociation of oxygen from oxyhaemoglobin, allowing oxygen to be delivered to tissues.

Shifts of Oxygen Dissociation Curve

• Shift to the Right: Indicates increased oxygen release from haemoglobin. Occurs due to:
  • Decreased pO₂
  • Increased pCO₂
  • Increased H⁺ concentration (decreased pH)
  • Increased temperature
  • Increased 2,3-DPG

This rightward shift due to increased pCO₂ is known as the Bohr effect, seen at tissue capillaries.

• Shift to the Left: Indicates increased oxygen binding. Occurs due to:
  • Decreased pCO₂
  • Decreased H⁺ concentration (increased pH)
  • Lower temperature
  • Presence of fetal haemoglobin (higher O₂ affinity)
• Oxygen Delivery:
  • About 5 mL of oxygen is delivered to tissues by 100 mL of oxygenated blood.

Transport of Carbon Dioxide

Carbon dioxide (CO₂) produced during cellular respiration diffuses from body cells into the blood and is transported to the lungs for elimination.

Forms of Transport of Carbon Dioxide

Carbon dioxide is transported in blood in three forms:

1. Transport as Bicarbonate Ions (Major Form)

About 70% of CO₂ is transported as bicarbonate ions (HCO₃⁻).

• At tissues, CO₂ diffuses into red blood cells
• CO₂ combines with water to form carbonic acid (H₂CO₃)
• This reaction is catalysed by the enzyme carbonic anhydrase (present in RBCs)
• Carbonic acid is unstable and dissociates into:
  • H₂CO₃ → H⁺ + HCO₃⁻
  • The reaction is slow in plasma but very fast inside RBCs due to carbonic anhydrase.

Chloride Shift (Hamburger’s Phenomenon)

• Most bicarbonate ions diffuse out of RBCs into plasma
• To maintain ionic balance, chloride ions (Cl⁻) move from plasma into RBCs
• This exchange is called chloride shift

At alveoli, the reaction reverses, chloride ions move out, bicarbonate re-enters RBCs, and CO₂ is released.

2. Transport as Carbamino-Haemoglobin

About 20–25% of CO₂ is transported bound to haemoglobin.

• CO₂ binds to amino groups of haemoglobin forming carbamino-haemoglobin (HbCO₂)
• Binding is favoured by:
  • High pCO₂
  • Low pO₂ (tissue conditions)
• At alveoli: Low pCO₂ and high pO₂ cause CO₂ to dissociate from haemoglobin

3. Transport as Dissolved CO₂

About 7% of CO₂ is transported dissolved directly in plasma due to its high solubility.

• Carbon Dioxide Delivery
  • 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

Haldane Effect

• The Haldane effect explains enhanced transport of CO₂.
• Binding of oxygen with haemoglobin reduces its affinity for CO₂
• Thus, oxygenation of blood in lungs promotes release of CO₂
• Deoxygenated haemoglobin in tissues carries more CO₂

“Haldane effect states that oxygenation of haemoglobin in lungs promotes release of CO₂ from blood.”

The Haldane effect is more important for CO₂ transport than the Bohr effect.

Regulation of Respiration

Breathing is regulated to maintain appropriate levels of O₂ and CO₂ in the blood.

Neural Regulation of Breathing

• Respiration is controlled by a group of neurons located in the medulla oblongata and pons.
• Respiratory Rhythm Centre
  • Located in the medulla oblongata
  • Acts as the main centre for generating breathing rhythm
• Dorsal Respiratory Group (DRG)
  • Mainly responsible for inspiration
• Ventral Respiratory Group (VRG)
  • Involved in both inspiration and expiration during forced breathing
• Pneumotaxic Centre
  • Located in the pons
  • Regulates the activity of the respiratory rhythm centre
  • Shortens the duration of inspiration
  • Helps adjust breathing rate

Chemical Regulation of Respiration

• Chemosensitive Area
  • Located near the respiratory rhythm centre
  • Highly sensitive to:
    – CO₂ concentration
    – Hydrogen ion (H⁺) concentration
  • Increase in CO₂ or H⁺ stimulates this area, sending signals to increase breathing rate and depth.
• Peripheral Chemoreceptors
  • Located in:
    • Carotid bodies
    • Aortic bodies
  • Detect changes in CO₂ and H⁺ levels
  • Send impulses to respiratory centres for corrective action
• Role of Oxygen
  • Oxygen plays a minor role in regulation of normal breathing
  • CO₂ and H⁺ are the primary regulators

Disorders of the Respiratory System

Asthma:

• Symptoms: Difficulty in breathing, wheezing.
• Cause: Inflammation of bronchi and bronchioles.

“In asthma, excessive mucus secretion clogs bronchi and bronchioles.”

Emphysema:

• A chronic respiratory disorder
• Symptoms: Chronic breathing problems.
• Cause: Damage to alveolar walls, reduced respiratory surface.
• Major Cause: Cigarette smoking.

“Destruction of alveolar walls leads to reduced respiratory surface area.”

Occupational Respiratory Disorders:

• Seen in workers exposed to industrial dust (grinding, stone-breaking, mining)
• Cause: Excessive dust exposure.
• Examples:
  1. Silicosis – due to silica dust
  2. Asbestosis – due to asbestos dust
• Effects: Inflammation, fibrosis (formation of fibrous tissue), lung damage.
• Prevention: Workers should wear protective masks.

Hypoxia:

• Condition of oxygen deficiency in tissues

Asphyxia:

• Oxygen level falls and CO₂ level rises
• Respiratory centre gets paralysed
• Breathing stops, leading to death

Bronchitis:

• Inflammation of bronchi
• Excess mucus production
• Causes persistent cough with greenish-yellow sputum
• Caused by infections, smoking, and air pollutants

COPD (Chronic Obstructive Pulmonary Disease):

• Collective term for chronic respiratory disorders
• Includes:
  – Chronic bronchitis
  – Emphysema
  – Chronic asthma

Chapter Summary

• Cells require oxygen for metabolic activities to release energy from glucose, amino acids and fatty acids.
• During these processes, carbon dioxide is produced as a waste product which must be continuously removed.
• Animals have evolved efficient respiratory mechanisms to ensure continuous supply of oxygen and elimination of carbon dioxide.
• In humans, this function is performed by a well-developed respiratory system consisting of air passages and a pair of lungs.

“Respiration involves breathing, exchange of gases, transport of gases by blood and utilisation of oxygen by cells for catabolic reactions with release of carbon dioxide.”

Steps in Respiration: Respiration involves a series of coordinated processes:

1. Breathing:
   • Inspiration: involves intake of atmospheric air into the lungs.
   • Expiration: involves release of CO₂-rich alveolar air.
2. Exchange of Gases:
   • Oxygen diffuses from alveoli into deoxygenated blood.
   • Carbon dioxide diffuses from blood into alveoli.
   • Gases are then transported by blood throughout the body, and further exchange occurs between oxygenated blood and body tissues.
3. Cellular Respiration:
   • Oxygen is utilised by cells for catabolic reactions to release energy, producing carbon dioxide as a by-product.

“Inspiration is an active process involving muscle contraction, whereas expiration is usually a passive process.”

Mechanism of Breathing:

• Breathing occurs due to pressure gradients created between the atmosphere and alveoli.
• Contraction and relaxation of the diaphragm and intercostal muscles change the volume of the thoracic cavity, leading to inspiration and expiration.
• A spirometer is used to measure respiratory volumes and capacities.

“Contraction of diaphragm and external intercostal muscles increases thoracic volume and decreases intrapulmonary pressure.”

“Except residual volume and functional residual capacity, all respiratory volumes can be measured using a spirometer.”

Exchange of Gases:

• Gas exchange occurs by simple diffusion at the alveoli and tissues.
• The process depends on partial pressure gradients of oxygen (pO₂) and carbon dioxide (pCO₂), solubility of gases, and thickness of the diffusion membrane.
• Oxygen diffuses from alveoli to blood and from blood to tissues, while carbon dioxide diffuses from tissues to blood and from blood to alveoli.

“The diffusion membrane consists of alveolar epithelium, capillary endothelium and basement membrane.”

“Higher solubility of CO₂ allows faster diffusion compared to oxygen.”

Transport of Gases:

“In blood, about 97% of oxygen is transported by haemoglobin and about 3% is transported dissolved in plasma.”

• Oxygen is mainly transported as oxyhaemoglobin.
  • In alveoli: High pO₂ helps O₂ bind to haemoglobin.
    • “In alveoli, low pCO₂ and low hydrogen ion concentration favour formation of oxyhaemoglobin.”
  • In tissues: Low pO₂ and high pCO₂ and H⁺ help O₂ dissociate from haemoglobin.
• Carbon dioxide transport:
  • Carbon dioxide is transported in three forms:
  • about 70% as bicarbonate ions formed with the help of carbonic anhydrase, 20–25% as carbamino-haemoglobin, and a small amount dissolved in plasma.
  • High pCO₂ in tissues facilitates CO₂ binding, whereas low pCO₂ in alveoli promotes its release.

“Haldane effect facilitates release of CO₂ in lungs due to oxygenation of haemoglobin.”

Regulation of Respiration:

• Breathing rhythm is regulated by the respiratory centre located in the medulla oblongata.
• The pneumotaxic centre in the pons modifies the breathing rate.
• Chemosensitive areas and peripheral chemoreceptors respond mainly to changes in CO₂ and hydrogen ion concentration, ensuring proper regulation of respiration.

“The role of oxygen in regulation of respiration is minimal under normal conditions.”