Breathing and Exchange of Gases

Breathing and Respiratory Organs

Importance of Oxygen and Carbon Dioxide:

• Organisms use oxygen (O2) to break down glucose, amino acids, fatty acids, etc.
• This breakdown provides energy for various activities.
• Carbon dioxide (CO2) is released as a waste product during these reactions.
• O2 needs to be continuously supplied to cells, and CO2 must be removed.

What is Breathing?

• Breathing is the exchange of O2 from the atmosphere with CO2 produced by cells.
• This process is also known as respiration.
• You can feel your chest moving up and down when you breathe.

Respiratory Organs

Different Animals Have Different Breathing Mechanisms:

• Lower Invertebrates:
  • Sponges, coelenterates, flatworms, etc., use simple diffusion over their body surface.
• Earthworms:
  • Use their moist cuticle for gas exchange.
• Insects:
  • Have tracheal tubes to transport air within their bodies.
• Aquatic Arthropods and Molluscs:
  • Use gills (branchial respiration) for gas exchange.
• Terrestrial Forms:
  • Use 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).

Human Respiratory System

Structure of the Respiratory System:

• External Nostrils:
  • Located above the upper lips.
  • Lead to the nasal chamber through the nasal passage.
• Nasal Chamber:
  • Opens into the pharynx (common passage for food and air).
• Pharynx:
  • Opens into the larynx, which leads to the trachea.
• Larynx (Sound Box):
  • Cartilaginous box for sound production.
  • Covered by the epiglottis during swallowing to prevent food entry.
• Trachea:
  • Straight tube extending to mid-thoracic cavity.
  • Divides into right and left primary bronchi at the 5th thoracic vertebra.
• Bronchi and Bronchioles:
  • Primary bronchi divide into secondary and tertiary bronchi, then into bronchioles.
  • Supported by incomplete cartilaginous rings.
  • End in terminal bronchioles leading to alveoli.
• Alveoli:
  • Thin, irregular-walled, vascularised structures for gas exchange.
  • Form the lungs along with bronchi and bronchioles.

Lungs:

• Structure:
  • Two lungs covered by a double-layered pleura with pleural fluid in between.
  • Outer pleural membrane contacts thoracic lining.
  • Inner pleural membrane contacts lung surface.
• Function:
  • Reduce friction on the lung surface.

Respiratory Parts:

• Conducting Part:
  • From external nostrils to terminal bronchioles.
  • Transports air, clears foreign particles, humidifies air, and adjusts air temperature.
• Exchange Part:
  • Alveoli and their ducts.
  • Site of O2 and CO2 diffusion between blood and air.

Thoracic Chamber:

• Structure:
  • Air-tight chamber formed by vertebral column (back), sternum (front), ribs (sides), and diaphragm (bottom).
• Function:
  • Changes in thoracic cavity volume reflect in lung cavity, essential for breathing.

Steps in Respiration:

1. Breathing (Pulmonary Ventilation):
   • Drawing in atmospheric air and releasing CO2-rich alveolar air.
2. Gas Diffusion:
   • O2 and CO2 diffuse across alveolar membrane.
3. Gas Transport:
   • Gases transported by blood.
4. Tissue Gas Diffusion:
   • O2 and CO2 diffuse between blood and tissues.
5. Cellular Respiration:
   • Cells use O2 for catabolic reactions, releasing CO2.

Mechanism of Breathing

Breathing Stages:

• Inspiration: Drawing in atmospheric air.
• Expiration: Releasing alveolar air.

How Air Moves In and Out:

• Pressure Gradient:
  • Air moves due to pressure differences between lungs and atmosphere.
  • Inspiration: Intra-pulmonary pressure < atmospheric pressure.
  • Expiration: Intra-pulmonary pressure > atmospheric pressure.

Muscles Involved:

• Diaphragm:
  • Contracts to increase thoracic volume (front to back).
• Intercostal Muscles:
  • External Intercostals:
    • Contract to lift ribs and sternum, increasing thoracic volume (side to side).
  • Internal Intercostals:
    • Relax during inspiration and contract during expiration.

Process of Inspiration:

1. Diaphragm contracts.
2. External intercostal muscles lift ribs and sternum.
3. Thoracic volume increases.
4. Pulmonary volume increases.
5. Intra-pulmonary pressure decreases.
6. Air moves into the lungs.

Process of Expiration:

1. Diaphragm relaxes.
2. Intercostal muscles relax.
3. Thoracic volume decreases.
4. Pulmonary volume decreases.
5. Intra-pulmonary pressure increases.
6. Air moves out of the lungs.

Additional Muscles:

• Help increase strength of breathing movements.

Breathing Rate:

• Healthy human breathes 12-16 times per minute.

Measuring Air Volume:

• Spirometer: Device used to estimate air volume in breathing and assess lung function.

Respiratory Volumes and Capacities

Respiratory Volumes:

• Tidal Volume (TV):
  • Air inspired or expired during normal respiration.
  • About 500 mL.
  • Healthy person breathes 6000 to 8000 mL of air per minute.
• Inspiratory Reserve Volume (IRV):
  • Extra air a person can inhale with a deep breath.
  • Averages 2500 to 3000 mL.
• Expiratory Reserve Volume (ERV):
  • Extra air a person can exhale with a deep breath out.
  • Averages 1000 to 1100 mL.
• Residual Volume (RV):
  • Air remaining in lungs after a deep exhale.
  • Averages 1100 to 1200 mL.

Pulmonary Capacities:

• Inspiratory Capacity (IC):
  • Total air a person can inhale after normal exhalation.
  • Includes TV + IRV.
• Expiratory Capacity (EC):
  • Total air a person can exhale after normal inhalation.
  • Includes TV + ERV.
• Functional Residual Capacity (FRC):
  • Air remaining in lungs after normal exhalation.
  • Includes ERV + RV.
• Vital Capacity (VC):
  • Maximum air a person can inhale after forced exhalation or exhale after forced inhalation.
  • Includes ERV + TV + IRV.
• Total Lung Capacity (TLC):
  • Total air in lungs after forced inhalation.
  • Includes RV + ERV + TV + IRV or VC + RV.

Exchange of Gases

Where Gas Exchange Happens:

• Alveoli: Main sites for gas exchange.
• Blood and Tissues: Gases also exchange here.

How Gas Exchange Happens:

• Simple Diffusion: Gases move based on pressure/concentration gradients.
• Important Factors:
  • Solubility of Gases: CO2 is more soluble than O2.
  • Membrane Thickness: Thinner membranes allow faster diffusion.

Partial Pressure:

• Definition: Pressure contributed by an individual gas in a mixture.
  • Oxygen: pO2
  • Carbon Dioxide: pCO2
• Gradients:
  • Oxygen: Moves from alveoli to blood to tissues.
  • Carbon Dioxide: Moves from tissues to blood to alveoli.

Diffusion Membrane Layers:

1. Thin Squamous Epithelium of Alveoli
2. Endothelium of Alveolar Capillaries
3. Basement Membrane:
   • Supports the epithelium and capillary endothelial cells.

• Thickness: Less than a millimeter, which is ideal for gas diffusion.

Key Points:

• Oxygen: Diffuses easily from alveoli to tissues.
• Carbon Dioxide: Diffuses easily from tissues to alveoli.

Transport of Gases

Blood as Transport Medium:

• 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:

• Haemoglobin:
  • Red pigment in RBCs.
  • Binds with O2 to form oxyhaemoglobin.
  • Each haemoglobin carries 4 O2 molecules.
  • Binding influenced by:
    • Partial pressure of O2 (pO2).
    • Partial pressure of CO2 (pCO2).
    • Hydrogen ion concentration (H+).
    • Temperature.
• Oxygen Dissociation Curve:
  • Sigmoid curve showing haemoglobin saturation with O2 against pO2.
  • In Alveoli:
    • High pO2, low pCO2, low H+, low temperature.
    • Favors oxyhaemoglobin formation.
  • In Tissues:
    • Low pO2, high pCO2, high H+, high temperature.
    • Favors oxygen release from oxyhaemoglobin.
• O2 Delivery:
  • 100 ml of oxygenated blood delivers about 5 ml of O2 to tissues.

Transport of Carbon Dioxide:

• Carbamino-Haemoglobin:
  • CO2 binds with haemoglobin.
  • Binding influenced by pCO2 and pO2.
  • In Tissues:
    • High pCO2, low pO2.
    • More CO2 binds to haemoglobin.
  • In Alveoli:
    • Low pCO2, high pO2.
    • CO2 released from haemoglobin.
• Carbonic Anhydrase Enzyme:
  • Converts CO2 + H2O into HCO3– and H+.
  • Reaction reverses in alveoli to release CO2 and H2O.
• CO2 Delivery:
  • 100 ml of deoxygenated blood delivers about 4 ml of CO2 to alveoli.

Regulation of Respiration

How Breathing is Controlled:

• Neural System: Controls and adjusts breathing rhythm based on body needs.
• Respiratory Rhythm Centre:
  • Located in the medulla region of the brain.
  • Main center for regulating breathing.
• Pneumotaxic Centre:
  • Located in the pons region of the brain.
  • Can adjust the activity of the respiratory rhythm centre.
  • Shortens inspiration duration, altering the breathing rate.

Chemosensitive Area:

• Location: Near the respiratory rhythm centre.
• Function: Sensitive to CO2 and hydrogen ions (H+).
  • Increase in CO2 and H+ activates this area.
  • Sends signals to adjust breathing to remove excess CO2 and H+.

Receptors in Aortic Arch and Carotid Artery:

• Function: Detect changes in CO2 and H+ levels.
• Action: Send signals to the respiratory rhythm centre for adjustments.

Oxygen’s Role:

• Significance: Not very important in regulating breathing rhythm.

Disorders of the Respiratory System

Asthma:

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

Emphysema:

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

Occupational Respiratory Disorders:

• Workers of Industries Affected: Grinding, stone-breaking.
• Cause: Excessive dust exposure.
• Effects: Inflammation, fibrosis (formation of fibrous tissue), lung damage.
• Prevention: Workers should wear protective masks.

Chapter Summary:

• Cells use oxygen for metabolism and produce energy.
• Carbon dioxide, which is harmful, is also produced.
• Animals have mechanisms to transport oxygen and remove carbon dioxide.
• Humans have a respiratory system with two lungs and air passages.

Steps in Respiration:

1. Breathing:
   • Inspiration: Taking in atmospheric air.
   • Expiration: Releasing alveolar air.
2. Exchange of Gases:
   • Between deoxygenated blood and alveoli.
   • Transport of gases by blood throughout the body.
   • Between oxygenated blood and tissues.
3. Cellular Respiration:
   • Utilisation of O2 by cells.

Mechanism of Breathing:

• Involves creating pressure gradients between the atmosphere and alveoli.
• Uses intercostal muscles and diaphragm.
• Spirometer measures air volumes involved in breathing.

Exchange of Gases:

• Occurs by diffusion at alveoli and tissues.
• Depends on partial pressure gradients of O2 (pO2) and CO2 (pCO2), solubility, and thickness of the diffusion surface.
• O2 diffuses from alveoli to deoxygenated blood and from oxygenated blood to tissues.
• CO2 diffuses from tissues to alveoli.

Transport of Gases:

• Oxygen is mainly transported as oxyhaemoglobin.
  • In alveoli: High pO2 helps O2 bind to haemoglobin.
  • In tissues: Low pO2 and high pCO2 and H+ help O2 dissociate from haemoglobin.
• Carbon dioxide transport:
  • 70% as bicarbonate (HCO3–) with carbonic anhydrase.
  • 20-25% as carbamino-haemoglobin.
  • High pCO2 in tissues binds CO2 to blood.
  • Low pCO2 in alveoli removes CO2 from blood.

Regulation of Respiration:

• Respiratory centre in the medulla controls rhythm.
• Pneumotaxic centre in the pons can change the rhythm.
• Chemosensitive area in the medulla responds to CO2 and H+ levels.