Breathing and Exchange of Gases: Comprehensive Human Physiology Study Notes

Unit 5: Human Physiology - Perspectives and Scope

• Methodological Approaches to Studying Life Forms:

  • Reductionist Approach: Historically, the study of life forms led to an increasing use of physico-chemical concepts and techniques. This approach typically employed surviving tissue models or cell-free systems.
  • Impact of Reductionism: This approach resulted in an explosion of knowledge in the field of molecular biology. For a time, molecular physiology became nearly synonymous with biochemistry and biophysics.
  • Current Realization: It is now understood that neither a purely organismic approach nor a purely reductionistic molecular approach is sufficient to reveal the total truth about biological processes or living phenomena.
  • Systems Biology: This perspective posits that all living phenomena are emergent properties resulting from interactions among components of the system under study.
  • Hierarchy of Emergence: Emergent properties are created within regulatory networks of molecules, supra molecular assemblies, cells, tissues, organisms, populations, and communities.

• Scope of Unit 5:

  • The unit describes major human physiological processes in cellular and molecular terms.
  • Topics covered include the exchange of gases, blood circulation, and locomotion and movement.
  • Coordination and regulation of body events are addressed at the organismic level.
  • Chapter List:
    • Chapter 14: Breathing and Exchange of Gases
    • Chapter 15: Body Fluids and Circulation
    • Chapter 16: Excretory Products and their Elimination
    • Chapter 17: Locomotion and Movement
    • Chapter 18: Neural Control and Coordination
    • Chapter 19: Chemical Coordination and Integration

Biography of Alfonso Corti (1822 – 1888)

• Background: Alfonso Corti was an Italian anatomist born in 1822.
• Scientific Career:
  • He began his career by studying the cardiovascular systems of reptiles.
  • Later, he transitioned his research focus to the mammalian auditory system.
• The Organ of Corti:
  • In 1851, Corti published a paper describing a specific structure located on the basilar membrane of the cochlea.
  • This structure contains hair cells responsible for converting sound vibrations into nerve impulses.
  • The structure was named the "organ of Corti" in his honor.
• Death: Corti passed away in 1888.

Introduction to Breathing and Respiration

• Necessity of Oxygen ($O_2$):
  • Organisms utilize $O_2$ to indirectly break down simple molecules such as glucose, amino acids, and fatty acids.
  • This breakdown is a catabolic process intended to derive energy for various cellular activities.
• Production of Carbon Dioxide ($CO_2$):
  • $CO_2$ is a harmful byproduct released during these catabolic reactions.
• Definition of Breathing:
  • The process of exchanging $O_2$ from the atmosphere with $CO_2$ produced by the cells is called breathing, commonly known as respiration.
  • Breathing is physically evident through the rhythmic movement of the chest.

Diversity of Respiratory Organs

• Variation in Mechanisms: Breathing mechanisms vary among different animal groups depending on their habitats and levels of organization.
• Lower Invertebrates:
  • Sponges, Coelenterates, and Flatworms: Exchange $O_2$ and $CO_2$ via simple diffusion over their entire body surface.
• Other Invertebrates:
  • Earthworms: Utilize their moist cuticle for gas exchange.
  • Insects: Use a network of tracheal tubes to transport atmospheric air directly within the body.
• Specialized Respiratory Structures:
  • Gills (Branchial Respiration): Vascularized structures used by most aquatic arthropods, molluscs, and fishes.
  • Lungs (Pulmonary Respiration): Vascularized bags used by terrestrial forms, including reptiles, birds, and mammals.
• Amphibians:
  • Frogs and other amphibians can respire through lungs.
  • They are also capable of cutaneous respiration, which involves gas exchange through their moist skin.

Anatomy of the Human Respiratory System

• Air Passages:
  • External Nostrils: A pair of openings located above the upper lips.
  • Nasal Chamber: Leads from the nostrils via the nasal passage.
  • Pharynx: The common passage for both food and air; the nasal chamber opens into it.
  • Larynx: A cartilaginous box situated at the opening of the trachea; known as the "sound box" because it assists in sound production.
  • Epiglottis: A thin, elastic cartilaginous flap that covers the glottis during swallowing to prevent food from entering the larynx.
  • Trachea: A straight tube extending to the mid-thoracic cavity. It divides into right and left primary bronchi at the level of the $5^{th}$ thoracic vertebra.
• The Bronchial Tree:
  • Primary bronchi divide repeatedly into secondary and tertiary bronchi.
  • These further divide into bronchioles, ending in very thin terminal bronchioles.
  • Structural Support: The trachea, primary, secondary, and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
• Alveoli:
  • Terminal bronchioles give rise to multiple very thin, irregular-walled, and vascularized bag-like structures called alveoli.
  • The branching network comprising bronchi, bronchioles, and alveoli forms the lungs.
• The Lungs and Pleura:
  • Humans have two lungs.
  • Lungs are enclosed by a double-layered pleura.
  • Pleural Fluid: Found between the pleural layers; it serves to reduce friction on the lung surface.
  • Pleural Membranes: The outer pleural membrane is in close contact with the thoracic lining, while the inner pleural membrane is in contact with the lung surface.
• Functional Divisions:
  • Conducting Part: Includes the path from external nostrils to the terminal bronchioles. Functions: Transports atmospheric air to alveoli, clears foreign particles, humidifies air, and brings air to body temperature.
  • Respiratory (Exchange) Part: Includes the alveoli and their ducts. Function: The site of actual diffusion of $O_2$ and $CO_2$ between blood and atmospheric air.

The Thoracic Chamber and Breathing Steps

• Anatomical Boundaries of the Thoracic Chamber:
  • Dorsal: Vertebral column.
  • Ventral: Sternum.
  • Lateral: Ribs.
  • Lower Side: Dome-shaped diaphragm.
• Mechanical Principle: The thoracic chamber is an air-tight chamber. Since we cannot directly alter pulmonary volume, the anatomical setup ensures that any volume change in the thoracic cavity is reflected in the pulmonary (lung) cavity.
• Five Steps of Respiration:
  1. Pulmonary Ventilation (Breathing): Drawing in atmospheric air and releasing $CO_2$ rich alveolar air.
  2. Alveolar Diffusion: Diffusion of $O_2$ and $CO_2$ across the alveolar membrane.
  3. Transport: Movement of gases via the blood.
  4. Tissue Diffusion: Diffusion of $O_2$ and $CO_2$ between blood and tissues.
  5. Cellular Respiration: Utilization of $O_2$ by cells for catabolic reactions and the resulting release of $CO_2$.

Mechanism of Breathing

• Pressure Gradients: Air movement is driven by pressure differences between the lungs and the atmosphere.
• Inspiration (Inhalation):
  • Occurs when intra-pulmonary pressure is less than atmospheric pressure (negative pressure).
  • Initiation: Contraction of the diaphragm increases thoracic volume in the antero-posterior axis.
  • Rib Movement: Contraction of external intercostal muscles lifts the ribs and sternum, increasing thoracic volume in the dorso-ventral axis.
  • Result: Overall increase in thoracic volume leads to an increase in pulmonary volume, decreasing pressure and forcing air into the lungs.
• Expiration (Exhalation):
  • Occurs when intra-pulmonary pressure is higher than atmospheric pressure.
  • Mechanism: Relaxation of the diaphragm and intercostal muscles returns the diaphragm and sternum to normal positions.
  • Result: Thoracic and pulmonary volume decreases, increasing intra-pulmonary pressure and expelling air.
• Additional Musculature: Abdominal muscles can be used to increase the strength of breathing.
• Clinical Measurements:
  • Average healthy human breathing rate: $12-16$ times/minute.
  • Spirometer: An instrument used to estimate the volume of air involved in breathing, aiding in clinical assessment of pulmonary functions.

Respiratory Volumes and Capacities

• Volumes:
  • Tidal Volume (TV): Volume of air inspired or expired during normal respiration. Value: approximately $500\,mL$ (approx. $6000-8000\,mL$ per minute).
  • Inspiratory Reserve Volume (IRV): Additional volume of air a person can inspire by a forcible inspiration. Average: $2500\,mL$ to $3000\,mL$.
  • Expiratory Reserve Volume (ERV): Additional volume of air a person can expire by a forcible expiration. Average: $1000\,mL$ to $1100\,mL$.
  • Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration. Average: $1100\,mL$ to $1200\,mL$.
• Capacities (Derived from Volumes):
  • Inspiratory Capacity (IC): Total air a person can inspire after a normal expiration ($TV + IRV$).
  • Expiratory Capacity (EC): Total air a person can expire after a normal inspiration ($TV + ERV$).
  • Functional Residual Capacity (FRC): Air remaining in lungs after a normal expiration ($ERV + RV$).
  • Vital Capacity (VC): Maximum air a person can breathe in after a forced expiration (or breathe out after a forced inspiration). Includes $ERV + TV + IRV$.
  • Total Lung Capacity (TLC): Total air accommodated in lungs after forced inspiration. Includes $RV + ERV + TV + IRV$ or $VC + RV$.

Exchange of Gases

• Primary Site: Alveoli are the primary sites, though exchange also occurs between blood and tissues.
• Mechanism: Simple diffusion based on pressure/concentration gradients.
• Factors Influencing Diffusion Rate:
  1. Partial Pressure: Pressure of an individual gas in a mixture, denoted as $pO_2$ and $pCO_2$.
  2. Solubility: $CO_2$ solubility is $20-25$ times higher than $O_2$, allowing much more $CO_2$ to diffuse per unit of partial pressure difference.
  3. Membrane Thickness: The diffusion membrane total thickness is much less than a millimetre ($1\,mm$).
• Partial Pressure Values ($mm\,Hg$):
  • Atmospheric Air: $pO_2 = 159$; $pCO_2 = 0.3$
  • Alveoli: $pO_2 = 104$; $pCO_2 = 40$
  • Deoxygenated Blood: $pO_2 = 40$; $pCO_2 = 45$
  • Oxygenated Blood: $pO_2 = 95$; $pCO_2 = 40$
  • Tissues: $pO_2 = 40$; $pCO_2 = 45$
• The Diffusion Membrane Layers:
  1. Thin squamous epithelium of alveoli.
  2. Endothelium of alveolar capillaries.
  3. Basement substance (basement membrane supporting the epithelium and the capillary endothelium).

Transport of Gases

• Medium: Blood serves as the transport medium.
• Oxygen ($O_2$) Transport:
  • $97\%$ is transported by Red Blood Cells (RBCs) as oxyhaemoglobin.
  • $3\%$ is carried in a dissolved state through plasma.
  • Haemoglobin (Hb): A red-colored iron-containing pigment. Each Hb molecule can carry a maximum of four $O_2$ molecules.
  • Binding Factors: Binding is primarily related to $pO_2$. It is interfered with by $pCO_2$, $H^+$ concentration, and temperature.
  • Oxygen Dissociation Curve: A sigmoid curve resulting from plotting percentage saturation of Hb against $pO_2$.
  • Conditions for Oxyhaemoglobin Formation (Alveoli): High $pO_2$, low $pCO_2$, lesser $H^+$ concentration, lower temperature.
  • Conditions for Dissociation (Tissues): Low $pO_2$, high $pCO_2$, high $H^+$ concentration, higher temperature.
  • Efficiency: Every $100\,mL$ of oxygenated blood delivers approximately $5\,mL$ of $O_2$ to tissues under normal conditions.
• Carbon Dioxide ($CO_2$) Transport:
  • $20-25\%$ is transported by RBCs as carbamino-haemoglobin. Binding occurs when $pCO_2$ is high and $pO_2$ is low.
  • $70\%$ is carried as bicarbonate ($HCO_3^-$).
  • $7\%$ is carried in a dissolved state through plasma.
  • Enzymatic Reaction: RBCs contain a high concentration of carbonic anhydrase (also present in minute quantities in plasma). This enzyme facilitates:
    • $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$
  • Efficiency: Every $100\,mL$ of deoxygenated blood delivers approximately $4\,mL$ of $CO_2$ to the alveoli.

Regulation of Respiration and Disorders

• Neural Regulation:
  • Respiratory Rhythm Centre: Located in the medulla oblongata; primarily responsible for regulation.
  • Pneumotaxic Centre: Located in the pons; moderates the rhythm centre's functions. It can reduce the duration of inspiration to alter the respiratory rate.
  • Chemosensitive Area: Adjacent to the rhythm centre; highly sensitive to $CO_2$ and $H^+$ ions. Activation leads to signals for elimination of these substances.
  • Peripheral Receptors: Found in the aortic arch and carotid artery; detect changes in $CO_2$ and $H^+$ and signal the rhythm centre.
  • Note: The role of oxygen in regulating respiratory rhythm is insignificant.
• Respiratory Disorders:
  • Asthma: Inflammation of bronchi and bronchioles causing wheezing and breathing difficulty.
  • Emphysema: Chronic disorder where alveolar walls are damaged, decreasing the respiratory surface. Major cause: cigarette smoking.
  • Occupational Respiratory Disorders: Found in industries like stone-breaking or grinding. Dust leads to inflammation and fibrosis (proliferation of fibrous tissues), causing lung damage. Prevention: wearing protective masks.

Questions & Discussion

• Exercises and Definitions:
  • Vital Capacity: The maximum volume of air a person can breathe in after a forced expiration.
  • Volume after normal breathing: The Functional Residual Capacity ($FRC = ERV + RV$).
  • Alveolar Site Specificity: Only the alveoli serve as exchange sites because the diffusion membrane there is extremely thin ($< 1\,mm$) and vascularized, unlike the thick-walled conducting parts.
  • Transport Mechanisms for $CO_2$: Primarily as bicarbonate ($70\%$), followed by carbamino-haemoglobin ($20-25\%$), and dissolved in plasma ($7\%$).
  • Partial Pressure Comparison: Atmospheric air has higher $pO_2$ and lower $pCO_2$ compared to alveolar air.
  • Inspiration Process: Initiated by diaphragm and external intercostal contraction, leading to increased thoracic volume and decreased intra-pulmonary pressure.
  • Hypoxia: A condition of oxygen deficiency reaching the tissues; often discussed in the context of high altitudes.
  • Tidal Volume Calculation: If $TV = 500\,mL$ and rate is $12-16$ breaths/min, the hourly volume is approx. $360-480\,Litres$.