Breathing and Exchange of Gases

• Introduction

  • Definition of Breathing vs. Respiration
  • Need for oxygen and removal of $CO_2$

• Respiratory Organs

  • Diversity in animals
    • General Body Surface: Sponges, Coelenterates, Flatworms
    • Skin/Cuticle: Earthworms, Amphibians (cutaneous respiration)
    • Tracheal Tubes: Insects
    • Gills (Branchial): Fishes, aquatic arthropods, molluscs
    • Lungs (Pulmonary): Reptiles, birds, mammals

• Human Respiratory System

  • Conducting Zone
    • Nostrils and Nasal Cavity
    • Pharynx and Larynx (Sound box)
    • Trachea (supported by cartilaginous rings)
    • Bronchi and Bronchioles
  • Respiratory/Exchange Zone
    • Alveoli (site of gas exchange)
  • Thoracic Chamber
    • Ribs, Sternum, and Vertebral column
    • Diaphragm (dome-shaped muscular partition)

• Mechanism of Breathing

  • Steps
    • Breathing/Pulmonary ventilation
    • Diffusion of gases across alveolar membrane
    • Transport of gases by blood
    • Diffusion of gases between blood and tissues
    • Cellular respiration (utilization of $O_2$)
  • Inspiration (Inhalation)
    • Diaphragm contracts (flattens)
    • External intercostal muscles contract (ribs move up and out)
    • Thoracic volume increases
    • Intra-pulmonary pressure decreases below atmospheric pressure
    • Air rushes into lungs
  • Expiration (Exhalation)
    • Diaphragm relaxes (becomes dome-shaped)
    • Intercostal muscles relax (ribs move down and in)
    • Thoracic volume decreases
    • Intra-pulmonary pressure increases above atmospheric pressure
    • Air is expelled out
  • Respiratory Volumes and Capacities
    • Tidal Volume (TV): ~500 mL per breath
    • Inspiratory Reserve Volume (IRV): 2500–3000 mL
    • Expiratory Reserve Volume (ERV): 1000–1100 mL
    • Residual Volume (RV): 1100–1200 mL
    • Vital Capacity (VC): TV + IRV + ERV
    • Total Lung Capacity (TLC): VC + RV

• Exchange of Gases

  • Sites of Exchange
    • Alveoli to Blood (lungs)
    • Blood to Tissues
  • Factors Affecting Diffusion
    • Pressure/Concentration gradient ($PO_2$ and $PCO_2$)
    • Solubility of gases ($CO_2$ is 20-25 times more soluble than $O_2$)
    • Thickness of the diffusion membrane (very thin squamous epithelium)
  • Partial Pressures ($mmHg$)
    • In Atmospheric air: $PO_2 = 159$, $PCO_2 = 0.3$
    • In Alveoli: $PO_2 = 104$, $PCO_2 = 40$
    • In Deoxygenated blood: $PO_2 = 40$, $PCO_2 = 45$
    • In Oxygenated blood: $PO_2 = 95$, $PCO_2 = 40$
    • In Tissues: $PO_2 = 40$, $PCO_2 = 45$

• Transport of Gases

  • Transport of Oxygen ($O_2$)
    • ~3% dissolved in plasma
    • ~97% bound to Hemoglobin in RBCs (Oxyhemoglobin)
    • Oxygen-Hemoglobin Dissociation Curve (Sigmoid curve)
    • Factors shifting curve to right (favoring dissociation in tissues): High $H^+$ (low pH), high $PCO_2$, high temperature, high 2,3-DPG
  • Transport of Carbon Dioxide ($CO_2$)
    • ~7% dissolved in plasma
    • ~20-25% as Carbaminohemoglobin (bound to hemoglobin)
    • ~70% as Bicarbonate ions ($HCO_3^-$) via Carbonic Anhydrase enzyme

• Regulation of Respiration

  • Respiratory Rhythm Center
    • Located in Medulla oblongata
    • Primarily controls basic rhythm
  • Pneumotaxic Center
    • Located in Pons
    • Can moderate the functions of the respiratory rhythm center (reduces duration of inspiration)
  • Chemosensitive Area
    • Highly sensitive to $CO_2$ and $H^+$ ions (located near rhythm center)
    • Aortic arch and carotid artery receptors detect changes in $O_2$ and $H^+$ concentrations

• Disorders of Respiratory System

  • Asthma: Spasm of bronchial muscles, difficulty in breathing, wheezing
  • Emphysema: Damage to alveolar walls, decreases surface area for exchange, major cause is cigarette smoking
  • Occupational Respiratory Disorders:
    • Silicosis, Asbestosis
    • Due to long-term exposure to dust in industries (stone-grinding, mining)
    • Leads to fibrosis (proliferation of fibrous tissue) and serious lung damage