Respiratory Physiology

• Function of Respiration


  • Gas Exchange between external environment & body to obtain O2 and eliminate CO2
  • Regulate acid-base balance
  • Respiration includes all events involved in gas exchange
• General organization of the respiratory system

  • An air pump for alveolar ventilation
  • A surface for gas exchange (alveoli)
  • A mechanism to carry oxygen and carbon dioxide in the blood
  • A circulatory system
  • A mechanism for locally regulating the distribution of air and blood flow
  • A mechanism for centrally regulating ventilation
• External & Internal Respiration

  1. External respiration: exchange of O2 and CO2 between atmosphere and body tissues

  2. Internal respiration: use of O2 in mitochondria to generate ATP by oxidative phosphorylation, with CO2 as the waste product
• Ventilation


  1. Main purpose: maintain optimal composition of alveolar gas

  2. Alveolus acts as a buffer compartment between atmosphere and capillary blood, with O2 removed by blood and CO2 added, replenished, and removed by ventilation
• Nonrespiratory Functions of Respiratory System

  • Filter against thrombi and emboli
  • Metabolic organ (e.g., converts Angiotensin I to Angiotensin II, produces surfactant)
  • Shock-absorber for the heart, enhances venous return
  • Alters the pH of blood
  • Route for water loss and heat elimination
  • Blood reservoir
  • Provides airflow for speech, singing, and other vocalizations
• Respiratory System


  • Consists of airways leading into the lungs, lungs, and structures in the thorax for producing movement of air through airways
• Respiratory Airways


  • Tubes that carry air between the atmosphere and alveoli, including nasal passages, pharynx, trachea, larynx, bronchi, bronchioles, and alveoli clustered at the ends of terminal bronchioles
• Airway diameter regulation


  • Regulated by smooth muscle innervation, circulating hormones, and local chemicals
• Conducting Zone


  • Leads inspired air to gas exchange regions, includes trachea and first 16 generations of airways without alveoli, blood-gas barrier, or gas exchange
• Respiratory Zone


  • Last 7 generations of airways, site of gas exchange with 300 million alveoli, where the blood-gas barrier is present
• Conducting Zone


  • Functions: distributes air evenly, warms and humidifies inspired air, defense mechanism with mucus
• Respiratory Zone


  • Large surface area, thin-walled with one layer of flattened Type I alveolar cells, total blood-gas barrier is 2 cells across
• Respiratory Zone


  • Large surface area (60-80 m2)
  • Thin-walled with one layer of flattened Type I alveolar cells
  • Total blood-gas barrier is 2 cells across (Alveolar epithelium, interstitial fluid, capillary endothelium)
  • Type II alveolar cells secrete surfactant
  • Alveolar macrophages guard lumen
  • Pores of Kohn permit airflow between adjacent alveoli (collateral ventilation)
• Alveoli


  • Saturated with water vapor
  • Moving staircase of mucus
• Lungs

  • Superior tip is apex, just deep to clavicle
  • Concave inferior surface resting on diaphragm is the base
  • Lung tissue (parenchyma) consists of airways, alveoli, blood vessels, elastic connective tissue
• Thorax


  • Thoracic cage consists of ribs, spine, sternum, thoracic vertebrae
  • Internal and external intercostal muscles connect the 12 rib pairs
  • Sternocleidomastoids and scalenes connect the head and neck to the first 2 ribs
  • Diaphragm is a dome-shaped skeletal muscle separating thoracic cavity from the abdominal cavity
• Pleural fluid


  1. Lubricates pleural surfaces

  2. Causes pleural surfaces to adhere together
• A pleural sac separates each lung from the thoracic wall
• Intrapleural fluid creates a slippery surface allowing lungs to slide against the thoracic wall
• Because of cohesive forces, when the chest expands, lungs are compelled to follow
• Pleural fluid acts as a lubricant and allows the lungs and chest to expand as a single unit
• Atmospheric pressure is the pressure exerted by the weight of the air in the atmosphere (760 mm Hg at sea level)
• Intrapulmonary pressure is the pressure inside the alveoli
• Intrapleural pressure is the pressure in the pleural fluid, normally less than intra-alveolar pressure
• Transmural pressure is the pressure difference across the wall (transpulmonary pressure = across lung wall = Palveolar - Pintrapleural)
• Transmural pressure gradient is an important reason lungs follow the chest
• Transpulmonary pressure makes it easier to accomplish lung expansion
• The transmural pressure gradient and intrapleural fluid's cohesiveness help keep the lung & chest from pulling away from each other except to the slightest degree
• Pneumothorax, or "air in chest," can lead to collapse of alveoli (atelectasis)
• If there is an opening in the chest wall, air enters the pleural space, causing the lungs and thorax to separate and assume their natural positions
• Common symbols in respiratory physiology denote physical quantities
• Lung collapse


  1. Transpulmonary pressure gradient lost

  2. Intrapleural fluid’s cohesiveness cannot hold lungs and wall

  3. Lungs and thorax separate and assume their natural positions

  4. Collapse of alveoli – atelectasis
• Primary symbols in respiratory physiology

  • P: pressure, tension or partial pressure of a gas
  • V: volume of a gas
  • F: fractional concentration of a gas
  • Q: volume of blood
  • C: content
• Secondary symbols in respiratory physiology

  • A: alveolar
  • a: arterial
  • B: barometric
  • D: dead space
  • E: expiratory
  • I: inspiratory
  • ip: pleural
  • v: venous
• Tertiary symbols in respiratory physiology

  • O2: oxygen
  • CO2: carbon dioxide
  • N2: nitrogen
• A dot (.) above a symbol denotes a rate
• To alter lung volumes, we need:

  
  

  1. Respiratory muscles to change size of thoracic cavity

  2. Overcome tissue elastance

  3. Overcome surface tension within alveoli
• Air flows down a pressure gradient
• Intra-alveolar pressure can be altered by changing the volume of the lungs
• Boyle’s Law: "the pressure and volume of a gas are inversely related"