lect 7 objs

• The human lungs are a pair of large, spongy organs optimized for gas exchange between blood and air
• Key features enabling gas exchange

  • An air “pump” for alveolar ventilation
  • A mechanism to carry oxygen and carbon dioxide in the blood - red blood cells
  • A large, thin surface for gas exchange - the anatomy of alveoli
  • A circulatory system with distinct differences from systemic circulation
  • A mechanism for locally regulating the distribution of air and blood flow
  • A mechanism for centrally regulating ventilation
• The respiratory system includes the airways leading into the lungs, the lungs themselves, and the structures of the thorax involved in producing movement
• Airways

  1. The airways consist of a series of branching tubes which become narrower, shorter, and more numerous as they penetrate deeper into the lung

  2. All airways divide dichotomously

  3. The trachea divides into 2 daughter airways: right and left main bronchi, which divide into lobar then segmental bronchi

  4. Terminal bronchioles divide into respiratory bronchioles (transitional zone) which have occasional alveoli budding from walls

  5. Alveolar ducts (respiratory zone) are completely lined with alveoli

  6. The diameter of airways gets progressively smaller, but total cross-sectional surface area increases

  7. Velocity of air flow is highest in trachea and lowest in terminal bronchioles

  8. To permit airflow in and out of the gas-exchange portions of the lungs, the entrance through the terminal bronchioles to the alveoli must remain open

  9. The trachea and larger bronchi are fairly rigid nonmuscular tubes encircled by cartilaginous rings that prevent compression

  10. The smaller bronchioles have no cartilage to hold them open, but parenchyma and elasticity of lung tissue help keep these airways open

  11. The only muscle within the lungs is the smooth muscle in the walls of the arterioles and walls of bronchioles, both of which are subject to control

  12. There is no muscle within the alveolar walls to cause them to inflate and deflate during breathing
• Lungs


  • Spongy tissue mostly occupied by air-filled spaces
  • Divided into lobes: right lung into three lobes, left lung into two
  • Left lung is smaller than the right due to the space occupied by the heart
• Thorax


  • Created by 12 pairs of ribs, thoracic vertebrae, and skeletal muscles bounding the thorax
  • Diaphragm muscle forms thoracic floor
  • Internal and external intercostal muscles connect the 12 rib pairs
  • Sternocleidomastoids and scalenes connect the head and neck to the first 2 ribs
• Pleural sac


  • Surrounds each lung
  • Secretes a small amount of fluid to create a slippery surface allowing movement of membranes as lungs move
• Blood-gas barrier


  1. Consists of alveolar epithelium, an interstitial space, and the capillary endothelium

  2. Type I cells allow gas exchange

  3. Type II cells secrete pulmonary surfactant

  4. Defensive alveolar macrophages are present
• Lungs have the most extensive capillary network of any organ
• Pulmonary capillaries occupy 70-80% of alveolar surface area
• Minute Pores of Kohn connect adjacent alveoli and allow air pressure throughout the lung to be equalized
• Pressure is a relative measure defined as differences between two compartments
• Atmospheric pressure (PB) = ambient, barometric pressure (760 mmHg at sea level); PB decreases as altitude increases
• Alveolar pressure (PA) = the total gas pressure in alveoli, will equilibrate with PB
• Intrapleural pressure (Pip) = pressure within the space between the visceral and parietal pleura (756 mmHg), slightly negative due to chest and lung forces
• Transmural pressure (PL) = pressure across the surface of lungs (PA-Pip), key to inflating lungs
• Recoil forces create a vacuum


  Closed cavity
• If air enters the pleural space


  Pip will equalize with PB, the transmural pressure gradient will be gone, the lungs will collapse, and the thoracic wall will spring out. This is a pneumothorax
• Common abbreviations used in pulmonary physiology

  • P = partial pressure
  • V = volume of gas
  • F = fractional concentration of a gas
  • Q = volume of blood
  • C = content
  • A = alveolar
  • a = arterial
  • B = barometric
  • D = dead space
  • E = expiratory
  • I = inspiratory
  • ip = pleural
  • v = venous
  • O2 = oxygen
  • CO2 = carbon dioxide
  • N2 = nitrogen
• Processes for optimal gas exchange

  • Ventilation - getting gas to the alveoli
  • Perfusion - removing gas from the alveoli by the blood
  • Diffusion - getting gas across alveolar walls
  • Control of breathing - regulating gas exchange
• The airways consist of a series of branching tubes which become narrower, shorter, and more numerous as they penetrate deeper into the lung
• Airway zones

  • Conducting zone (no alveoli) - trachea, bronchi, bronchioles
  • Respiratory zone (alveoli) - respiratory bronchioles, alveolar ducts & sacs
• There are 300 million alveoli in lungs creating a total surface area of about 75 m2. Alveoli are small, thin-walled inflatable air sacs encircled by pulmonary capillaries. An alveolus has a single layer of thin exchange epithelium and is the site of gas exchange. Air flows between adjacent alveoli via pores of Kohn
• Types of cells in alveoli

  • Type I alveolar cells: Very thin, allowing gas exchange
  • Type II alveolar cells: Thicker, secrete surfactant to ease lung expansion
  • Alveolar macrophages: Protect and defend
• Atmospheric pressure (PB) = 760 mmHg at sea level, decreases as altitude increases
• Intra-alveolar pressure (PA) will equilibrate with atmospheric pressure
• Intrapleural pressure (Pip) = 756 mmHg; recoil forces create a vacuum (“-4”); a closed cavity
• Transmural pressure (PL): pressure across the lungs (PA-Pip); key to inflating lungs
• Intrapleural fluid’s cohesiveness and the transmural pressure gradient (most important) hold the lungs and thoracic wall in tight apposition even though the lungs are smaller
• PA = 760 mmHg, pushes out vs Pip of 756 mmHg
• PB = 760 mmHg, pushes in vs Pip
• Pleural space has slightly negative pressure because chest is pulling out, lungs are pulling in, and there’s no extra fluid to fill expanded space
• Pneumothorax: air enters pleural cavity, pressure equalizes with atmospheric pressure, transmural pressure gradient is gone, lungs collapse, thoracic wall springs out
• Respiratory Physiology lecture


  March 12 & 19, 2024
• Lecturer: 'Deborah A. Podolin, PhD'
• Email: [e-mail]
• Boyle's Law


  Describes the relationship between the pressure and volume of a gas
• Muscles and pressures associated with breathing

  Inspiration results from the contraction of the diaphragm and intercostal muscles (an active process). Expiration results from the relaxation of the diaphragm and intercostal muscles (a passive process)
• Pressures during breathing


  Before inspiration, intrapleural pressure is negative, alveolar pressure is equal to the atmosphere. During inspiration, intrapleural pressure becomes more negative, alveolar pressure becomes negative. End of inspiration/beginning of expiration, alveolar pressure is equal to the atmosphere. During expiration, alveolar pressure starts to become positive to atmospheric
• The concepts that dictate blood movement in the cardiovascular system are very similar to the concepts dictating air movement through the lung