respiratory system

• Respiratory system


  Functions to obtain oxygen from the environment and excrete carbon dioxide from cells, in both an internal / cellular manner and an external manner. It also helps to regulate pH, protect against pathogens and irritating substances, as well as facilitate vocalisation.
• Respiratory system portions


  • Organised into 2 parts; the conducting portion and respiratory portion
• Conducting portion


  1. Consists of cavities and tubes that conduct air into the lungs

  2. Functions to warm, humidify, and filter air
• Conducting portion components

  • Trachea
  • Primary bronchi
  • Secondary bronchi
  • Tertiary bronchi
  • Bronchioles (from this point no cartilage is present)
  • Terminal bronchioles
• Larger airways in conducting portion

  Rigid and non-muscular, supported by cartilage to prevent collapse
• Bronchioles


  No cartilage, surrounded by rings of smooth muscle
• Respiratory portion


  Site of gas exchange
• Respiratory portion components


  • Respiratory bronchioles
  • Alveoli
• Bronchioles


  Surrounded by rings of smooth muscle, which contracts and relaxes to change the resistance to airflow
• Smooth muscle


  Responsive to the autonomic nervous system
• Parasympathetic nerves


  Release ACh which causes bronchoconstriction and mucous secretion to reduce the bronchiole diameter and increase resistance
• Sympathetic nerves


  Release adrenaline which causes bronchodilation to increase the bronchiole diameter and decrease resistance
• Respiratory portion


  • Optimised for gas exchange as it has a rich blood supply, a large surface area (approximately 145m2) and a short pathlength for diffusion
• Pulmonary artery


  Branches into capillary beds that form a meshwork around each alveolus
• Pleura


  A double membrane sac which lines the inside of the chest cavity and covers the surface of the lungs. There are 2 layers – the parietal (outer) layer and the visceral (inner) layer. The visceral layer is closely associated with the lung surface and the two layers glide smoothly past each other thanks to pleural fluid acting as a lubricant and reducing friction between the pleural surfaces and the lungs
• Muscles involved in quiet breathing

  • External intercostals (inspiration)
  • Diaphragm (inspiration)
  • Passive expiration relies on the natural recoil of the lungs
• Muscles involved in forceful breathing

  • Sternocleidomastoids (inspiration)
  • Scalene muscles (inspiration)
  • Internal intercostals (expiration)
  • Abdominal muscles (expiration)
• Boyle's Law


  P1V1 = P2V2. It states that when there is a volume decrease, pressure increases, and when there is a volume increase, pressure decreases.
• Air flows


  From areas of higher pressure to low pressure
• Inspiration


  Occurs when the pressure in the lungs is less (volume increased) than the atmospheric pressure, causing air to flow into the lungs
• Expiration


  Occurs when the pressure in the lungs is greater (volume decreased) than the atmospheric pressure, causing air to flow out of the lungs
• Intrapleural space


  Negative pressure (-4mmHg less than atmospheric pressure) created by the opposing forces acting of the lungs and chest wall (inward recoil of lungs and outward recoil of chest wall). It helps the lungs maintain normal expansion (prevents them from collapsing), facilitates lung compliance (reduces the work required for inspiration), and assists airflow (creates pressure gradient between atmosphere and alveoli)
• Lung volumes


  • RV (residual volume)
  • ERV (expiratory reserve volume)
  • V1 / TV (tidal volume)
  • IRV (inspiratory reserve volume)
• RV (residual volume)


  Volume of air left in lungs after MAXIMAL exhalation, can't be directly measured, average volume = 1200mL
• ERV (expiratory reserve volume)


  Extra volume of air that can be forcefully exhaled after the end of normal expiration, average = 1100mL
• V1 / TV (tidal volume)

  Volume of air that moves during a single inspiration or expiration, average = 500mL
• IRV (inspiratory reserve volume)


  Extra volume of air that can be maximally inspired over and above the typical resting tidal volume, average = 3000mL
• Lung capacities


  • Inspiratory capacity (IC)
  • Vital capacity (VC)
  • Total lung capacity (TLC)
  • Functional residual capacity (FRC)
• Inspiratory capacity (IC)


  Maximum volume of air that can be inspired at the end of a normal quiet expiration, IC = IRV + TV (av. 3500 mL)
• Vital capacity (VC)


  Maximum volume of air moved out during a single breath following a MAXIMAL inspiration, VC = IRV + TV + ERV // VC = IC + ERV (av. 4500mL)
• Total lung capacity (TLC)


  Maximum volume of air that the lungs can hold, TLC = VC + RV (av. 5700mL)
• Functional residual capacity (FRC)

  Volume of air in the lungs at end of normal passive expiration, FRC = ERV + RV (av 2200mL)
• Forced vital capacity (FVC)

  Maximal amount of air that can be exhaled upon a maximal inhalation followed by a forced exhalation
• Forced expiratory volume in 1 second (FEV1)


  Volume that leaves the airways in the first second of a FVC test
• FEV1/FVC ratio


  Important determinant of pulmonary function, normally >80%, <80% can indicate an obstructive lung disease
• Total pulmonary ventilation


  Total volume of air moved in and out of the lungs in one minute, calculated by multiplying the tidal volume with the ventilation rate
• Total alveolar ventilation


  Volume of air that enter the alveoli and contribute to gas exchange, calculated by multiplying the ventilation rate with the tidal volume minus the anatomic dead space volume (conducting portion of airways, usually 150mL in volume)
• Pulmonary surfactant


  Mixture of lipids and proteins produced by type 2 alveolar cells, lines all air-liquid interfaces, disrupts cohesive force of water molecules and reduces surface tension to maintain alveolar stability
• Inspiration is always active (always requires energy) and thus it requires work
• Elastance


  Relates to how readily the lungs rebound after being stretched, lungs tend to collapse after being stretched due to the elastic connective tissue and surface tension