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• Respiration


  Physiological process of breathing in oxygen and breathing out carbon dioxide for the liberation of energy that is required for the sustenance of living cells
• Phases of respiration

  • Inspiration (inhalation)
  • Expiration (exhalation)
• Types of respiration

  • Internal respiration
  • External respiration
• Functions of respiration

  • Ventilation (breathing)
  • Gaseous exchange between air and blood, and between blood and other body tissues
  • Oxygen utilization by the tissues for energy liberation
• Conducting zone

  Anatomical structures through which air passes before reaching the respiratory zone. Gaseous exchange does not occur in this zone.
• Respiratory zone


  Where gaseous exchange occurs
• Structures of the anatomical (conducting) zone

  • Mouth
  • Nose
  • Pharynx
  • Larynx
  • Trachea
  • Primary bronchi
  • Successive branchings of the bronchioles up to (and including) the terminal bronchioles
• Structures of the respiratory zone

  • Respiratory bronchioles
  • Alveoli
• Alveoli

  • Sac-like structures that confer on the respiratory zone the ability to allow for gaseous exchange
  • There are an estimated 300 million alveoli
  • Provide a large surface area (60 to 80 square meters) for diffusion of gases
  • Each alveolus is only one cell-layer thick
• Air-blood barrier

  Structural barrier of about 2 μm thick, between air and blood, made up of the alveolar cell membrane and the pulmonary capillary endothelial cell membrane
• The basement membranes of type 1 alveolar cells and capillary endothelial cells fuse in such a way that the diffusion distance between them is only about 0.3 μm thick, about 1/100th the width of a human hair
• Alveolar wall

  • Not fragile but strong enough to withstand high stress during heavy exercise and high lung inflation
  • Great tensile strength provided by the fused membranes of the air-blood barrier which are composed of type IV collagen proteins
• Types of alveolar cells

  • Type I alveolar cells (95 - 97% of total lung alveolar population), primarily dedicated to gaseous exchange
  • Type II alveolar cells (remaining percentage), dedicated for the secretion or production of pulmonary surfactant
• Functions of the anatomical (conducting) zone

  • Warming and humidification of inspired air
  • Filtering and cleaning
• The mucus lining of the conducting zones are moved at a rate of 1 to 2cm per minute by cilia projections from the top of the epithelial cells that lines the conducting zone
• There are about 300 cilia per cell that beat in a coordinated fashion to move mucus toward the pharynx, where it can either be swallowed or expectorated
• Particles larger than 6 μm do not normally enter the respiratory zone of the lungs. These particles usually serve as irritants of the respiratory tracts that may initiate a respiratory protective reflex

  
• Respiratory protective reflexes

  • Sneezing reflex
  • Coughing reflex
  • Swallowing or deglutition reflex
• Pleural membranes


  Two layers of wet epithelial membranes that envelop the structures in the central region of the thoracic cavity
• Normally, the lungs fill the thoracic cavity so that the visceral pleural constantly pushes against the parietal pleural. Therefore, there is little or no air between these pleural membranes under physiological conditions; although there is a potential space (also called intra-pleural space) between them

  
• Lung compliance


  The ability to distend or respond to stretch, defined as change in lung volume per change in trans-pulmonary pressure (ΔV/ΔP)

  
• Lung elasticity


  The tendency of the lungs to return to its initial size after being distended. The lungs are normally stuck to the chest wall, so they are always in a state of elastic tension
• Surface tension


  A phenomenon at the surface of liquid that is caused by intermolecular forces. Both elastic resistance and the surface tension that is exerted by the alveolar fluid are forces that act to resist lung distension
• Pneumothorax


  The presence of gas in the intra-pleural space with a resultant collapse of the lungs (one or both lungs, depending on the degree of chest injury that allows for air leak into the intra-pleural space)

  
• Atelectasis


  The deflation or collapse of the alveoli or lungs

  
• Types of atelectasis

  • Compressive atelectasis
  • Resorptive/obstructive atelectasis
  • Contraction atelectasis

  
• Pulmonary surfactants


  Substances secreted by type II alveolar cells that act on the surface of alveolar fluids to prevent alveolar and lung collapse following an elastic recoil

  
• Surfactants consist of phospholipids, primarily phosphatidylcholine and phosphatidylglycerol, together with hydrophobic surfactant proteins. They are interspersed between water molecules and water-air interface thereby reducing the hydrogen bonds between water molecules at the surface to bring about a drastic reduction of surface tension
• The ability of surfactant to lower surface tension improves as the alveoli get smaller during expiration. This may be because the surfactant molecules become more concentrated as the alveoli get smaller
• Surfactant prevents the alveoli from collapsing during expiration. Even after a forceful expiration, the alveoli remains open and a residual volume of air remains in the lungs
• Surfactants


  Consist of phospholipids, primarily phosphatidylcholine and phosphatidylglycerol, together with hydrophobic surfactant proteins. They are interspersed between water molecules and water-air interface thereby reducing the hydrogen bonds between water molecules at the surface to bring about a drastic reduction of surface tension.
• As a result of the effect of pulmonary surfactant, the surface tension of the alveoli is negligible.
• The ability of surfactant to lower surface tension


  Improves as the alveoli get smaller during expiration
• Surfactant prevents the alveoli from collapsing during expiration.
• Even after a forceful expiration, the alveoli remains open and a residual volume of air remains in the lungs.

  
• Premature babies are sometimes born with lungs that lack sufficient surfactant and their alveoli are collapsed as a result. This condition is called Respiratory Distress Syndrome (RDS).
• RDS occurs in about 60% of babies born at less than 28 weeks, 30% of babies born at 28 to 34 weeks, and less than 5% of babies born after 34 weeks of gestation.
• The risk of RDS can be assessed by analysis of amniotic fluid (surrounding the fetus), and mothers can be given exogenous corticosteroids to accelerate the maturation of their fetus's lungs.

  
• Acute Respiratory Distress Syndrome (ARDS)


  A type of distress syndrome in non-infants, where inflammation causes increased capillary and alveolar permeability that lead to the accumulation of a protein-rich fluid in the lungs.

  
• This decreases lung compliance and is accompanied by a reduced surfactant, which further lowers compliance. The blood leaving the lungs, as a result, has an abnormally low oxygen concentration (a condition called hypoxemia).