Respiratory System

• Functions of the respiratory system

  • Gas exchange
  • Filtration of inspired air
  • Blood pH regulation
  • Phonation
  • Olfaction
  • Water and heat elimination (expired air)
  • Intake of oxygen
  • Elimination of carbon dioxide
• Parts of the respiratory system

  • Upper respiratory system (Nose, pharynx and associated structures)
  • Lower respiratory system (Larynx, trachea, bronchi and lungs)

  
• Alveoli


  Major sites of gas exchange
• Alveolar sacs

  Within these are 2 or more alveoli

  
• Types of alveolar cells

  • Type I pneumocyte (Main site of gas exchange)
  • Type II pneumocyte (Produce surfactant)
  • Alveolar macrophages
  • Fibroblasts

  
• Alveolar and capillary walls

  • Very thin membrane (0.5 µm) consisting of 4 membranes: Type I and Type II pneumocytes, alveolar macrophages, epithelial basement membrane, capillary basement membrane, endothelial cells lining the capillary
• In most vessels

  Causes dilation of blood vessels to increase O2 delivery
• In pulmonary blood vessels

  Causes vasoconstriction (ventilation-perfusion coupling) to allow pulmonary blood to be diverted to well-ventilated areas
• Exercise-induced pulmonary hemorrhage
• Surfactant production in premature infants
• Airway obstruction in short-nosed dogs
• Gas exchange

  1. Pulmonary ventilation (movement of air into and out of the lungs)

  2. External respiration (exchange of gases between the lungs and pulmonary capillaries)

  3. Internal respiration (exchange of gases between systemic capillaries and tissue)
• Boyle's Law

  At constant temperature and pressure, an inverse relationship between volume and pressure exist: P1V1 = P2V2
• Pressure inside a closed container

  Decreases as the volume of the container increases, and vice versa
• Pressure inside the lungs

  Decreases as volume of the lungs increases
• Air moves into the lungs

  As a result of decrease in air pressure within the lungs
• Main muscles for quiet inhalation

  • Diaphragm (descends downward)
  • External intercostal muscles (increase horizontal dimensions of thoracic cavity, increase volume of the thorax, expand volumes of lungs, decrease pressure inside the lungs, inspiration)

  
• As inspiration begins

  Pressure drops (from 754 mmHg to 744 mmHg) resulting in: volume of thoracic cavity increases, alveolar pressure decreases, air flows from area of high pressure (atmosphere) to area of low pressure (alveoli)
• Increase force of inspiration

  • Involves other muscles such as sternocleidomastoid muscles, scalene muscles, pectoralis minor muscles
• Expiration

  1. Passive process (elastic recoil of the lungs, relaxation of inspiratory muscles)

  2. Intrapulmonary pressure increases, air leaves the lungs down its partial pressure gradient

  3. Strong expiratory effort (glottis closed, assists in defecation, micturition or parturition)
• Elastic fibers


  Tendency of structure to return to its initial size after being stretched during inhalation, inwardly directed force due to surface tension from alveolar fluid
• Expiration

  Diaphragm relaxes, external intercostals relax allowing ribs to move dorsally and caudally, decrease volume of thoracic cavity reduces lung volume, alveolar pressure increases (2 mmHg above AP), air flows out of the lungs to an area of lower pressure
• Surfactant

  Coats inside surface of alveoli, has a surface tension that produces inwardly directed force causing alveoli to assume the smallest possible diameter, surface tension must be exceeded, surfactant reduces surface tension of fluid lining the alveoli making alveolar expansion easier
• Lung compliance


  Refers to distensibility of lungs, measures ease at which lungs expand under pressure, high lung compliance means lungs expand easily due to elasticity of lung's tissue and decreased alveolar surface tension from surfactant
• Tidal volume


  Volume of air inspired and expired, about 70% of tidal volume reaches the lungs, remaining portion is found in airways and anatomic dead space where gas exchange does not occur
• Conducting airways

  • Nose
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles
  • Terminal bronchioles

  
• Alveolar dead space


  Alveoli that are poorly perfused with blood, suboptimal gas exchange

  
• Physiologic dead space

  Anatomic and alveolar dead spaces
• Panting helps in humidifying inhaled air and cooling of body
• Minute volume


  Volume of air inhaled and exhaled each minute, calculated as TV x Respiration Rate
• Alveolar ventilation rate


  Portion of tidal volume that actually reaches the site of gas exchange

  
• Inspiratory reserve volume

  Additional volume of air inhaled during forceful inhalation above normal tidal volume
• Expiratory reserve volume

  Additional volume of air exhaled during forceful expiration
• Residual volume


  Air remaining in the lungs after forceful expiration
• Lung volumes and capacities

  • Tidal volume
  • Inspiratory reserve volume
  • Expiratory reserve volume
  • Residual volume
  • Inspiratory capacity
  • Functional residual capacity
  • Vital capacity
  • Total lung capacity
• Inspiratory capacity


  Amount of air an animal can breath distending the lungs to the maximum amount
• Functional residual capacity

  Amount of air that remains in the lungs at the end of normal expiration
• Total lung capacity


  Maximum volume to which the lungs can be expanded with the greatest possible effort
• Vital capacity


  Maximum amount of air expelled from the lungs after first filling the lungs to their maximum extent then expiring to the maximum extent
• States of breathing

  • Eupnea (normal, quiet breathing)
  • Dyspnea (difficult breathing)
  • Apnea (cessation of breathing)
  • Hyperpnea (increased depth or rate of breathing or both)
  • Polypnea (rapid, shallow breathing)