L42 respiratory system anatomy

• Upper respiratory system


  • Nasal cavity
  • Pharynx
  • Larynx
• Roles of upper respiratory system

  • Humidify and warm air
  • Defence: mucus and cillia
  • Sensory and speech
• Lower respiratory system


  • Trachea
  • Primary bronchi
  • Lungs
• Roles of lower respiratory system

  • Gas exchange
  • Defence
  • Metabolism
• Airway resistance


  Resistance of the flow of air through the respiratory tract during inhalation and expiration
• Airway resistance


  Proportional to 1/radius^4
• Airway resistance is summative
• Huge numbers of respiratory bronchioles
• Small airways make a small contribution to total resistance
• Conducting zone bronchioles have the largest influence on airway resistance
• Lung epithelium specialisation


  • Bronchi- ciliated, goblet, glandular
  • Bronchioles- ciliated, non-ciliated, goblet, club cells
  • Alveoli- squamous, cuboidal
• Goblet cells


  Produce the normal mucous film which has two layers - the sol layer and the gel layer. The gel layer traps and removes inhaled foreign particles to protect the underlying cells.
• The respiratory unit


  • Massive network of airways and air sacs (around 500 million) to maximise surface area (50-100 m2)
  • Surface area of alveoli around 80 m2 roughly the same as ½ a tennis court
• Alveolar epithelial cells (pneumocytes)


  • Type 1 - Very large surface area, cover 95% of alveolus, involved in gas exchange
  • Type II - Granular & roughly cuboidal, majority of cells, secrete pulmonary surfactant, precursors of type I
• Surfactant


  Produced by type II alveolar epithelial cells, lipid (90%) - mostly phospholipid, proteins - SP-A, -B, -C, -D, reduces surface tension, prevents alveolar collapse, has innate immunity function, used to treat Infant Respiratory Distress Syndrome
• Lungs at birth


  • Lung epithelium develops in the last trimester of pregnancy
  • Maturation stimulated by corticosteroids in premature babies
  • Lung is fluid filled in foetus - must rapidly empty
  • Surge in corticosteroids and catecholamines at birth
  • Activation of absorptive channels like Epithelial Sodium Channels (ENaC)
  • Pressure changes (squeeze through birth canal)
  • Caesarean section – reduced drive for fluid absorption
• Blood supply to the lungs

  • Pulmonary circulation - Artery flows directly from the right ventricle, LOW oxygen, HIGH flow, LOW pressure, Capillaries pass around the alveoli, Pulmonary vein returns oxygenated blood to left heart
  • Bronchial circulation - From aorta (left ventricle), HIGH pressure, Supplies oxygen and nutrients the conducting airways, Not involved systemic respiration processes, Only 2% of cardiac outflow
• Ventilation


  • Movement of air from environment into the lungs (breathing)
  • Automatic and controlled by CNS
• Muscle innervation


  • Somatic motor nerves - Phrenic (innervates diaphragm), Intercostal
  • Autonomic nerves - Bronchial (supply smooth muscle & secretory cells, branch from vagus, reflex bronchospasm and mucus secretion, important in asthma)
• Sensory output


  • Sensory (afferent) nerves FROM the airway epithelium and smooth muscles signal to brain
  • Autonomic fibres TO glandular epithelium and smooth muscle from CNS
• Autonomic nervous system control of airways

  • Parasympathetic branches of the vagus nerve - Excitatory (e) and inhibitory (i)
  • Very little sympathetic innervation
  • b-adrenoceptors on airway smooth muscle are stimulated by circulating adrenaline
• Sensory afferent pathways from the lung

  • Myelinated - Slowly adapting stretch receptors (SAR), Rapidly adapting stretch receptors (RAR)
  • Unmyelinated - Pulmonary and bronchial C fibres
• Reflexes from sensory afferents

  • SAR - Elicit reflexes: Shortened inspiration, Hering-Breuer reflex

  RAR - Elicit reflexes: Cough, Bronchoconstriction, Mucus secretion

  C fibres - Reflex bronchoconstriction & mucus secretion
• Cough reflex


  1. Stimulation of irritant receptors

  2. Sensory nerves conveys signal to medulla

  3. Motor nerves signal to skeletal muscles

  4. Glottis closes

  5. Abdominal and internal intercostal muscles contract rapidly

  6. Intrapulmonary pressure rises

  7. Glottis opens

  8. COUGH
• Pleural membranes


  • Double membrane surrounding the lungs
  • Left & right pleura anatomically distinct
  • Normally very small amounts of pleural fluid between membranes (few ml)
  • Serous fluid
  • Lubricant
• Ventilation


  • Fluid in the pleural sac helps the lung wall "stick" to the inside of the thorax
  • Contraction of the diaphragm and the external intercostal muscles
  • Increased lung volume
  • Internal pressure falls (Boyle's Law)
  • Air drawn into lungs
  • Exhalation passive at rest
  • Elastic recoil
• Airway resistance is increased in many diseases
• Causes of increased airway resistance

  • Smooth muscle contraction
  • Obstruction with mucus
  • Collapse
  • Structural changes - Smooth muscle growth, Fibrosis
• Metabolic functions of the lung

  • Vascular cells activate angiotensin I to pressor angiotensin II

  Angiotensin Converting Enzyme (ACE)

  Inactivates vasoactive substances like bradykinin and serotonin

  Inactivates circulating hormones like prostaglandins and norepinephrine

  Fibrinolytic function

  Club cells detoxify inhaled substances