Respiratory

• Pleural membranes


  • Membrane called pleura surrounds the lungs
  • Forms a pleural cavity around each lung: Parietal pleural lining thoracic cavity, diaphragm and mediastinum and continues with Visceral pleura lining lungs
• Pleural cavity


  • Contains fluid
  • Lubricant and holds pleural membranes together
• Mechanics of breathing (ventilation)

  1. Inspiration (inhalation)

  2. Expiration (exhalation)
• Inspiration


  • Air moves from higher to lower pressure
  • Atmospheric pressure exerted by air on all objects in contact with it and forces that air into lungs
  • When pressure inside lungs decreased below atmospheric pressure, air flows in from atmospheric
• Maximal inspiration


  • Requires contraction of several muscles other than the diaphragm (pectoralis minor, sternocleidomastoid, scalenes), to enlarge the thoracic cavity even more
  • Due to surface tension between the two layers of the pleura, as the thoracic cavity expands, the lungs expand with it
  • Surface tension is not advantageous in the alveoli; it would tend to cause alveolar collapse
  • Surfactant keeps the alveoli inflated, preventing collapse
• Diaphragm and external intercostal muscles

  • They expand the thoracic cavity for normal inspiration
  • Diaphragm contracts and moves downwards, enlarging the thoracic cavity and triggering breathing
  • External intercostals contract to move ribs and sternum upward and outward
  • Muscle contraction results in pressure in lungs falling to 2 mm below atmospheric pressure, causing air to rush into the lungs
• Normal expiration


  • Passive process of elastic recoil of the muscle and lung tissue which forms surface tension within alveoli
  • Diaphragm and external intercostal muscles relax and the lungs recoil, decreasing the volume of the thoracic cavity
  • As the diaphragm recoils, abdominal organs spring back to original shape, which pushes the diaphragm upward
  • As the lungs recoil, the pleura and chest wall are pulled inward
  • Increased surface tension in the alveoli decreases their volume and intra-alveolar pressure increases to about 1 mm Hg above atmospheric pressure, causing air to rush out of the lungs into the atmosphere
• Expiration


  1. Atmospheric air pressure equals alveolar air pressure

  2. External intercostal muscles relax – ribs move down and in

  3. Diaphragm relaxes – flattened dome shape

  4. Lung volume decrease alveolar air pressure increases

  5. Alveolar air pressure greater than atmospheric pressure

  6. Air moves out of the lungs

  7. A passive process in quiet breathing
• Respiratory volumes


  • Tidal volume (TV)
  • Inspiratory reserve volume (IRV)
  • Expiratory reserve volume (ERV)
  • Residual volume (RV)
• Spirometry


  Measures various air volumes that move in and out of the lungs due to different degrees of effort
• Respiratory cycle


  One inspiration followed by expiration
• Control of breathing


  • Normal breathing is rhythmic, involuntary event even when muscles are under voluntary control
  • Brain and the nerve supply to the respiratory system control breathing by causing inspiration and expiration and adjusting rate and depth of breathing
• Factors affecting breathing


  • Chemicals
  • Lung tissue stretching
  • Emotional state
  • Level of physical activity
• Central chemoreceptors


  • Chemo sensitive areas associated with the respiratory centres in medulla oblongata
  • Sensitive to changes in the concentration of CO2 and H+ ions in the cerebrospinal fluid, which represents blood concentrations
  • If CO2 or H+ ion concentrations rise (pH decreases), the central chemoreceptors signal the respiratory centre, and breathing rate and depth increase
  • As more CO2 is exhaled, blood and CSF levels fall, and breathing returns to normal
• Oxygen


  Required by cells to break down nutrition to release energy and produce ATP. Carbon dioxide is a product of nutrient breakdown which has to be excreted from the body.
• Respiratory system


  • Obtains oxygen from the atmosphere, and removes carbon dioxide from the body cells
  • Consists of tubes that filter, warm, and moisturise incoming air, and transport it into the gas exchange areas, and microscopic air sacs that exchange gases
  • Other functions: removes particles from incoming air, regulates temperature and water content of the air, provides vocal sounds, regulates blood pH, and helps in sense of smell
  • Works closely with cardiovascular system to achieve requirements
• Respiratory tract


  Oro-laryngo-pharyngeal airway
• Pharynx


  • Area behind the nose, mouth and larynx through which air and food pass to the trachea and oesophagus respectively
• Larynx


  • Composed of a framework of muscles and cartilage bound by elastic tissue. Larger cartilages include the Thyroid, Cricoid and Epiglottic cartilages
  • Contains two pairs of vocal folds consisting of muscle and connective tissue, covered with mucous membrane. Enables human voice sound.
• Trachea


  • Sits in front of the oesophagus and below larynx
  • C shaped rings of cartilage maintain structure
  • Branches into right and left bronchi to each lung
  • Bronchi branch into lobar bronchi, segmental bronchi, terminal bronchioles and alveoli
• Bronchial tree


  • Cartilage lessens in smaller tubular structures and is absent in bronchioles
  • Smooth muscle increases as cartilage diminishes, enabling changes in bronchiole diameter
  • Bronchodilation: increase in bronchiole diameter during fight or flight or sympathetic stimulation
  • Bronchoconstriction: decrease in bronchiole diameter during allergic responses or asthma
  • Mucous membranes filter, warm and humidify incoming air
• Alveoli


  • Consist of simple squamous epithelium which conducts rapid gas exchange with capillaries
  • Respiratory bronchioles, terminal bronchioles and alveolar ducts have smooth muscle
  • Bronchodilation occurs when smooth muscle relaxes, allowing more air to enter
  • Bronchoconstriction occurs when smooth muscle contracts, reducing air entering