Physiology

• Respiration


  The process of supplying oxygen (O2) to and removing carbon dioxide (CO2) from the tissues
• External respiration


  Exchange of gases between the lungs (external environment) and the blood (internal environment)
• Internal respiration


  Exchange of gases between the blood and the cells
• Cellular respiration


  Oxidation of glucose (aerobic respiration) to produce energy
• Aerobic respiration


  Glucose + oxygen —> carbon dioxide + water + energy
• Respiratory system - structures

  • The air conducting passages - nose, pharynx, larynx, bronchi and bronchioles
  • The respiratory surfaces (lungs) - respiratory bronchioles, alveolar ducts and alveoli
• Air conducting passages


  • Walls contain cartilage to maintain patency
  • Smooth muscle to regulate diameter of bronchioles
  • Ciliated epithelium to continue filtering air
• Respiratory bronchioles, alveolar ducts and alveoli

  • Endothelial layer 1 cell thick surrounded by pulmonary capillaries also 1 cell thick to facilitate diffusion
  • Epithelial cells produce surfactant to maintain patency of alveoli
• Proximity of organs


  Main organs of respiratory and cardiovascular systems within the thoracic cavity
• Structural relationship


  Minimal distance for blood to travel between the two systems
• Mechanism of breathing


  1. Thoracic cavity lined by pleura exerting a negative pressure

  2. Elastic lung tissue

  3. Stimulation by nerves from respiratory centre causes contraction of respiratory muscles
• Pleura


  Double serous membrane with serous fluid in the potential space between the layers, exerting a negative pressure
• Respiratory cycle


  1. Inspiration - active process, contraction of muscles

  2. Expiration - passive process, elastic recoil of lungs

  3. Pause
• CNS control of respiration

  • Inspiratory centre sends stimulus to inspiratory muscles
  • Expiratory centre sends impulses to expiratory muscles during active expiration
  • Neural connections link inspiratory and expiratory centres
  • Apnoeustic centre stimulates inspiratory centre, must be inhibited for expiration
  • Pneumotaxic centre inhibits apnoeustic and inspiratory centres, facilitates expiration
• Autonomic regulation in the lung

  • Dual innervation affects submucosal glands and blood vessels
  • Only parasympathetic innervation affects airway smooth muscle
• Inspiration


  1. External intercostal muscles contract, ribs and sternum move upwards and outwards, diaphragm contracts and flattens

  2. Increased capacity of thorax, reduced pressure in pleural cavity, increased 'suction pull' on elastic lung tissue, air drawn in from atmosphere
• Expiration


  1. External intercostal muscles relax, ribs and sternum move downwards and inwards, diaphragm relaxes and ascends

  2. Decreased capacity of thorax, increased pressure in pleural cavity, decreased 'suction pull' on lung tissue, air forced out into atmosphere
• Forced breathing


  1. Inspiration - muscles of neck contract, moves first rib upwards and sternum upwards and forwards

  2. Expiration - internal intercostal muscles contract to actively aid ascent of diaphragm
• Central chemoreceptors


  • Located in medulla, sensitive to pH and pCO2 (not pO2)
• Peripheral chemoreceptors


  • Aortic body and carotid bodies, sensitive to changes in PO2
• Mechanoreceptors


  • Stretch receptors provide info on lung volume, rapid adapting irritant receptors important for cough reflex
• Compliance


  The ease with which the lungs and thorax can be expanded
• Factors affecting compliance


  • High compliance - easily stretched elastic tissue, reduced surface tension from surfactant
  • Low compliance - resistance to expansion
• Airway resistance


  • Depends on pressure difference between alveoli and atmosphere, and resistance of bronchi and bronchioles
  • Regulated by degree of contraction of airway smooth muscle
• Surfactant


  Detergent-like substance secreted by type 2 alveolar cells, reduces surface tension in alveoli
• Terms for breathing patterns

  • Eupnoea (normal breathing)
  • Apnoea (absence of breathing)
  • Hyperpnoea (overbreathing/hyperventilation)
  • Hypopnoea (underbreathing/hypoventilation)
• Lung volumes


  • Tidal volume
  • Vital capacity
  • Residual volume
  • Total lung capacity
• Minute volume of respiration (MVR)


  Total air taken into the lungs in 1 minute, calculated as tidal volume x breathing rate
• Anatomical dead space


  Air from regions where no gas exchange occurs (airways and unperfused alveoli)
• Alveolar ventilation


  Volume of air reaching the alveoli per minute, calculated as (tidal volume - dead space) x breathing rate
• Gaseous exchange


  Gases move from an area of high pressure (contraction) to an area of low pressure
• Calculating gas partial pressures

  1. Determine % of gas in mixture (py)

  2. Use atmospheric pressure (pA)

  3. Use water vapour pressure (pW)

  4. py = % y x (pA - pW)/100 (mmHg)
• Note: each gas in mix behaves as if others do not exist
• External respiration


  Exchange of O2 and CO2 between air in the alveoli and the blood in the pulmonary capillaries
• Gaseous exchange


  Gases move from an area of high pressure (contraction) to an area of low pressure
• Gas partial pressures


  To calculate the partial pressure exerted by a gas we need to know the % of the gas in a mixture (py), atmospheric pressure (pA) and water vapour pressure (pW)
• Calculating partial pressure of a gas

  py = % y x (pA - pW)/100 (mmHg)
• Alveolar air


  • 14% O2, pA (variable approx. 760 mmHg), pW (variable, approx. 47 mmHg)
  • pO2 = 14 x (760-47)/100 = 99.8 mmHg
• Each gas in a mix behaves as if others do not exist
• External respiration


  Exchange of O2 and CO2 between air in the alveoli and the blood in the pulmonary capillaries