chapter 6

• Respiratory system


  System that allows gases from the environment to enter the bronchial tree, facilitates gas exchange, and permits gases in the lungs to be eliminated
• Functions of the respiratory system

  • Allow gases from the environment to enter the bronchial tree through inspiration by expanding the thoracic volume
  • Allow gas exchange to occur at the respiratory membrane, so that oxygen diffuses into the blood while carbon dioxide diffuses into the bronchial tree
  • Permit gases in the lungs to be eliminated through expiration by decreasing the thoracic volume
• General anatomy of the respiratory system

  External nares → nasal cavity → nasopharynx → laryngopharynx → larynx → trachea → primary bronchi → lungs (secondary bronchi → tertiary bronchi → bronchioles → alveolar sacs → alveoli)
• Lungs

  • Cone-shaped organs located in the thoracic cavity
  • Thoracic cavity is lined with parietal pleura, while the surface of lungs is covered with visceral pleura
  • Pleural cavity between the two pleural membranes is filled with pleural fluid to minimize friction and provide surface tension
  • Surfactant secreted by the lungs facilitates surface tension
• The respiratory tract - bronchial tree

  1. Trachea to tertiary bronchi lined with ciliated pseudostratified columnar epithelium, smooth muscle and cartilage rings

  2. Bronchioles lined with cuboidal epithelium

  3. Alveolar ducts to alveoli lined with simple squamous epithelium

  4. Diaphragm under involuntary control to facilitate control of thoracic volume
• Bronchial tree


  • Tree-like branching tubes extended from the trachea
  • Primary bronchi external to the lungs, rest embedded in lung tissues
  • Diameters of tubes from primary to tertiary bronchi large, supported by cartilage rings
  • Diameter at bronchioles down to 1mm, no cartilage rings, lined with cuboidal cells
  • From alveolar duct to alveoli, lining tissue is simple squamous epithelium for gas exchange
• Gas law


  Gas molecules always diffuse from higher pressure area to lower pressure area
• Boyle's law

  Pressure and volume are inversely related (with temperature constant), pressure increases in smaller volume and decreases in larger volume
• Inspiration (inhalation)


  1. Active process where nerve impulses from medulla oblongata cause contraction of diaphragm and external intercostal muscles

  2. Thoracic volume increases, decreasing intra-alveolar pressure below atmospheric pressure

  3. Gases move from environment into lungs
• Expiration (exhalation)


  1. Passive process where elastic tissues of lungs and diaphragm recoil to original position

  2. Thoracic volume decreases, raising intra-alveolar pressure above atmospheric pressure

  3. Gases move from lungs into environment
• Pulmonary ventilation - inspiration

  1. Contraction of diaphragm pulls down, enlarging intrapleural cavity

  2. Elevation of ribs also expands intrapleural cavity

  3. Decreased intrapleural pressure causes air flow into lungs
• Pulmonary ventilation - expiration

  1. Diaphragm relaxes, ribs pulled down

  2. Increased intrapleural pressure causes air flow out of lungs

  3. Normal quiet breathing accomplished by diaphragm movement

  4. Ventilation can increase up to 100 liters per minute during maximal exercise
• Lung capacities

  • Tidal volume (TV)
  • Inspiratory reserve volume (IRV)
  • Expiratory reserve volume (ERV)
  • Inspiratory capacity (IC)
  • Vital capacity (VC)
  • Residual volume (RV)
  • Total lung capacity (TLC)
• Anatomic dead space


  Amount of air in bronchial tree not involved in gas exchange, due to obstruction or damage
• Alveolar dead space


  Amount of air in alveolar ducts/sacs not involved in gas exchange, due to poor blood flow or long diffusion distances
• Physiologic dead space

  Total amount of air in lungs not involved in gas exchange (anatomic + alveolar dead space)
• Factors affecting normal breathing

  • Stretching in the lungs and thoracic walls
  • O2 level in the blood
  • CO2 level in the blood
  • H+ level in the blood
• Normal breathing is affected by


  • Stretching of lungs/thoracic walls, rise in O2, decrease in CO2/H+ inhibits breathing
  • Relaxing of lungs/thoracic walls, decrease in O2, rise in CO2/H+ stimulates breathing
• Respiratory centers


  • Rhythmicity area in medulla oblongata sets basic rhythm of inspiration and expiration
  • Pneumotaxic area in pons sets depth, duration, and rate of breathing
• Hyperventilation


  Decreases carbon dioxide concentration
• Low blood PO2

  • Increases alveolar ventilation (peripheral chemoreceptors in the carotid bodies & aortic bodies detect low O2 concentrations)
• High blood PCO2

  • Increases alveolar ventilation
• High CSF, H+ ion concentration

  • Increases breathing rate and alveolar ventilation (CO2 combines with water to form carbonic acid, which in turn, releases H+ ions in CSF)
• Changing alveolar ventilation

  Affects PO2 & PCO2 in the alveoli
• Normal breathing


  • Rhythmic, involuntary action regulated by the respiratory centers in the pons and medulla oblongata of the brain stem
• Rhythmicity area in the medulla oblongata


  • Sets the basic rhythm of inspiration and expiration, and is subdivided into the dorsal respiratory group (which controls normal breathing) and the ventral respiratory group (which controls forceful, voluntary breathing)
• Pneumontaxic area in the pons


  • Sets the depth, duration, and rate of breathing by influencing the dorsal respiratory group
• Central Chemoreceptors

  • Associated with the respiratory centers, stimulation increases alveolar ventilation (CO2 combines with water to form carbonic acid, which in turn, releases H+ ions in CSF)
• Peripheral Chemoreceptors


  • In the carotid bodies & aortic bodies, sense low O2 concentration, when O2 concentration is low, alveolar ventilation increases
• PCO2

  Medullary chemoreceptors are sensitive to the pH of CSF, diffusion of CO2 from the blood into CSF lowers the pH of CSF by forming carbonic acid
• pH

  Peripheral chemoreceptors are stimulated by decreased blood pH independent of the effect of blood CO2, chemoreceptors in the medulla are not affected by changes in blood pH because H+ cannot cross the blood brain barrier
• PO2


  Low blood PO2 augments the chemoreceptor response to increases in blood PCO2 and can stimulate ventilation directly when the PO2 falls below 50 mmHg
• Ventilation

  The amount of air moved in and out of the lungs during each minute, increases in metabolism is accompanied by increases in ventilation (due to increase in plasma CO2)
• Respiratory membrane


  • Formed by the walls of alveoli and capillaries where they are both made of simple squamous epithelium, thin enough to allow diffusion of gases called gas exchange to occur
• Dalton's Law


  Gases (particularly O2 and CO2) always diffuse from high pressure to low pressure, each gas in a mixture of gases produces its own pressure called partial pressure, and the sum of all partial pressures is the total pressure of that gas mixture
• External Respiration


  Occurs in the lungs to oxygenate the blood and remove CO2 from the deoxygenated blood (O2 diffuses from the alveoli into capillaries, while CO2 diffuses from the capillaries into alveoli)
• Internal Respiration (Tissue Respiration)


  Occurs in the body tissues to provide O2 to tissue cells and remove CO2 from the cells (O2 diffuses from the capillaries into tissue cells, while CO2 diffuses from tissue cells into capillaries)
• Alveolar Gas Exchange


  • Gas exchanges between the air and the blood occur within the alveoli, diffusion through the respiratory membrane (O2 diffuses from the alveolar air into the blood; CO2 diffuses from the blood into the alveolar air)
• Pulmonary Capillaries

  Alveolar PO2 = 104mmHg, Pulmonary capillaries PO2 = 40mmHg (O2 enters capillaries), Alveolar PCO2 = 40mmHg, Pulmonary capillaries PCO2 = 45 mmHg (CO2 enters alveoli)
• Systemic Capillaries

  Systemic capillaries PO2 = 104mmHg, Tissues PO2 = >40mmHg (O2 enters tissues), Systemic capillaries PCO2 = 40mmHg, Tissues PCO2 = < 45 mmHg (CO2 enters capillary)