Unit 7.7

• Gas Exchange Surfaces


  The events that result in gas exchange between the body's cells and environment
• Respiration


  1. Ventilation (breathing in and out)

  2. External Respiration (gas exchange between air and blood in lungs)

  3. Internal Respiration (gas exchange between blood and tissue fluid)

  
• Organisms are either small enough to allow gas exchange across their membranes, or have developed specialized tissues to allow diffusion of gases
• Aquatic animals


  • Expend more energy carrying out gas exchange (25% for fish, 1-2% for humans)
  • Water is more dense, and a given volume of water only carries a fraction of air
• Small aquatic organisms (hydra, planarian)


  • Have a large surface area compared to body size, so most cells can exchange gases directly with environment
• Small terrestrial invertebrates (earthworm)


  • Use body surface for respiration because capillaries come close to the surface
• Aquatic invertebrates & vertebrates

  • Use gills to extract oxygen from water
  • Gills are highly vascularized
  • Use countercurrent exchange (water flows in opposite direction of oxygen-depleted blood)
• Insects


  • Have inefficient circulation (hemolymph fluid just contained within the hemocoel cavity)
  • Overcome inefficiency with tracheal system (trachea -> tubules -> tracheoles -> ~0.1um tubes)
  • Deliver air directly to cells without entering the blood
  • Air sacs act like bellows to move air into and out from the trachea tubes

  
• Terrestrial vertebrates


  • Lungs are vascularized outgrowths from the lower pharyngeal region
  • Lungs can be subdivided into smaller passageways and spaces
  • Human lungs have surface area of ~70m2 (50x skin surface area)
  • Air has a drying effect, so lungs are moistened to keep them from drying out

  
• Human Lungs & Respiration

  1. Air moves from nose -> pharynx -> trachea -> bronchi -> lungs

  2. Filtered, warmed, and humidified in this process

  3. At body temp and saturated with water once it reaches lungs

  4. Larynx and trachea permanently held open by cartilage rings to facilitate air movement

  5. When swallowing, larynx rises, glottis closed by epiglottis, and soft palate covers nasal passages

  
• Breathing - Inspiration


  1. Creation of negative pressure in the lungs

  2. Rib cage elevated

  3. Diaphragm contracts and lowers

  4. Thoracic pressure decreases to less than atmospheric pressure

  5. Atmospheric pressure forces air into the lungs

  
• Breathing - Expiration


  1. Creation of positive pressure in the lungs

  2. Rib cage lowered

  3. Diaphragm rises

  4. Thoracic pressure increases to more than atmospheric pressure

  5. Forces air out of the lungs

  
• Ventilation


  • Terrestrial vertebrates (except birds) use tidal ventilation (air moves in and out same route, new air mixes with used air)
  • Birds use one-way ventilation (fresh air never mixes with used air)
• Modifications of Breathing in Humans


  • Normally 12-20 ventilations per minute
  • Controlled by respiratory center in medulla oblongata
  • Can be influenced by nervous or chemical input

  
• Gas Exchange and Transport


  • Gases diffuse from areas of higher to lower partial pressure
  • Ventilation causes alveoli to have higher oxygen and lower carbon dioxide partial pressure than blood in pulmonary capillaries
• Transport of Oxygen and Carbon Dioxide

  1. External Respiration (oxygen combines with hemoglobin in RBCs)

  2. Internal Respiration (oxyhemoglobin gives up oxygen to tissues, carbon dioxide enters blood and binds to hemoglobin or is transported as bicarbonate ion)

  
• Disorders of the Upper Respiratory Tract


  • Cold (mild viral infection)
  • Strep Throat (acute pharyngitis caused by Streptococcus pyogenes)

  
• Disorders of the Lower Respiratory Tract


  • Choking
  • Acute bronchitis
  • Chronic bronchitis
  • Asthma (airways sensitive to irritants, causing spasms)
• Disorders Affecting the Lungs


  • Pneumonia (viral, bacterial, or fungal infection)
  • Pulmonary tuberculosis (caused by Mycobacterium tuberculosis)
  • Pulmonary fibrosis (fibrous tissue buildup)
  • Emphysema (alveoli distended, reduced surface area)
  • Lung cancer (begins with thickening and callusing of airway cells)