Respiration on Land

• Respiration on land


  Specialized gas exchange system of birds is associated with the O2 demands of flying, their endothermic metabolism & unusual pulmonary architecture

  
• Three major evolutionary trends

  • Higher cellular metabolic rates
  • Larger multicellular animals
  • Shift from breathing water to air
• Tracheae


  Openings (spiracles) at body surface branch into a series of air-filled tubes (tracheae & tracheoles) which facilitate diffusion to all cells of the body
• Book lung


  • Haemolymph flows between layers of air pockets (lamellae) for gas exchange

  
• Arachnids


  • Air-filled cavity (atrium) in underside of abdomen
• Water is dense and viscous with low O2 availability


  Aerial respiration facilitates rates of gas exchange at lower metabolic cost
• Gills not suited for aerial respiration as they collapse & lamellae stick together
• Air breathing fish


  • Most specialized air breathers found in tropical freshwater subject to drought & hypoxia
• Amphibious air breather


  • Mudskipper
• Bimodal (aquatic air) breather


  • Catfish (gills & lungs (modified swim bladder))

  
• Lung fish


  • Burrow into mud & estivate during dry season
• Basal fish had lungs

  
• Amphibian lungs


  • Simple membranous lungs, Positive pressure buccal pump forces air into lungs
• Reptile lungs


  • Anterior vascularized portion, Posterior nonvascularized portion (air sacs), Gas exchange within ediculi & faveoli

  
• Reptile respiration


  Aspirate into lungs by a negative intrapulmonary pressure, Incomplete diaphragm membrane (lizards, snakes & crocs), Septate with partitions to increase surface area
• Chuckwalla lizard


  • Contraction of diaphragmaticus muscle, Contraction of abdominal muscles
• Chelonians (turtles & tortoises)


  • Rigid carapace limits expansion capacity of chest, Non-muscular septum between thoracic & abdominal cavity, Exhalation & breath holding caused by limb contraction, Extension of pectoral & pelvic limbs aspirates air into lungs

  
• Mammalian respiration


  Contraction of muscular diaphragm and movement of ribcage
• Mammalian lungs


  • Branching of trachea into smaller bronchi & branchioles, Many alveoli at end of branchioles

  
• Bird lungs


  • Complex series of thin walled air sacs, Parallel cylindrical tubes (parabronchi) function as gas exchange surface, Capillaries extend radially, Bellow mechanism ensures unidirectional air flow

  
• Bird respiration


  Inhalation: 1. fresh air bypasses lungs to posterior air sacs, 2. fresh air moves out of posterior air sacs into lung, 3. anterior air sacs fill with stale air, 4. stale air exits from anterior air sac

  Exhalation: 1. ?, 2. ?, 3. ?, 4. ?

  
• Models of gas exchange

  • Crosscurrent (eg. bird lung), Tidal ventilation (Uniform "pool" eg. Mammal lung), Countercurrent (eg. fish gills)

  
• Birds & mammals (endotherms) have greater O2 demand
• Similarities in anatomical structure and air flow suggest these features are plesiomorphic for Archosauria and may have evolved a hundred million years before the origin of birds