Gas Exchange

• Oxygen is needed in tissues for aerobic respiration to occur and to extract ATP from food
• Carbon dioxide must be released to prevent physiological pH in tissues from being very acidic
• In plants, the carbon dioxide that is released as a by-product of cellular respiration may again be taken up for the process of photosynthesis
• Respiratory organs in plants and animals


  • They are specialized to perform an important role in gas exchange

  
• Plants do not breathe, they only respire
• Stomata


  Openings in the leaf that allow plants to obtain oxygen directly from the air
• Respiratory organ


  A specialized organ within the body of animals where gas exchange occurs (e.g. lungs, gills)
• No respiratory organ is present in plants, the tissues of a leaf are adapted for photosynthesis and gas exchange
• The carbon dioxide produced in animals during respiration is released to the atmosphere
• The carbon dioxide produced during plant respiration may be used by the plant for carrying out photosynthesis
• There is no respiratory pigment in plants, whereas in animals respiratory pigments like hemoglobin are present
• Plants do not breathe, they only respire. Animals usually breathe in the air for carrying out cellular respiration. Plants lack a respiratory system which is usually found in animals.
• Plants


  • The leaf obtains oxygen directly from the air through the stomata
  • Stems and roots also take in oxygen
• Animals


  • Oxygen is taken in through special openings (like nostrils or gill clefts) into the respiratory organ
• Animals


  • There is a respiratory organ (like lungs, gills, etc.) present within the body, where exchange of gases occurs
  • No such respiratory organ is present in plants
• Plants


  • The tissues of a leaf are adapted for photosynthesis and gas exchange
• Carbon dioxide produced in animals during respiration


  Released to the atmosphere
• Carbon dioxide produced during plant respiration


  May be used by the plant for carrying out photosynthesis
• Plants


  • No respiratory pigment
  • Gas exchange occurs through the process of respiration where free energy is released and transiently stored in a compound, ATP, which can be readily utilized for the maintenance and development of the plant
• Animals


  • Respiratory pigments play an important role in transporting oxygen to the cells
  • Have to take in oxygen and expel carbon dioxide to sustain cellular respiration and stay alive
• Terrestrial animals and aquatic animals

  • Face different challenges in performing gas exchange
• Respiratory surface or organ

  • Part of an animal's body where gases are exchanged with the environment
  • Must be moist, large enough, and protected from desiccation
• Partial pressure


  The pressure exerted by a particular gas in a mixture of gases
• Frick's Law states that the amount of diffusion of a gas across a membrane is proportional to the surface area and the difference in partial pressure between the two sides and inversely proportional to the thickness of the membrane.
• Surface-to-volume ratio


  • As an animal grows, the surface area increases at a lesser rate than its volume, making diffusion of gases into the interior more difficult
• Ventilation


  The movement of the respiratory medium (air or water) over the respiratory surface
• Perfusion


  The flow of blood to alveolar capillaries
• Respiratory Pigments or Proteins

  • Adaptations of animals for gas exchange include respiratory pigments that bind and transport gases
  • The respiratory pigment of vertebrates is hemoglobin while that of invertebrates (e.g. arthropods and mollusks) is hemocyanin
  • Blood cannot carry sufficient oxygen and carbon dioxide in dissolved form to meet the body's requirements; hemoglobin helps enhance its capacity

  
• Structures for gas exchange in plants

  • Stomata in leaves, lenticels in stems, root hairs in aerial roots and pneumatophores or lateral roots of mangroves
• Respiratory surfaces or organs in invertebrates

  • Integumentary exchange
  • External gills
  • Tracheal system in arthropods

  
• Respiratory organs in vertebrates


  • External gills
  • Internal gills
  • Lungs

  
• Lungs in vertebrates


  • Internal respiratory surfaces shaped as a cavity or sac
  • Provide a membrane for gaseous exchange
  • Require a circulatory system to transport gases to the rest of the body

  
• How air moves in and out of the lungs

  1. Air moves by bulk flow into and out of the lung

  2. Gases diffuse across the inner respiratory surfaces of the lungs

  3. Pulmonary circulation allows the diffusion of dissolved gases across lung capillaries

  4. Oxygen diffuses from blood -> internal fluid -> cells, carbon dioxide pathway is in reverse

  5. All lungs receive deoxygenated blood from the heart and return oxygenated blood to the heart

  
• Breathing mechanisms in vertebrates

  • Amphibians: Positive pressure breathing
  • Birds: System of air sacs as blower to keep air flowing through the lungs in one direction
  • Mammals: Negative pressure breathing

  
• Air entry and exit

  • Through nasal cavities where air is filtered by hair or cilia, warmed by blood vessels and moistened with mucus

  
• Oxygen transport


  1. Oxygen diffuses down a pressure gradient from the lungs into the blood plasma -> red blood cells -> binds to hemoglobin

  2. Hemoglobin gives up its oxygen in tissues where partial pressure of oxygen is low, blood is warmer, partial pressure of carbon dioxide is higher, and pH level is lower
• Carbon dioxide transport


  1. Carbon dioxide diffuses down its partial pressure gradient from the tissues into the blood plasma and red blood cells to air in alveoli

  2. 7% is dissolved in plasma, 23% binds with hemoglobin to form carbaminohemoglobin, and 70% is in bicarbonate form

  3. Bicarbonate and carbonic acid formation is enhanced by the enzyme carbonic anhydrase in red blood cells
• Coordination of air flow with blood flow

  1. Gas exchange in the alveoli is most efficient when air flow equals the rate of blood flow

  2. Local controls within the lungs correct imbalances in air and blood flow by constricting or dilating both bronchioles and arterioles

  
• Nervous system control of respiration

  Monitors oxygen and carbon dioxide levels and regulates the rate and depth of breathing
• Respiratory adaptations to extreme conditions

  • Animals in high altitudes have larger hearts and lungs, and hemoglobin with high affinity for binding oxygen
  • Diving animals have unusually high hematocrits and muscles with high amounts of myoglobin
  • Humans at higher altitude develop more alveoli and capillary network in the lungs
  • Humans during diving, the heartbeat slows, and circulation is reduced to all parts of the body except the brain