gas exchange animals

• Aerobic


  Requiring oxygen
• Gas exchange


  The exchange of oxygen (O2) and carbon dioxide (CO2) molecules between an organism and its environment
• All aerobic single-cell and multi-cell species must exchange O2 and CO2 molecules with the environment
• Cellular respiration


  The process by which cells convert oxygen and nutrients into ATP, the primary energy currency of the cell
• Aerobic species require O2 molecules to enable cells to synthesize ATP molecules by cellular respiration
• CO2 molecules produced during cellular respiration must exit the cell and the body to maintain homeostasis
• CO2 accumulation is toxic
• Aerobic, single-cell microbes


  • The surface area of the cell membrane is relatively large compared to the small volume of cytosol it surrounds
  • Diffusion of gas molecules across the cell membrane satisfies their gas exchange needs
  • Every part of the cell surface contacts the surrounding air or water to facilitate gas diffusion
  • Diffusion is sufficient for the intracellular distribution of gas molecules
• Thin, multi-cell, aerobic, aquatic organisms

  • Diffusion of gas molecules across the cell membrane of each cell satisfies their gas exchange needs
  • All, or most, of the body's cells directly contact the surrounding water to facilitate the exchange of O2 and CO2 by diffusion
  • Diffusion is sufficient to drive the intracellular distribution of O2 and CO2
• All animals are aerobic and must exchange O2 and CO2 molecules with the surrounding environment
• Respiratory organs


  • They are connected to a circulatory system that quickly distributes O2 molecules to all the body's cells
  • They have highly folded or highly branched gas exchange membranes to increase the surface area available for gas exchange by diffusion
  • They have moist gas exchange membranes so gas molecules must dissolve in the moisture layer to diffuse across the membrane
  • They have thin gas exchange membranes that consist of one cell layer to reduce the distance gas molecules must diffuse
• Ventilation


  1. Delivery of fresh air or fresh water with high O2 and low CO2 content to the environmental surface of the respiratory organ

  2. This creates the concentration gradients of O2 and CO2 that drive the diffusion of O2 molecules into the body and CO2 molecules from the body

  
• Perfusion


  1. Delivery of blood (fluid) by the circulatory system to the capillaries of the respiratory organ

  2. Capillaries deliver blood with high CO2 and low O2 content to the inner surface of the gas exchange membrane

  3. This creates the concentration gradients of O2 and CO2 that drive the diffusion of O2 molecules into the body and CO2 molecules from the body

  
• Characteristics shared by all respiratory systems

  • Respiratory organs must be ventilated
  • Respiratory organs must be perfused
  • Respiratory organs must be linked to a circulatory system that quickly delivers oxygen (O2) from the respiratory organ to the body's cells and carbon dioxide (CO2) from the body's cells to the respiratory organ

  
• Tracheal system in insects

  • It is an internal system of inter-connected, hollow, thin, tubes and air sacs that enables gas exchange with the body's cells
  • It is a highly branched system that permeates the entire body
  • Most cells are near the smallest air tubes called tracheoles
  • Metabolically active tissues and organs are near large air sacs containing O2
  • Air enters and exits the body through the spiracles at the surface

  
• Muscle contraction while flying or running

  Increases air movement to increase gas exchange
• External gills in aquatic animals

  • They directly contact the water to facilitate O2 and CO2 exchange by diffusion
• Internal gills in aquatic animals

  • They remove ~80-85% of oxygen (O2) from the water
  • Fresh water flowing over the gills has a higher O2 concentration compared to blood in the capillaries due to counter current exchange
• Bird "circulatory lungs"


  • They ensure that fresh inhaled air flows in one direction ("circulates")
  • Inhaled fresh air with maximal O2 content and minimal CO2 content contacts the gas exchange membranes
  • Inhaled fresh air with high O2 content does not mix with stale air with low O2 content

  
• Breathing in birds


  1. Birds must inhale and exhale twice for the inhaled fresh air to circulate through the system

  2. Inhaled fresh air with maximal O2 content passes through the porous air tubes in the lungs to enable O2 to diffuse into the blood in adjacent capillaries and CO2 to diffuse from the blood in the capillaries into the air

  
• Human lungs

  • ~250-300 million alveoli in each lung create a large surface area (~35-50 meter2) for gas exchange by diffusion
  • During breathing, the inhaled fresh air with high O2 content mixes with the stale air with low O2 content in each lung, reducing gas exchange efficiency compared to circulatory bird lungs

  
• Gas exchange in human lungs

  1. O2 molecules diffuse from the air in alveoli into blood in the capillaries

  2. CO2 molecules in the blood in capillaries diffuse into the air in alveoli

  
• Human lungs


  • The short distance (0.5-1.5 uM) between each alveolus and the surrounding capillaries reduces the distance O2 and CO2 must diffuse, increasing the rate of O2 and CO2 diffusion

  
• Breathing


  Inhalation: 1) Lung volume increases, 2) Lung pressure decreases, 3) Fresh air flows in

  Exhalation: 1) Lung volume decreases, 2) Lung pressure increases, 3) Stale air flows out

  
• Breathing


  • The alternating contraction and relaxation of the diaphragm and intercostal (rib) muscles change the volume, and pressure, of the thoracic cavity to enable air to enter, and exit, each lung

  
• Breathing control


  • The medulla oblongata and pons regions of the mid-brain regulate breathing by sending nerve impulses to instruct the diaphragm and intercostal (rib) muscles to contract
  • Breathing control is autonomous (automatic), we do not consciously need to remember to inhale or exhale

  
• Various stimuli


  Digestion, fight or flight response, high altitude (low O2 levels), physical activity, pregnancy (progesterone), temperature increase, touch (love/romance/pain), toxins and drugs (alcohol) can instruct the medulla oblongata and pons to adjust the rate of breathing
• At rest, tidal volume is 500 ml and breathing rate is 12/minute. During exercise, tidal volume is 1000 ml and breathing rate is 35-45/minute.