Gas Exchange

• Carbon dioxide may appear to be a waste product of respiration in plant cells


  But carbon dioxide may be a by-product because it is used in photosynthesis
• Plant cells must have carbon dioxide available in them while the oxygen gas must be eliminated
• Gas exchange


  An important process in the metabolism of energy, and an essential prerequisite for life
• Diffusion through a moist membrane


  1. The fundamental process of gas exchange

  2. Diffusion is the movement of molecules in the direction following the concentration gradient, from a region of greater concentration to a region of lower concentration

  3. Molecules travel through cell membranes in living systems, which are constantly moistened by fluid
• Plants


  • They exchange their gases with the atmosphere
  • Water moves through the tissues of aquatic plants and provides the means for the exchange of carbon
  • Air enters the tissues of terrestrial plants, and the gases diffuse through the moisture that bathes the inner cells
• Gas exchange in plants


  1. An abundant supply of carbon dioxide must be available in the leaves of the plant

  2. Oxygen from photosynthesis must be released

  3. Gases do not pass through the leaf cuticle; they pass through pores called stomata in the epidermis

  4. Stomata are numerous and usually open during the day when the rate of photosynthesis is highest

  5. The opening and closure of stomata are due to physiological changes in the surrounding guard cells
• Gas exchange in animals


  • Oxygen and carbon dioxide diffuses across moist membranes
  • The exchange happens directly with the environment in simple animals
  • The exchange between the environment and the blood happens with complex organisms, such as mammals
  • The blood transports oxygen to deeply embedded cells and transports carbon dioxide out of the body
• Gas exchange in simple animals


  • Oxygen and carbon dioxide are transferred by earthworms directly through their skin
  • Oxygen diffuses into tiny blood vessels on the surface of the skin where it mixes with hemoglobin, a red pigment
  • Hemoglobin loosely binds to oxygen and brings it across the bloodstream of the animal
  • Hemoglobin transports carbon dioxide back to the blood
• Gas exchange in terrestrial arthropods


  • They have a set of openings on the surface of the body called spiracles
  • Spiracles open into tiny air tubes called tracheae, which grow into fine branches that reach into all areas of the body of the arthropod
• Gas exchange in fish


  • They use external extensions of the surface of their body, called gas exchange gills
  • Gills are tissue flaps which are richly supplied by blood vessels
  • This attracts water into its mouth and through the gills as a fish swims
  • Oxygen spreads through the gill's blood vessels from the water, while carbon dioxide exits the blood vessels and enters the water flowing through the gills
• Gas exchange in terrestrial vertebrates


  • They have well-developed respiratory systems with lungs
  • In their lungs, frogs swallow air, where oxygen diffuses the blood to bind with hemoglobin in the red blood cells
  • By means of their skin, amphibians can exchange gases as well
  • Reptiles have folded lungs to provide increased surface space for gas exchange
  • Rib muscles aid the expansion of the lungs and protect the lungs from damage
• Gas exchange in birds


  • They have in their lungs large air spaces called air sacs
  • The rib cage spreads apart when a bird inhales, and a partial vacuum is created in the lungs
  • Air flows into the lungs and then into the air sacs, where much of the exchange of gas takes place
  • This method is the adaptation of birds to the rigors of flight and their enormous metabolic requirements
• Gas exchange in mammals


  • Their lungs are classified into millions of microscopic air sacs called alveoli
  • A rich network of blood vessels for transporting gases surrounds each alveolus
  • Mammals have a dome-shaped diaphragm that separates the thorax from the abdomen, providing a separate the chest cavity for breathing and blood circulating
  • The diaphragm contracts and flattens to create a partial vacuum in the lungs during inhalation
  • With air, the lungs fill, and gas exchange follows
• Gas exchange between the external atmosphere and the circulatory system


  • The primary feature of the respiratory system
  • This exchange combines the oxygenation of blood with the removal of carbon dioxide and other metabolic waste from circulation in humans and other mammals
• Gas exchange at the molecular level in the alveoli


  1. The alveolar epithelial tissue is extremely thin and permeable, facilitating the exchange of gas between the air inside the lungs and the blood stream capillaries

  2. Air moves due to variations in pressure, where air flows from high-pressure areas to low-pressure areas
• Key respiratory system organs

  • Provide oxygen for cellular respiration to body tissues
  • Extract carbon dioxide from the waste product
  • Help preserve acid-base balance
  • Also used for non-vital purposes, such as odor detection, voice generation, and stress, such as during childbirth or coughing
• Parts of the respiratory system

  • Mouth and nose
  • Sinuses
  • Pharynx (throat)
  • Trachea
  • Bronchial tubes
  • Lungs
  • Diaphragm
  • Ribs
  • Alveoli
  • Bronchioles
  • Capillaries
  • Lung lobes
  • Pleura
  • Cilia
  • Epiglottis
  • Larynx (voice box)