gas exchange

• When a cell increases in size, the diffusion pathway gets longer, making diffusion slower from the outer cell surface to the center of the cell
• Diffusion may not meet the cell's needs when it becomes very large, such as supplying nutrients and removing waste
• Gas exchange: the process by which oxygen reaches cells and carbon dioxide is removed from them
• Ventilation: the process of moving the respiratory medium (air or water) over the respiratory surface to maintain a concentration gradient, replacing stale air high in CO2 with fresh oxygenated air
• Respiration: a series of chemical reactions that results in the release of energy in the form of ATP
• Amoeba is a small unicellular organism with a larger surface area to volume ratio than a large multicellular organism
• As size increases, the surface area to volume ratio decreases
• Unicellular organisms like Amoeba have extremely large surface area to volume ratios and exchange gases across their whole surface via diffusion
• Simple multicellular organisms like Flatworm have evolved a flattened shape to increase their surface area to volume ratio, allowing them to exchange gases directly with the environment via diffusion
• Earthworm, a simple multicellular organism, exchanges gases directly with the environment through its moist surface, aided by a large surface area to volume ratio
• Larger multicellular organisms require a specialized gas exchange surface to meet their higher metabolic oxygen demand
• Many animals and plants have evolved specialized gas exchange surfaces like gill lamellae in fish, alveoli in mammal lungs, and tracheoles in insects
• To achieve maximum diffusion rate, all respiratory surfaces must be thin, moist, permeable to gases, and have a large surface area for efficient gas exchange
• Insects have a tracheal system for gas exchange, where oxygen diffuses directly into cells from fluid-filled tracheoles, aided by an extensive blood supply and haemoglobin
• Fish have different gas exchange mechanisms: cartilaginous fish use parallel flow, while bony fish use counter-current flow for efficient gas exchange
• Counter-current flow in bony fish maintains a higher oxygen concentration gradient along the gill lamellae, making it more efficient than parallel flow in cartilaginous fish
• Amphibians have adaptations for gas exchange, using moist skin for diffusion when inactive and lungs when active
• The human breathing system includes structures like rings of cartilage to prevent airways from collapsing and the pleural cavity that encloses the lungs
• The trachea has cartilage and ciliated epithelium with goblet cells for efficient gas exchange
• Trachea:

  • Low magnification shows cartilage and ciliated epithelium containing goblet cells
  • High magnification shows goblet cells producing and secreting mucus to trap microorganisms, with cilia wafting to move the mucus up and out of the trachea
• Bronchiole:

  • Microscopic transverse section shows bronchiole
  • Lung tissue:
  • Healthy lung tissue shows many intact alveoli air sacs
  • Emphysema lung tissue shows breakdown of alveoli air sac walls and drastically reduced gas exchange surface area
• Ventilation of the human lungs:

  • Inspiration involves negative pressure breathing
  • External intercostal muscles and ribs contract, raising the ribs up and out
  • Expiration involves muscles relaxing, moving the ribs down and in
  • Outer pleural membrane is pulled outwards, reducing pressure in the pleural cavity
  • Inner pleural membrane is pulled outwards
  • Lungs and alveoli: Lung surface is drawn out, causing alveoli to expand; Lungs move in, alveoli deflate
  • Pressure in alveoli is lower than atmospheric pressure so air moves in, and higher than atmospheric so air moves out
• Pressure changes during inspiration:

  • Diaphragm flattens and external intercostal muscles contract, causing the rib cage to move up
  • Outer pleural membrane moves outwards, lowering the pressure in the pleural cavity
  • Inner pleural membrane pulls on the lungs, increasing the volume of the lungs/alveoli, decreasing the pressure in the alveoli
  • Pressure in the alveoli is below atmospheric pressures, so air moves in
• Gas Exchange in Alveoli:

  • Alveoli are suitable for gas exchange due to:
  • Surfactant: reduces surface tension, prevents alveoli from sticking together and collapsing
• Respiratory Pigment:

  • Increases the oxygen-carrying capacity of the blood, e.g., Haemoglobin
• • Pleural cavity: contains pleural fluid acting as a lubricant
  • Epiglottis: flap of skin preventing food from entering the trachea
  • Pleural membranes: act as a lubricant allowing friction-free movement against the inner wall of the thorax
• Structure and Function:

  • Intercostal muscle contracts, pulling ribs up and out
  • Bronchiole: smaller tubes branching from bronchi
  • Larynx: box-shaped structure above trachea, contains vocal cords
  • Alveoli: main site of gas exchange, large surface area
  • Bronchi: trachea splits into two of these
  • Surfactant: prevents alveoli from sticking together and collapsing, reduces surface tension
  • Trachea: tube held open by rings of C-shaped cartilage
  • Ribs: moved by intercostal muscles, altering the size of the thorax
  • Diaphragm: dome-shaped muscle that alters the volume of the thorax
• Structures in the Breathing System:

  • Epiglottis, Diaphragm, Surfactant, Pleural fluid, Alveoli, Trachea, Larynx, Bronchiole, Ribs, External intercostal muscles, Bronchi, Pleural cavity
• Gas exchange:

  • Process by which oxygen reaches cells and carbon dioxide is removed from them
• Ventilation:

  • Process of moving the respiratory medium (air or water) over the respiratory surface to maintain a concentration gradient, replacing stale air high in CO2 with fresh oxygenated air
• Respiration:

  • Series of chemical reactions resulting in the release of energy in the form of ATP
• Respiratory pigment:

  • Molecule increasing the oxygen-carrying capacity of the blood, e.g., haemoglobin
• Tracheae:

  • System of branched chitin-lined air tubes in insects
• Ends of tracheoles:

  • Site of gas exchange in insects
• Spiracles:

  • Holes in an insect’s exoskeleton allowing exchange of gases and reducing water loss
• Gill Lamellae:

  • Site of gas exchange in fish
• Alveoli:

  • Site of gas exchange in mammals
• Parallel flow:

  • Gas exchange system where blood in the gill capillaries circulates in the same direction as water flowing over the gills
• Counter-current flow:

  • Gas exchange system where blood in the gill capillaries circulates in the opposite direction to water flowing over the gills
• Operculum:

  • Bony structure in bony fish providing a protective covering for the gill