Gas Exchange in Humans

• Air passes from the mouth into the trachea (also called the windpipe)
• The trachea divides into two branches called "bronchi", with one bronchus of each lung
• Each bronchus divides further into smaller tubes called bronchioles
• At the end of each bronchiole, there is a group of tiny air sacs. These air sacs have bulges in them called alveoli to increase their surface area and absorb oxygen gas from outside air that has been sucked in
• We need to get oxygen from the air into the bloodstream, and we need to remove waste carbon dioxide from the bloodstream into the air

  This process is called gas exchange
• air sac is called alveoli
• alveoli is surrounded by capillaries
• Alveoli are surrounded by tiny blood vessels called capillaries
• The oxygen molecule passes across from the air into the blood via a process called diffusion
• Carbon Dioxide pathway :

  Up the bronchioles ─> bronchus ─> trachea, and out of the mouth
• There are approximately 300 million alveoli in each lung
• Cilia on the trachea and bronchi help waft mucus into the mouth, where it can be swallowed, to be killed by your stomach acid.
• Goblet cells (which are found in between the cilia) secrete mucus
• Bronchoscopy : A procedure that allows a doctor to examine the lungs using a thin, flexible tube.
• Larynx is the voice box
• The diaphragm is a sheet of muscle
• Diffusion is the movement of particles across a selectively permeable membrane from an area of high concentration to an area of low concentration.

  • i.e. movement down a concentration gradient
• Diffusion of Gases Across Alveolar Membrane

  • from a high concentration to a low concentration
  • across a partially permeable membrane
• trachea connects the lungs to the mouth
• Lungs do NOT contain muscles, so they are unable to expand on their own.

  However, lots of things happen at the same time to force air into the lungs

  Firstly, the intercostal muscles contract, forcing the ribs upwards, and outwards

  Then, the diaphragm contracts and flattens

  These two movements INCREASE the volume of the thorax.

  This chain of events draws breath into the lungs
• Air is drawn into the lungs as a result of muscular contraction
• When we breathe out :

  • intercostal muscles relax
  • diaphragm relaxes and domes upwards again
  • the volume of the torso is decreased, so air is forced out of the lungs
• oxygen needs to get into our bloodstream, and it does this by gaseous exchange
• gaseous exchange occurs in the alveoli
• waste carbon dioxide diffuses OUT of the bloodstream / capillaries, and into the alveoli, ready to be breathed out

  oxygen diffuses INTO the bloodstream from the alveoli
• The alveoli enable the oxygen from the air we breathe to diffuse into our blood, and the carbon dioxide to diffuse out of our blood. This is gaseous exchange.
• Features of alveoli:

  • Enormous surface area of around 90 metres squared (as they are rounded structures)
  • Moist lining for dissolving gasses
  • Very thin walls
  • Good blood supply
• How is smoking bad for us?

  • Heat and chemicals from cigarette smoke can destroy the cilia, so dust and germs get into the lungs
  • Bronchioles may become infected by Bronchitis (disease)
  • Mucus from damaged cilia also collects in the alveoli, damaging the walls of the alveoli, reducing their capacity for gas exchange. This disease is called Emphysema.
  • Can cause Lung cancer
  • Nicotine can cause an increase in blood pressure, which increases the chances of suffering from a heart attack, or a stroke.
• Why do organisms need energy? (1)

  • Muscular contraction : muscles allow the body / parts of the body to move
  • protein synthesis : energy is needed in the reactions that join amino acids together to make proteins
  • cell division : needed for growth and repair in the body
• Why do organisms need energy? (2)

  • active transport : needed so that ions and molecules can be transported across a cell membrane against a concentration gradient
  • the passage of nerve impulses : needed so that the body can respond to changes in environment, and to coordinate its actions and responses
  • the maintenance of a constant body temperature : enzymes and body processes work better at a warm body temperature, which may be hotter than the environment
• Aerobic respiration = using oxygen to break down food molecules
• Glucose is the molecule normally used for respiration – it is the main respiratory substrate. 
• Glucose is oxidised to release the energy that is locked in its chemical bonds.

  It happens in mitochondria, which are tiny structures in the cells of the body
• The word equation for AEROBIC RESPIRATION

  glucose    +    oxygen    →    carbon dioxide   +   water + energy released
• What is Anaerobic Respiration?

  Unlike aerobic respiration, anaerobic respiration does not need oxygen. 

  It is the release of a relatively small amount of energy in cells by the breakdown of food substances in the absence of oxygen.
• Anaerobic Respiration in Muscles in Humans/Animals

  glucose → lactic acid

  C6H12O6 → 2C3H6O3
• Anaerobic respiration :

  Glucose is not completely broken down, so much less energy is released than during aerobic respiration.

  There is a buildup of lactic acid in the muscles during vigorous exercise.