Gas Exchange

• • Single-celled organisms absorb and release gases by diffusion through their cell-surface membranes
  • They have a relatively large surface area, a thin surface and a short diffusion pathway - so there's no need for a specialised gas system
• Gas exchange in fish

  • Water containing oxygen, enters the fish through its mouth and it passes out through the gills
  • Each gill is made of lots of thin plates called gill filaments which give a large surface area for exchange of gases - increases the rate of diffusion
  • The gill filaments are covered in lots of lamellae, which increases the surface area even more
  • The lamellae have lots of blood capillaries and a thin surface layer of cell to speed up diffusion, between the water and the blood
• The counter-current system

  • In the gills of a fish, blood flows through the lamellae in one direction and water flows over them in the opposite direction
  • This is a counter-current system meaning that the water with a relatively high oxygen conc always flows next to the blood with a lower conc of oxygen. This maintains a steep conc gradient between the water and the blood so as much oxygen as possible diffuses from the water into the blood
• Gas exchange in dicotyledonous plants

  • Plants need CO2 for photosynthesis, which produces O2 as a waste gas. They need O2 for respiration, which produces CO2 as a waste gas.
  • The main gas exchange surface is the surface of the mesophyll cells in the leaf - have a large surface area
  • Gases move in and out through special pores in the epidermis called stomata. The stomata can open to allow exchange of gases, and close if the plant is losing too much water
  • Guard cells control the opening and closing of the stomata
• Gas exchange in insects

  • Terrestrial insects have microscopic air-filled pipes called tracheae which they use for gas exchange
  • Air moves into the tracheae through pores on the surface called spiracles
  • Oxygen travels down the concentration gradient towards the cells. The tracheae branch off into smaller tracheoles which have thin, permeable walls and go to individual cells - diffusing directly into respiring cells
  • Carbon dioxide from the cells move down its own conc gradient towards the spiracles to be released into the atmosphere
  • Insects use rhythmic abdominal movements to move air in and out of the spiracles
• • If insects are loosing too much water, they close their spiracles using muscles.
  • They also have a waterproof, waxy cuticle all over their body and tiny hairs around their spiracles, both which reduce evaporation
• • Plants stomata are usually kept open during the day to allow gaseous exchange
  • Water enters the guard cells, making them turgid, which opens the stomatal pore
  • If the plant starts to get dehydrated the guard cell lose water and become flaccid which closes the pore
• Some plants are specially adapted for life in warm, dry or windy habitats where water loss is a problem. These plants are called xerophytes

  • Stomata sunk in pits to trap water vapour reducing the conc gradient of water between the leaf and the air. This reduces the evaporation of water from the leaf
  • A layer of hairs on the epidermis to trap water vapour round the stomata
  • Curled leaves with the stomata inside , protecting them from the wind
  • A reduced number of stomata, so there are fewer places for water to escape
  • Thicker waxy cuticles on leaves and stems to reduce evaporation
• Human gas exchange system

  • As you breathe in, air enters the trachea (windpipe). The trachea splits into two bronchi - one bronchus leading to each lung
  • Each bronchus then branches off into smaller tubes called the bronchioles
  • The bronchioles end in small air sacs called the alveoli where gases are exchanged
  • The ribcage, intercostal muscles and diaphragm all work together to move air in and out
• • Ventilation consists of inspiration (breathing in) and expiration (breathing out)
  • Its controlled by the movements of the diaphragm, internal and external intercostal muscles and ribcage
• Inspiration

  • During inspiration the external intercostal and diaphragm muscles contract
  • This causes the ribcage to move upwards and outwards and the diaphragm to flatten increasing the volume of the thoracic cavity
  • As the volume of the thoracic cavity increases the lung pressure decreases to below atmospheric pressure
  • Air will always flow from an area of higher pressure to an area of lower pressure so air flows down the trachea and into the lungs.
  • Inspirations an active process as it requires energy
• Expiration

  • During expiration the external intercostal and diaphragm muscles relax
  • The ribcage moves downwards and inwards, and the diaphragm curves upwards again forming a dome shape
  • The volume of the thoracic cavity decreases causing the air pressure to increase to above atmospheric pressure
  • Air is forced down the pressure gradient and out of the lungs
• Forced expiration

  • Normal expiration is a passive process - it doesn't require energy
  • During forced expiration, the external and intercostal muscles relax and internal intercostal muscles contract, pulling the ribcage further down and in
  • During this time, the movement of the two sets of intercostal muscles is said to be antagonistic
• Lungs contain millions of microscopic air sacs where gas exchange occurs - called alveoli

  The alveoli are surrounded by a network of capillaries
• • The wall of each alveolus is made from a single layer of thin, flat cells called alveolar epithelium
  • The walls of the capillaries are made from capillary endothelium
  • The walls of each alveolus contain a protein called elastin
  • Elastin is elastic - it helps the alveoli to return to their normal shape after inhaling and exhaling air
• Air moves down the trachea, bronchi and bronchioles into the alveoli. This movement happens down a pressure gradient

  • Oxygen then moves into the blood where it can be transported around the body - this movement happens down a diffusion gradient
  • Carbon dioxide moves down its own diffusion and pressure gradients, but in the opposite direction to oxygen so that it can be breathed out
• Gas exchange in the alveoli

  • Oxygen diffuses out the alveoli, across the alveolar epithelium and capillary endothelium, and into a compound called haemoglobin in the blood
  • Carbon dioxide diffuses into the alveoli from the blood
• Trachea -> Bronchi -> Bronchioles -> Alveoli -> Alveolar epithelium -> Capillary endothelium -> Blood
• Factors affecting the rate of diffusion

  Alveoli have features that speed up the rate of diffusion so gases can be exchanged quickly:

  • A thin exchange surface — the alveolar epithelium is only one cell thick. This means there’s a short diffusion pathway (which speeds up diffusion).
  • A large surface area — there are millions of alveoli.This means there’s a large surface area for gas exchange.
  • There’s also a steep concentration gradient of oxygen and carbon dioxide between the alveoli and the capillaries, which increases the rate of diffusion. This is constantly maintained by the flow of blood and ventilation