Exchange

• insects have an internal network of tubes called tracheae. they are supported by strengthened rings to stop them from collapsing
• the tracheae divide into smaller dead - end tubes called tracheoles. the tracheoles extend throughout all the body tissues of the insect
• when cells are respiring, oxygen is used up and so its concentration towards the end of the tracheoles falls. this creates a diffusion gradient that causes gaseous oxygen to diffuse form the atmosphere along the tracheae and tracheoles to the cells. carbon dioxide is produced which diffuses along the tracheoles and into the atmosphere.
• mass transport- the contraction of muscles in insectss can squeeze the trachea enabling mass movements of air in and out. This further speeds up the exchange of respiratory gases
• the ends of tracheoles are filled with water during periods of major activity, the muscle cells carry out anaerobic respiration. this produces lactate which is soluble and lowers the water potential of the muscle cells. water moves into the cells from tracheoles via osmosis. the water in the end decreases in volume and air is drawn further into them. the final diffusion pathway is gas = more rapid
• gases enter and leave the tracheae through tiny pores called spiracles on the body surface. the spiracles can be opened and closed by a valve. when they're open, water can evaporate from them. insects keep them closed mostly to prevent this
• it is limited because it relies mostly on diffusion to exchange gases between the environment and the cells. however for this to be effective, insects need a short diffusion pathway, which is why insects are small. so their size is limited.
• how are insects adapted to reduce water loss?

  they have a waxy, waterproof cuticle made of chitin which is impermeable to gases. they also have spiracles which close during the day, and hairs around the spiracles to reduce evapouration by trapping moisture
• how are insects adapted for gas exchange

  • tracheoles have thin walls so the diffusion pathway is short
  • the tracheoles are branched so they have a large surface area
  • the tracheae provide lots of oxygen so there is rapid diffusion into cells
  • movement of insects causes fluid in the ends of tracheoles to travel out by osmosis so there is faster diffusion
  • mass transport
• what is tissue fluid?

  the environment around the cells of multicellular organisms
• the size and metabolic rate of an organism will affect the amount of each material that is exchanged
• how do exchanges take place [other than heat]

  • passively, by diffusion or osmosis
  • actively, by active transport
• how do you calculate the surface area and ratio of a sphere

  surface area: 4πr^2

  volume: 4/3 πr^3
• features of specialised exchange surfaces

  1. a large surface area to the volume of the organism which increases rate of exchange
  2. very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
  3. selectively permeable to allow selected materials to cross
  4. movement of the environmental medium to maintain a diffusion gradient
  5. a transport system to ensure the movement of the internal medium to maintain a diffusion gradient
• how does gas exchange occur in single celled organisms

  they are small and therefore have a large surface area to volume ratio. oxygen is absorbed by diffusion across their body surface, which is covered only by a cell surface membrane. in the same way, carbon dioxide from respiration diffuses out across their body surface.
• describe the structure of the gills

  thev gills are made up of gill filaments. the gill filaments are stacked in a pile and at the right angels to the filaments are gill lamellae, which increase the surface area of the gills. water is taken in through the mouth and forced over the gills and out through an opening on each side of the body.
• what is the countercurrent exchange principle

  blood and water that flow over the gill lamellae do so in opposite directions. this arrangement means that:

  • blood that is already well loaded with oxygen meets water, which has its masimum concentration of oxygen. therefore diffusion of oxygen from the water to the blood takes place
  • blood with little oxygen in it meets water which has had most of its oxygen removed. diffusion of oxygen from the water in the blood takes place
• how are plants and insect gas exchange similar

  • no living cell is far from the external air, and therefore a source of oxygen and carbon dioxide
  • diffusion takes place in the gas phase, which makes it more rapid than if it were in water
• what adaptations do leaves have for gaseous exchange

  • many small pores, called stomata, and so no cell is far from a stoma and therefore the diffusion pathway is short
  • numerous interconne.cting air spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells
  • large surface area of mesophyll cells for rapid diffusion
• what are stomata

  minute pores that occur mainly, but no5 exclusively, on the leaves, especially the underside.

  each stoma is surrounded by a pair of guard cells.

  these cells can open and close the stomatal pore.

  in this way they can control the rate of gaseous exchange

  this is important because terrestrial organisms lose water by evapouration.

  plants close stomata at times when water loss would be excessive
• what are xerophytes

  plants that are adapted to living in areas where water is in short supply
• what modifications do plants have to limit water loss

  • a thick cuticle
  • rolling up of leaves
  • hairy leaves
  • stomata in pits or grooves
  • a reduced surface area to volume ratio of leaves
• why is the volume of oxygen absorbed and the volume of carbon dioxide that must be removed large in mammals?

  • they are relatively large organisms with a large volume of living cells
  • they maintain a high body temperature which is related to them having high metabolic and respiratory rates
• why are mammalian lungs inside the body

  • air is not dense enough to support and protect these delicate structures
  • the body as a whole would otherwise lose a great deal of water and dry out
• what does the ribcage do?

  supports and protects the lungs
• what are the lungs

  a pair of lobed structures made up of a series of highly branched tubules, called bronchioles, which end in tiny air sacs called alveoli
• what is the trachea

  a flexible airway that is supported by rings of cartilage. the catilage prevents the treachea collapsing as the air pressure inside falls when breathing in. the tracheal walls are made up of muscle, lined with cilated epithelium and goblet cells
• what are the bronchi

  two divisions of the trachea, each leading to one lung. they are similar in structure to the trachea and, like the trachea, they also produce muscus to trap dirt particles and have cilia that move the dirt-laden mucus towards the throat. the larger bronchi are supported by cartilage, although the amount of cartilage is reduced as the bronchi get smaller
• what are the bronchioles

  a series of branching subdivisions of the bronchi. their walls are made of muscle lined with epithelial cells. this muscle allows them to constrict so that they can control the flow of air in and out of the alveoli
• what are the alveoli

  minute air - sacs at the end of the bronchioles. between the alveoli there are some collagen and elastic fibres. the alveoli are lined with epithelium. the elastic fibres allow the alveoli to stretch as they fill with the air when breathing in. they then spring back during breathing out in order to expel the carbon dioxide-rich air. the aveolar membrane is the gas exchange surface
• what is breathing / ventilation

  maintaining diffusion of gases across the aveolar epithelium, air is constantly moved in and out of the lungs
• what is inspiration

  when the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs
• what is expiration

  when the air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs
• what sets of muscles bring about the pressure changes within the lungs

  • the diaphragm, which is a sheet of muscle that separates the thorax from the abdomen
  • the intercostal muscles, which lie between the ribs
• outline the process of inspiration

  this is an active process

  1. the external intercostal muscles contract, and the internal intercostal muscles relax
  2. the ribs are pulled upwards and outwards, increasing the volume of the thorax
  3. the diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax
  4. the increased thorax volume results in reduction of pressure in the lungs
  5. atmospheric pressure is now greater than pulmonary pressure, and air is forced into the lungs
• outline the process of expiration

  breathing out is a largely passive process

  1. the internal intercostal muscles contract, while the external intercostal muscles relax
  2. the ribs move downwards and inwards, decreasing the volume of the thorax
  3. the diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration. the volume of the thorax is further decreased
  4. the decreased volume of the thorax increases the pressure in the lungs
  5. the pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs
• what happens during normal quiet breathing

  the recoil of elastic tissue in the lungs is the main cause of air being forced out. only under more strenuous conditions such as exercise do the various muscles play a major part
• how do you calculate pulmonary ventilation rate

  pulmonary ventilation rate is the total volume of air that is moved into the lungs during 1 minute.

  tidal volume x breathing rate
• what is the site of gas exchange in mammals

  the epithelium of the alveoli
• why will diffusion of gases between the alveoli and the blood will be very rapid?

  • red blood cells are slowed as they pass through pulmonary capillaries, more time for diffusion
  • the distance between the alveolar air and red blood cells is reduced as the red blood cells are flattened
  • the walls of both alveoli and capillaries are very thin, short diffusion pathway
  • aveoli and pulmonary capillaries have a large total surface area
  • breathing movements ventilate lungs, blood circulates, steep concentration gradient
  • blood flow through pulmonary capillaries maintains a concentration gradient