3.1.1 exchange surfaces

• Why do multicellular organisms need exchange surfaces?

  - large surface area to volume ratio

  - there's a big distance between cells deep within the body and the outside environment so they can't rely on diffusion alone

  - high metabolic rates so they use up oxygen and glucose faster
• Why don't single-celled organisms need exchange surfaces?

  substances can diffuse directly in and out of the cell across the cell surface membrane as there is a small diffusion distance
• what are the features of an efficient exchange surface

  - large surface area

  - thin layers

  - blood supply

  - ventilation
• give an example of a cell with a large surface area

  root hair cells:

  - hair-like structures that stick out into the soil - each branch of a root is covered in millions of microscopic hairs

  - large surface are increases rate of absorbtion of water by osmosis and mineral ions by active transport
• give an example of a cell with a thin layer

  alveoli:

  - one cell thick layer of epithelium cells

  - there is a short diffusion distance for O2 to diffuse into the blood and CO2 to diffuse out
• give an example of a cell with a good blood supply/ ventilation (human)

  alveoli:

  - surrounded by a capillary network giving each alveolus its own blood supply: the blood constantly takes away O2 and brings more CO2

  - lungs are ventilated so air is constantly replaced

  - this helps maintain concentration gradients of O2 and CO2
• give an example of a cell with a good blood supply/ ventilation

  fish gills:

  - O2 and CO2 are exchanged between the fish's blood and the surrounding water

  - large network of capillaries maintains blood supply

  - they are well-ventilated as fesh water constantly passes over them

  - this helps maintain concentration gradients of O2 and CO2
• role of rings of cartilage in gas exchange?

  in the walls of the trachea and bronchi:

  - provides support

  - strong, but flexible - stops trachea and bronchi collapsing when you breathe in and the pressure drops
• role of smooth muscle in gas exchange?

  in the walls of trachea, bronchi, bronchioles:

  controls the diameter of the lumen:

  - during exercise the smooth muscle relaxes = increases lumen diameter --> less resistance to airflow and air can move in and out the lungs more easily

  - smooth muscle contracts if there's a harmful substance in the air --> reduces lumen diameter = less airflow
• role of elastic fibres in gas exchange?

  in the walls of the trachea, bronchi, bronchioles and alveoli:

  inhalation - lungs inflate and elastic fibres are stretched

  exhalation - fibres recoil to help push air out
• role of goblet cells in gas exchange?

  lines the walls of the trachea, bronchi and bronchioles. secretes mucus which traps microorganisms and dust particles stopping them from reaching the alveoli
• role of ciliated epithelial cells in gas exchange?

  lines the walls of the trachea, bronchi and bronchioles. cilia beat the mucus to waft it up away from the lungs to the throat to be swallowed to prevent lung infections
• why is ventilation needed

  to maintain concentration gradient for gas exchange in alveoli
• how is ventilation controlled

  the movements of the diaphragm, internal and external intercostal muscles and ribcage
• Describe inhalation

  1) external intercostal muscles and diaphragm muscles contract so diaphragm flattens this causes the ribcage to move upwards and outwards

  2) the volume of the thorax increases

  3) lung pressure decreases to below atmospheric pressure

  4) this causes air to flow into the lungs
• describe exhalation

  1) external intercostal muscles and diaphragm muscles relax

  so diaphragm curves again this causes the ribcage to move downwards and inwards

  2) the volume of the thorax decreases

  3) lung pressure increases above atmospheric pressure

  4) this causes air to be forced out of the lungs
• what type of process is exhalation

  normal expiration is a passive process - doesn't require energy

  during forced expiration, the internal intercostal muscles contract to pull the ribcage down and in
• what type of process is inhalation

  active - requires energy
• what is tidal volume

  the volume of air in each breath at rest
• what is vital capacity

  the maximum volume of air that can be breathed in or out
• what is residual volume

  the volume of air that's always in the lungs so they don't collapse
• what is oxygen uptake

  the rate at which an organism uses up oxygen
• how can you measure volume in lungs

  spirometer
• how do spirometers work

  1) the person breathes through a tube connected to the oxygen chamber

  2) as the person breathes in and out, the lid of the chamber moves up and down

  3) these movements are recorded by a pen attached to the lid - this writes on a rotating drum, creating a spirometer trace
• what is a spirometer

  a machine with an oxygen-filled chamber with a movable lid
• why does the overall volume of gas in a spirometer decrease over time

  - the spirometer contains soda lime which absorbs CO2

  - we use up the oxygen from the tank, while the CO2 breathed out is absorbed

  - it decreases by volume of oxygen used by the participant