AS OCR Biology - Chapter 4 - Exchange

Exchange Surfaces -

• All cells need constant supply of oxygen and nutrients, and need to deposit of waste materials; carbon dioxide etc.
• These substances are obtained from or released to the external environment.
• The bigger an organism the bigger the surface area and volume become. Except the volume becomes much bigger than the surface area.  So, to make sure all cells get their requirements, a different sort of gas exchange is needed.

Surface Area : Volume Ratio   =   Surface Area / Volume

Features of exchange surfaces;

•  
  • Large surface area.
  • Thin exchange surface.
  • Diffusion gradient - concentrations different so that diffusion occurs.
  • Cells on surface need protection from drying out

Mammalian Lungs -

• Large surface area provided by the millions of tiny alveoli present in the lungs.
• Thin exchange surfaces - alveolar walls and capillary walls are only one cell thick.
• Diffusion gradient - Breathing in oxygen-rich and carbon dioxide-low air means that the oxygen diffuses into the blood cells that are oxygen-poor and the large amount of carbon dioxide in the blood moves into the lungs to be taken away.
• Protection from drying out - alveoli deep inside lungs, and a fluid is secreted that keeps the alveoli moist.

Plant Leaves -

• Large surface area - plants have many leaves, creates fairly large SA : Vol ratio. Air spaces in leaves - means many cells are exposed to air.
• Thin - plant leaves only a few cells thick. Gases reach most cells directly from air. Stomata present allowing direct contact of air spaces inside leaf with air outside. Gases diffuse directly.
• Diffusion gradient - use of carbon dioxide lowers concentration of it in leaf. Oxygen diffuses into leaves at night, and leaves it again in the daylight.
• Drying out - Stomata can close to prevent movement of gases. Waxy cuticle present on leaves which reduces water loss from cells on leaf surface.

Epithelial Tissue in Airway -

• Ciliated cells and goblet cells situated on a basement membrane.
• Goblet cells secrete a mucus that contains sugar molecules.
• This mucus not only keeps the cells moist, but also trap dust and bacteria to stop them getting into the alveoli and causing problems.
• The cilia wave the mucus to the top of the trachea where it is disposed of by swallowing.

Other Tissues -

• Cartilage - tough tissue that supports the trachea and bronchi, bronchioles don't contain cartilage. Very strong like bone, but more flexible than bone.
• Smooth muscle - found in trachea, bronchi and bronchioles. Contracts slowly and steadily; involuntary muscle. Changes diameter of bronchioles for exercise they become bigger.
• Elastic fibres - found in walls of all airways, helps tubes expand, especially alveoli that need to expand to take in as much air as possible.
• Macrophages - patrol alveolar surfaces. Look out for harmful substances, which they will then engulf.

Mechanism of Breathing In -

• Contraction of external intercostal muscles - rib cage moves upwards and outwards - increasing volume of thorax.
• Contraction of muscle in the diaphragm pulls the diaphragm lower - increasing volume of thorax
• Pressure in thorax falls with increas in volume caused by rib and diaphragm movements. Air flows in down pressure gradient.

Mechanism of Breathing Out -

• Elastic fibres recoil pushing air out, this also causes the pressure in the thorax to rise making the air flow out anyway.

Spirometer -

• As the person breathes out into the spirometer, a soda lime container absorbs the carbon dioxide and the float moves up. This is attached to a pen which draws on a kymograph.
• As the person breathes in again, the float goes down drawing a downwards line on the kymograph.
• They continue to do this until a good result has been taken.
• The end diagram looks like zig-zags.

\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

Keywords -

• Tidal volume - volume of air breathed in and out in one breath.
• Vital capacity - most amount of air that can be breathed in and out in one time.
• Expiratory reserve volume - the extra air breathed out in a breath.
• Inspiratory reserve volume - the extra air breathed in in a breath.