Biology AS F211 (Exchange surfaces and breathing)

Pretty much what it says in the title.....

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Surface area to volume ratio

As the volume of an object increases the ratio of surface are to volume becomes smaller. This means that multicellular organisms which have a high volume need specialised surfaces in order to get what they need. This is because as the surface area decreases the distance inbetween cells in the middle increases and this makes diffusion much harder.

In a specialised area the amount of surface area can be increased by use of specific techniques like infolding.

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Efficien exchange surfaces

Features of an efficient exchange surface....

- High surface area, this allows more space for the molecules to pass through.

- Thin barrier, this reduces the diffusion distance.

- Fresh supply of molecule on one side, this increases the concentration difference.

- Quick removal of molecules on one side.

(The final three are important for helping maintain a steep concentration gradient)

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Adaptations of the lungs

The adaptations of the lungs to aid with diffusion...

- They have a very large Surface area, this is due to microvili and alveoli.

- A permeable barrier, which allows passage of oxygen and carbon dioxide.

- Thin barrier, the wall of the alveoli is one cell thick. It has squamous cells (these are flattened cells).

- It also maintains a steep diffusion gradient it does this by the blood transport system and the continual ventialtion of the lungs.

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Process of breathing


- The diaphragm contracts, pushing the organs down.

- The intercostal muscles pulling the ribs up and out.

(These two processes cause the volume of the thorax to increase, lowering the pressure inside the lungs)

- Air moves into lungs.


- Diaphragm relaxes

- Intercostal muscles relax

(Volume of thorax decreases increasing pressure inside)

-Air is forced out.

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Roles of tissues within the lungs

- Cartilage, The cartilage plays a structural role keeping airways open, these line the trachea and bronchi in C-shape rings to allow for some movement and flexbility.

- Smooth muscle, Controls the flow of air by constrict the airways and making the lumen narrower.

- Elastic fibres, when the airway is constricted it cannot widen without help,so when constrict it deforms the elastic fibres which then recoil returning it to its original shape.

- Goblet cells, These produce mucus which can trap bacteria and other particles from the air.

- Ciliated epithelium, These waft the mucus to the back of the throat where it can be swallowed and whatever it contains destroyed.

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Lung volume, and associated terms

Tidal Volume - This is the volume of air moved in and out with each breath when at rest.

Vital capacity - This is the largest amount of air that can be moved in and out of the lungs at any one time.

Residual volume - This is the amount of air which remains in the lungs even after each breath.

Dead space - This is a popular computer game. Its also the amount of gas which remains in the bronchioles, bronchi and trachea. In this gas there is no echange between it and the blood.

Inspiratory reserve volume - The amount over which you can normally inhale that is kept permanently in reserve.

Expiratory reserve volume - The amount which you can breathe out more than you usually would that is kept permanently in reserve.

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