CB8a-b
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- Created on: 12-03-21 19:34
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- CB8a-b
- CB8a Efficient transport and exchange
- Animals need circulatory system because all cells need a supply of oxygen to stay alive, but diffusion is too slow to get it to the cells
- Surface area : volume ratio is important because the larger a cell's surface area, the more of a substance can diffuse into and out of it in a certain time. However, if a cell's volume is too big, the cell cannot fill up with all the materials it needs quickly enough.
- The lungs are adapted to allow gas exchange. These adaptations include:
- firstly, they are round instead of flat so they have an enormous surface area,
- secondly, they have a moist lining which allows gases to dissolve.
- Thirdly, they have very thin walls
- finally they have a very good blood supply.
- CB8b Factors affecting diffusion
- Arteries take blood away from the heart, they have a narrow tube with a thick layer of elastic and muscle fibres.
- Veins carry bloods back to the heart, they have a wide tube with a thin, flexible wall.
- Capillaries are in tissues. They have a very narrow tube with a wall only one cell thick to allow faster diffusion of substances into and out of the capillary
- In the circulatory system, blood flows away from the heart into arteries. These divide into narrow capillaries, which form fine networks running through tissues. Blood returns to the heart in veins.
- When it beats, the heart squirts blood into arteries under high pressure. Artery walls are thick to withstand this sudden increase in pressure, but it makes them stretch. A wave of stretching passes along the walls, we feel this as a pulse.
- After stretching, muscle and elastic fibres in the artery walls cause the arteries to contracts again. The stretching and contracting of arteries makes the blood flow more smoothly.
- Blood flows under low pressure in veins and so they only need thin walls. As you move muscles in your skeleton help to bush blood along the veins. Veins contain valves to prevent blood flowing the wrong way.
- After stretching, muscle and elastic fibres in the artery walls cause the arteries to contracts again. The stretching and contracting of arteries makes the blood flow more smoothly.
- CB8a Efficient transport and exchange
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