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Diffusion
The movement of molecules or ions from a region of their high concentration to a region of their low concentration down a concentration gradient
Why do some organisms not need a circulatory system?
Single Celled organisms/ jellyfish/ flatworms are:
•small
• have a high surface area: volume ratio
•have a short diffusion distance
This means diffusion is sufficient to provide all cells with oxygenated blood fast enough to meet demand for aerobic respiration.
they may also have a low metabolic rate and a low demand for oxygen
describe an open circulatory system
•No blood vessels
•blood is held in cavity, organs are bathed in blood
•diffusion takes place between blood and organs
•blood is under low pressure
describe a Closed circulatory system
blood is held within blood vessels; blood is under high pressure
describe a single closed circulatory system
• Blood flows through the heart once when making a complete circuit.
• Blood is at a low pressure when reaching the body capillaries
• (in a fish circulatory system) the heart pumps deoxygenated blood to the gills, where gas exchange takes place
• Oxygenated blood leaves the gills and flows (not pumped) around the rest of the body before returning to the heart
describe a double closed circulatory system
• blood flows through the heart twice in each complete circulation
•blood is at a high pressure when reaching body capillaries
• (in a human circulatory system) the heart pumps blood to the lungs to be oxygenated, then returns to the heart where oxygenated blood is pumped to the body through the arteries. The blood returns to the body through veins.
Explain the advantages of a double closed circulatory system
•Oxygenated and deoxygenated blood cannot mix, so tissues recieve maximum oxygenated blood for aerobic respiration
• Blood can be pumped at different pressures by each side of the heart. The lower pressure in the lungs allows blood to travel more slowly, maximising the efficiency of gas exchange. It can also travel at a high pressure in the body allowing for more efficient oxygenation of tissues.
•Repressurisation- blood gets an extra boost when it returns from the lungs to allow the heart to pump oxygenated blood faster to the body.
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