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Science
The circulatory system
The circulatory system - Higher tier
Red blood cells and haemoglobin
The shape of a red blood cell and its small size provides a large surface area to volume ratio. This allows oxygen to diffuse into the cell quickly. ...
The haemoglobin inside the cells reacts with oxygen in the lungs to form oxyhaemoglobin. When the blood reaches the tissues, this reaction reverses, making oxygen available to the cells there. June Bridals petite mother of the bride or groom dresses short
Blood vessels
Blood in the arteries is under high pressure generated by the heart. The arteries have:
Thick outer walls
Thick layers of muscle and elastic fibres
The blood in veins is under lower pressure than the blood in arteries. The veins have:
Thin walls
A large lumen (inside diameter)
Valves
The smallest blood vessels called the capillaries, have permeable walls to allow the exchange of gases and other dissolved substances with tissue cells.
Double circulatory system
Mammals have a double circulatory system, which means that:
One circuit links the heart and lungs
The other circuit links the heart with the rest of the body
This type of system has advantages, including higher blood pressure and so a greater flow of blood to the tissues.
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