why do we have a circulatory system

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. artery Thick-walled muscular tube, which carry blood away from the heart. is under high pressure generated by the heart.

The arteries have:
The blood in veins is under lower pressure than the blood in arteries. The veins have: The smallest blood vessels called the capillaries, have permeable walls to allow the exchange of gases and other dissolved substances with tissue cells.

Mammals have a double circulatory system, which means that: This type of system has advantages, including higher blood pressure and so a greater flow of blood to the tissues. Now try a - higher tier. Living cells need to absorb nutrient A substance that provides sustenance to living organisms. and oxygen, and to release waste products, such as carbon dioxide.

Some animals such as the single-celled amoeba - are small enough to do this by diffusion from their surface, and do not require a circulatory system. But larger, multicellular animals need a circulatory system to transport substances to and from their cells. Some animals, such as insects, have an open circulatory system. Their blood flows freely through their body cavity, carrying nutrients to their cells.

Oxygen is delivered directly to the insects tissues through tiny tubes that open to the outside. vertebrate Vertebrates are animals that have a backbone. They include fish, mammals, reptiles, birds and amphibians. have closed circulatory systems. Their blood flows through blood vessels ( artery Thick-walled muscular tube, which carries blood away from the heart. , vein Thin-walled, valved tubes which carry blood back to the heart. and capillary Capillaries are the smallest blood vessels in the body, connecting the smallest arteries to the smallest veins. ) and they have a heart to push the blood around their body.

The human circulatory system is a closed system.

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