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Artery walls are thick and have three layers. The first layer on the inside are the epithelial cells which are smooth, so blood can flow easily. The next layer in the middle is made of smooth muscle tissue. The next layer, the outer layer, is made up of flexible connective tissue, which gives it flexibility.

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When blood leaves the heart it travels through the arteries.

1. Blood pumps from the right ventricle to the lungs and the left ventricle to the aorta where small arteries branch. The coronary arteries carry blood to the heart. Other branches distribute blood throughout your body to places like the brain, intestines and other organisms.

2. It regulates the blood flow. It makes sure the blood goes to all the places it needs to go. The arteries act like control gates.

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The red part with the layers are the arteries.

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One thin layer of epithelial cells, so materials can pass easily through them.

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Materials are exchanged between the blood and body's cells.

The materials such as glucose and oxygen, come from the arteries. The materials are passed by blood into the cells. One way the materials are exchanged is called diffusion where molecules move from high to low concentration.

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The part with the skinnier and half red and half blue tubes are the capillaries

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Blood cells flowing through the capillary

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Veins have three layers just like the arteries. The layer of epithelial cells on the inside, smooth muscle tissue and flexible connective tissue. Even though they might have the same type of layer the veins layers are much thinner.

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After the blood moves through the capillaries it enters larger vessels called the veins which carry blood back to the heart.

By the time the blood gets to the veins the hearts pushing force has less effect than it does in the arteries. Though several other things help the blood to flow.

1. Since they are located near the skeletal muscles, when you contact your muscles it pushes the blood.

2. Larger veins have valves to prevent blood from flowing backwards and to help them go forward

3. Breathing movement which pushes a squeezing pressure against the veins in your chest which forces the blood to move towards the heart.

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These are the valves making sure the blood goes to the heart.