By: Shelby Lipham

 The circulatory system is made up of the muscles and vessels that help control the flow of blood around the body. This process is called circulation.  The main parts of the system are the arteries, veins, capillaries, and heart.

 The heart is a muscle that sends blood around your body. It is sort of like a pump. The right side of your heart receives blood from the body and pumps it to the lungs. The left side of the heart does the exact opposite: It receives blood from the lungs and pumps it out to the body.

 Blood is transported around the body through tubes called blood vessels. The main types of blood vessels are arteries, veins, and capillaries.

 Arteries carry blood away from the heart to the rest of the body. They have very strong and thick muscular walls that can withstand the high pressure of blood as it is pumped out of the heart. Artery walls have three thick layers: - a thick outer layer of collagen fibers  - a thick middle layer of elastic and muscle fibers arranged in rings  - a thin lining layer

 The walls of major arteries are very elastic, allowing them to stretch as the heart pumps blood into them. Blood flows quickly along arteries, in pulses.  The aorta is the largest artery in the body. The blood leaving the aorta is full of oxygen.

 Veins carry blood back to the heart. They have thinner walls than arteries, because they carry blood at a lower pressure. Their walls also have three layers:  - a thin outer layer of collagen fibers  - a thin middle layer, containing few muscle fibers  - a thin lining layer  Blood flows slowly along veins, without a pulse.

 Unlike the arteries and veins, capillaries are very thin and fragile. The capillaries are actually only one epithelial cell thick.  They are so thin that blood cells can only pass through them in single file. The exchange of oxygen and carbon dioxide takes place through the thin capillary wall. The red blood cells inside the capillary release their oxygen which passes through the wall and into the surrounding tissue.

You can wrap your blood vessels around the equator about two times! About 8 million blood cells die in the human body every second, along with 8 million born! With a tiny droplet of blood, there are around 5 million red blood cells! The heart beats around 3 billion times in the average person’s life! It takes about 20 seconds for a red blood cell to circle through the human body!

 Ballard, Carol. Heart & blood. Oxford: Heinemann Library, Print.  "Circulatory System." ThinkQuest : Library. N.p., n.d. Web. 19 Mar <  Brynie, Faith Hickman. 101 questions about blood and circulation, with answers straight from the heart. Brookfield, Conn.: Twenty-First Century Books, Print.  "Your Heart & Circulatory System." KidsHealth - the Web's most visited site about children's health. N.p., n.d. Web. 19 Mar <