Cardiovascular System

Fact: The heart pumps your blood through 60,000 miles of blood vessels each day!
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Functions

Your circulatory system is powered by your heart. It moves blood throughout your body which carries oxygen and nutrients to various organs within it. Blood also contains hormones and leukocytes. Leukocytes produce antibodies which help the body fight off infections. In addition to its chemical and biological properties, the circulation of blood also helps maintain a constant body temperature.

What's in your blood?

Blood performs many important functions in your body, but what are the contents in your blood that performs all these functions? Your blood is made up of the following:
  1. Plasma - liquid medium of blood. Circulates nutrients and removes waste products.
  2. erythrocytes (red blood cells) - transports oxygen throughout the body.
  3. leukocytes (white blood cells) - fights infections by producing antibodies and through phagocytosis.
  4. thrombocytes (platelets) - assist in the coagulation of blood and hemostasis (cell growth).

Veins, Arteries, and Capillaries: 

Veins - the common misconception is that veins only carry deoxygenated blood. This is untrue, veins are blood vessels that carry blood to the heart. Most of the time it is deoxygenated, except in the case of pulmonary and umbilical veins which carry oxygenated blood to the heart.
Arteries - arteries are more muscular than veins and do not have valves like veins. Inversely, arteries do not carry blood to the heart but away from it.
Capillaries - Unlike veins and arteries, capillaries do not function on their own. They form interweaving networks to provide nutrients and carry away wastes. They are usually only one cell thick and connect arteries and veins.

Tracing a Drop of Blood Through the Heart

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http://image.wistatutor.com/content/feed/u2044/systemic%20circulation%20diagram.jpg
Deoxygenated blood from lower and upper body travel to the right atrium. Once it travels through the tricuspid valve, it is in the right ventricle. Once in the right ventricle, it is pumped out through the pulmonary valve into the pulmonary artery into the lungs. The lungs oxygenate the blood and it then travels back to the heart via the left atrium through the mitral valve into the left ventricle. The left ventricle then pump the blood out through the aortic valve into the aorta to be used by the body.