Circulatory System

The circulatory system includes your heat, blood vessels and blood.
Blood is a special body fluid that delivers necessary nutrients and oxygen to cells in the body.  

In other words, Blood is the body's transport system powered by the heart. Blood circulates around the body in tubes called blood vessels. Blood leaves the heart in large blood vesels called arteries, and it returns in vessels called veins. As the blood travels around the body, it passes through organs along the way.

Arteries split into smaller branches and eventually turn into capillaries. These capillaries can be finer then a strand of hair. Branches of capillaries lead into veins, which join together and get bigger on the way back to the heart.

From Red to Blue
Did you know the colour of blood depends on how much oxygen it contains?
Oxygen rich blood, typically in arteries, is a very bright red.
Oxygen poor blood, typically in veins, is a dark purplish / red colour.

One drop of blood contains 5 million red blood cells, 7,000 white blood cells and other elements such as water, sugars, salts, vitamins and proteins. Red blood cells transport oxygen from your lungs to the rest of your body. White blood cells help fight diseases. 

The average adult has just under 6 litres of blood while a newborn baby has only a cupful. As you grow, the amount of blood in your body increases. By the time a baby turns ten years old, it would have about two litres of blood in its system.

The Heart
Your heart is a pump that moves blood around your whole body. Each heart beat squirts a small cupful of blood and then refills for the next heart beat. One half pumps blood through your lungs, and the other half pumps blood around the rest of the body.
The heart is located in the middle of your chest in between your two lungs. 

Muscles need extra blood when you're active, so your heart speeds up. It beats about 70 times a minute when you are resting, but that can increase to about 200 times a minute if you are running.

Further Reading