Arteries, Capillaries, and Veins

    Arteries carry oxygen-rich blood out of the heart and throughout the body to the tissues and every cell of the body. The arteries are also known as arterioles.
There are two main artery systems. One is the Pulmonary arteries and the other is the Systemic arteries. 
 The Pulmonary arteries carry the oxygen-rich blood from the right ventricle to the lungs.  
Arteries are tough on the outside and smooth on the inside, this makes it easier for the blood
 to flow through.

    Capillaries are the smallest tubes, capillaries are also called lymph capillaries.
They connect arteries and veins together, they merge to form larger lymph vessels.
There are three different kinds of capillaries: the Continuous capillaries, the Fenestrated capillaries, and the Sinusoidal capillaries.
Continuous capillaries have the the thickest endothelial walls.
The continuous capillaries carry water and ions which are groups of atoms.
Fenestrated capillaries have windows that allow larger molecules to go in and out.
 Sinusoidal capillaries have the greatest amount of permeability. This lets the red blood cells and proteins through the endothelial walls.
    Veins carry blood by lower blood pressure.  Like arteries veins have three layers but the layers are thinner containing less tissue. This makes the veins weaker then the arteries.
Veins bring blood to the heart, the veins receive blood from the capillaries.
It is important for the veins to move carbon dioxide-rich blood along the body in the proper direction, this is what the valves are for.  Valves are like gates that allow the blood to move in one direction.
The blood in the veins are red but because of the reflection of the light the dark red appears to be blue.  The walls are very thin the the rich blood is visible through the skin.