Ahh... blood. The red stuff that flows in your arteries and veins. It is made up of red blood cells, white blood cells, platelets and plasma.
Blood flows in arteries, arterioles, capillaries, venules and veins around the body and is pumped by the heart through the arteries and capillaries. Blood returns through the veins by passive movement, aided by muscle contraction and one-way valves. In the human body, blood flows around in a double circulatory system, passing through the heart twice in one circuit as it is pumped through the lungs and then through the rest of the body.
Possibly the best way to think of blood is as a medium. It is a medium for the transport of oxygen and nutrients to the tissues, removal of waste products including carbon dioxide and the transport of hormones from endocrine organs to their target tissues.
As it carries white blood cells, antibodies and complement around the body, it acts as a medium for the immune system too.
Blood carries around its own clotting (coagulation) system which serves to stop bleeding in case of injury. This is where platelets and clotting factors come into play.
Plasma is the fluid that makes up the rest of the blood that is not the blood cells or platelets. Most stuff that the blood takes around is dissolved in or bound to plasma.
The average normal adult male would have about 5 liters of blood flowing in his body.
Sometimes you can tell things just by looking at blood. Venous blood is darker than arterial blood because most of the oxygen has been sucked out of it. The blood of someone with carbon monoxide poisoning is bright cherry red. Once in a while, you see milky blood from someone with a sky high cholesterol or triglyceride level.