How to Tell Crystal From Glass. Here are some quick tips on how to distinguish classic crystal from plain glass. 1  Hold up a glass to the light. 2  If the glass acts as a prism and you see a rainbow you are holding crystal. 3  If not, it is just plain glass. 4  When struck, crystal produces a musical ring.
Blood pressure is controlled by tiny muscles that line the inside of your blood vessels. These muscles allow your arteries to operate like soft rubber tubes, that expand with each beat of your heart. When these muscles throughout the vascular system [the arteries and veins that carry blood] expand, blood pressure drops.
Some of the carbon dioxide in your blood is transferred into the air that is in your lungs. This controls the levels of oxygen and carbon dioxide in your bloodstream. See also the separate leaflet called The Heart and Blood Vessels for more information on how blood is pumped to the lungs and to the rest of your body.