The traffic that flows along the passages of your circulatory system is blood. If you’re an average person, you have about 6 quarts of blood perpetually circulating through your body. (In soft-drink terms, your body has 16 Coke cans’ worth of blood in it.)
We usually think of blood as a runny, red substance. However, if you broke blood down to its basic components, you’d start with a straw-colored liquid called plasma, which consists of water, dissolved nutrients, and a few other ingredients you’ll learn about in a moment. The red color of blood comes from the oxygen-carrying red blood cells.
Despite popular lore, blood is never blue (even when it’s inside your body, and even in royalty). When your blood is fully stocked up with oxygen, it’s bright red. Oxygen-depleted blood turns a purple-tinged, dark-red color. Due to the way your skin refracts light, your veins may look blue, but the blood inside is always some shade of red.
Your blood’s complex cocktail of substances includes:
• Oxygen and sugar. Your circulatory system delivers the essential nutrients and fuel every cell in your body needs to produce energy.
• Waste products. Your circulatory system takes potentially poisonous substances from the cells that excrete them to their appropriate disposal site. For example, it carries carbon dioxide to your lungs and delivers other toxins to your kidneys and liver.
• Hormones. These signaling chemicals let one body part influence another in ways both subtle and profound, and they travel from source to destination through your blood. Often, your brain starts the cascade of hormone release. (For example, to trigger your fight-or-flight response, your body uses hormones that speed up your heart, dilate your pupils, and constrict your blood vessels. On a much more extended timescale, your body releases growth hormones that reshape your body and start the changes of puberty.)
• Germ fighters. Your blood is the battleground on which your immune system confronts bacteria, viruses, and tumors.
• Clotting compounds. Your blood is precious stuff. Fortunately, it has its own loss-prevention system: platelets that plug holes in your skin, and a tough protein called fibrin that forms a reinforcing mesh around the platelets. Together, these compounds prevent your entire blood supply from draining out of a minor cut.
Source of Information : Oreilly - Your Body Missing Manual