Blood as one of the body’s defense mechanisms- In some cultures, blood is a powerful symbol, a mystifying substance, a wonder medicine, or even a talisman, capable of giving life, supporting life, purifying objects, and placating gods. Sharing blood and shedding blood are considered the ultimately expression of friendships and loyalty. The second most common tissue in our body, second only to the skin, and it comprises about 7% of the body’s weight. The blood’s familiar color is due to the hemoglobin component of the red blood cells. In many cultures, blood is a powerful symbol or talisman, capable of giving life, purifying objects, and placating the gods. Shared blood is considered the strongest of bonds between people, and shedding blood is the ultimate sacrifice. Blood is certainly central to our survival, an organ we can’t do without. It’s the second most common tissue in the body, skin being the first, and comprises about 7% of a ’s body weight. All animals use blood cells to transport oxygen, but not all animals’ red blood cells are the same. Humans make red blood cells in their bone marrow. Before the cells enter the bloodstream to do their job, they lose their nuclei, which keeps them from dividing.

When your body needs more —they wear out and die after about 120 days—your bone marrow must produce them. Birds, on the other hand, have hollow bones without marrow. Their red blood cells retain their nuclei, and therefore can reproduce by dividing. The shapes of RBC differ between animals as well. Human red blood cells are doughnut shaped, while those of some other animals, like camels and penguins, have a more oval appearance. Whichever shape they take, an important characteristic of red blood cells is their flexibility: their shapes make them pliable enough to squeeze through the tiny capillaries where oxygen is exchanged for carbon dioxide. The primary functions of the blood are: it carries oxygen to all body tissues; it protects humans from viral and bacterial infections or invaders; and heals wounds. The components of the blood are the following: red blood cells; white blood cells which is composed of granulocytes, T cells, and macrophages, important for immune system; platelets, that helps healing wounds; complement proteins for fighting infections. The average adult has 5-6 quarts of blood. Blood makes up about 7% of your body’s weight. In one day, your blood travels nearly 12,000 miles. About half of your blood volume is water. There is no known medical substitute for blood. One teaspoon of blood contains about 25 million red blood cells. The cells in your blood have a variety of lifespans. The red blood cells have 120 days lifespan; granulocytes have 6 hours; and some macrophages and some T cells last for decades. But blood contains other types of cells as well. About one percent of the cells in your blood are white blood cells. These make up the most important part of your immune system. Unlike red blood cells, white blood cells can leave your circulatory system and move out into other types of tissues to fend off invading bacteria and viruses. They rush to the site of an injury, seek out foreign cells and particles, and engulf them. There are many different types of white cells; some live less than a day, but others stay with you for much of your life. It’s worth marveling at the scope of your white blood cells’ effectiveness: every day, you’re exposed to thousands of germs—bacteria and viruses—but the vast majority never succeed in infecting you. An invading cell does slip through just about every other day. But thanks to the quick response of your immune system, you actually get a cold or flu far less often than that. When you do get a cold, you usually get over it in a few days. That’s your immune system at work, too. Just as there are different types of white blood cells, there are also many different types of invading bacteria and viruses. But your white cells form an agile and adaptive swarm keeping this ever-changing set of microscopic trespassers at bay.