The immune system protects the human body against germs, which are microorganisms that cause sickness and disease. There are four major types of germs— harmful bacteria (pathogens), viruses, fungi, and protozoa. This defense system begins with the skin, which stops germs from getting into your blood or tissues. If germs get into your body, for example through your nose or mouth, white blood cells called phagocytes and lymphocytes attack them. Phagocytes scout out and destroy invaders, and long-living lymphocytes remember the invaders and release chemicals called antibodies to make the body resistant, or immune, to them.
White blood cells live in the bloodstream, lymphatic system, and spleen. The lymphatic system (or lymph system, for short) is a far-reaching network that extends throughout your entire body. A clear liquid called lymph runs throughout the system, washing the body’s cells with nutrients and water and detecting and removing pathogens. Lymph is filtered through the lymph nodes, and then passes into the body’s bloodstream.
Although everyone eats different foods, doctors and nutritionists (people who plan food and nutrition programs) generally agree that a healthy diet is one loaded with fruits, vegetables, whole grains, and fat-free or low-fat milk and milk products; one that includes lean meats, poultry, fish, beans, eggs, and nuts; and one that is low in saturated fats (like those fats found in butter), trans fats (also called partially hydrogenated oils, found in foods like French fries and donuts), cholesterol, salt (sodium), and added sugars.
Foods like processed cheese that squirts from a can, frosted pastries filled with sugary jam, and deep-fried fish, nuggets, or French fries do not provide the types of nutrients your body needs to grow, and are sometimes called “empty” foods. For example, 1 ounce of potato chips has 152 calories and 10 grams of fat (3 grams of which are saturated fat).
As the young red blood cell grows and takes on an adult form in the marrow of the bone, it loses its nucleus, and it increases its production of hemoglobin. Hemoglobin is the red pigment, or color of blood, and contains iron, combined with protein. (Oxygen combined with iron is red; the more oxygen iron has bound to it, the redder it is.)
When blood passes through the lungs, oxygen attaches itself to the hemoglobin of the red cells. From there, the red cells carry the oxygen through the arteries and the capillaries to all other cells of the body. The arteries appear reddish because the iron in the blood gives up its oxygen to the cells that need it as the red blood cells travel throughout the body. By the time the blood is back on its way to the heart and then to the lungs it has less than half as much oxygen as it did before. The veins, therefore, do not get as much oxygen as the other tissues and they appear bluish.
As people age, their bodies change in many ways that affect the ways their cells and organ systems function. These changes occur little by little, progress over time, and are different for every man and woman. We all lose height as we age, and by the time we reach 80 years old, our height may have decreased by two inches (five centimeters). Changes in posture, in the growth of our spine’s vertebrae, and joint changes all contribute to our loss of height. With aging, the hair follicles produce less melanin, the pigment that gives hair its color.
Hair becomes lighter, gray, and eventually turns white. The nails also change with aging: they grow slower, may become dull and brittle, and may become yellowed and opaque. With aging, the outer skin layer (epidermis) thins and the number of pigment-containing cells (called melanocytes) decreases, but the remaining melanocytes increase in size. Aging skin thus appears thinner, more pale, and translucent. Changes in the connective tissue reduce the skin’s strength and elasticity, resulting in a wrinkly, leathery skin.
Germs are everywhere! Most germs spread through the air, invading our homes, pets, and family, and sometimes they make us sick. Besides your bathroom toilet and the kitchen sink, everyday items like shopping-carts, restaurant menus, computer keyboards, and the shower curtain contain germs. These items contain bacteria, mold, and rhinoviruses (instigators of the common cold) that can lead to sickness. In fact, cold and flu viruses can survive for 18 hours on hard surfaces.
Common household items can be swabbed with a disinfectant wipe easily before use in order to prevent germs from spreading. Washing your hands with soap and water, using a hand sanitizer, and avoiding touching your face with your hands after using these items also helps keep germs away from you. To eliminate dust mites—those little critters that live in your bed sheets and feed on dead skin cells—don’t make your bed for a while. Studies have found that dust mites need humidity levels above 50 percent to survive and cannot live in the arid conditions of an unmade bed.
A popular summer-camp rhyme is “Leaves of three, let it be.” Poison ivy, the threeleaved plant that grows wild in all regions of the United States, is aggravating to the skin, though not lethal if swallowed unless you are very allergic. After brushing up against the poison ivy plant, a red rash usually develops.
Rubbing the rash will not spread poison ivy to other parts of the body (or to another person) unless urushiol oil—the sticky, resin-like substance that causes the rash—has been left on your hands. Other plants that usually irritate the skin upon contact include cowhage, poison oak, poison sumac, rengas tree, and trumpet vine.
In the United States you are considered a grown-up when you reach the age of 18. You are no longer legally connected with your parents, and you are entitled to the rights—and expected to fulfill the duties—of an adult American citizen. (You may vote and be called for military service, for instance.)
There is a good chance, though, that when you are 18 your body has not yet reached full maturity. Many people continue to grow for a few more years. Most are fully grown—at least in height—by the time they are 20 years old, though boys may keep on growing until they are 23.
Absolutely! Strong, healthy teeth help you speak clearly, chew harder vegetables and meats, and help you look your best. Brushing your teeth helps prevent plaque, a clear film that sticks to your teeth. The sticky film acts like a magnet for bacteria and sugar. Bacteria eats the sugar on your teeth, breaking it down into acids that deteriorate tooth enamel, causing holes called cavities. Plaque also causes the gum disease gingivitis, which make your gums red, swollen, and sore. At around age six, you lose your baby teeth and a larger set of teeth begin to surface. Eventually, 32 new teeth will line your growing jaws, the last coming in around the age of 18.
These permanent teeth will perform all of your eating tasks for the rest of your life, so they are worth taking care of! Your four front teeth (on top and bottom) are sharp incisors that cut and tear off food when you bite, along with your four pointed canine teeth. The flat-topped bicuspids (premolars) and molars near the back of your mouth crush and chew your food.
The fastest growing hairs on the human body are men’s beard hairs. If the average male never trimmed his beard, it would grow to almost 30 feet (9 meters) long in his lifetime.
In early England, one man would challenge another to a duel by slapping his face with a glove. The challenge was a serious matter of honour, and if the slapped man did not accept it, he would be branded a coward. Having a chip on your shoulder was kind of an early Wild West equivalent of the glove slap, though generally less mortal in nature.
Boys and men would place a woodchip on their shoulder, challenging anyone who dared knock it off to a fistfight. So, if a man had a “chip on his shoulder,” he was clearly in an aggressive mood and spoiling for a fight.