“Dog days” are the hot, humid days of summer that usually take place in the Northern Hemisphere in July and August typically between July 3 and August 11. The days get their name from the dog star Sirius of the constellation Canis Major.
At this time of year, Sirius, the brightest visible star, rises in the east at the same time as the Sun in the northern hemisphere. Ancient Egyptians believed that the heat of this brilliant star added to the Sun’s heat to create this hot weather and they blamed the star for everything from withering droughts to sickness.
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.
Scientists do not know exactly why people need sleep, but studies show that sleep is necessary for survival. Sleep appears to be necessary for the nervous system to work properly. While too little sleep one night may leave us feeling drowsy and unable to concentrate the next day, a long period of too little sleep leads to poor memory and physical performance.
Hallucinations (seeing things that aren’t really there), vision problems, and mood swings may develop if sleep deprivation continues.
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.
The word “smog” was first used in London during the early 1900s to describe the combination of smoke and fog. Today, the term “smog” is used to describe a mixture of pollutants, primarily made up of ground-level ozone. Ozone can be beneficial or harmful depending on its location. The ozone located high above the surface in the stratosphere protects human health and the environment, but ground-level ozone is responsible for the choking, coughing, and stinging eyes associated with smog.
Smog-forming pollutants come from many sources, such as automobile exhaust, power plants, factories, and many consumer products, including paints, hair spray, charcoal starter fluid, solvents, and even plastic popcorn packaging. In many American cities, at least half of the pollutants come from cars, buses, trucks, and boats. Scientists estimate that about 90 million Americans live in areas with ozone levels above the standards for health safety.
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.
Human beings communicate through language, a complicated system of vocal symbols that our complex brains allow us to learn after we are born. But we also communicate through our bodies and senses. Our organ of touch is our skin, covering the outside of our bodies. (Nerve endings under the surface of skin give us our sense of touch.) Hugging and kissing are ways to share love and caring through touch. When you were born, well before you knew language and could understand caring words, you were learning about love through your sense of touch. As a newborn, when everything was frighteningly new, you immediately experienced the comfort of touch when you were held in your mother’s arms, feeling the warmth of her body and the beat of her heart, sensations familiar to you when you were inside her womb. You were held close when you first learned about food and about how good it felt to have milk in your empty stomach.
Your parents’ caring hands kept you clean and dressed in dry clothes when you could not yet do those things for yourself. So, from your earliest days, you learned that someone’s touch usually made you feel comfortable and safe. Loving and caring about special people in our lives is a feeling inside that is hard to describe in words. But hugs and kisses make it easy to show that love—and their message is clear. Giving hugs and kisses feels as good as getting them. (Because the lips have an extra supply of nerve endings, kissing is an especially intense way to touch.) The human need to share affection through touch is something we all experience throughout our lives.
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.
Dams, which are structures that hold back water, have been built since ancient times. They are usually made of earth, rock, brick, or concrete—or a combination of these things. They are constructed to control the flow of water in a river, and they are built for a number of reasons. One reason is to prevent flooding. Heavy rains in high country may cause water levels in a river to rise. As the river flows downhill, it may overflow its banks, flooding communities located downstream. A dam can prevent this by stopping or slowing rushing water, allowing it to be released at a controlled rate. Dams are also frequently used to store water for general use and farming. When a river’s flow is restricted by a dam, water often spreads out behind the dam to form a lake or reservoir in the river valley. That water can then be used as needed, preventing water shortages and crop damage during long periods of dry weather.
A great number of dams today are used to make electricity. Such hydroelectric dams are built very tall, to create a great difference in the height of the water level behind and in front of it. High water behind a dam passes through gates in the dam wall that allow it to fall to the river far below. As the water falls, it flows past huge blades called turbines; the turbines run generators that make electricity. One of the world’s largest and most productive hydroelectric dams is the Hoover Dam, located on the Colorado River between Nevada and Arizona. Built in the 1930s, it is 726 feet (221 meters) high and 1,244 feet (379 meters) long. Its reservoir (Lake Mead)—the world’s largest— supplies water to several states, allowing huge regions of naturally dry terrain in southern California, Arizona, and Mexico to flourish. Many modern dams are used for all three purposes: flood control, water storage, and hydroelectric power.
Water is vital to the survival of everything on the planet and is limited in supply. Earth might seem like it has abundant water, but in fact only 1 percent is available for human use. While the population and the demand on freshwater resources are increasing (each person uses about 12,000 gallons of water every year), supply remains the same. Water is constantly being cleaned and recycled through Earth’s water cycle, yet we still need to conserve it because people use up Earth’s freshwater faster than it can naturally be replenished.
When you use water wisely, you help the environment. You save water for fish and animals, help preserve drinking water supplies, and ease the job of wastewater treatment plants—the less water you send down the drain, the less work these plants have to do to make water clean again. When you use water wisely, you also save the energy that your water supplier uses to treat and move water to you, and the energy your family uses to heat your water. Your family pays for the water you use, so if you use less water, you’ll have more money left to spend on other things.