Scheckel - Ask a science teacher, how everyday stuff really works : why dont we feel the Earth spin? How do airplanes stay in the air? What makes blood red? and 247 other things youve always wondered about

This book is dedicated to my wife, Ann, for her suggestions, constant encouragement, joy, and inspiration.

And to my deceased parents, Alvin and Martha Scheckel, who made me what I am today.

There is no real consensus on the number of cells in the human body. Estimates put the number between ten trillion and one hundred trillion. A trillion is a million millionits a word that crops up when we talk about the size of our national debt! The number of cells depends on the size of the person: bigger person, more cells. Also, the number of cells in our body keeps changing as old cells die and new ones form.

Cells are so small that most can only be seen through a microscope. Every cell is made from an already existing cell. Each cell in the body behaves like a little factory and has two major components, the cytoplasm and the nucleus. The cytoplasm contains the structures that consume and transform energy and perform many of the cells specialized functions, including storing and transporting cellular materials, breaking down waste, and producing and processing proteins. The nucleus is the control center and contains the genetic information that allows cells to reproduce. The mitochondrion (plural mitochondria) in the cell is the factory where food and oxygen combine to make energy. Human cells and other animal cells have a membrane that holds the contents together. This membrane is thin, allowing nutrients to pass in and waste products to pass out. Food is the energy the cell needs. Each cell needs oxygen to burn (metabolize) the nutrients released from food.

The body has some cells that do not experience cell division. And red blood cells and outer skin cells have cytoplasm but do not have a nucleus.

In the cell, the process is called respiration. Oxygen breaks down the food into small pieces. The oxidizing of the food molecules is turned into carbon dioxide and water. Water makes up about two-thirds of the weight of the cell. The energy released is used for all the activities of the cell. The cell membrane has receptors that allow the cell to identify surrounding cells. Different kinds of cells release different chemicals, each of which causes certain other types of nearby cells to react in certain ways. Within each of these different cells are found twenty different types of organelles, or structures.

Slightly over two hundred different kinds of cells make up the human body. The shape and size of each type of cell is determined by its function. Muscle cells come in many different forms and have many different functions. Blood cells are unattached and move freely through the bloodstream. Skin cells divide and reproduce quickly. Some cells in the pancreas produce insulin, others produce pancreatic juice for digestion. Mucus is produced in cells in the lining of the lung. Our lungs also contain alveolar cells that are responsible for taking in gas from the blood. The cells that line the intestine have extended cell membranes to increase the surface area, helping them absorb more food. Cells in the heart have a large number of mitochondria to help them process a lot of energy, because they have to work very hard.

Nerve cells generate and conduct electrical impulses; for the most part, they do not divide. Each nerve cell has a specific place in our nervous system. Nerve cells outside of the brain are very long and have the task of passing signals between the brain and the rest of the body, allowing us to move our muscles and sense the world around us. The rest of our nerve cellsabout one hundred billion of our bodys cellsare brain cells.

Brain cells are the most important cells in our bodies. It is our brain that defines who we are. Brain cells in children under five do have the ability to reproduce, to some extent. However, we are naturally losing brain cells all the time. The best estimate of normal brain cell loss is put at nine thousand per day. That may seem like a large number, but remember that the brain has 100 billion cells, so a nine-thousand-cell loss per day is not that great. Inhalants, such as glue, gasoline, and paint thinner, cause brain cell loss at thirty times the normal rate. Excessive alcohol use is a big contributor to brain cell damage.

Cells that all do the same job make up tissue, such as bone, skin, or muscle. Groups of different types of cells make up the organs of the body. Different organs grouped together form a system, such as the digestive system or the circulatory system. All the systems working together make up a healthy human body.

Cells live, of course, but cells also die. Liver cells last about a year and half. Red blood cells live for 120 days. Skin cells are good for 30 days. White blood cells survive for thirteen days. And it turns out that the great majority of cells in the human body are bacterial cells, and most are beneficial. It is hard to believe that the average adult loses close to 100 million cells every minute. The good news is that the body, through cell division, is replacing those lost 100 million cells every minute. And in any case, even 100 million cells is only a small fraction of the trillions of cells that make up our bodies.

Babies get sick more often than older children or adults because their immune systems are not fully developed and functioning at full capacity. The common cold, which is an infection of the respiratory system caused by a virus, is the most frequent malady. Doctors say that normal, healthy babies get up to about seven colds before they reach their first birthday. Another common affliction is the flu, caused by a different family of viruses, which bring on high fever, chills, fatigue, and sometimes digestive symptoms like vomiting and diarrhea, in addition to the respiratory symptoms of a cold.

Another reason babies get sick so often is that they are frequently around other children, often siblings, and this exposes them to viruses and bacteria in school and daycare. Children in schools and daycare get more colds, runny noses, and ear infections than children cared for at home. However, their earlier exposure to these diseases also leads them to develop immunity earlier.

Babies are also curious about the wide, wonderful world they are born into. So they will stick anything and everything into their mouth as a means of exploring that world. You can imagine the enormous amount of germs that ride along.

Furthermore, babies have not developed the immunity to the many different viruses that cause colds, because they havent had time to acquire the antibodies to fight off viruses. Babies do have some of their mothers antibodies when they are born, which were transmitted through the placenta during pregnancy. This kind of immunity isnt permanent, but breastfeeding can extend it, because many of the mothers antibodies are present in her milk. This is why breast-fed babies tend to have fewer colds and flu symptoms than bottle-fed babies. Babies, like other people, also develop their own antibodies in response to germs they are exposed to; in fact, its a mistake to try to eliminate all pathogens from a babys environment.

Winter is the toughest time for babies, because it is the season when colds spread nationwide. Also, in winter people spend more time indoors, where viruses are more likely to spread from one person to another. The less humid air of indoor heating dries nasal passages, which allows viruses to thrive.