How many cells are in your body?

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 million—it’s 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 cell’s 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 cells—about one hundred billion of
our body’s cells—are 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.

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