The Epic Interactive Encyclopedia 1998/Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso/H/Human_body/INFOTEXT

Text File | 1992-09-04 | 4KB | 85 lines

The human body is made up of about two
thousand million cells. There are about two
hundred basic types of cell. Where two or
more types of cell are found together, this
is called a tissue. An example is brain
tissue, which has both brain cells and
supporting cells. Organs are formed of more
than two types of tissue found together. An
example is the liver, which has liver tissue,
connective tissue, biliary tissue and
vascular tissue in it. Two or more organs are
grouped together to form systems, such as the
cardiovascular system, which is composed of
the heart, the arteries and the veins. The
main systems of the body are (1) the nervous
system (the brain, the spinal cord and the
nerves), (2) the cardiovascular system (the
heart, the arteries, the veins and the
blood), (3) the respiratory system (the lungs
and airways), (4) the digestive system (the
mouth, the oesophagus or gullet, the stomach,
the pancreas, the liver, the gall-bladder,
the small intestine and the large intestine),
(5) the urinary system (the kidneys and
bladder), (6) the muscular system (all of the
muscles), (7) the skeletal system (all of the
bones and bone marrow), (8) the integumentary
system (the skin, nails and hair), (9) the
reproductive system (in males the testicles,
and penis; in women the ovaries, Fallopian
tubes, uterus, cervix and vagina), (10) the
immune system (the white blood cells,
tonsils, spleen and lymph nodes), (11) the
endocrine system (the pancreas, the pituitary
gland, the adrenal glands, and so on). All of
these systems work together to produce a
healthy person. Some systems can afford to
lose a single organ, for example, a person
can survive comfortably on one kidney, or
even one lung. Loss of a complete system
almost always means that the person cannot
survive. Organs are made up of millions of
cells. Cells are very small, usually only a
few microns wide, although they vary greatly
in shape. Muscle cells, for example, are long
and thin, and may be several centimetres
long. Nerve cells may also be very long. Each
cell, regardless of where it is from or what
job it performs, has some basic components
which are common to all cells. The nucleus of
the cell is the black spot seen in cells
under the microscope. The nucleus is where
the genetic material DNA is stored, and it is
the nucleus which tells the cell which type
of cell to be. All types of cell in our
bodies have exactly the same DNA, but all are
using it slightly differently: it is as if
they all had the same copy of a book, but
were reading it at different pages. Some
cells, such as the red blood cell, have no
nucleus at all. Other parts of the cell are
too small to be seen in the light microscope.
These include the mitochondria, which are
small components which produce energy for the
cell; the endoplasmic reticulum, which
produces complex proteins which the cell
needs to make; and lysosomes, which break
down anything which the cell absorbs. All
cells are surrounded by a very delicate
membrane, which regulates very closely the
nutrients passing into and out of the cell.
The membrane also carries some proteins which
signal the body that the cell is part of it.
Any cell not carrying these proteins or
carrying the wrong ones will be attacked and
destroyed by the white blood cells. Nearly
all types of cell are able to divide and
produce two new smaller cells. The two small
cells soon grow to the size of the adult
cell, and may divide in their turn. In this
way, old cells which die or are damaged are
replaced by fresh healthy ones.

Subject by: Dr Aidan M. O'Donnell