Cells
The body can be studied from its
simplest to its most complex level, beginning with the cell, the basic unit
of living organisms Cells carry out metabolism, the sum of all of the physical and chemical activities
that occur in the body. Providing the
energy for metabolic reactions is the chemical ATP (adenosine
triphosphate), commonly described as
the energy compound of the cell. The main categories of organic compounds
in the body are:
• Proteins, which include the enzymes, some hormones, and
structural materials.
• Carbohydrates, which include sugars and starches. The main
carbohydrate is the sugar glucose, which
circulates in the blood to provide
energy for the cells.
• Lipids, which include fats. Some hormones are derived from
lipids, and adipose (fat) tissue is designed
to
store lipids.
Within the cytoplasm that fills the cell are subunits called organelles,
each with a specific function.
All body functions derive from the
activities of billions of specialized cells. The nucleus is the control
region of the cell. It contains the chromosomes, which carry genetic information
Each human cell, except for the sex
cells, contains 46 chromosomes. The chromosomes are composed of a complex
organic substance, DNA (deoxyribonucleic acid), which is organized into separate
units called genes.
Genes control the formation of enzymes, the catalysts needed for metabolic reactions. To help
manufacture
enzymes, the cells use a compound
called RNA (ribonucleic
acid), which is
chemically related
to DNA.
When a body cell divides, by the
process of mitosis, the chromosomes are doubled and then
equally distributed
to the two daughter cells Sex cells
(egg and sperm) divide by another process (meiosis)
that halves the chromosomes in
preparation for fertilization.
Tissues
Cells are
organized into four basic types of tissues that
perform specific functions • Epithelial (ep-i-THE -le-al) tissue covers and protects body
structures and lines organs, vessels, and
cavities.
• Connective
tissue supports and binds body structures. It contains fibers and other
nonliving material
between the
cells. Included are adipose (fat) tissue, cartilage, bone and blood
• Muscle tissue
(root my/o) contracts to produce movement. There
are three types of muscle tissue:
• Skeletal or
voluntary muscle moves the skeleton. Skeletal muscle is discussed in greater
detail in
Chapter 20.
• Cardiac muscle
forms the heart. It functions without conscious control and is described as
involuntary.
• Smooth, or
visceral, muscle forms the walls of the abdominal organs; it is also
involuntary.
• Nervous tissue
(root neur/o) makes up the brain, spinal cord, and
nerves. It coordinates and controls
body responses by
the transmission of electrical impulses
The simplest
tissues are membranes. Mucous membranes secrete mucus, a thick fluid that lubricates surfaces
and protects
underlying tissue. Serous membranes, which secrete a thin, watery fluid, line
body cavities
and cover organs.
Tissues are
arranged into organs, which serve specific functions. The organs, in turn, are
grouped
into systems.
Each of the body systems is discussed in Part 3. Bear in mind, however, that
the body functions
as a whole—no
system is independent of the others. They work together to maintain the body’s
state of internal
stability, termed homeostasis.
Biopsy is the
removal and examination of living tissue to determine a diagnosis. The
term is also
applied to the
specimen itself. Biopsy comes from the Greek word bios, meaning "life,” plus
opsis,
meaning "vision.”
Together they mean the visualization of living tissue. Some other terms that
apply to cells and tissues come from Latin. In vivo means "in the living body,” as contrasted with in vitro,which literally means "in glass” and refers to procedures
and experiments done in the laboratory, as compared with studies done in living
organisms. In situ means "in its original place,” and is used to refer to
tumors that have not spread.
In
toto means "whole” or "completely,”
as referring to a structure or organ removed totally from the body. Postmortem literally means "after death,” as in
referring to an autopsy performed to determine the cause of death.
