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
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.
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
• 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
• 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.