Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

288 CHAPTER 7
Information, too, is transferred by two different mechanisms: by the nervous
system, in which electrical impulses pass along nerves at a rate of meters per second,
and by hormones that are secreted by endocrine (ductless) glands, such as the
thyroid, directly into the blood and are carried by the blood to the specific receptor
organs that respond to these hormones.
Metabolism: Tissue Building and Energy Conversion
Cells, from which all tissues and organs are made, go through life cycles of their
own. They are born from relatively undifferentiated progenitor cells—such as the
basal layer in the skin and the stem cells in the bone marrow—go through a period
of maturation, grow old, die, and are sloughed off. The time for cellular death,
called apoptosis, and the instructions for the synthesis of new tissue is contained
in the information encoded in the DNA molecules within the cells. In addition to
building tissue, the body also synthesizes protein molecules such as enzymes and
hormones that serve specific functions. These undifferentiated cells are much more
sensitive to radiation damage than the mature cells that develop from them. The
raw materials used in this tissue-building process comes from the food and water
we consume. The energy necessary to drive these vital processes comes from the
release of the intramolecular bonding energy that is stored in the chemical bonds
of the food. The processes by which the complex food molecules are disassembled
and then reassembled into cellular material and specialized proteins, and by which
energy that is stored in the food is converted into useful energy are collectively called
metabolism.
The food and drink that we consume—proteins, carbohydrates, fats, and water—
supply the materials for the manufacture of new tissue and for the synthesis of specialized
molecules. In the process called metabolism, the foodstuffs are broken down
into their constituent subunits—amino acids, sugars, fatty acids, and glycerols—and
then these units are reassembled into the cellular constituents needed for building
tissues and organs. Metabolic processes include a number of oxidation–reduction
reactions that result in the transfer of energy stored in the intramolecular bonds
of the foodstuffs into energy-consuming reactions that drive all the vital processes.
Oxygen for these oxidation reactions is brought into the body through the respiratory
system. Useful energy is supplied mainly by the carbohydrates and fats, while
the proteins supply most of the material for the synthesis of new protein. Whereas
the initial steps in the metabolism of the different nutrients differ, eventually all the
subunits are integrated into a common metabolic pool from which new molecules
are synthesized.
All the chemical reactions in the metabolic process can occur only in solution.
To this end, the human body is about 60% water by weight (about 42 L water in the
reference person), of which about two-thirds is contained within the cells of the body
and is therefore called intracellular fluid. The remaining one-third is outside the
cells and is called extracellular fluid. About one-third of the extracellular fluid is in
the blood and the remainder is in the interstices between the cells, thereby providing
the cells with an aquatic environment. These various spaces can be thought of as
compartments. Thus, we have the vascular compartment, which contains the blood;
the interstitial compartment; and the intracellular compartment. Water enters the
system via the GI tract and leaves by way of the kidneys as urine, by way of the GI