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<span style="color:purple">Section I. Introduction</span>
1. The General & Cellular Basis of Medical Physiology
In unicellular organisms, all vital processes occur in a single cell. As the evolution of multicellular organisms has progressed, various cell groups have taken
over particular functions. In humans and other vertebrate animals, the specialized cell groups include a gastrointestinal system to digest and absorb food; a
respiratory system to take up O2 and eliminate CO2; a urinary system to remove wastes; a cardiovascular system to distribute food, O2, and the products of
metabolism; a reproductive system to perpetuate the species; and nervous and endocrine systems to coordinate and integrate the functions of the other
systems. This book is concerned with the way these systems function and the way each contributes to the functions of the body as a whole.
This chapter presents general concepts and principles that are basic to the function of all the systems. It also includes a short review of fundamental aspects of
cell physiology. Additional aspects of cellular and molecular biology are considered in the relevant chapters on the various organs.
Organization of the Body
The cells that make up the bodies of all but the simplest multicellular animals, both aquatic and terrestrial, exist in an "internal sea" of extracellular fluid (ECF)
enclosed within the integument of the animal. From this fluid, the cells take up O2 and nutrients; into it, they discharge metabolic waste products. The ECF is
more dilute than present-day seawater, but its composition closely resembles that of the primordial oceans in which, presumably, all life originated.
In animals with a closed vascular system, the ECF is divided into two components: the interstitial fluid and the circulating blood plasma. The plasma and the
cellular elements of the blood, principally red blood cells, fill the vascular system, and together they constitute the total blood volume. The interstitial fluid is
that part of the ECF that is outside the vascular system, bathing the cells. The special fluids lumped together as transcellular fluids are discussed below. About
a third of the total body water (TBW) is extracellular; the remaining two-thirds are intracellular (intracellular fluid).
In the average young adult male, 18% of the body weight is protein and related substances, 7% is mineral, and 15% is fat. The remaining 60% is water. The
distribution of this water is shown in Figure 1-1.
The intracellular component of the body water accounts for about 40% of body weight and the extracellular component for about 20%. Approximately 25% of
the extracellular component is in the vascular system (plasma = 5% of body weight) and 75% outside the blood vessels (interstitial fluid = 15% of body weight).
The total blood volume is about 8% of body weight.
Measurement of Body Fluid Volumes
It is theoretically possible to measure the size of each of the body fluid compartments by injecting substances that will stay