Understanding the functions of the organ systems requires profound knowledge of basic cellular mechanisms. Although each organ system differs in its overall function, all are undergirded by a common set of physiologic principles.
The following basic principles of physiology are introduced in this chapter: body fluids, with particular emphasis on the differences in composition of intracellular fluid and extracellular fluid; creation of these concentration differences by transport processes in cell membranes; the origin of the electrical potential difference across cell membranes, particularly in excitable cells such as nerve and muscle; generation of action potentials and their propagation in excitable cells; transmission of information between cells across synapses and the role of neurotransmitters; and the mechanisms that couple the action potentials to contraction in muscle cells.
Volume and Composition of Body Fluids, 1
Characteristics of Cell Membranes, 4
Transport Across Cell Membranes, 5
Diffusion Potentials and Equilibrium Potentials, 14
Resting Membrane Potential, 18 Action Potentials, 19
Synaptic and Neuromuscular Transmission, 26
Skeletal Muscle, 34
Smooth Muscle, 40
Challenge Yourself, 44
These principles of cellular physiology constitute a set of recurring and interlocking themes. Once these principles are understood, they can be applied and integrated into the function of each organ system.
VOLUME AND COMPOSITION OF BODY FLUIDS Distribution of Water in the Body Fluid Compartments
In the human body, water constitutes a high proportion of body weight. The total amount of fluid or water is called total body water, which accounts for 50% to 70% of body weight. For example, a 70-kilogram (kg) man whose total body water is 65 % of his body weight has 45.5 kg or 45.5 liters (L) of water (1 kg water « 1 L water). In general, total body water correlates inversely with body fat. Thus total body water is a higher percentage of body weight when body fat is low and a lower percentage when body fat is high. Because females have a higher percentage of adipose tissue than males, they tend to have less body water. The distribution of water among body fluid compartments is described briefly in this chapter and in greater detail in Chapter 6.
Total body water is distributed between two major body fluid compartments: intracellular fluid (ICF) and extracellular fluid (ECF) (Fig. 1.1). The ICF is contained within the cells and is two-thirds of total body water; the ECF is outside the cells and is one-third of total body water. ICF and ECF are separated by the cell membranes.
ECF is further divided into two compartments: plasma and interstitial fluid. Plasma is the fluid circulating in the blood vessels and is the smaller of the two ECF