EVALUATION OF THE EXTRACELLULAR VOLUME
Life most likely began in or very near an ocean where the predominant cation was sodium (Na+). That is, life is supported by a solution high in Na+. Life itself developed from single cells where potassium (K+) would become the predominant cation of the cytoplasm. From that ancient time to present day animal cells, the intracellular fluid (ie, the ICF, the first space of life) remains high in K+ while the extracellular fluid (ie, the ECF, the second space of life support) remains high in Na+.
Initially, the cyclical ebb and flow of the ocean provided nutrients via diffusion, but as organisms became larger and more complex, another system was needed to deliver nutrients and excrete wastes. Ultimately, this became the cardiovascular system consisting of the heart—a pump to provide the driving force (ie, blood pressure) for transport—and the blood vessels (permeabilities) through which the nutrients flow. This system also features circulation into an excretory system (ie, kidney), where key regulation of ECF volume (ie, sodium balance) occurs.
Life on land results in a constant loss of water and salt. Therefore, the fundamental challenge terrestrial animals (including humans) face is the ability to ingest and conserve adequate water and salt to maintain body tonicity and ECF volume, respectively. Disorders of body tonicity—discussed in Chapter 3 are disturbances in water balance that relate to sodium concentration. That is, hypernatremia usually occurs in states of net free water loss (ie, dehydration) and hyponatremia occurs in states of relative free water excess.
In contradistinction, disorders of ECF volume—the subject of this chapter—are disorders of sodium balance and relate to total body sodium content. A person is said to be euvolemic when the volume of isotonic fluid in their ECF, as judged from the history and physical examination, is optimal. That is, the ECF is large enough to ensure adequate arterial perfusion of the tissues but without evidence of interstitial fluid volume expansion (edema) or depletion. Hypovolemia exists when a net loss of sodium (eg, via skin, gut, or kidney) leads to ECF volume depletion. On the other hand, hypervolemia exists when a net gain of sodium leads to ECF volume overload. Hypervolemia is usually a result of organ dysfunction (eg, congestive heart failure, liver cirrhosis, kidney failure) but may also occur with aggressive volume resuscitation as seen during the treatment of early sepsis.
The distribution of body fluid volumes in health are given in Table 2–1. With reference to the ECF volume, a steady state will exist whenever the Na+ excretion rate (output) equals the Na+ ingestion rate (input). In normal individuals, this steady state occurs when their ECF volume is judged to be euvolemic.