As the major osmotically active solute in cells, the amount of cellular K.+ is the major determinant of the amount of water in (and, therefore, the volume of) the ICF compartment, in the same way that extracellular Na+ is a major determinant of ECF volume. When cells lose K.+ (and accompanying anions), they also lose water and shrink,- the converse is also true.
The distribution of K+ across plasma membranes—that is, the ratio of intracellular to extracellular K+ concentrations—is the major determinant of the resting membrane potential of cells and, hence, their excitability (see Chapter 3). Disturbances of K+ balance often produce altered excitability of nerves and muscles. Low plasma [K+] leads to membrane hyperpolarization and reduced excitability, muscle weakness is a common symptom. Excessive plasma K+ levels lead to membrane depolarization and increased excitability. High plasma K+ levels cause cardiac arrhythmias and, eventually, ventricular fibrillation, usually a lethal event.
K+ balance is linked to acid-base balance in complex ways (see Chapter 25). K+ depletion, for example, can lead to metabolic alkalosis, and K+ excess to metabolic acidosis. A primary disturbance in acid-base balance can also lead to abnormal K+ balance.
K+ affects the activity of enzymes involved in carbohydrate metabolism and electron transport. K+ is needed for tissue growth and repair. Tissue breakdown or increased protein catabolism result in a loss of K+ from cells.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.