This chapter describes water as the principal constituent of the human body and is essential for life. In healthy individuals, the volume and osmolality of the tissue fluids are maintained within closely defined limits. This process is mainly concerned with the mechanisms that regulate the quantity of water that is present in the body and maintain its distribution between the different body compartments. The chapter cites some common disorders of electrolyte balance and the detailed mechanisms by which the osmolality of the body fluids is regulated. Broadly speaking, body water is distributed between the intracellular fluid or ICF and the extracellular fluid or ECF.
Chapter
Fluid and electrolyte balance
Chapter
The transport functions of the plasma membrane
This chapter looks at the plasma membrane, wherein the region outside the cell or the extracellular compartment is separated from the inside of the cell or intracellular compartment. It shows how the physical separation allows each cell to regulate its internal composition independently of other cells. Chemical analysis has shown that the composition of the intracellular fluid is very different to that of the extracellular fluid, noting that it is rich in potassium ions but relatively poor in both sodium ions and chloride ions. The chapter highlights the mechanisms responsible for establishing and maintaining the difference in ionic composition between the intracellular and the extracellular compartments. The chapter outlines the ways in which cells utilize ionic gradients to perform their essential physiological roles and the mechanisms by which proteins and other large molecules cross the cell membrane.
Chapter
Body fluid regulation: principles and processes
This chapter evaluates body fluid regulation of animals. The bulk of the body mass of animals is water, distributed between intracellular fluid (ICF) inside the cells and extracellular fluid (ECF) outside the cells. In aquatic animals, the osmotic concentration of fluids on either side of their permeable external epithelia influences the rate of water flow by osmosis. Body fluid regulation depends on solute and water transport across epithelial cells via paracellular pathways between the cells passing through ‘tight’ junctions at the apical border and a transcellular route across apical and basolateral membranes, passing through the cell cytoplasm. Cell volume regulation is also required for normal cell function in animals. Cell volume regulatory responses result from changes in the cell content of inorganic osmolytes and/or organic osmolytes that are typically compatible solutes, i.e. do not interfere with the functioning of enzymes and other cellular proteins.
