The human organism consists of 50–60% water; this per centage is slightly higher in children than in adults and men than in women. Water is distributed in two compartments, intracellular and extracellular, which differ in solute composition. The intracellular compartment contains about two- thirds of the total pool of water (intracellular fluid, ICF). In contrast, the extracellular compartment contains the remaining one-third of water (extracellular fluid, ECF) and can be further distinguished into extravascular or interstitial (connective, bone, serous cavities, and secretions) and intravascular (plasma) (Fig. 1). The intra- and extracellular compartments, although separated by cell membranes and capillary walls, are in dynamic equilibrium. The concentration of solutes in a liquid is termed “osmolality” and is expressed in milliosmoles per kilogram of water (mOsm/kg). Sodium (Na⁺) and the anions Cl⁻ and HCO₃⁻ represent the primary solutes of ECF, whereas potassium (K⁺) and organic phosphorus esters (adenosine triphosphate (ATP), creatine phosphate, and phospholipids) represent the primary solutes of ICF. The solutes in each compartment determine the effective osmolality of the compartment. The cell membrane and capillary wall are highly permeable barriers to water, which pass from one compartment to another to maintain osmotic balance. In particular, the water movement through the capillary wall depends on Starling’s forces (hydrostatic pressure and colloid osmotic pressure), whereas that through the cell membranes depends on the difference in osmolality between ICF and ECF.

Fig1. Distribution of water in the body. (Copyright EDISES 2021. Reproduced with permission)

The composition of extracellular liquids can vary within narrow limits; water and the solutes dissolved play an essential role in cellular metabolic reactions, so excessive variations in extracellular liquids composition may lead to severe cell function alterations. It is, therefore, essential that the body’s hydroelectrolytic content is kept constant according to the law of conservation of matter: everything that enters the body must leave it, if the body does not need it.

Daily, our organism acquires from the outside about 2 liters of water and food that contain NaCl, K⁺, H⁺, Ca2⁺, HCO₃⁻, and phosphate ions. The body has several ways to excrete water and ions, mainly the kidneys; a small amount is also lost through stool, sweating, and respiration. In addition to physiological mechanisms, behavioral mechanisms, such as thirst and appetite for salt, also play an important role.

The maintenance of hydroelectrolytic balance is critical for the homeostasis. Water and Na+ are critical for the volume and osmolality of ECF. Consequently, their alterations can result in cell volume changes, either shriveling or swelling, thus compromising cell function. Potassium (K⁺) is, on the other hand, important for the membrane potential of excitable cells, and, its alterations can, therefore, compromise cardiac, muscular, and nervous functions.

K⁺, which is more concentrated within the cell, tends to leave by passive diffusion. In contrast, Na+ tends, by passive diffusion, to enter the cell; the Na+/K+ATP-dependent pump actively extracts Na+ from the cell and reintroduces K+, thus allowing the maintenance of the different concentration of these solutes between the two compartments. The regulation of hydroelectrolytic balance is highly complex. It involves not only renal and behavioral responses but also the cardiovascular and respiratory systems, which are under the control of the nervous system.

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مواضيع ذات صلة

Volume and Osmolality of Extracellular and Intracellular Fluids in Abnormal States

Regulation Of Fluid Exchange And Osmotic Equilibrium Between Intracellular And Extracellular Fluid

BODY FLUID COMPARTMENTS

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وظيفة الجسم المضاد

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الجسم المضاد Antibody

دور الأنسجة الليمفاوية في تكوين المناعة المكتسبة

المناعة Immunity

الهيبارين Heparin