Electrolyte homeostasis

Fluid and electrolyte homeostasis is maintained by feedback mechanisms, hormones, and many organ systems and is necessary for the body s normal physiologic functions. Disorders of sodium and water, calcium, phosphorus, potassium, and magnesium homeostasis are addressed separately in this chapter. 

Sixty percent of total body water is distributed intracellularly, and 40% is contained in the extracellular space, 

Adding an isotonic solution to the extracellular fluid (ECF) does not change intracellular volume. Adding a hypertonic solution to the ECF decreases cell volume, whereas adding a hypotonic solution increases it (Table 78-1). Hypernatremia and hyponatremia can be associated with conditions of high, low, or normal ECF sodium and volume. Both conditions are most commonly the result of abnormalities of water metabolism. 

5% desctrose in water ECF, extracellular fluid ICF, intracellular fluid. 

Symptoms of hypernatremia are primarily caused by decreased neuronal ceU volume and can include weakness, restlessness, confusion, and coma. 

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Metabolic complications of EN most commonly include disorders of fluid and electrolyte homeostasis and hyperglycemia. More severely ill patients require more frequent monitoring than those who are more stable (see Table 98-9). Both dehydration and fluid overload can occur with tube feeding. Careful monitoring of fluid inputs and outputs as well as body weight is important. Dehydration may be due either to excessive fluid... 

Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) are members of a family of so-called natriuretic peptides, synthesized predominantly in the cardiac atrium, ventricle, and vascular endothelial cells, respectively (G13, Y2). ANP is a 28-amino-acid polypeptide hormone released into the circulation in response to atrial stretch (L3). ANP acts (Fig. 8) on the kidney to increase sodium excretion and glomerular filtration rate (GFR), to antagonize renal vasoconstriction, and to inhibit renin secretion (Ml). In the cardiovascular system, ANP antagonizes vasoconstriction and shifts fluid from the intravascular to the interstitial compartment (G14). In the adrenal cortex, ANP is a powerful inhibitor of aldosterone synthesis (E6, N3). At the hypothalamic level, ANP inhibits vasopressin secretion (S3). It has been shown that some of the effects of ANP are mediated via a newly discovered hormone, called adreno-medullin, controlling fluid and electrolyte homeostasis (S8). The diuretic and blood pressure-lowering effect of ANP may be partially due to adrenomedullin (V5). 

L3. Laragh, J. H., Atrial natriuretic hormone, the renin-aldosterone axis, and blood pressure-electrolyte homeostasis. N. Engl. J. Med. 313, 1330-1340 (1985). 

Schell, D. A., Vari, R. C., and Samson, W. K., Adrenomedullin A newly discovered hormone controlling fluid and electrolyte homeostasis. Trends Endocrinol. Metab. 7,7-13 (1996). 

Li+ also has effects upon the peripheral renin-angiotensin axis which is involved in fluid-electrolyte homeostasis. Renin is released from the kidney into the blood, where it cleaves angiotensin (a Ca2+-mobilizing hormone) from angiotensinogen angiotensin I (AI) is converted to... 

Hypertension is more common and more severe in African Americans than in those of other races. Differences in electrolyte homeostasis, glomerular filtration rate, sodium excretion and transport mechanisms, plasma renin activity, and BP response to plasma volume expansion have been noted. 

Eccrine glands can be found on the entire body surface of humans, except for the lips, external ear canal, clitoris, and labia minora. These glands play an important role in thermoregulation, which is necessary for fluid and electrolyte homeostasis. They secrete a milky or oily odorless liquid which produces the characteristic body smell after metabolism through surface bacteria of the skin. 

Among antihypertensives, those targeting the renin-angiotensin system are among the best studied. The renin-angiotensin system consists of a two-enzyme cascade that is involved in the regulation of blood pressure and electrolyte homeostasis. 

Taken together under physiological conditions the renin angiotensin system maintains volume and electrolyte homeostasis as well as the blood pressure. Under pathological conditions, like heart failure, it increases blood pressure and fluid retention, thereby enhancing pre- and afterload of the heart. 

Ang II exerts important actions at vascular smooth muscle, adrenal cortex, kidney, heart, and brain. Through these actions, the renin-angiotensin system plays a key role in the regulation of fluid and electrolyte balance and arterial blood pressure. Excessive activity of the renin-angiotensin system can result in hypertension and disorders of fluid and electrolyte homeostasis. 

The treatment of HHS centers around aggressive rehydration and restoration of glucose and electrolyte homeostasis the rate of correction of these variables must be monitored closely. Low-dose insulin therapy may be required. 

NSAIDs can interfere with fluid and electrolyte homeostasis, thereby causing edema, hyponatremia, hyperkalemia, and blunting of the natriuretic effects of diuretics (48,49). 

Of all the potentially serious adverse effects of NSAIDs, functional renal insufficiency, which is probably the most common, results from the hemodynamic changes secondary to inhibition of prostaglandin synthesis. Renal insufficiency and interference with fluid and electrolyte homeostasis occur in disease states in which prostaglandins are significant determinants of renal function, such as those characterized by hypovolemic states due to salt... 

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