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Fluid and Electrolyte PART Disorders VI These forces favor movement into the interstitial space at the arterial Chapter 68 ends of the capillaries. The decreased hydrostatic forces and increased oncotic forces, which result from the dilutional increase in albumin concentration, cause movement of fluid into the venous ends of the Electrolyte and Acid-Base capillaries. Overall, there is usually a net movement of fluid out of the intravascular space to the interstitial space, but this fluid is returned to Disorders the circulation via the lymphatics. An imbalance in these forces may cause expansion of the interstitial volume at the expense of the intravascular volume. In children with hypoalbuminemia, the decreased oncotic pressure of the intravascular 68.1 Composition of Body Fluids fluid contributes to the development ofedema . Loss of fluid from the Larry A. Greenbaum intravascular space may compromise the intravascular volume, placing the child at risk for inadequate blood flow to vital organs. This is especially TOTAL BODY WATER likely in diseases in which capillary leak occurs because the loss of Total body water (TBW) as a percentage of body weight varies with albumin from the intravascular space is associated with an increase in the age (Fig. 68.1). The fetus has very high TBW, which gradually decreases albumin concentration in the interstitial space, further compromising the to approximately 75% of birthweight for a term infant. Premature infants oncotic forces that normally maintain intravascular volume. In contrast, have higher TBW than term infants. During the 1st yr of life, TBW with heart failure, there is an increase in venous hydrostatic pressure decreases to approximately 60% of body weight and remains at this from expansion of the intravascular volume, which is caused by impaired level until puberty. At puberty, the fat content of females increases more pumping by the heart, and the increase in venous pressure causes fluid than that in males, who acquire more muscle mass than females. Because to move from the intravascular space to the interstitial space. Expansion fat has very low water content and muscle has high water content, by of the intravascular volume and increased intravascular pressure also the end of puberty, TBW in males remains at 60%, but TBW in females cause the edema that occurs with acute glomerulonephritis. decreases to approximately 50% of body weight. The high fat content in overweight children causes a decrease in TBW as a percentage of ELECTROLYTE COMPOSITION body weight. During dehydration, TBW decreases and thus is a smaller The composition of the solutes in the ICF and ECF are very different percentage of body weight. (Fig. 68.3). Sodium (Na+) and chloride (Cl−) are the dominant cation and anion, respectively, in the ECF. The sodium and chloride concentra- FLUID COMPARTMENTS tions ([Na+], [Cl−]) in the ICF are much lower. Potassium (K+) is the TBW is divided between 2 main compartments: intracellular fluid most abundant cation in the ICF, and its concentration ([K+]) within (ICF) and extracellular fluid (ECF). In the fetus and newborn, the the cells is approximately 30 times higher than in the ECF. Proteins, ECF volume is larger than the ICF volume (Fig. 68.1). The normal organic anions, and phosphate are the most plentiful anions in the ICF. postnatal diuresis causes an immediate decrease in the ECF volume. The dissimilarity between the anions in the ICF and the ECF is largely This is followed by continued expansion of the ICF volume, which determined by the presence of intracellular molecules that do not cross results from cellular growth. By 1 yr of age, the ratio of ICF volume to the cell membrane, the barrier separating the ECF and the ICF. In ECF volume approaches adult levels. The ECF volume is approximately contrast, the difference in the distribution of cations—Na+ and K+—relies 20–25% of body weight, and the ICF volume is approximately 30–40% on activity of the Na+,K+-adenosine triphosphatase (ATPase) pump and of body weight, close to twice the ECF volume (Fig. 68.2). With puberty, membrane ion channels. the increased muscle mass of males causes them to have a higher ICF The difference in the electrolyte compositions of the ECF and the volume than females. There is no significant difference in the ECF ICF has important ramifications in the evaluation and treatment of volume between postpubertal females and males. electrolyte disorders. Serum concentrations of electrolytes—[Na+], [K+], The ECF is further divided into the plasma water and the interstitial and [Cl−]—do not always reflect total body content. Intracellular [K+] fluid (see Fig. 68.2). The plasma water is 5% of body weight. The blood is much higher than the serum concentration. A shift of K+ from the volume, given a hematocrit of 40%, is usually 8% of body weight, although intracellular space (ICS) can maintain a normal or even an elevated it is higher in newborns and young infants; in premature newborns it serum [K+] despite massive losses of K+ from the ICS. This effect is seen is approximately 10% of body weight. The volume of plasma water can in diabetic ketoacidosis, in which significant K+ depletion is masked be altered by pathologic conditions, including dehydration, anemia, by transmembrane shift of K+ from the ICF to the ECF. Therefore, for polycythemia, heart failure, abnormal plasma osmolality, and hypoal- K+ and phosphorus, electrolytes with a high intracellular concentration, buminemia. The interstitial fluid, normally 15% of body weight, can serum level may not reflect total body content. Similarly, the serum increase dramatically in diseases associated with edema, such as heart calcium concentration ([Ca2+]) does not reflect total body content of failure, protein-losing enteropathy, liver failure, nephrotic syndrome, Ca2+, which is largely contained in bone and teeth (see Chapter 64). and sepsis. An increase in interstitial fluid also occurs in patients with ascites or pleural effusions. OSMOLALITY There is a delicate equilibrium between the intravascular fluid and The ICF and the ECF are in osmotic equilibrium because the cell the interstitial fluid. The balance between hydrostatic and oncotic forces membrane is permeable to water. If the osmolality in 1 compartment regulates the intravascular volume, which is critical for proper tissue changes, then water movement leads to a rapid equalization of osmolality, perfusion. The intravascular fluid has a higher concentration of albumin with a shift of water between the ICS and extracellular space (ECS). than the interstitial fluid, and the consequent oncotic force draws water Clinically, the primary process is usually a change in the osmolality of into the intravascular space. The maintenance of this gradient depends the ECF, with resultant shift of water into the ICF if ECF osmolality on the limited permeability of albumin across the capillaries. The decreases, or vice versa if ECF osmolality increases. The ECF osmolality hydrostatic pressure of the intravascular space, which is caused by the can be determined and usually equals ICF osmolality. Plasma osmolality, pumping action of the heart, drives fluid out of the intravascular space. normally 285-295 mOsm/kg, is measured by the degree of freezing-point 389 Downloaded for sumitr sutra ([email protected]) at Khon Kaen University Faculty of Medicine- library from ClinicalKey.com by Elsevier on March 11, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved. Chapter 68 ◆ Electrolyte and Acid-Base Disorders 389.e1 Keywords total body water fluid compartments edema intracellular fluid extracellular fluid osmolality hyperglycemia pseudohyponatremia point-of-care testing Downloaded for sumitr sutra ([email protected]) at Khon Kaen University Faculty of Medicine- library from ClinicalKey.com by Elsevier on March 11, 2020. For personal use only. No other uses without permission. Copyright ©2020. Elsevier Inc. All rights reserved. 390 Part VI ◆ Fluid and Electrolyte Disorders 100 PLASMA INTRACELLULAR 90 Cations Anions Cations Anions 80 Total body 70 water (TBW) eight 60 Naϩ (140) ClϪ (104) Kϩ (140) PhosϪ (107) 50 Intracellular fluid (ICF) 40 % Body W 30 20 Extracellular fluid (ECF) Ϫ 10 HCO3 (24) Ϫ 0 Prot (40) 2 468 6312 6912 15 Adult Birth ProtϪ (14) Months Years ϩ K (4) ϩ Na (13) Ϫ HCO3 (10) Age Caϩ (2.5) Other (6) Mgϩ (1.1) PhosϪ (2) Mgϩ (7) ClϪ (3) Fig. 68.1 Total body water, intracellular fluid, and extracellular fluid as a percentage of body weight as a function of age. (From Winters Fig. 68.3 Concentrations of the major cations and anions in the RW: Water and electrolyte regulation. In Winters RW, editor: The body intracellular space and the plasma, expressed in mEq/L. fluids in pediatrics, Boston, 1973, Little, Brown.) Urea is not only confined to the ECS because it readily crosses the cell membrane, and its intracellular concentration approximately equals its extracellular concentration. Whereas an elevated [Na+] causes a shift of water from the ICS, with uremia there is no osmolar gradient between the two compartments and consequently no movement of water. The only exception is during hemodialysis, when the decrease in extracellular urea is so rapid that intracellular urea does not have time to equilibrate. Intracellular This disequilibrium syndrome may result in shift of water into brain (30-40%) cells and leads to severe symptoms. Ethanol, because it freely crosses cell membranes, is another ineffective osmole. In the case of glucose, the effective osmolality can be calculated as follows: Effective osmolality =×21[]Na +[]glucose 8 Theeffective osmolality (also called the tonicity) determines the osmotic force that is mediating the shift of water between the ECF and the ICF. Hyperglycemia causes an increase in the plasma osmolality because it is not in equilibrium with the ICS. During hyperglycemia, there is shift of water from the ICS to the ECS.
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