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The Regulation of Fluid and Electrolyte Balance 2424 George A CHAPTER The Regulation of Fluid and Electrolyte Balance 2424 George A. Tanner, Ph.D. CHAPTER OUTLINE ■ FLUID COMPARTMENTS OF THE BODY ■ CALCIUM BALANCE ■ WATER BALANCE ■ MAGNESIUM BALANCE ■ SODIUM BALANCE ■ PHOSPHATE BALANCE ■ POTASSIUM BALANCE ■ URINARY TRACT KEY CONCEPTS 1. Total body water is distributed in two major compart- filtration rate, angiotensin II and aldosterone, intrarenal ments: intracellular water and extracellular water. In an av- physical forces, natriuretic hormones and factors such as erage young adult man, total body water, intracellular wa- atrial natriuretic peptide, and renal sympathetic nerves. ter, and extracellular water amount to 60%, 40%, and 20% Changes in these factors may account for altered Naϩ ex- of body weight, respectively. The corresponding figures for cretion in response to excess Naϩ or Naϩ depletion. Estro- an average young adult woman are 50%, 30%, and 20% of gens, glucocorticoids, osmotic diuretics, poorly reabsorbed body weight. anions in the urine, and diuretic drugs also affect renal Naϩ 2. The volumes of body fluid compartments are determined excretion. by using the indicator dilution method and this equation is: 9. The effective arterial blood volume (EABV) depends on the Volume ϭ Amount of indicatorϵConcentration of indicator degree of filling of the arterial system and determines the at equilibrium. perfusion of the body’s tissues. A decrease in EABV leads 3. Electrical neutrality is present in solutions of electrolytes; to Naϩ retention by the kidneys and contributes to the de- that is, the sum of the cations is equal to the sum of the an- velopment of generalized edema in pathophysiological ions (both expressed in milliequivalents). conditions, such as congestive heart failure. 4. Sodium (Naϩ) is the major osmotically active solute in ex- 10. The kidneys play a major role in the control of Kϩ balance. tracellular fluid (ECF), and potassium (Kϩ) has the same Kϩ is reabsorbed by the proximal convoluted tubule and role in the intracellular fluid (ICF) compartment. Cells are the loop of Henle and is secreted by cortical collecting duct typically in osmotic equilibrium with their external environ- principal cells. Inadequate renal Kϩ excretion produces hy- ment. The amount of water in (and, hence, the volume of) perkalemia and excessive Kϩ excretion produces hy- cells depends on the amount of Kϩ they contain and, simi- pokalemia. larly, the amount of water in (and, hence, the volume of) 11. Calcium balance is regulated on both input and output the ECF is determined by its Naϩ content. sides. The absorption of Ca2ϩ from the small intestine is 5. Plasma osmolality is closely regulated by arginine vaso- controlled by 1,25(OH)2 vitamin D3, and the excretion of pressin (AVP), which governs renal excretion of water, and Ca2ϩ by the kidneys is controlled by parathyroid hormone by habit and thirst, which govern water intake. (PTH). 6. AVP is synthesized in the hypothalamus, released from the 12. Magnesium in the body is mostly in bone, but it is also an posterior pituitary gland, and acts on the collecting ducts important intracellular ion. The kidneys regulate the of the kidney to increase their water permeability. The ma- plasma [Mg2ϩ]. jor stimuli for the release of AVP are an increase in effec- 13. Filtered phosphate usually exceeds the maximal reabsorp- tive plasma osmolality (detected by osmoreceptors in the tive capacity of the kidney tubules for phosphate (TmPO4), anterior hypothalamus) and a decrease in blood volume and about 5 to 20% of filtered phosphate is usually ex- (detected by stretch receptors in the left atrium, carotid si- creted. Phosphate reabsorption occurs mainly in the proxi- nuses, and aortic arch). mal tubules and is inhibited by PTH. Phosphate is an im- 7. The kidneys are the primary site of control of Naϩ excre- portant pH buffer in the urine. Hyperphosphatemia is a tion. Only a small percentage (usually about 1%) of the fil- significant problem in chronic renal failure. tered Naϩ is excreted in the urine, but this amount is of 14. The urinary bladder stores urine until it can be conve- critical importance in overall Naϩ balance. niently emptied. Micturition is a complex act involving 8. Multiple factors affect Naϩ excretion, including glomerular both autonomic and somatic nerves. 403 404 PART VI RENAL PHYSIOLOGY AND BODY FLUIDS major function of the kidneys is to regulate the volume, Interstitial fluid composition, and osmolality of the body fluids. The and lymph water A (15% body fluid surrounding our body cells (the ECF) is constantly re- Intracellular water weight; 10.5 L) Plasma water newed and replenished by the circulating blood plasma. (40% body weight; 28 L) (5% body The kidneys constantly process the plasma; they filter, re- weight; 3.5 L) absorb, and secrete substances and, in health, maintain the internal environment within narrow limits. In this chapter, we begin with a discussion of the fluid compartments of the body—their location, magnitude, and composition. Then we consider water, sodium, potassium, calcium, magne- sium, and phosphate balance, with special emphasis on the role of the kidneys in maintaining our fluid and electrolyte balance. Finally, we consider the role of the ureters, urinary bladder, and urethra in the transport, storage, and elimina- tion of urine. Extracellular water (20% body weight; 14 L) FLUID COMPARTMENTS OF THE BODY Total body water (60% body weight; 42 L) Water is the major constituent of all body fluid compart- FIGURE 24.1 Water distribution in the body. This dia- ments. Total body water averages about 60% of body gram is for an average young adult man weigh- weight in young adult men and about 50% of body weight ing 70 kg. In an average young adult woman, total body water is in young adult women (Table 24.1). The percentage of 50% of body weight, intracellular water is 30% of body weight, body weight water occupies depends on the amount of adi- and extracellular water is 20% of body weight. pose tissue (fat) a person has. A lean person has a high per- centage and an obese individual a low percentage of body weight that is water because adipose tissue contains a low percentage of water (about 10%), whereas most other tis- ter is in the ICF, and one third is in the ECF (Fig. 24.1). sues have a much higher percentage of water. For example, These two fluids differ strikingly in terms of their electrolyte muscle is about 75% water. Newborns have a low percent- composition. However, their total solute concentrations age of body weight as water because of a relatively large (osmolalities) are normally equal, because of the high water ECF volume and little fat (see Table 24.1). Adult women permeability of most cell membranes, so that an osmotic dif- have relatively less water than men because, on average, ference between cells and ECF rapidly disappears. they have more subcutaneous fat and less muscle mass. As The ECF can be further subdivided into two major sub- people age, they tend to lose muscle and add adipose tissue; compartments, which are separated from each other by the hence, water content declines with age. endothelium of blood vessels. The blood plasma is the ECF found within the vascular system; it is the fluid portion of the blood in which blood cells and platelets are suspended. Body Water Is Distributed in The blood plasma water comprises about one fourth of the Several Fluid Compartments ECF or about 3.5 L for an average 70-kg man (see Fig. 24.1). The interstitial fluid and lymph are considered together be- Total body water can be divided into two compartments or cause they cannot be easily separated. The water of the in- spaces: intracellular fluid (ICF) and extracellular fluid terstitial fluid and lymph comprises three fourths of the (ECF). The ICF is comprised of the fluid within the trillions ECF. The interstitial fluid directly bathes most body cells, of cells in our body. The ECF is comprised of fluid outside and the lymph is the fluid within lymphatic vessels. The of the cells. In a young adult man, two thirds of the body wa- blood plasma, interstitial fluid, and lymph are nearly iden- tical in composition, except for the higher protein concen- tration in the plasma. An additional ECF compartment (not shown in Fig. 24.1), Average Total Body Water as a Percent- TABLE 24.1 the transcellular fluid, is small but physiologically important. age of Body Weight Transcellular fluid amounts to about 1 to 3% of body weight. Age Men Both Sexes Women Transcellular fluids include cerebrospinal fluid, aqueous hu- mor of the eye, secretions of the digestive tract and associated 0–1 month 76 organs (saliva, bile, pancreatic juice), renal tubular fluid and 1–12 months 65 bladder urine, synovial fluid, and sweat. In these cases, the 1–10 years 62 10–16 years 59 57 fluid is separated from the blood plasma by an epithelial cell 17–39 years 61 50 layer in addition to a capillary endothelium. The epithelial 40–59 years 55 52 layer modifies the electrolyte composition of the fluid, so that 60 years and older 52 46 transcellular fluids are not plasma ultrafiltrates (as is intersti- tial fluid and lymph); they have a distinct ionic composition. From Edelman IS, Leibman J. Anatomy of body water and electrolytes. Am J Med 1959;27:256–277. There is a constant turnover of transcellular fluids; they are continuously formed and absorbed or removed. Impaired for- CHAPTER 24 The Regulation of Fluid and Electrolyte Balance 405 mation, abnormal loss from the body, or blockage of fluid re- Cellular water cannot be determined directly with any moval can have serious consequences. indicator. It can, however, be calculated from the differ- ence between measurements of total body water and extra- cellular water.
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