The Cellular Environment: Fluids and Electrolytes, Acids and Bases PDF

CHAPTER 3...

CHAPTER 3 The Cellular Environment: Fluids and Electrolytes, Acids and Bases Alexa K. Doig and Sue E. Huether http://evolve.elsevier.com/McCance/ Review Questions and Answers Animations Online Course: Module 2 CHAPTER OUTLINE Distribution of Body Fluids, 103 Alterations in Potassium, Calcium, Phosphate, AGING and Distribution of Body Fluids, 104 and Magnesium Balance, 114 Water Movement Between ICF and ECF, 105 Potassium, 114 Water Movement Between Plasma and Interstitial Fluid, 105 Calcium and Phosphate, 119 Alterations in Water Movement, 105 Magnesium, 122 Edema, 105 Acid-Base Balance, 122 Sodium, Chloride, and Water Balance, 108 Hydrogen Ion and pH, 122 Sodium and Chloride Balance, 108 Buffer Systems, 123 Water Balance, 109 Acid-Base Imbalances, 126 Alterations in Sodium, Chloride, and Water Balance, 109 Isotonic Alterations, 109 Hypertonic Alterations, 110 Hypotonic Alterations, 112 The cells of the body live in a fluid environment that requires how alterations occur and the body’s ability to compensate or electrolyte and acid-base concentrations maintained within correct the disturbance is important for comprehending many a very narrow range. A balance is maintained by an integra- pathophysiologic conditions. tion of renal, hormonal, and neural functions. Changes in the composition of electrolytes affect electrical potentials of excit- atory cells and cause shifts of fluid from one compartment to DISTRIBUTION OF BODY FLUIDS another that can affect cell function. Fluid fluctuations also The fluids of the body are distributed among functional com- affect blood volume and therefore blood pressure. Alterations partments, or spaces, and provide a transport medium for in pH (measure of the acidity or alkalinity of a solution) dis- cellular and tissue function. Water moves freely among body rupt the cellular function of enzyme systems and can cause cell compartments and is distributed by osmotic and hydrostatic injury. Disturbances in fluid and electrolyte or acid-base bal- forces. Two thirds of the body’s water is intracellular fluid (ICF) ance are common and can be life threatening. Understanding and one third is in the extracellular fluid (ECF) compartments. 103 104 UNIT I The Cell The two main ECF compartments are the interstitial fluid and compositions. The percentage of TBW varies with the amount the intravascular fluid, the latter being the blood plasma. Other of body fat and age. Because fat is water repelling (hydropho- ECF compartments include the lymph and the transcellular flu- bic), very little water is contained in adipose cells. Individuals ids, such as the saliva, intestinal, biliary, hepatic, pancreatic, and with more body fat have proportionately less TBW and tend to cerebrospinal fluids; sweat; urine; and pleural, synovial, perito- be more susceptible to fluid imbalances that cause dehydration. neal, pericardial, and intraocular fluids (Table 3-1). The sum of fluids within all compartments constitutes the total body water (TBW) (Table 3-2). The volume of TBW is AGING AND DISTRIBUTION OF BODY FLUIDS usually expressed as a percentage of body weight in kilograms. The distribution and amount of TBW change with age (see The standard value for TBW is 60% of the weight of a 70-kg Table 3-3). In newborn infants, TBW is about 75% to 80% adult male, which is equivalent to 42 L of fluid (Table 3-3). The of body weight because infants store less fat. In the immedi- rest of the body weight is composed of fat and fat-free solids, ate postnatal period, a physiologic loss of body water occurs, particularly bone. equivalent to about 5% of body weight as the infant adjusts to a Although daily fluid intake may fluctuate widely, the body new environment. Infants are particularly susceptible to signifi- regulates water volume within a relatively narrow range. The cant changes in TBW because of their high metabolic rate and primary sources of body water are drinking of fluids, ingestion potential for evaporative fluid loss attributable to their greater of water in food, and derivation of water from oxidative metab- body surface area in proportion to total body size. Loss of fluids olism. Normally, the largest amounts of water are lost through from diarrhea can represent a significant proportion of body renal excretion. Lesser amounts are eliminated through the weight in infants. Renal mechanisms that regulate fluid and stool and through vaporization from the skin and lungs (insen- electrolyte conservation may not be mature enough to counter sible water loss) (Table 3-4). the losses, so dehydration can develop rapidly. Although the amount of fluid within the various compart- During childhood, TBW slowly decreases to 60% to 65% of ments is relatively constant, exchange of solutes (e.g., salts) and body weight. At adolescence the percentage of TBW approaches water occurs between compartments to maintain their unique adult proportions, and gender differences begin to appear. Males eventually have a greater percentage of body water as a function of increasing muscle mass. Females have more body TABLE 3-1 APPROXIMATE fat and less muscle as a function of estrogens and therefore have CONCENTRATIONS less body water. OF ELECTROLYTES With increasing age the percentage of TBW declines further IN TRANSCELLULAR FLUIDS still. The decrease is caused in part by an increased amount of fat NA+ K+ CL− HCO3 FLUID (mEq/L) (mEq/L) (mEq/L) (mEq/L) Saliva 33 20 34 40 TABLE 3-3 TOTAL BODY WATER* Gastric juice* 60 9 84 0 IN RELATION TO BODY WEIGHT Bile 149 5 101 45 BODY TBW (%) TBW (%) TBW (%) Pancreatic 141 5 77 92 BUILD ADULT MALE ADULT FEMALE INFANT juice Ileal fluid 129 11 116 29 Normal 60 50 70 Cecal fluid 80 21 48 22 Lean 70 60 80 Cerebrospinal 141 3 127 23 Obese 50 42 60 fluid *NOTE: TBW (total body water) is a percentage of body weight. Sweat 45 5 58 0 *The Cl− concentration exceeds the Na+, K+ concentration by 15 mEq/L in gastric juice. This largely represents the secretions of hydro- TABLE 3-4 NORMAL WATER GAINS chloric acid by parietal cells. AND LOSSES (70-KG MAN) DAILY INTAKE DAILY OUTPUT (ML) (ML) TABLE 3-2 DISTRIBUTION Drinking 1400-1800 Urine ≈60% 1400-1800 OF BODY WATER ≈60% Water in food 700-1000 Stool ≈2% 100 PERCENTAGE ≈30% OF BODY WEIGHT VOLUME (L) Water of 300-400 Skin ≈10% 300-500 Intracellular fluid (ICF) 40 28 oxidation Extracellular fluid (ECF) 20 14 ≈10% Interstitial (15) (11) Lungs ≈28% 600-800 Intravascular (5) (3) TOTAL Total body water (TBW) 60 42 2400-3200 2400-3200  CHAPTER 3 The Cellular Environment: Fluids and Electrolytes, Acids and Bases 105 and a decreased amount of muscle and by a reduced ability to The movement of fluid back and forth across the capillary regulate sodium and water balance. With older age the kidneys wall is called net filtration and is best described by the Starling becomes less efficient at conserving sodium and therefore have hypothesis: difficulty concentrating the urine. Insensible water loss through Net filtration = (Forces favoring filtration) − the skin may increase and thirst perception may be impaired. (Forces opposing filtration) The normal reduction of TBW in older adults becomes clini- cally important when the body is under stress, such as develop- Forces favoring filtration = Capillary hydrostatic ment of fever or dehydration; loss of body fluids at such times pressure and interstitial oncotic pressure can be severe and life threatening.1 Forces opposing filtration = Capillary oncotic pressure Water Movement Between ICF and ECF and interstitial hydrostatic pressure The movement of water between ICF and ECF compartments is primarily a function of osmotic forces. (Osmosis and other Normally the interstitial forces are negligible because only a mechanisms of passive transport are discussed in Chapter 1.) very small percentage of plasma proteins crosses the capillary Water moves freely by diffusion through the lipid bilayer cell membrane and interstitial fluid moves into cells or is drawn membrane and through aquaporins, a family of water chan- back into the plasma. Thus the major forces for filtration are nel proteins that provide permeability to water.2 The osmolal- within the capillary. ity (number of osmoles of solute per kilogram of fluid [Osm/ As the plasma flows from the arterial to the venous end of the kg]) of TBW is normally at equilibrium. Sodium is responsible capillary, the force of hydrostatic pressure facilitates the move- for the osmotic balance of the ECF space. Potassium main- ment of water across the capillary membrane. Oncotic pressure tains the osmotic balance of the ICF space. The osmotic force remains fairly constant because plasma proteins normally do not of ICF proteins and other nondiffusible substances is balanced cross the capillary membrane. At the arterial end of the capillary, by the active transport of ions out of the cell. Water crosses hydrostatic pressure is greater than capillary oncotic pressure and cell membranes freely so the osmolality of TBW is normally at water filters into the interstitial space. Because of oncotic forces, equilibrium. Normally the ICF is not subject to rapid changes some water moves back into the capillary, but the net effect is loss in osmolality but when ECF osmolality changes, water moves of water from the capillary. This loss of water from the plasma from one compartment to another until osmotic equilibrium is decreases the hydrostatic pressure within the capillary; thus at reestablished (see Figure 3-6, p. 110). the venous end of the capillary, oncotic pressure exceeds hydro- static pressure. Fluids then are attracted back into the circula- Water Movement Between Plasma tion, balancing the movement of fluids between the plasma and and Interstitial Fluid the interstitial space. The overall effect is filtration at the arterial The distribution of water and the movement of nutrients and end and reabsorption at the venous end (Figure 3-1). Interstitial waste products between the plasma in the tissue capillaries and hydrostatic pressure promotes the movement of about 10% of interstitial spaces occur as a result of changes in hydrostatic the interstitial fluid along with small amounts of protein into the pressure and osmotic forces at the arterial and venous ends of lymphatics, which eventually returns to the circulation. the capillary. Water, sodium, and glucose move readily across An important factor in capillary filtration of fluid is the the capillary membrane. The plasma proteins maintain the integrity of the capillary membrane. Changes in membrane effective osmolality (concentration of solutes per kilogram of permeability may permit the escape of plasma proteins into the solution), do not cross the capillary membrane, and generate interstitial space. The normal relationship defined by the Star- plasma oncotic pressure. Albumin is the plasma protein that is ling hypothesis is altered with the osmotic movement of water primarily responsible for the plasma oncotic pressure because it into the interstitial space, causing tissue edema. has the highest concentration. Osmotic forces within the capil- lary are balanced by the hydrostatic pressure, which is primarily ALTERATIONS IN WATER MOVEMENT determined by blood pressure and blood volume. As plasma flows from the arterial to the venous end of the Edema capillary, four forces determine if fluid moves out of the capil- Edema is the excessive accumulation of fluid within the inter- lary and into the interstitial space (filtration) or if fluid moves stitial spaces. It is often a problem of fluid distribution and back into the capillary from the interstitial space (reabsorption): does not necessarily indicate a fluid excess. In some conditions, 1. Capillary hydrostatic pressure (blood pressure) facili- sequestered fluids can cause both edema and intravascular tates the outward movement of water from the capillary dehydration. The pathophysiologic process of edema is related to the interstitial space. to an increase in the forces favoring fluid filtration from the 2. Capillary (plasma) oncotic pressure osmotically attracts capillaries or lymphatic channels into the tissues. The four most water from the interstitial space back into the capillary. common mechanisms are: 3. Interstitial hydrostatic pressure facilitates the inward move- 1. Increased capillary hydrostatic pressure ment of water from the interstitial space into the capillary. 2. Decreased plasma oncotic pressure 4. Interstitial oncotic pressure osmotically attracts water 3. Increased capillary membrane permeability from the capillary into the interstitial space. 4. Lymphatic obstruction (Figure 3-2) 106 UNIT I The Cell Capillary (fluid movement Cell by net filtration) (fluid movement pressures by osmosis) Arteriole Intracellular osmotic pressure Interstitial Capillary osmotic pressure hydrostatic pressure Filtrat ion Interstitial hydrostatic pressure Capillary oncotic Lymphatics pressure o n pti s or Reab Venule Arterial Capillary Pressures Venous Capillary Pressures Capillary hydrostatic pressure 35 mmHg Capillary hydrostatic pressure 18 mmHg Interstitial fluid hydrostatic 2 mmHg Interstitial fluid hydrostatic 1 mmHg pressure pressure Net hydrostatic pressure 33 mmHg Net hydrostatic pressure 17 mmHg Capillary oncotic pressure 24 mmHg Capillary oncotic pressure 25 mmHg Interstitial fluid oncotic pressure 0 mmHg Interstitial fluid oncotic pressure 0 mmHg Net oncotic pressure 24 mmHg Net oncotic pressure 25 mmHg Net filtration pressure 19 mmHg Net filtration pressure 28 mmHg FIGURE 3-1 Capillary Filtration Forces. Water, electrolytes, and small molecules exchange freely between the vascular compartment and the interstitial space at the site of capillaries and small venules. The rate and amount of exchange are driven by the physical forces of hydrostatic and oncotic pressures and the permeability and surface area of the capillary membranes. The two opposing hydrostatic pressures are capillary hydrostatic pressure and interstitial hydrostatic pressure. The two oppos- ing oncotic pressures are capillary oncotic pressure and interstitial oncotic pressure. The forces that favor filtration from the capillary are capillary hydrostatic pressure and interstitial oncotic pressure, and the forces that oppose filtration are capillary oncotic pressure and interstitial hydrostatic pressure. The sum of their effects is known as net filtration pressure (NFP). In the example of normal exchange illustrated here, a small amount of fluid moves to the lymph vessels, which accounts for the net filtration difference between the arterial and venous ends of the capillary. PATHOPHYSIOLOGY. Increased capillary hydrostatic pressure can are common causes of venous obstruction. Right congestive heart result from venous obstruction or sodium and water retention. failure, renal failure, and cirrhosis of the liver are conditions asso- Venous obstruction causes hydrostatic pressure to increase behind ciated with excessive sodium and water retention, which in turn the obstruction, pushing fluid from the capillaries into the intersti- cause volume overload, increased venous pressure, and edema. tial spaces. Venous blood clots, hepatic obstruction, right heart fail- Decreased plasma oncotic pressure results from losses or ure, tight clothing around the extremities, and prolonged standing diminished production of plasma albumin. Decreased oncotic  CHAPTER 3 The Cellular Environment: Fluids and Electrolytes, Acids and Bases 107 Increased capillary permeability Decreased synthesis of (burns, inflammation) plasma proteins (cirrhosis, malnutrition) Increased loss of plasma proteins (nephrotic syndrome) Loss of plasma proteins Increased tissue Increased plasma Na! to interstitial space oncotic pressure and H2O retention (dilution of plasma proteins) Decreased transport of capillary filtered protein Decreased capillary Edema oncotic pressure Lymph obstruction Fluid movement into tissues Increased capillary hydrostatic pressure (venous obstruction, salt and water retention, heart failure) FIGURE 3-2 Mechanisms of Edema Formation. attraction of fluid within the capillary causes fluid to move into the interstitial space, resulting in edema. Decreased synthesis of plasma protein and decreased oncotic pressure may occur with liver disease or protein malnutrition. Losses of plasma proteins occur with glomerular diseases of the kidney (nephrotic syn- drome), hemorrhage, and serous drainage from open wounds or burns. Increased capillary permeability is usually associated with inflammation and the immune response. (Immunity is discussed in Chapters 7, 8, and 9; inflammation is discussed in Chapters 7 and 9.) These responses are often the result of trauma such as burns or crushing injuries, neoplastic disease, allergic reactions, and infection. Excess amounts of fluid escape from the plasma to the interstitial space and produce edema. This type of edema is often very severe because of loss of proteins from the vascular space, which decreases capillary oncotic pressure and increases interstitial oncotic pressure. Lymphatic obstruction occurs when the lymphatic channels are blocked because of infection or tumor. Proteins and fluids FIGURE 3-3 Pitting Edema. (From Bloom A, Ireland J: Color atlas of diabetes, are not reabsorbed and accumulate in the interstitial space, ed 2, St Louis, 1992, Mosby.) causing lymphedema. Lymphedema of the arm or leg also can occur after surgical removal of axillary or femoral lymph nodes, respectively, for treatment of cancer.3 edematous fluid in tissues overlying bony prominences. A pit CLINICAL MANIFESTATIONS. Edema may be localized or general- will be left in the skin; hence the term pitting edema (Figure 3-3). ized. Some localized edema is usually limited to the site of tissue Edema is usually associated with swelling and puffiness, injury, as in a sprained joint. Local edema can also occur within tight-fitting clothes and shoes, and limited movement of the particular organs, causing, for example, cerebral edema in the affected area. Weight gain can be significant. The accumulation brain and pulmonary edema in the lungs. Edema of specific of fluid increases the distance required for nutrients, oxygen, organs, such as the brain, lung, or larynx, can be life threatening. and wastes to move between capillaries and cells in the tissues. Generalized edema is manifested by a more uniform distribu- Increased tissue pressure also may diminish capillary blood tion of fluid in interstitial spaces throughout the body. Depen- flow, leading to ischemia. Therefore, wounds heal more slowly dent edema, in which fluid accumulates in gravity-dependent and formation of pressure sores increases (see Chapter 46). areas of the body, might appear in the feet and legs when stand- As edematous fluid accumulates, it is trapped in a “third ing and in the sacral area and buttocks when supine. Depen- space” (i.e., the interstitial space) and dehydration can develop dent edema can be identified by using the fingers to press away as a result of this sequestering of fluid. Such sequestration 108 UNIT I The Cell occurs with severe burns, in which large amounts of vascular gradients established by changes in salt concentration, sodium fluid are lost to the interstitial spaces, reducing plasma volume balance and water balance are intimately related. Sodium is and causing shock (see Chapter 48). regulated by the renal effects of aldosterone from the adrenal EVALUATION AND TREATMENT. Specific conditions causing cortex and natriuretic peptides from the heart. Water balance is edema require diagnosis. Edema may be treated symptom- primarily regulated by antidiuretic hormone (ADH; also known atically until the underlying disorder is corrected. Supportive as arginine-vasopressin) from the posterior pituitary. measures include elevating edematous limbs, using compres- sion stockings or devices, avoiding prolonged standing, restrict- Sodium and Chloride Balance ing salt intake, and taking diuretics. Sodium accounts for 90% of the ECF cations (positively charged ions). The distribution of electrolytes in body compartments is summarized in Table 3-5 and the concentration of electrolytes is SODIUM, CHLORIDE, AND WATER BALANCE summarized in Table 3-1. As the most abundant ECF cation, along The kidneys and hormones have a central role in maintaining with its constituent anions (negatively charged ions) chloride sodium and water balance. Because water follows the osmotic and bicarbonate, sodium regulates extracellular osmotic forces and therefore regulates water balance. Sodium is important in other body functions, including maintenance of neuromuscular TABLE 3-5 DISTRIBUTION OF irritability for conduction of nerve impulses (in conjunction with ELECTROLYTES IN BODY potassium and calcium), regulation of acid-base balance (through COMPARTMENTS sodium bicarbonate and sodium phosphate), participation in cel- EXTRACELLULAR INTRACELLULAR lular chemical reactions, and transport of substances across the FLUID (mEq/L) FLUID (mEq/L) cellular membrane (see Chapter 1). The kidney, in conjunction with neural and hormonal medi- Cations ators, maintains normal serum sodium concentration within Sodium 142 10 a narrow range (135 to 145 mEq/L) primarily through renal Potassium 5 156 tubular reabsorption. The average dietary intake of sodium Calcium 5 4 ranges from 5 to 6 g/day; the minimal daily requirement of Magnesium 2 26 sodium is 500 mg. Sweating depletes sodium and water volume TOTAL 154 196 and increases the body’s sodium requirement. Anions Hormonal regulation of sodium balance is mediated by Bicarbonate 24 12 aldosterone, a mineralocorticoid (steroid) synthesized and Chloride 104 4 secreted from the adrenal cortex as the end product of the renin- Phosphate 2 40-95 angiotensin-aldosterone system (Figure 3-4) (also see Chapters Proteins 16 54 21 and 37). When circulating blood pressure and renal blood Other anions 8 31-86 flow, or serum sodium concentrations, are reduced, renin, an TOTAL _____ ____________ enzyme secreted by the juxtaglomerular cells of the kidney, is 154 196 (average) released. Renin stimulates the formation of angiotensin I, an ↓ BP ↓ Serum Na! ↓ ECF ↑ Urine Na! Adrenal ↓ Renal perfusion cortex Kidney Lungs (juxtaglomerular cells) n-converting Angioteennsziyme Kidney Liver Angiotensin II Angiotensin I Aldosterone Renin Angiotensinogen ↑ Sodium and Arterioles water retention Blood Vasoconstriction vessel ↑ Extracellular fluid ↑ Blood pressure FIGURE 3-4 The Renin-Angiotensin-Aldosterone System. BP, Blood pressure; ECF, extracellular fluid; Na, sodium. (From Lewis et al: Medical-surgical nursing: assessment and management of clinical problems, ed 8, St. Louis, 2011, Mosby.)  CHAPTER 3 The Cellular Environment: Fluids and Electrolytes, Acids and Bases 109 inactive polypeptide. Angiotensin-converting enzyme (ACE) ADH is regulated by a feedback mechanism. The restoration in pulmonary vessels converts angiotensin I to angiotensin II. of plasma osmolality, blood volume, and blood pressure then Angiotensin II has two major functions: it causes vasoconstric- inhibits ADH secretion. tion, which elevates systemic blood pressure, and it stimulates With fluid loss (dehydration) (e.g., from vomiting, diarrhea, the secretion of aldosterone. Aldosterone promotes sodium and or excessive sweating), a decrease in blood volume and blood water reabsorption by the proximal tubules of the kidneys, thus pressure often occurs. Baroreceptors (volume/pressure sensi- conserving sodium, blood volume, and blood pressure. Aldo- tive receptors) (stretch receptors that are sensitive to changes sterone also stimulates secretion (and therefore excretion) of in arterial volume and pressure) also stimulate the release of potassium by the distal tubule of the kidney, reducing potas- ADH. Baroreceptors are located in the right and left atria and sium concentrations in the ECF. The restoration of sodium large veins, and in the aorta, pulmonary arteries, and carotid levels, blood volume, and renal perfusion then inhibits further sinus. When arterial and atrial pressure drops baroreceptors release of renin. signal the hypothalamus to release ADH. The reabsorption Natriuretic peptides are hormones that include atrial natri- of water mediated by ADH then promotes the restoration of uretic peptide (ANP) produced by myoendocrine atrial cells, plasma volume and blood pressure (see Figure 3-5). ADH also brain natriuretic peptide (BNP—named brain since it was stimulates arterial vasoconstriction. first discovered in porcine brain) produced by myoendocrine ventricular cells, and urodilatin (an ANP analog) synthesized ALTERATIONS IN SODIUM, CHLORIDE, within the kidney. ANP and BNP are released when there is an increase in transmural atrial pressure caused by increased intra- AND WATER BALANCE atrial volume as may occur with heart failure.4 ANP and BNP Alterations in sodium and water balance are closely related. increase sodium and water excretion by the kidneys, which low- Water imbalances may develop because of changes in osmotic ers blood volume and pressure. Urodilatin is released from dis- gradients caused by gain or loss of salt. Likewise, sodium imbal- tal tubular kidney cells when there is increased arterial pressure ances occur with alterations in body water volume. Generally and increased renal blood flow. These hormones are natural the alterations can be classified as changes in tonicity, or the antagonists to the renin-angiotensin-aldosterone system. The change in concentration of electrolytes in relation to water (see restoration of lower atrial pressure then inhibits further release Chapter 1). Alterations can therefore be classified as isotonic, of ANP and BNP. hypertonic, or hypotonic (Table 3-6 and Figure 3-6). Chloride is the major anion in the ECF and provides elec- troneutrality, particularly in relation to sodium. Chloride Isotonic Alterations transport is generally passive and follows the active transport The term isotonic refers to a solution that has the same concen- of sodium so that increases or decreases in chloride are propor- tration of solutes as the plasma. Isotonic alterations occur when tional to changes in sodium. Chloride concentration tends to changes in TBW are accompanied by proportional changes vary inversely with changes in the concentration of bicarbonate (HCO3− ), the other major ECF anion. ↑ Plasma ↓ Plasma osmolality volume Water Balance One manner by which water balance is regulated is through the perception of thirst. Thirst is a sensation that stimulates water- Detection by brain Detection by drinking behavior. Thirst is experienced when water loss equals osmoreceptors volume receptors 2% of an individual’s body weight or when there is an increase in osmolality. Dry mouth, hyperosmolality, and plasma volume ↑ Thirst and depletion activate hypothalamic osmoreceptors. The action of fluid intake Hypothalamus the osmoreceptors then causes thirst. Drinking water restores plasma volume and dilutes the ECF osmolality. Water balance also is directly regulated by antidiuretic hor- Pars nervosa of posterior pituitary mone (arginine-vasopressin), which is secreted when plasma osmolality increases or circulating blood volume decreases and blood pressure drops (Figure 3-5). Increased plasma osmolality ADH occurs with a water deficit or sodium excess in relation to water. The increased osmolality stimulates hypothalamic osmorecep- tors. In addition to causing thirst, the stimulated osmorecep- Renal water retention tors signal the posterior pituitary to release ADH. The action of ADH is to increase the permeability of renal tubular cells to water, increasing water reabsorption and promoting the res- toration of plasma volume and blood pressure. Urine concen- ↓ Plasma ↑ Plasma osmolality volume tration increases, and the reabsorbed water decreases plasma osmolality, returning it toward normal. Like most hormones, FIGURE 3-5 The Antidiuretic Hormone (ADH) System. 110 UNIT I The Cell in the amounts of electrolytes and water. For example, if an hemorrhage, severe wound drainage, and excessive diaphore- individual loses pure plasma or ECF, fluid volume is depleted sis (sweating). Loss of extracellular volume results in weight but the number and type of electrolytes (e.g., sodium) and the loss, dryness of skin and mucous membranes, decreased urine osmolality remain within a normal range. Excessive amounts output, increased hematocrit value, and symptoms of hypo- of isotonic body fluids can result from excessive administra- volemia. Indicators of hypovolemia include a rapid heart rate tion of intravenous normal saline (0.9% NaCl) or oversecre- and flattened neck veins, and can present with a normal or tion of aldosterone with renal retention of both sodium and decreased blood pressure. In severe states, hypovolemic shock water. Isotonic fluid loss results in hypovolemia. Causes include (severe hypotension) can occur (see Chapter 48). Isotonic fluid excesses result in hypervolemia. Causes include excessive administration of intravenous fluids, hypersecretion of aldosterone, the effects of drugs such as cortisone, or renal TABLE 3-6 WATER AND SOLUTE failure. Weight gain and a decrease in hematocrit level and IMBALANCES plasma protein concentration caused by the diluting effect of TONICITY MECHANISM excess plasma volume will occur. The neck veins may distend, Isotonic (isoosmolar) Gain or loss of extracellular fluid (ECF) and the blood pressure increases. Increased capillary hydro- imbalance resulting in a concentration equivalent to a static pressure leads to edema formation. If the plasma volume 0.9% sodium chloride (salt) solution (normal is great enough, pulmonary edema and heart failure develop. saline); no shrinking or swelling of cells Hypertonic Imbalance that results in an ECF Hypertonic Alterations (hyperosmolar) concentration >0.9% salt solution; that is, Hypertonic fluid alterations develop when the osmolality of the imbalance water loss or solute gain; cells shrink in a ECF is elevated above normal (greater than 294 mOsm). The hypertonic fluid most common causes are an increased concentration of ECF Hypotonic (hypoosmolar) Imbalance that results in an ECF