Chapter 3

The Cellular Environment

Fluids and Electrolytes, Acids and Bases

Alexa K. Doig and Sue E. Huether

http://evolve.elsevier.com/McCance/

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The cells of the body live in a fluid environment that requires electrolyte and acid-base concentrations maintained within a very narrow range. A balance is maintained by an integration of renal, hormonal, and neural functions. Changes in the composition of electrolytes affect electrical potentials of excitatory cells and cause shifts of fluid from one compartment to another that can affect cell function. Fluid fluctuations also affect blood volume and therefore blood pressure. Alterations in pH (measure of the acidity or alkalinity of a solution) disrupt the cellular function of enzyme systems and can cause cell injury. Disturbances in fluid and electrolyte or acid-base balance are common and can be life threatening. Understanding how alterations occur and the body’s ability to compensate or correct the disturbance is important for comprehending many pathophysiologic conditions.

Distribution of Body Fluids

The fluids of the body are distributed among functional compartments, or spaces, and provide a transport medium for cellular and tissue function. Water moves freely among body compartments and is distributed by osmotic and hydrostatic forces. Two thirds of the body’s water is intracellular fluid (ICF) and one third is in the extracellular fluid (ECF) compartments. The two main ECF compartments are the interstitial fluid and the intravascular fluid, the latter being the blood plasma. Other ECF compartments include the lymph and the transcellular fluids, such as the saliva, intestinal, biliary, hepatic, pancreatic, and cerebrospinal fluids; sweat; urine; and pleural, synovial, peritoneal, pericardial, and intraocular fluids (Table 3-1).

TABLE 3-1

APPROXIMATE CONCENTRATIONS OF ELECTROLYTES IN TRANSCELLULAR FLUIDS

FLUID	Na⁺ (mEq/L)	K⁺ (mEq/L)	Cl⁻ (mEq/L)	HCO3− (mEq/L)
Saliva	33	20	34	40
Gastric juice^∗	60	9	84	0
Bile	149	5	101	45
Pancreatic juice	141	5	77	92
Ileal fluid	129	11	116	29
Cecal fluid	80	21	48	22
Cerebrospinal fluid	141	3	127	23
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 hydrochloric acid by parietal cells.

The sum of fluids within all compartments constitutes the total body water (TBW) (Table 3-2). The volume of TBW is usually expressed as a percentage of body weight in kilograms. The standard value for TBW is 60% of the weight of a 70-kg adult male, which is equivalent to 42 L of fluid (Table 3-3). The rest of the body weight is composed of fat and fat-free solids, particularly bone.

TABLE 3-2

DISTRIBUTION OF BODY WATER

	PERCENTAGE OF BODY WEIGHT	VOLUME (L)
Intracellular fluid (ICF)	40	28
Extracellular fluid (ECF)	20	14
Interstitial	(15)	(11)
Intravascular	(5)	(3)
Total body water (TBW)	60	42

TABLE 3-3

TOTAL BODY WATER ^∗ IN RELATION TO BODY WEIGHT

BODY BUILD	TBW (%) ADULT MALE	TBW (%) ADULT FEMALE	TBW (%) INFANT
Normal	60	50	70
Lean	70	60	80
Obese	50	42	60

^∗NOTE: TBW (total body water) is a percentage of body weight.

Although daily fluid intake may fluctuate widely, the body regulates water volume within a relatively narrow range. The primary sources of body water are drinking of fluids, ingestion of water in food, and derivation of water from oxidative metabolism. Normally, the largest amounts of water are lost through renal excretion. Lesser amounts are eliminated through the stool and through vaporization from the skin and lungs (insensible water loss) (Table 3-4).

TABLE 3-4

NORMAL WATER GAINS AND LOSSES (70-kg MAN)

	DAILY INTAKE (ml)		DAILY OUTPUT (ml)
Drinking ≈60%	1400-1800	Urine ≈60%	1400-1800
Water in food ≈30%	700-1000	Stool ≈2%	100
Water of oxidation ≈10%	300-400	Skin ≈10%	300-500
		Lungs ≈28%	600-800
total	2400-3200		2400-3200

Although the amount of fluid within the various compartments is relatively constant, exchange of solutes (e.g., salts) and water occurs between compartments to maintain their unique compositions. The percentage of TBW varies with the amount of body fat and age. Because fat is water repelling (hydrophobic), very little water is contained in adipose cells. Individuals with more body fat have proportionately less TBW and tend to be more susceptible to fluid imbalances that cause dehydration.

Aging and Distribution of Body Fluids

The distribution and amount of TBW change with age (see Table 3-3). In newborn infants, TBW is about 75% to 80% of body weight because infants store less fat. In the immediate postnatal period, a physiologic loss of body water occurs, equivalent to about 5% of body weight as the infant adjusts to a new environment. Infants are particularly susceptible to significant changes in TBW because of their high metabolic rate and potential for evaporative fluid loss attributable to their greater body surface area in proportion to total body size. Loss of fluids from diarrhea can represent a significant proportion of body weight in infants. Renal mechanisms that regulate fluid and electrolyte conservation may not be mature enough to counter the losses, so dehydration can develop rapidly.

During childhood, TBW slowly decreases to 60% to 65% of body weight. At adolescence the percentage of TBW approaches 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 fat and less muscle as a function of estrogens and therefore have less body water.

With increasing age the percentage of TBW declines further still. The decrease is caused in part by an increased amount of fat and a decreased amount of muscle and by a reduced ability to regulate sodium and water balance. With older age the kidneys becomes less efficient at conserving sodium and therefore have difficulty concentrating the urine. Insensible water loss through the skin may increase and thirst perception may be impaired. The normal reduction of TBW in older adults becomes clinically important when the body is under stress, such as development of fever or dehydration; loss of body fluids at such times can be severe and life threatening.¹

Water Movement Between ICF and ECF

The movement of water between ICF and ECF compartments is primarily a function of osmotic forces. (Osmosis and other mechanisms of passive transport are discussed in Chapter 1.) Water moves freely by diffusion through the lipid bilayer cell membrane and through aquaporins, a family of water channel proteins that provide permeability to water.² The osmolality (number of osmoles of solute per kilogram of fluid [Osm/kg]) of TBW is normally at equilibrium. Sodium is responsible for the osmotic balance of the ECF space. Potassium maintains the osmotic balance of the ICF space. The osmotic force of ICF proteins and other nondiffusible substances is balanced by the active transport of ions out of the cell. Water crosses cell membranes freely so the osmolality of TBW is normally at equilibrium. Normally the ICF is not subject to rapid changes in osmolality but when ECF osmolality changes, water moves from one compartment to another until osmotic equilibrium is reestablished (see Figure 3-6, p. 110).

Water Movement Between Plasma and Interstitial Fluid

The distribution of water and the movement of nutrients and waste products between the plasma in the tissue capillaries and interstitial spaces occur as a result of changes in hydrostatic pressure and osmotic forces at the arterial and venous ends of the capillary. Water, sodium, and glucose move readily across the capillary membrane. The plasma proteins maintain the effective osmolality (concentration of solutes per kilogram of solution), do not cross the capillary membrane, and generate plasma oncotic pressure. Albumin is the plasma protein that is primarily responsible for the plasma oncotic pressure because it has the highest concentration. Osmotic forces within the capillary are balanced by the hydrostatic pressure, which is primarily determined by blood pressure and blood volume.

As plasma flows from the arterial to the venous end of the capillary, four forces determine if fluid moves out of the capillary and into the interstitial space (filtration) or if fluid moves back into the capillary from the interstitial space (reabsorption):

1. Capillary hydrostatic pressure (blood pressure) facilitates the outward movement of water from the capillary to the interstitial space.

2. Capillary (plasma) oncotic pressure osmotically attracts water from the interstitial space back into the capillary.

3. Interstitial hydrostatic pressure facilitates the inward movement of water from the interstitial space into the capillary.

4. Interstitial oncotic pressure osmotically attracts water from the capillary into the interstitial space.

The movement of fluid back and forth across the capillary wall is called net filtration and is best described by the Starling hypothesis:

Net filtration=(Forces favoring filtration)−(Forces opposing filtration)

Forcesfavoringfiltration=Capillaryhydrostaticpressure and interstitial oncotic pressure

Forcesopposingfiltration=Capillary oncotic pressureand interstitial hydrostatic pressure

Normally the interstitial forces are negligible because only a very small percentage of plasma proteins crosses the capillary membrane and interstitial fluid moves into cells or is drawn back into the plasma. Thus the major forces for filtration are within the capillary.

As the plasma flows from the arterial to the venous end of the capillary, the force of hydrostatic pressure facilitates the movement of water across the capillary membrane. Oncotic pressure remains fairly constant because plasma proteins normally do not cross the capillary membrane. At the arterial end of the capillary, hydrostatic pressure is greater than capillary oncotic pressure and water filters into the interstitial space. Because of oncotic forces, some water moves back into the capillary, but the net effect is loss of water from the capillary. This loss of water from the plasma decreases the hydrostatic pressure within the capillary; thus at the venous end of the capillary, oncotic pressure exceeds hydrostatic pressure. Fluids then are attracted back into the circulation, balancing the movement of fluids between the plasma and the interstitial space. The overall effect is filtration at the arterial end and reabsorption at the venous end (Figure 3-1). Interstitial hydrostatic pressure promotes the movement of about 10% of the interstitial fluid along with small amounts of protein into the lymphatics, which eventually returns to the circulation.

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 opposing 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.

An important factor in capillary filtration of fluid is the integrity of the capillary membrane. Changes in membrane permeability may permit the escape of plasma proteins into the interstitial space. The normal relationship defined by the Starling hypothesis is altered with the osmotic movement of water into the interstitial space, causing tissue edema.

Alterations in Water Movement

Edema

Edema is the excessive accumulation of fluid within the interstitial spaces. It is often a problem of fluid distribution and does not necessarily indicate a fluid excess. In some conditions, sequestered fluids can cause both edema and intravascular dehydration. The pathophysiologic process of edema is related to an increase in the forces favoring fluid filtration from the capillaries or lymphatic channels into the tissues. The four most common mechanisms are:

1. Increased capillary hydrostatic pressure

2. Decreased plasma oncotic pressure

3. Increased capillary membrane permeability

4. Lymphatic obstruction (Figure 3-2)

FIGURE 3-2 Mechanisms of Edema Formation.

Pathophysiology

Increased capillary hydrostatic pressure can result from venous obstruction or sodium and water retention. Venous obstruction causes hydrostatic pressure to increase behind the obstruction, pushing fluid from the capillaries into the interstitial spaces. Venous blood clots, hepatic obstruction, right heart failure, tight clothing around the extremities, and prolonged standing are common causes of venous obstruction. Right congestive heart failure, renal failure, and cirrhosis of the liver are conditions associated with excessive sodium and water retention, which in turn cause volume overload, increased venous pressure, and edema.

Decreased plasma oncotic pressure results from losses or diminished production of plasma albumin. Decreased oncotic 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 syndrome), 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 are not reabsorbed and accumulate in the interstitial space, 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

Clinical Manifestations

Edema may be localized or generalized. Some localized edema is usually limited to the site of tissue injury, as in a sprained joint. Local edema can also occur within particular organs, causing, for example, cerebral edema in the brain and pulmonary edema in the lungs. Edema of specific organs, such as the brain, lung, or larynx, can be life threatening.

Generalized edema is manifested by a more uniform distribution of fluid in interstitial spaces throughout the body. Dependent edema, in which fluid accumulates in gravity-dependent areas of the body, might appear in the feet and legs when standing and in the sacral area and buttocks when supine. Dependent edema can be identified by using the fingers to press away edematous fluid in tissues overlying bony prominences. A pit will be left in the skin; hence the term pitting edema (Figure 3-3).

FIGURE 3-3 Pitting Edema. (From Bloom A, Ireland J: *Color atlas of diabetes,* ed 2, St Louis, 1992, Mosby.)

Edema is usually associated with swelling and puffiness, tight-fitting clothes and shoes, and limited movement of the affected area. Weight gain can be significant. The accumulation of fluid increases the distance required for nutrients, oxygen, and wastes to move between capillaries and cells in the tissues. Increased tissue pressure also may diminish capillary blood flow, leading to ischemia. Therefore, wounds heal more slowly and formation of pressure sores increases (see Chapter 46).

As edematous fluid accumulates, it is trapped in a “third space” (i.e., the interstitial space) and dehydration can develop as a result of this sequestering of fluid. Such sequestration occurs with severe burns, in which large amounts of vascular fluid are lost to the interstitial spaces, reducing plasma volume and causing shock (see Chapter 48).

Evaluation and Treatment

Specific conditions causing edema require diagnosis. Edema may be treated symptomatically until the underlying disorder is corrected. Supportive measures include elevating edematous limbs, using compression stockings or devices, avoiding prolonged standing, restricting salt intake, and taking diuretics.

Sodium, Chloride, and Water Balance

The kidneys and hormones have a central role in maintaining sodium and water balance. Because water follows the osmotic gradients established by changes in salt concentration, sodium balance and water balance are intimately related. Sodium is regulated by the renal effects of aldosterone from the adrenal cortex and natriuretic peptides from the heart. Water balance is primarily regulated by antidiuretic hormone (ADH; also known as arginine-vasopressin) from the posterior pituitary.

Sodium and Chloride Balance

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 summarized in Table 3-1. As the most abundant ECF cation, along with its constituent anions (negatively charged ions) chloride and bicarbonate, sodium regulates extracellular osmotic forces and therefore regulates water balance. Sodium is important in other body functions, including maintenance of neuromuscular irritability for conduction of nerve impulses (in conjunction with potassium and calcium), regulation of acid-base balance (through sodium bicarbonate and sodium phosphate), participation in cellular chemical reactions, and transport of substances across the cellular membrane (see Chapter 1).

TABLE 3-5

DISTRIBUTION OF ELECTROLYTES IN BODY COMPARTMENTS

	EXTRACELLULAR FLUID (mEq/L)	INTRACELLULAR FLUID (mEq/L)
Cations
Sodium	142	10
Potassium	5	156
Calcium	5	4
Magnesium	2	26
total	154	196
Anions
Bicarbonate	24	12
Chloride	104	4
Phosphate	2	40-95
Proteins	16	54
Other anions	8	31-86
total	_____ 154	____________ 196 (average)

The kidney, in conjunction with neural and hormonal mediators, maintains normal serum sodium concentration within a narrow range (135 to 145 mEq/L) primarily through renal tubular reabsorption. The average dietary intake of sodium ranges from 5 to 6 g/day; the minimal daily requirement of sodium is 500 mg. Sweating depletes sodium and water volume and increases the body’s sodium requirement.

Hormonal regulation of sodium balance is mediated by aldosterone, a mineralocorticoid (steroid) synthesized and secreted from the adrenal cortex as the end product of the renin-angiotensin-aldosterone system (Figure 3-4) (also see Chapters 21 and 37). When circulating blood pressure and renal blood flow, or serum sodium concentrations, are reduced, renin, an enzyme secreted by the juxtaglomerular cells of the kidney, is released. Renin stimulates the formation of angiotensin I, an inactive polypeptide. Angiotensin-converting enzyme (ACE) in pulmonary vessels converts angiotensin I to angiotensin II. Angiotensin II has two major functions: it causes vasoconstriction, which elevates systemic blood pressure, and it stimulates the secretion of aldosterone. Aldosterone promotes sodium and water reabsorption by the proximal tubules of the kidneys, thus conserving sodium, blood volume, and blood pressure. Aldosterone also stimulates secretion (and therefore excretion) of potassium by the distal tubule of the kidney, reducing potassium concentrations in the ECF. The restoration of sodium levels, blood volume, and renal perfusion then inhibits further release of renin.

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.)

Natriuretic peptides are hormones that include atrial natriuretic peptide (ANP) produced by myoendocrine atrial cells, brain natriuretic peptide (BNP—named brain since it was first discovered in porcine brain) produced by myoendocrine ventricular cells, and urodilatin (an ANP analog) synthesized within the kidney. ANP and BNP are released when there is an increase in transmural atrial pressure caused by increased intra-atrial volume as may occur with heart failure.⁴ ANP and BNP increase sodium and water excretion by the kidneys, which lowers blood volume and pressure. Urodilatin is released from distal tubular kidney cells when there is increased arterial pressure and increased renal blood flow. These hormones are natural antagonists to the renin-angiotensin-aldosterone system. The restoration of lower atrial pressure then inhibits further release of ANP and BNP.

Chloride is the major anion in the ECF and provides electroneutrality, particularly in relation to sodium. Chloride transport is generally passive and follows the active transport of sodium so that increases or decreases in chloride are proportional to changes in sodium. Chloride concentration tends to vary inversely with changes in the concentration of bicarbonate (HCO3−), the other major ECF anion.

Water Balance

One manner by which water balance is regulated is through the perception of thirst. Thirst is a sensation that stimulates water-drinking behavior. Thirst is experienced when water loss equals 2% of an individual’s body weight or when there is an increase in osmolality. Dry mouth, hyperosmolality, and plasma volume depletion activate hypothalamic osmoreceptors. The action of the osmoreceptors then causes thirst. Drinking water restores plasma volume and dilutes the ECF osmolality.

Water balance also is directly regulated by antidiuretic hormone (arginine-vasopressin), which is secreted when plasma osmolality increases or circulating blood volume decreases and blood pressure drops (Figure 3-5). Increased plasma osmolality occurs with a water deficit or sodium excess in relation to water. The increased osmolality stimulates hypothalamic osmoreceptors. In addition to causing thirst, the stimulated osmoreceptors 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 restoration of plasma volume and blood pressure. Urine concentration increases, and the reabsorbed water decreases plasma osmolality, returning it toward normal. Like most hormones, ADH is regulated by a feedback mechanism. The restoration of plasma osmolality, blood volume, and blood pressure then inhibits ADH secretion.

FIGURE 3-5 The Antidiuretic Hormone (ADH) System.

With fluid loss (dehydration) (e.g., from vomiting, diarrhea, or excessive sweating), a decrease in blood volume and blood pressure often occurs. Baroreceptors (volume/pressure sensitive receptors) (stretch receptors that are sensitive to changes in arterial volume and pressure) also stimulate the release of ADH. Baroreceptors are located in the right and left atria and large veins, and in the aorta, pulmonary arteries, and carotid sinus. When arterial and atrial pressure drops baroreceptors signal the hypothalamus to release ADH. The reabsorption of water mediated by ADH then promotes the restoration of plasma volume and blood pressure (see Figure 3-5). ADH also stimulates arterial vasoconstriction.

Alterations in Sodium, Chloride, and Water Balance

Alterations in sodium and water balance are closely related. Water imbalances may develop because of changes in osmotic gradients caused by gain or loss of salt. Likewise, sodium imbalances occur with alterations in body water volume. Generally the alterations can be classified as changes in tonicity, or the change in concentration of electrolytes in relation to water (see Chapter 1). Alterations can therefore be classified as isotonic, hypertonic, or hypotonic (Table 3-6 and Figure 3-6).

TABLE 3-6

WATER AND SOLUTE IMBALANCES

TONICITY	MECHANISM
Isotonic (isoosmolar) imbalance	Gain or loss of extracellular fluid (ECF) resulting in a concentration equivalent to a 0.9% sodium chloride (salt) solution (normal saline); no shrinking or swelling of cells
Hypertonic (hyperosmolar) imbalance	Imbalance that results in an ECF concentration >0.9% salt solution; that is, water loss or solute gain; cells shrink in a hypertonic fluid
Hypotonic (hypoosmolar) imbalance	Imbalance that results in an ECF <0.9% salt solution; that is, water gain or solute loss; cells swell in a hypotonic fluid

Isotonic Alterations

The term isotonic refers to a solution that has the same concentration of solutes as the plasma. Isotonic alterations occur when changes in TBW are accompanied by proportional changes in the amounts of electrolytes and water. For example, if an individual loses pure plasma or ECF, fluid volume is depleted but the number and type of electrolytes (e.g., sodium) and the osmolality remain within a normal range. Excessive amounts of isotonic body fluids can result from excessive administration of intravenous normal saline (0.9% NaCl) or oversecretion of aldosterone with renal retention of both sodium and water. Isotonic fluid loss results in hypovolemia. Causes include hemorrhage, severe wound drainage, and excessive diaphoresis (sweating). Loss of extracellular volume results in weight loss, dryness of skin and mucous membranes, decreased urine output, increased hematocrit value, and symptoms of hypovolemia. Indicators of hypovolemia include a rapid heart rate and flattened neck veins, and can present with a normal or decreased blood pressure. In severe states, hypovolemic shock (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 failure. Weight gain and a decrease in hematocrit level and plasma protein concentration caused by the diluting effect of excess plasma volume will occur. The neck veins may distend, and the blood pressure increases. Increased capillary hydrostatic pressure leads to edema formation. If the plasma volume is great enough, pulmonary edema and heart failure develop.

Hypertonic Alterations

Hypertonic fluid alterations develop when the osmolality of the ECF is elevated above normal (greater than 294 mOsm). The most common causes are an increased concentration of ECF sodium (hypernatremia) or a deficit of ECF free water. In both instances the hypertonicity of the ECF attracts water from the intracellular space, causing ICF dehydration. A primary increase in the amount of ECF sodium causes an osmotic attraction of water and symptoms of hypervolemia. In contrast, a hypertonic state caused primarily by free water loss leads to hypovolemia (Table 3-7).

TABLE 3-7

CAUSES AND CONSEQUENCES OF HYPERTONIC IMBALANCES

CAUSATIVE FACTOR	MECHANISM	ECF EFFECTS	ICF EFFECTS
Increased sodium (hypernatremia)	Excessive hypertonic salt solutions Intravenous hypertonic sodium Saline-induced abortions Selected infant formulas Hyperaldosteronism Cushing syndrome	Hypervolemia Weight gain Bounding pulse Increased blood pressure Edema Venous distention Neuromuscular symptoms Muscle weakness Seizures	Intracellular dehydration Thirst Fever Decreased urine output Shrinkage of brain cells Confusion Coma Cerebral hemorrhage
Water deficit	Water deprivation Confusion or coma Inability to communicate Loss of thirst Water loss Watery diarrhea Diabetes insipidus Excessive diuresis Excessive diaphoresis	Hypovolemia Weight loss Weak pulses Postural hypotension Tachycardia	Intracellular dehydration See above
Other factors	Hyperglycemia	Initial dilutional hyponatremia Polyuria Polydipsia Weight loss Hypovolemia Late hypernatremia	Intracellular dehydration See above

ECF, Extracellular fluid; ICF, intracellular fluid.

Hypernatremia

Water Deficit