27. ELECTROLYTES IN BODY FLUIDS

27. ELECTROLYTES IN BODY FLUIDS

• The ions formed when electrolytes dissolve and dissociate serve four general functions in the body. 

(1) Because they are largely confined to particular fluid compartments and are more numerous
than nonelectrolytes, 

• certain ions control the osmosis of water between fluid compartments. 

(2) Ions help maintain the acid–base balance required for normal cellular activities. 

(3) Ions carry electrical current, 

• which allows production of action potentials and graded potentials. 

(4) Several ions serve as cofactors needed for optimal activity of enzymes.


Concentrations of Electrolytes in Body Fluids

To compare the charge carried by ions in different solutions,

•  the concentration of ions is typically expressed in units of milliequivalents per liter (mEq/liter).
• These units give the concentration of cations or anions in a given volume of solution.

• One equivalent is the positive or negative charge equal to the amount of charge in one mole of H+ ;
• a milliequivalent is onethousandth of an equivalent. 

•  a mole of a substance is its molecular weight expressed in grams. 

For ions such as sodium (Na +), potassium (K+ ), and bicarbonate (HCO3¯ ), which have a single positive or negative charge, 

• the number of mEq/liter is equal to the number of mmol/liter. 

For ions such as calcium (Ca2+ ) or phosphate (HPO4 2¯ ), which have two positive or negative charges, 

• the number of mEq/liter is twice the number of mmol/liter.

 The chief difference between the two extracellular fluids—blood plasma and interstitial fluid—is that 

• blood plasma contains many protein anions, 
• in contrast to interstitial fluid, which has very few. 

• Because normal capillary membranes are virtually impermeable to proteins, only a few plasma proteins leak out of blood vessels into the interstitial fluid. 
• This difference in protein concentration is largely responsible for the blood colloid osmotic pressure exerted by blood plasma.
• In other respects, the two fluids are similar.

The electrolyte content of intracellular fluid differs considerably from that of extracellular fluid. 

In extracellular fluid, 

• the most abundant cation is Na+ , 
• and the most abundant anion is Cl ¯. 

In intracellular fluid, 

• the most abundant cation is K+ , and
• the most abundant anions are proteins and phosphates (HPO4 2 ¯).

• By actively transporting Na+ out of cells and K + into cells, sodium–potassium pumps (Na +/K+ ATPase) play a major role in maintaining the high intracellular concentration of K+ and high extracellular concentration of Na+ .

Sodium

• Sodium ions (Na +) are the most abundant ions in extracellular fluid, 
• accounting for 90% of the extracellular cations.
• The normal blood plasma Na+ concentration is 136–148 mEq/liter.

• Na + plays a pivotal role in fluid and electrolyte balance because it accounts for almost half of the osmolarity of extracellular fluid (142 of about 300 mOsm/liter).

• The flow of Na + through voltage-gated channels in the plasma membrane also is necessary for the generation and conduction of action potentials in neurons and muscle fibers. 
• The typical daily intake of Na+ in North America often far exceeds the body’s normal daily requirements, due largely to excess dietary salt.

• The kidneys excrete excess Na +, but they also can conserve it during periods of shortage.

The Na + level in the blood is controlled by 

1. aldosterone, 
2. antidiuretic hormone (ADH), and
3.  atrial natriuretic peptide (ANP).

• Aldosterone increases renal reabsorption of Na+ . When the blood plasma concentration of Na  + drops below 135 mEq/liter, a condition called hyponatremia, 
• ADH release ceases. 
• The lack of ADH in turn permits greater excretion of water in urine and restoration of the normal Na+ level in ECF. 

• Atrial natriuretic peptide (ANP) increases Na + excretion by the kidneys when Na+ level is above normal, a condition called hypernatremia.

CLINICAL CONNECTION 

Indicators of Na+ Imbalance 

• If excess sodium ions remain in the body because the kidneys fail to excrete enough of them, water is also osmotically retained. 

The result is

• increased blood volume, 
• increased blood pressure, and
•  edema, an abnormal accumulation of interstitial fluid. 

1. Renal failure and
2.  hyperaldosteronism (excessive aldosterone secretion) are two causes of Na+ retention.

• Excessive urinary loss of Na+ , by contrast, causes excessive water loss, 
• which results in hypovolemia, an abnormally low blood volume.

Hypovolemia related to Na + loss is most frequently due 

• to the inadequate secretion of aldosterone associated with adrenal insufficiency
• or overly vigorous therapy with diuretic drugs.

Chloride

• Chloride ions (Cl ¯) are the most prevalent anions in extracellular fluid. 
• The normal blood plasma Cl¯ concentration is 95–105 mEq/liter. 

• Processes that increase or decrease renal reabsorption of sodium ions also affect reabsorption of chloride ions. 

Potassium

• Potassium ions (K+ ) are the most abundant cations in intracellular fluid (140 mEq/liter). 

• Conversely, when blood plasma K+ concentration is low, aldosterone secretion decreases and less K+ is excreted in urine. 

• Because K+ is needed during the repolarization phase of action potentials, abnormal K levels can be lethal. 
• For instance, hyperkalemia (above-normal concentration of K+ in blood) can cause death due to ventricular fibrillation.

Bicarbonate

• Bicarbonate ions (HCO3¯  ) are the second most prevalent extracellular anions. 
• Normal blood plasma HCO3¯  concentration is 22–26 mEq/liter in systemic arterial blood 
• and 23–27 mEq/ liter in systemic venous blood. 

HCO3 ¯concentration increases as blood flows through systemic capillaries 

• because the carbon dioxide released by metabolically active cells combines with water to form carbonic acid; 
• the carbonic acid then dissociates into H + and HCO3¯ . 

• The kidneys are the main regulators of blood HCO3¯ concentration.
• The intercalated cells of the renal tubule can either form HCO3¯
• and release it into the blood when the blood level is low  
• or excrete excess HCO3¯ in the urine when the level in blood is too high. 

Calcium

• Because such a large amount of calcium is stored in bone, it is the most abundant mineral in the body. 
• About 98% of the calcium in adults is located in the skeleton and teeth, 
• where it is combined with phosphates to form a crystal lattice of mineral salts.

•  In body fluids, calcium is mainly an extracellular cation (Ca2+). 

• The normal concentration of free or unattached  Ca2+in blood plasma is 4.5–5.5 mEq/liter.
• About the same amount of Ca2+ is attached to various plasma proteins. 

Besides contributing to the hardness of bones and teeth, 
 
Ca2+ plays important roles in 

• blood clotting, 
• neurotransmitter release, 
• maintenance of muscle tone, 
• and excitability of nervous and muscle tissue.

• The most important regulator of  Ca2+concentration in blood plasma is parathyroid hormone (PTH) 
•  A low level of  Ca2+in blood plasma promotes release of more PTH, 
• which stimulates osteoclasts in bone tissue to release calcium (and phosphate) from bone extracellular matrix. 
• Thus, PTH increases bone resorption. 

• Parathyroid hormone also enhances reabsorption of  Ca2+from glomerular filtrate through re-nal tubule cells and back into blood, 
• and increases production of calcitriol (the form of vitamin D that acts as a hormone), 
• which in turn increases  Ca2+absorption from food in the gastrointestinal tract. 

 calcitonin (CT) produced by the thyroid gland 

• inhibits the activity of osteoclasts, 
• accelerates Ca2+ deposition into bones, 
• and thus lowers blood Ca2+ levels.

Phosphate

• About 85% of the phosphate in adults is present as calcium phosphate salts, which are structural components of bone and teeth. The remaining 15% is ionized. 
• Three phosphate ions

1. H₂PO₄⁻
2. HPO₄²⁻
3. PO₄³⁻

•  are important intracellular anions. 
• At the normal pH of body fluids,  HPO₄²⁻ is the most prevalent form. 

• Phosphates contribute about 100 mEq/liter of anions to intracellular fluid. 
• HPO₄²⁻  is an important buffer of H , both in body fluids and in the urine. 

• Although some are “free,” most phosphate ions are covalently bound to organic molecules such as lipids (phospholipids), proteins, carbohydrates, nucleic acids (DNA and RNA), and adenosine triphosphate (ATP).

• The normal blood plasma concentration of ionized phosphate is only 1.7–2.6 mEq/liter. 

• The same two hormones that govern calcium homeostasis—parathyroid hormone (PTH) and calcitriol— also regulate the level of  HPO₄²⁻  in blood plasma. 

PTH

• stimulates resorption of bone extracellular matrix by osteoclasts,
• which releases both phosphate and calcium ions into the bloodstream.

In the kidneys,

•  however, PTH inhibits reabsorption of phosphate ions while stimulating reabsorption of calcium ions by renal tubular cells. 
• Thus, PTH increases urinary excretion of phosphate 
• and lowers blood phosphate level. 

Calcitriol 

• promotes absorption of both phosphates and calcium from the gastrointestinal tract. 

• Fibroblast growth factor 23 (FGF 23) is a polypeptide paracrine (local hormone) that also helps regulate blood plasma levels of  HPO₄²⁻ . 

This hormone decreases HPO₄²⁻ blood levels by 

• increasing  HPO₄²⁻ exeretion by the kidneys and 
• decreasing absorption of  HPO₄²⁻ by the gastrointestinal tract.

Magnesium

• In adults, about 54% of the total body magnesium is part of bone matrix as magnesium salts. 
• The remaining 46% occurs as magnesium ions (Mg²⁺ ) 
• in intracellular fluid (45%) and 
• extracellular fluid (1%). 

• Mg²⁺ is the second most common intracellular cation (35 mEq/liter). 

1. Functionally, Mg²⁺ is a cofactor for certain enzymes needed for the metabolism of carbohydrates and proteins and for the sodium–potassium pump. 
2. Mg²⁺ is essential for normal neuromuscular activity, 
3. synaptic transmission, and
4. myocardial functioning. 
5. In addition, secretion of parathyroid hormone (PTH) depends on  Mg²⁺.

• Normal blood plasma Mg2 concentration is low, only 1.3–2.1 mEq/liter. 

• Several factors regulate the blood plasma level of  Mg²⁺ by varying the rate at which it is excreted in the urine. 

1. The kidneys increase urinary excretion of Mg²⁺ in response to 

• hypercalcemia,
• hypermagnesemia, 
• increases in extracellular fluid volume decreases in parathyroid hormone, 
• and acidosis. 

• The opposite conditions decrease renal excretion of Mg²⁺ .

People at risk for fluid and electrolyte imbalances include 

• those who depend on others for fluid and food, such as infants, the elderly, and the hospitalized; 
• individuals undergoing medical treatment that involves intravenous infusions, drainages or suctions, and urinary catheters; 
• and people who receive diuretics,

                experience excessive fluid losses 

• and require increased fluid intake, 

              or experience fluid retention 

• and have fluid restrictions.

Finally, 

• athletes and military personnel in extremely hot environments, 
• postoperative individuals, 
• severe burn or trauma cases, 
• individuals with chronic diseases (congestive heart failure, diabetes, chronic obstructive lung disease, and cancer), 
• people in confinement, 
• and individuals with altered levels of consciousness who may be unable to communicate needs or respond to thirst 

            - are also subject to fluid and electrolyte imbalances.

BLOOD ELECTROLYTE IMBALANCES
EXCESS
ELECTROLYTE*	NAME AND CAUSES	SIGNS AND SYMPTOMS
Sodium (Na+) 136–148 mEq/liter	Hypernatremia may occur with dehydration, waterdeprivation, or excessive sodium in diet or intravenous fluids; causes hypertonicity of ECF, which pulls water out of body cells into ECF, causing cellular dehydration.	Intense thirst, hypertension, edema, agitation, and convulsions.
Chloride (Cl−) 95–105 mEq/liter	Hyperchloremia may result from dehydration due to water loss or water deprivation; excessive chloride intake; or severe renal failure, hyper aldosteronism, certain types of acidosis, and some drugs.	Lethargy, weakness, metabolic acidosis, and rapid, deep breathing.
Potassium (K+) 3.5–5.0 mEq/liter	Hyperkalemia may be  due to excessive potassium intake, renal failure, aldosterone deficiency, crushing injuries to body tissues, or transfusion of hemolyzed blood.	Irritability, nausea, vomiting, diarrhea, muscular weakness; can cause death by inducing ventricular fibrillation
Calcium (Ca2+) Total = 9–10.5 mg/dL; ionized = 4.5–5.5 mEq/liter	Hypercalcemia may result from hyperparathyroidism, some cancers, excessive  intake of vitamin D, and Paget’s disease of bone.	Lethargy, weakness, anorexia, nausea, vomiting, polyuria, itching, bone pain, depression, confusion, paresthesia, stupor, and coma.
Phosphate(HPO42−) 1.7–2.6 mEq/liter	Hyperphosphatemia occurs when the kidneys fail to excrete excess phosphate, as happens in renal failure; can also  result from increased intake of phosphates or destruction of body cells, which releases phosphates into the blood.	Anorexia, nausea, vomiting, muscular weakness, hyperactive reflexes, tetany, and tachycardia.
Magnesium (Mg2+) 1.3–2.1 mEq/liter	Hypermagnesemia occurs in renal failure or due, to increased intake of Mg2+, such as Mg2+ -containing antacids; also occurs in aldosterone deficiency and hypothyroidism.	Hypotension, muscular weakness or paralysis nausea, vomiting, and altered mental functioning.

BLOOD ELECTROLYTE IMBALANCES
DEFICIENCY
ELECTROLYTE*	NAME AND CAUSES	SIGNS AND SYMPTOMS
Sodium (Na+) 136–148 mEq/liter	Hyponatremia  may be due to  decreased sodium intake;  increased sodium loss through vomiting, ,  diarrhea,  aldosterone deficiency,  or taking certain diuretics;  and excessive water intake.	Muscular weakness  dizziness, and hypotension;  tachycardia and shock  mental confusion  stupor, and coma
Chloride (Cl−) 95–105 mEq/liter	Hypochloremia  may be due to  excessive vomiting,  overhydration,  aldosterone deficiency,  congestive heart failure,  and therapy with certain diuretics such as furosemide (Lasix®).	Muscle spasms  metabolic alkalosis,  shallow respirations,  hypotension, and tetany
Potassium (K+) 3.5–5.0 mEq/liter	Hypokalemia  may result from  excessive loss due to vomiting or diarrhea,  decreased potassium intake,  hyperaldosteronism,  kidney disease, and  therapy with some diuretics	Muscle fatigue,  flaccid paralysis,  mental confusion,  increased urine output,  shallow respirations,  and changes in the electrocardiogram, including flattening of. the T wave.
Calcium (Ca2+) Total = 9–10.5 mg/dL; ionized = 4.5–5.5 mEq/liter	Hypocalcemia  may be due to  increased calcium loss,  reduced calcium intake,  elevated levels of phosphate, or hypoparathyroidism.	Numbness and tingling of the fingers;  hyperactive reflexes,  muscle cramps tetany, and convulsions;  bone fractures;  spasms of laryngeal muscles that can cause death by asphyxiation.
Phosphate(HPO42−) 1.7–2.6 mEq/liter	Hypophosphatemia  may occur through  increased urinary losses,  decreased intestinal absorption,  or increased utilization.	Confusion, seizures coma,  chest and muscle pain,  numbness and tingling of the fingers,  decreased coordination,  memory loss, and lethargy.
Magnesium (Mg2+) 1.3–2.1 mEq/liter	Hypomagnesemia may be due to  intake or excessive loss in urine or feces;  also occurs in   alcoholism,   malnutrition,   diabetes mellitus, and  diuretic therapy.	Weakness, irritabilitytetany, delirium inadequate convulsions, confusion, anorexia, nausea, vomiting, paresthesia, and cardiac arrhythmias.