Fluid, Electrolyte, and Acid-Base Balance Chapter 19 Fluid, Electrolyte, and Acid-Base Balance

Fluid Compartments

Fluid Components Most of the body’s water (about 65%) resides inside cells; this is called intracellular fluid (ICF). The remaining 35%, called extracellular fluid (ECF), resides outside cells; this includes the fluid between the cells inside tissue (interstitial fluid), as well as the fluid within vessels as blood plasma and lymph. Other extracellular fluids (cerebrospinal fluid, synovial fluid in the joints, vitreous and aqueous humors of the eye, and digestive secretions) are called transcellular fluid.

Fluid Movement

Fluid Movement Intracellular and extracellular fluid continually mingle as fluid passes through the semipermeable membrane surrounding each compartment. The concentration of solutes within each compartment determines the amount and direction of flow. If the concentration of electrolytes (and therefore the osmolarity) of tissue fluid increases, water moves out of the cells and into the tissues (shown in figure on left). If the osmolarity of tissue fluid declines, water moves out of the tissues and into the cells (shown in figure on right).

Question What determines the amount and direction of fluid that flows between body compartments? The volume of fluid in one of the compartments The concentration of solutes The pH of the fluid The concentration of hydrogen ions in the fluid

Balancing Gains and Losses

Balancing Gains and Losses Normally, the amount of water gained and lost by the body each day is equal. (An adult gains and loses about 2,500 mL fluid each day.) Most fluid intake occurs through eating and drinking; the cells produce a fair amount of water as a byproduct of metabolic reactions. (This is called metabolic water.) Fluid is lost through the kidneys (as urine), the intestines (as feces), the skin (by sweat as well as diffusion), and the lungs (through expired air).

Regulation of Intake and Output Water loss varies in amount. To maintain balance, the body uses mechanisms to adjust intake and output. Water loss varies with environmental temperature and physical activity.

Mechanisms to Increase Intake Physical changes stimulate the thirst center in the hypothalamus. Salivation decreases, causing dry mouth and thirst. Water consumption occurs.

Mechanisms to Decrease Output Physical changes stimulate the hypothalamus. This stimulates the posterior pituitary to secrete antidiuretic hormone (ADH). ADH prompts the kidneys to reabsorb water and produce less urine. Fluid loss slows until water is ingested. The same factors that trigger increased fluid intake (decreased blood pressure, increased sodium concentration, and increased osmolarity) also trigger mechanisms to decrease urine output, which are listed here.

Question What effect does antidiuretic hormone (ADH) have on the body? It stimulates the thirst center. It produces a dry mouth. It prompts the kidneys to excrete more water. It prompts the kidneys to reabsorb more water.

Disorders of Water Balance Can result from an abnormality in Fluid volume Fluid concentration Distribution of fluid between compartments

Fluid Deficiency Volume depletion: Results from blood loss or the loss of both water and sodium Dehydration: Results when the body eliminates more water than sodium

Dehydration

Dehydration In dehydration, besides a loss of fluid, the concentration of sodium (and the osmolarity) of the extracellular fluid increases. The increase in osmolarity prompts the shifting of fluid from one compartment to another in an effort to balance the concentration of sodium. Dehydration results from consuming an inadequate amount of water to cover the amount of water lost. Other causes include diabetes mellitus and the use of diuretics. When severe, fluid deficiency can lead to circulatory collapse.

Fluid Excess Kidneys usually compensate by producing more urine. Specific type includes water intoxication.

Fluid Excess Because kidneys usually compensate, fluid excess is rarer than fluid deficit. One cause is renal failure, in which both sodium and water are retained and the extracellular fluid (ECF) remains isotonic. Another type is water intoxication, which can occur if someone consumes an excessive amount of water or if someone replaces heavy losses of water and sodium with just water. This causes the amount of sodium in the ECF to drop; water moves into the cells, causing them to swell. Complications of either type of fluid excess include pulmonary or cerebral edema.

Fluid Accumulation Involves the accumulation of fluid between compartments Edema: Fluid accumulation in interstitial spaces, causing tissue swelling Although fluid can accumulate in any organ or tissue, it typically affects the lungs, brain, and dependent areas (such as the legs).

Question Which condition results when the body eliminates more water than sodium? Edema Dehydration Water intoxication Volume depletion

Electrolyte Balance Crucial for proper body functioning Major cations: Na+, K+, Ca+, H+ Major anions: Cl−, HCO3−, Pi Electrolytes drive chemical reactions, affect distribution of the body’s water content, and determine a cell’s electrical potential.

Sodium Main electrolyte in extracellular fluid Determines the volume of total body water Influences how body water is distributed Plays a key role in depolarization Sodium accounts for 90% of the osmolarity of extracellular fluid. Because it plays a key role in depolarization, it is crucial for proper nerve and muscle function.

Sodium Regulation ↓ Serum Na+ Serum osmolarity ↓ Aldosterone prompts renal tubules to reabsorb Na+. Antidiuretic hormone (ADH) is suppressed → kidneys secrete water. Serum Na+ levels increase.

Sodium Regulation ↑ Serum Na+ Serum osmolarity ↑ Antidiuretic hormone (ADH) causes kidneys to reabsorb water. ADH stimulates thirst. Serum Na+ levels decline.

Sodium Imbalances Hypernatremia Hyponatremia Plasma concentration greater than 146 mEq/L Indicates fluid deficit Usually self-corrects by triggering thirst Plasma concentration less than 139 mEq/L Results from excess body water Usually corrected by excretion of excess water

Potassium Imbalances The most dangerous of all electrolyte imbalances Hyperkalemia Hypokalemia Plasma concentration above 5.0 mEq/L May occur suddenly or gradually Makes nerve and muscle cells irritable Plasma concentration less than 3.5 mEq/L May result from diuretics, vomiting, or chronic diarrhea Makes cells less excitable

Potassium Imbalances Potassium is the chief cation of intracellular fluid; it works with sodium for nerve and muscle function. Aldosterone regulates serum levels of potassium (just as it does sodium). Increasing potassium levels stimulate the adrenal cortex to secrete aldosterone, which causes the kidneys to excrete potassium as they reabsorb sodium.

Calcium Imbalances Hypercalcemia Hypocalcemia Plasma concentration greater than 5.8 mEq/L May result from hyperparathyroidism, hypothyroidism, alkalosis Inhibits depolarization Plasma concentration less than 4.5 mEq/L May result from hypoparathyroidism, hyperthyroidism, acidosis, diarrhea Increases excitation of nerves and muscles

Question What is the main cation of extracellular fluid? Potassium Sodium Chloride Calcium

Acid-Base Balance Influences homeostasis pH of blood ranges from 7.35 to 7.45 Even slight deviations can be fatal. The pH of a solution is determined by its concentration of hydrogen ions. The body uses chemical and physiological buffers to keep acids and bases in balance.

Chemical Buffers Include bicarbonate, phosphate, and protein buffer systems Use weak base to bind H+ ions and weak acid to release them Bicarbonate buffer system is the main buffering system; it uses bicarbonate and carbonic acid as shown in this equation: CO2 + H2O → H2CO3 → H+ + HCO3-. The equation moves to the right when the body needs to lower pH and to the left when it needs to raise pH.

Physiological Buffers Include respiratory and urinary systems Lungs expel CO2 to lower pH. Kidneys expel H+ ions to lower pH.

Respiratory Control of pH Central chemoreceptors in the brainstem detect a decline in pH from an accumulation of CO2. They signal the respiratory centers to increase the rate and depth of breathing. The lungs blow off CO2. Less CO2 is available to combine with water to form carbonic acid; the concentration of H+ ions decreases and pH rises.

View animation on “Renal control of pH” Kidneys expel H+ ions and reabsorb bicarbonate. This is the most powerful buffer system. It is also the slowest to respond. View animation on “Renal control of pH”

Question When the body’s pH rises above normal, which reaction would occur first? Respiratory rate would increase. Kidneys would excrete hydrogen ions. Hydrogen would bind with bicarbonate. Hydrogen would bind with carbonic acid.

Acid-Base Imbalances Respiratory imbalances result from an excess or deficiency of CO2. Metabolic imbalances result from an excess or deficiency of bicarbonate.

Acid-Base Imbalances CAUSES OF ACID GAIN (Acidosis)   CAUSES OF ACID GAIN (Acidosis) CAUSES OF ACID LOSS (Alkalosis) RESPIRATORY Retention of CO2 (hypoventilation—such as from emphysema or pneumonia—as well as apnea) Loss of CO2 (hyperventilation) METABOLIC Increased production of acids (such as ketone bodies in diabetes mellitus or lactic acid in anaerobic metabolism) Consumption of acidic drugs (such as aspirin) Inability of the kidneys to excrete H+ ions Loss of bicarbonate (such as from chronic diarrhea) Loss of gastric juices (such as through vomiting or suctioning) Excessive ingestion of bicarbonates (such as antacids)

Compensation for Acid-Base Imbalances Respiratory system responds to metabolic disturbances by adjusting ventilation. Renal system responds by adjusting the rate of H+ ion excretion.

Question Which electrolyte disturbance would result from acidosis? Hypernatremia Hyponatremia Hyperkalemia Hypokalemia