NSG-2254/2264-01(23/FA): Chapter 42: Fluid, electrolyte, and acid-base balance Vocab and Notes

Isotonic (Vocab)

A fluid with the same tonicity as normal blood is called isotonic.

Hypotonic (Vocab)

A hypotonic solution is more dilute than the blood.

Antidiuretic hormone

ADH regulates the osmolality of the body fluids by influencing how much water is excreted in urine. It is synthesized by neurons in the hypothalamus that release it from the posterior pituitary gland. ADH circulates in the blood to the kidneys, where it acts on the collecting ducts (Huether et al., 2020). Its name—antidiuretic hormone—tells you what it does. It causes renal cells to resorb water, taking water from the renal tubular fluid and putting it back in the blood. This action decreases urine volume, concentrating the urine while diluting the blood by adding water to it (see Fig. 42.6A).

Atrial natriuretic peptide (ANP)

ANP also regulates ECV by influencing how much sodium and water are excreted in urine. Cells in the atria of the heart release ANP when they are stretched (e.g., by an increased ECV). ANP is a weak hormone that increases the loss of sodium and water in the urine (see Fig. 42.6C). Thus ANP opposes the effect of aldosterone (Huether et al., 2020).

Movement of water and electrolytes

Active transport, diffusion, osmosis, and filtration are processes that move water and electrolytes between body compartments. These processes maintain equal osmolality in all compartments while allowing for different electrolyte concentrations.

Blood colloid osmotic pressure (oncotic pressure) (Vocab)

Blood colloid osmotic pressure, also called oncotic pressure, is an inward-pulling force caused by blood proteins that helps move fluid from the interstitial area back into capillaries.

Colloids (blood, albumin) (Vocab)

Blood contains albumin and other proteins known as colloids. These proteins are much larger than electrolytes, glucose, and other molecules that dissolve easily. Most colloids are too large to leave capillaries in the fluid that is filtered; therefore they remain in the blood. Because they are particles, colloids exert osmotic pressure.

Diffusion

Diffusion is passive movement of electrolytes or other particles down a concentration gradient (from areas of higher concentration to areas of lower concentration). Within a body compartment electrolytes diffuse easily by random movements until the concentration is the same in all areas. However, diffusion of electrolytes across cell membranes requires proteins that serve as ion channels. For example, when a sodium channel in a cell membrane is open, Na+ diffuses passively across the cell membrane into the ICF because concentration is lower in the ICF. Opening of ion channels is tightly controlled and plays an important part in muscle and nerve function.

Clinical dehydration

ECV deficit and hypernatremia often occur at the same time; this combination is called clinical dehydration. The ECV is too low, and the body fluids are too concentrated. Clinical dehydration is common with gastroenteritis or other causes of severe vomiting and diarrhea when people are unable to replace their fluid output with enough intake of dilute sodium-containing fluids. Signs and symptoms of clinical dehydration are those of both ECV deficit and hypernatremia.

Extracellular volume deficit (Vocab)

Extracellular volume deficit is present when there is insufficient isotonic fluid in the extracellular compartment.

Electrolyte imbalances

Factors such as diarrhea, endocrine disorders, and medications that disrupt electrolyte homeostasis cause electrolyte imbalances. Electrolyte intake greater than electrolyte output or a shift of electrolytes from cells or bone into the ECF causes plasma electrolyte excess. Electrolyte intake less than electrolyte output or shift of electrolyte from the ECF into cells or bone causes plasma electrolyte deficit.

Fluid balance

Fluid homeostasis is the dynamic interplay of three processes: fluid intake and absorption, fluid distribution, and fluid output (Felver, 2021c). To maintain fluid balance, fluid intake must equal output. Because some of the normal daily fluid output (e.g., urine, sweat) is a hypotonic salt solution, people must have an equivalent fluid intake of hypotonic sodium-containing fluid (or water plus foods with some salt) to maintain fluid balance (intake equal to output).

Electrolyte

Fluid in the body compartments contains mineral salts known technically as electrolytes. An electrolyte is a compound that separates into ions (charged particles) when it dissolves in water.

Fluid intake

Fluid intake occurs orally through drinking but also through eating because most foods contain some water. Food metabolism creates additional water. Average fluid intake from these routes for healthy adults is about 2300 mL, although this amount can vary widely, depending on exercise habits, preferences, and the environment (Table 42.2). Other routes of fluid intake include IV, rectal (e.g., enemas), and irrigation of body cavities that can absorb fluid.

Filtration

Fluid moves into and out of capillaries (between the vascular and interstitial compartments) by the process of filtration (Fig. 42.4). Filtration is the net effect of four forces, two that tend to move fluid out of capillaries and small venules and two that tend to move fluid back into them. Hydrostatic pressure is the force of the fluid pressing outward against a surface. Similarly, capillary hydrostatic pressure is a relatively strong outward-pushing force that helps move fluid from capillaries into the interstitial area. Interstitial fluid hydrostatic pressure is a weaker opposing force that tends to push fluid back into capillaries.

Active transport

Fluids in different body compartments have different concentrations of electrolytes that are necessary for normal function. For example, concentrations of Na+, Cl−, and HCO3− are higher in the ECF, whereas concentrations of K+, Mg2+, and phosphate are higher in the ICF. Cells maintain their high intracellular electrolyte concentration by active transport. Active transport requires energy in the form of adenosine triphosphate (ATP) to move electrolytes across cell membranes against the concentration gradient (from areas of lower concentration to areas of higher concentration). One example of active transport is the sodium-potassium pump, which moves Na+ out of a cell and K+ into it, keeping the ICF lower in Na+ and higher in K+ than the ECF.

Acid-base balance

For optimal cell function the body maintains a balance between acids and bases. Acid-base homeostasis is the dynamic interplay of three processes: acid production, acid buffering, and acid excretion. Normal acid-base balance is maintained with acid excretion equal to acid production. Acids release hydrogen (H+) ions; bases (alkaline substances) take up H+ ions. The more H+ ions that are present, the more acidic is a solution.

Hydrostatic pressure (HP) (Vocab)

Hydrostatic pressure is the force of the fluid pressing outward against a surface.

Hypocalcemia (Vocab)

Hypocalcemia is abnormally low calcium concentration in the blood (see Table 42.5). The physiologically active form of calcium in the blood is ionized calcium. Total blood calcium also contains inactive forms that are bound to plasma proteins and small anions such as citrate. Factors that cause too much ionized calcium to shift to the bound forms cause symptomatic ionized hypocalcemia (Kyle et al., 2018). People who have acute pancreatitis frequently develop hypocalcemia because calcium binds to undigested fat in their feces and is excreted. This process decreases absorption of dietary calcium and also increases calcium output by preventing reabsorption of calcium contained in GI fluids. Hypocalcemia increases neuromuscular excitability, the basis for its signs and symptoms.

Fluid imbalances

If disease processes, medications, or other factors disrupt fluid intake or output, imbalances sometimes occur (Felver, 2021c). For example, with diarrhea there is an increase in fluid output, and a fluid imbalance (dehydration) occurs if fluid intake does not increase appropriately. There are two major types of fluid imbalances: volume imbalances and osmolality imbalances (Fig. 42.7). Volume imbalances are disturbances of the amount of fluid in the extracellular compartment. Osmolality imbalances are disturbances of the concentration of body fluids. Volume and osmolality imbalances occur separately or in combination.

interstitial fluid (IF) (Vocab)

Interstitial fluid is located between the cells and outside the blood vessels.

Intravascular fluid (IVF) (Vocab)

Intravascular fluid is the liquid part of the blood (i.e., the plasma).

Arterial blood gases (ABGs) (Vocab)

Laboratory tests of a sample of arterial blood called arterial blood gases (ABGs) are used to monitor a patient's acid-base balance.

Metabolic acidosis (Vocab)

Metabolic acidosis occurs from an increase of metabolic acid or a decrease of base (bicarbonate). The kidneys are unable to excrete enough metabolic acids, which accumulate in the blood, or bicarbonate is removed from the body directly as with diarrhea (see Table 42.7). In either case the blood HCO3− decreases, and the pH falls (Ferrari et al., 2017; Huether et al., 2020). With an increase of metabolic acids, blood HCO3− decreases because it buffers metabolic acids. Similarly, when patients have conditions that cause the removal of HCO3−, the amount of HCO3− in the blood decreases.

Metabolic alkalosis (Vocab)

Metabolic alkalosis occurs from a direct increase of base HCO3− or a decrease of metabolic acid, which increases blood HCO3− by releasing it from its buffering function. Common causes include vomiting and gastric suction (see Table 42.7). The respiratory compensation for metabolic alkalosis is hypoventilation. The decreased rate and depth of respiration allow carbonic acid to increase in the blood, as seen by an increased PaCO2. The need for oxygen may limit the degree of respiratory compensation for metabolic alkalosis. Because HCO3− crosses the blood-brain barrier with difficulty, neurological signs and symptoms are less severe or even absent with metabolic alkalosis.

Osmolality (Vocab)

Osmolality of a fluid is a measure of the number of particles per kilogram of water.

Renin-angiotensin-aldosterone system

The RAAS regulates ECF volume by influencing how much sodium and water are excreted in urine. It also contributes to regulation of blood pressure (BP). Specialized cells in the kidneys release the enzyme renin, which acts on angiotensinogen, an inactive protein secreted by the liver that circulates in the blood. Renin converts angiotensinogen to angiotensin I, which is converted to angiotensin II by other enzymes in the lung capillaries (Huether et al., 2020). Angiotensin II has several functions, one of which is vasoconstriction in some vascular beds. The important fluid homeostasis functions of angiotensin II include stimulation of aldosterone release from the adrenal cortex.

Acid excretion

The body has two acid-excretion systems: lungs and kidneys. The lungs excrete carbonic acid; the kidneys excrete metabolic acids.

Excretion of metabolic acids

The kidneys excrete all acids except carbonic acid. They secrete H+ into the renal tubular fluid, putting HCO3− back into the blood at the same time. If there are too many H+ ions in the blood, renal cells move more H+ ions into the renal tubules for excretion, retaining more HCO3− in the process. If there are too few H+ ions in the blood, renal cells excrete fewer H+ ions.

Acidosis (Vocab)

The term acidosis describes a condition that tends to make the blood relatively too acidic.

Alkalosis (Vocab)

The term alkalosis describes a condition that tends to make the blood relatively too basic (alkaline).

Fluid distribution

The term fluid distribution means the movement of fluid among its various compartments. Fluid distribution between the extracellular and intracellular compartments occurs by osmosis. Fluid distribution between the vascular and interstitial parts of the ECF occurs by filtration.

Fluid (Vocab)

The term fluid means water that contains dissolved or suspended substances such as glucose, mineral salts, and proteins.

Hypovolemia (Vocab)

The term hypovolemia means decreased vascular volume and often is used when discussing ECV deficit.

Anion gap (Vocab)

To help identify the specific cause, health care providers and the laboratory calculate the anion gap, a reflection of unmeasured anions in plasma. You calculate anion gap by subtracting the sum of plasma concentrations of the anions Cl− and HCO3− from the plasma concentration of the cation Na+.

Osmosis (Vocab)

Water moves across cell membranes by osmosis, a process by which water moves through a membrane that separates fluids with different particle concentrations (Fig. 42.3). Cell membranes are semipermeable, which means that water crosses them easily, but they are not freely permeable to many types of particles, including electrolytes such as sodium and potassium. These semipermeable cell membranes separate interstitial fluid from ICF. The fluid in each of these compartments exerts osmotic pressure, an inward-pulling force caused by particles in the fluid. The particles already inside the cell exert ICF osmotic pressure, which tends to pull water into the cell. The particles in the interstitial fluid exert interstitial fluid osmotic pressure, which tends to pull water out of the cell. Water moves into the compartment that has a higher osmotic pressure (inward-pulling force) until the particle concentration is equal in the two compartments.

Excretion of carbonic acid

When you exhale, you excrete carbonic acid in the form of CO2 and water. If the PaCO2 (i.e., level of CO2 in the blood) rises, the chemoreceptors trigger faster and deeper respirations to excrete the excess. If the PaCO2 falls, the chemoreceptors trigger slower and shallower respirations so that more of the CO2 produced by cells remains in the blood and makes up the deficit. These alterations in respiratory rate and depth maintain the carbonic acid part of acid-base balance. People who have respiratory disease may be unable to excrete enough carbonic acid, which causes the blood to become more acidic and blood CO2 to increase. If an increased respiratory rate is unable to correct the problem, the kidneys begin some compensatory excretion of metabolic acid.

Electrolyte balance

You can best understand electrolyte balance by considering the three processes involved in electrolyte homeostasis: electrolyte intake and absorption, electrolyte distribution, and electrolyte output.

Hypernatremia (Vocab)

also called water deficit, is a hypertonic condition. Two general causes make body fluids too concentrated: loss of relatively more water than salt, or gain of relatively more salt than water. When the interstitial fluid becomes hypertonic, water leaves cells by osmosis, and they shrivel. Signs and symptoms of hypernatremia are those of cerebral dysfunction, which arise when brain cells shrivel. Hypernatremia may occur in combination with ECV deficit; this combined disorder is called clinical dehydration.

Hyponatremia (Vocab)

also called water excess or water intoxication, is a hypotonic condition. It arises from gain of relatively more water than salt or loss of relatively more salt than water. The excessively dilute condition of interstitial fluid causes water to enter cells by osmosis, causing the cells to swell. Signs and symptoms of cerebral dysfunction occur when brain cells swell.

Vascular access devices (VADs) (Vocab)

are catheters or infusion ports designed for repeated access to the vascular system.

Buffers (Vocab)

are pairs of chemicals that work together to maintain normal pH of body fluids. If there are too many free H+ ions, a buffer takes them up so that they no longer are free. If there are too few, a buffer can release H+ ions to prevent an acid-base imbalance. Buffers work rapidly, within seconds.

Respiratory alkalosis

arises from alveolar hyperventilation; the lungs excrete too much carbonic acid (CO2 and water). The PaCO2 falls, creating a deficit of carbonic acid in the blood, which increases pH (see Table 42.7). Respiratory alkalosis usually is short lived; therefore the kidneys do not have time to compensate. When the pH of blood, CSF, and ICF increases acutely, cell membrane excitability also increases, which can cause neurological symptoms such as excitement, confusion, and paresthesia. If the pH rises high enough, central nervous system (CNS) depression can occur.

Respiratory acidosis (Vocab)

arises from alveolar hypoventilation; the lungs are unable to excrete enough CO2. The PaCO2 rises, creating an excess of carbonic acid in the blood, which decreases pH (Table 42.7). The kidneys compensate by increasing excretion of metabolic acids in the urine, which increases blood bicarbonate. This compensatory process is slow, often taking 24 hours to show clinical effect and 3 to 5 days to reach steady state. Decreased cerebrospinal fluid (CSF) pH and intracellular pH of brain cells cause decreased level of consciousness.

Intracellular fluid (ICF) (Vocab)

fluid inside cells In adults, ICF comprises approximately two-thirds of total body water.

Extracellular fluid (ECF) (Vocab)

fluid outside cells ECF makes up approximately one-third of total body water. ECF has two major divisions (intravascular fluid and interstitial fluid) and a minor division (transcellular fluids).

The osmolality imbalances are called

hypernatremia and hyponatremia.

Hypertonic (Vocab)

hypertonic solution is more concentrated than normal blood.

Venipuncture (Vocab)

is a technique in which a vein is punctured through the skin by a sharp rigid stylet (e.g., metal stylet). The stylet is partially covered either with a plastic catheter or a needle attached to a syringe.

Hypercalcemia (Vocab)

is abnormally high calcium concentration in the blood. Hypercalcemia results from increased calcium intake and absorption, shift of calcium from bones into the ECF, and decreased calcium output (see Table 42.5). Patients with some types of cancers such as lung and breast cancers often develop hypercalcemia because some cancer cells secrete chemicals into the blood that are related to parathyroid hormone. When these chemicals reach the bones, they cause shift of calcium from bones into the ECF. This weakens bones, and the person sometimes develops pathological fractures (i.e., bone breakage caused by forces that would not break a healthy bone). Hypercalcemia decreases neuromuscular excitability, the basis for its other signs and symptoms, the most common of which is lethargy.

Hypermagnesemia (Vocab)

is abnormally high magnesium concentration in the blood. End-stage renal disease causes hypermagnesemia unless the person decreases magnesium intake to match the decreased output. Signs and symptoms are caused by decreased neuromuscular excitability, with lethargy and decreased deep tendon reflexes being most common.

Hyperkalemia (Vocab)

is abnormally high potassium ion concentration in the blood. Its general causes are increased potassium intake and absorption, shift of potassium from cells into the ECF, and decreased potassium output. People who have oliguria (decreased urine output) are at high risk of hyperkalemia from the resultant decreased potassium output unless their potassium intake also decreases substantially. Understanding this principle helps you remember to check urine output before you administer IV solutions containing potassium. Hyperkalemia can cause muscle weakness, potentially life-threatening cardiac dysrhythmias, and cardiac arrest.

Hypomagnesemia (Vocab)

is abnormally low magnesium concentration in the blood. Its general causes are decreased magnesium intake and absorption, shift of plasma magnesium to its inactive bound form, and increased magnesium output. Signs and symptoms are similar to those of hypocalcemia because hypomagnesemia also increases neuromuscular excitability.

Hypokalemia (Vocab)

is abnormally low potassium concentration in the blood. It results from decreased potassium intake and absorption, a shift of potassium from the ECF into cells, and an increased potassium output. Hypokalemia causes muscle weakness, which becomes life threatening if it includes respiratory muscles. It can also cause potentially life-threatening cardiac dysrhythmias.

Extracellular volume excess (Vocab)

occurs when there is too much isotonic fluid in the extracellular compartment. Intake of sodium-containing isotonic fluid has exceeded fluid output. For example, when you eat more salty foods than usual and drink water, you may notice that your ankles swell or rings on your fingers feel tight and you gain 1 kg (2 lb) or more overnight.

Osmotic pressure (Vocab)

osmotic pressure, an inward-pulling force caused by particles in the fluid.

Crystalloids (Vocab)

salts that dissolve readily into true solutions.