chapter 25 fluids and electrolytes

What percentage of the human body is fluid by weight

Between 45%-75%, depending on age and body composition

How does age affect body fluid percentage

Infants have the highest; older adults have the lowest; children and adults are intermediate

Why do men typically have a higher percentage of body fluid than women

Men have more muscle (which holds water), while women have more adipose tissue (which holds less)

What are the two main fluid compartments

Intracellular (ICF)—within cells; Extracellular fluid (ECF)— outside cells

What are the two major subdivisions of extracellular fluid

Interstitial fluid (surrounds cells) and plasma (within blood vessels)

How does fluid move between compartments

By osmosis, driven by differences in solute concentration (especially electrolytes)

What happens to water movement after fluid intake

Water moves from ECF to ICF until equilibrium is restored

What are the main sources of fluid intake

Preformed water (food/drink; ~2300 mL/day) and metabolic water (cellular respiration ~200 mL/day)

What are the main routes of fluid loss

Urine, sweat, expired air, feces, and cutaneous transpiration

What is fluid imbalance

When fluid output ≠ fluid intake.

What is volume depletion

Loss of both water and solutes; total body fluid decreased but osmolarity stays normal

→ Causes: bleeding, vomiting, diarrhea, burns.

What is volume excess

Gain of both water and solutes; total fluid increases, osmolarity normal

→ Causes: renal failure, excessive IV fluids.

What is dehydration

More water lost than solutes → ECF becomes hypertonic → water moves from cells into ECF.

What are common causes of dehydration

Sweating, insufficient water intake, diabetes, excessive alcohol

What is hypotonic hydration (water intoxication)

More water than solutes gained → ECF becomes hypotonic → water moves into cells → cells swell.

What is fluid sequestration

Fluid accumulates in a specific location (not available for circulation) — e.g., edema, ascites, pleural effusion

What are nonelectrolytes

Molecules that do not dissociate in solution (e.g., glucose, urea)

What are electrolytes

Compounds that dissociate into ions in solution, conducting electricity (e.g., Na⁺, Cl⁻, K⁺).

How is electrolyte concentration measured

In milliequivalents per liter (mEq/L)

Why are electrolytes important

Regulate fluid balance, neuromuscular activity, and acid-base balance

What are the major electrolytes in ECF

Sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻).

What are the major electrolytes in ICF

Potassium (K⁺), phosphate (PO₄³⁻), and magnesium (Mg²⁺).

What hormone primarily regulates sodium balance

Aldosterone (increases Na⁺ reabsorption and water retention).

What is the function of ADH (antidiuretic hormone)

Increases water reabsorption in kidneys, reducing urine volume

What does ANP (atrial natriuretic peptide) do

Inhibits Na⁺ and water reabsorption → lowers blood volume and pressure.

What does hypernatremia mean

High Na⁺ concentration; can cause cellular dehydration.

What does hyponatremia mean

Low Na⁺ concentration; causes cellular swelling

What is the main intracellular cation

Potassium (K⁺).

What happens in hyperkalemia

High K⁺ levels → can cause cardiac arrhythmias or cardiac arrest.

What happens in hypokalemia

Low K⁺ levels → muscle weakness, paralysis, and irregular heartbeats.

Where is calcium mostly stored

99% in bones in teeth

What are functions of calcium

Muscle contraction, neurotransmitter released, second messenger, blood clotting

What is hypercalcemia

High blood calcium levels → may cause muscle weakness, kidney stones.

What is hypocalcemia

Low calcium levels → causes muscle spasms or tetany.

What is the normal blood pH range

7.35-7.45

What does acidosis mean

Arterial pH < 7.35

What does alkalosis mean

Arterial pH > 7.45

Why is acid-base important

Enzyme function and metabolic processes depend on stable pH

What systems regulate acid-base balance>

Buffers, respiratory system, and kidneys

What are the main buffer systems in the body

Bicarbonate, phosphate, and protein buffer systems

What is respiratory acidosis

Retention of CO₂ due to hypoventilation → ↑H⁺, ↓pH.

What are causes of respiratory acidosis

Hypoventilation, airway obstruction, lung disease (asthma, emphysema, pneumonia)

Why are infants more susceptible to respiratory acidosis

Smaller lungs and lower residual volume

What is respiratory alkalosis

Excessive loss of CO₂ due to hyperventilation → ↓H⁺, ↑pH.

Common causes of respiratory alkalosis

Anxiety, high altitude, fever, oxygen deficiency

What is metabolic acidosis

Decrease in HCO₃⁻ or gain of H⁺ → ↓pH.

Causes of metabolic acidosis

Diarrhea (loss of HCO₃⁻), renal failure, lactic acidosis, ketoacidosis

What is metabolic alkalosis

Increase in HCO₃⁻ or loss of H⁺ → ↑pH.

Causes of metabolic alkalosis

Vomiting, diuretic overuse, excess antacid intake

What is compensation

The body’s attempt to restore normal pH after an acid-base disturbance

What are types of compensation

Respiratory and renal compensation

What is complete compensation

pH returns to normal; both systems have corrected the imbalance

What is partial compensation

pH is still abnormal, but body is adjusting

What is uncompensated

No correction has begun; pH remains normal

What is renal compensation

Kidneys excrete or retain H⁺ and HCO₃⁻ to balance pH; slower but more effective.

What is respiratory compensation

Changes in ventilation alter CO₂ levels to influence pH; faster but limited by oxygen needs.

During metabolic acidosis, how does the body compensate

Increases respiratory rate (hyperventilation) → lowers CO₂ → raises pH.

During metabolic alkalosis, how does the body compensate

Decreases respiratory rate (hypoventilation) → raises CO₂ → lowers pH.

During respiratory acidosis, how do kidneys compensate

Excrete more H⁺ and reabsorb more HCO₃⁻

During respiratory alkalosis, how do the kidneys compensate

Retain H⁺ and excrete more HCO₃⁻

What values are included in the ABG test

pH, PCO₂, and HCO₃⁻.

What ABG pattern indicates respiratory acidosis with renal compensation

↓pH, ↑PCO₂, ↑HCO₃⁻

What ABG pattern indicates respiratory alkalosis with renal compensation

↑pH, ↓PCO₂, ↓HCO₃⁻.

What ABG pattern indicates metabolic acidosis with respiratory compensation

↓pH, ↓HCO₃⁻, ↓PCO₂.

What ABG pattern indicates metabolic alkalosis with respiratory compensation

↑pH, ↑HCO₃⁻, ↑PCO₂.