Chapter 27 - Fluid, Electrolytes, and Acid/Base

What percentage of an adult male’s body is fluid compared to to solids

About 60% fluid (water) and 40% solids (organic/inorganic materials)

What percentages of an adult female’s body is fluid compared to solids

About 50% fluid and 50% solids, due to higher adipose tissue content

Why do males have more intracellular water content than females

Testosterone increases muscle mass → more intracellular fluid (ICF)

Estrogen promotes adipose storage → less intracellular water

What are the major fluid compartments of the body

Intracellular fluid (ICF): inside cells (~⅔ of body water)

Extracellular fluid (ECF): outside cells (~⅓ of body water)

What are the subdivisions of extracellular fluid (ECF)

Interstitial fluid: between cells (~80% of ECF)

Plasma: in blood vessels (~20% of ECF)

Which ions dominate the ICF

K⁺ (potassium) and HPO₄²⁻ (phosphate)

Which ions dominate the ECF 

Na⁺ (sodium) and Cl⁻ (chloride)

Which compartment has the highest protein content

ICF, followed by plasma, then interstitial fluid

What maintains the difference between Na⁺ and K⁺ concentrations inside and outside cells

The sodium-potassium pump (Na⁺/K⁺ ATPase)

What are typical daily water gains

Digestive absorption: 2200 mL

Metabolic water: 300 mL

➡ Total = ~2500 mL/day

What are typical daily water losses

Urine: 1200 mL

Evaporation (skin/lungs): 1150 mL

Feces: 150 mL

What principle explains water following solute movement

Osmosis: water moves toward higher solute concentration

What happens when ECF loses water

ECF becomes hypertonic

Water shifts out of cells (ICF → ECF)

ICF volume decreases until osmotic equilibrium restores

What does the body measure directly — solute quantity or concentration

Concentration (osmolarity), not total solute amount

What triggers ADH (antidiuretic hormone) release

Increased ECF osmolarity → osmoreceptors in hypothalamus stimulate ADH → kidneys reabsorb water

What restores homeostasis when Na⁺ levels rise 

Increased ADH secretion

Increased thirst

Water retention → dilutes Na⁺ concentration

What restores homeostasis when Na⁺ levels fall

ADH secretion decreases

Thirst suppressed

Kidneys excrete more water

How does the body respond to increased ECF volume (fluid gain)

Cardiac muscle cells release natriuretic peptides →

↑ Na⁺ and water loss in urine

↓ Thirst

↓ ADH, aldosterone, and sympathetic output

How does the body respond to decreased ECF volume (fluid loss)

Renin release → Angiotensin II → Aldosterone activation

↑ Na⁺ and water retention

↑ ADH release and thirst

↑ Cardiac output & vasoconstriction

What is the normal range of K⁺ concentration in blood

3.5–5.0 mEq/L

What happens in hypokalemia (<2 mEq/L)

Muscle weakness → paralysis → cardiac arrhythmia (fatal)

What causes hypokalemia

Diuretics (increase urine volume)

Aldosteronism (excess aldosterone → K⁺ loss)

What happens in hyperkalemia (>7 mEq/L)

Cardiac arrhythmias and death

What causes hyperkalemia

Kidney failure

Low blood pH (acidosis)

Drugs blocking Na⁺ reabsorption → ↓ K⁺ excretion

What does the suffix “-emia” indicate

A condition involving something in the blood

Define hyper-

Hyper-: too much of a substance (e.g., hyperkalemia = too much K⁺)

Define hypo-

Hypo-: too little of a substance (e.g., hyponatremia = too little Na⁺