Chapter 25 – Fluid, Electrolyte & Acid-Base Balance

Key vocabulary covering fluid compartments, electrolyte regulation, hormonal control, and acid-base balance from Chapter 25 lecture notes.

Intracellular Fluid (ICF)

Fluid inside cells; accounts for about two-thirds of total body water.

Extracellular Fluid (ECF)

All body fluid outside cells; includes interstitial fluid, blood plasma and specialized fluids.

Interstitial Fluid (IF)

ECF found in the microscopic spaces between tissue cells; ~12 L in adults.

Blood Plasma

Liquid component of blood; ~3 L; the only ECF with abundant soluble proteins.

Osmolality

Total concentration of solute particles per kilogram of water (≈280-300 mOsm in body fluids).

Osmosis

Movement of water across a semipermeable membrane from lower to higher osmolality.

Hydrostatic Pressure

Physical force of a fluid pushing against a surface; drives fluid out of capillaries.

Colloid Osmotic Pressure (COP)

Pulling force exerted by plasma proteins that draws water into the capillaries.

Obligatory Water Loss

Unavoidable loss of water via skin, lungs, feces and minimum urine to excrete wastes.

Facultative Water Loss

Adjustable water loss regulated by hormones at the nephron (mostly ADH).

Sensible Water Loss

Measurable fluid loss, e.g., urine and feces.

Insensible Water Loss

Unmeasurable fluid loss through skin evaporation and expired air.

Volume Depletion

Isotonic fluid loss exceeds isotonic fluid gain; plasma osmolality unchanged.

Volume Excess

Isotonic fluid gain exceeds loss; plasma osmolality unchanged.

Dehydration

Loss of water without equal loss of solute, producing hypertonic ECF.

Hypotonic Hydration

Excess water intake or renal failure dilutes ECF, causing cellular swelling.

Fluid Sequestration

Normal total body water but abnormal distribution, e.g., edema.

Edema

Atypical accumulation of interstitial fluid resulting in tissue swelling.

Thirst Mechanism

Hypothalamic process that drives water intake when plasma osmolality or angiotensin II rises.

Osmoreceptor

Hypothalamic neuron that senses ECF osmolality changes and regulates ADH release.

Baroreceptor

Stretch receptor in vessels/heart that detects blood pressure changes.

Electrolyte

Substance that dissociates into ions in water and conducts electricity.

Nonelectrolyte

Molecule that does not dissociate in water (e.g., glucose); exerts less osmotic power.

Sodium (Na⁺)

Major ECF cation; chief determinant of plasma osmolality and blood volume.

Hypernatremia

Above-normal blood sodium concentration (>145 mEq/L).

Hyponatremia

Below-normal blood sodium concentration (<135 mEq/L).

Potassium (K⁺)

Principal ICF cation; critical for resting membrane potential.

Hyperkalemia

Elevated blood potassium (>5.0 mEq/L); risk of cardiac arrhythmias.

Hypokalemia

Low blood potassium (<3.5 mEq/L); leads to muscle weakness and arrhythmias.

Chloride (Cl⁻)

Most abundant ECF anion; usually follows Na⁺ to maintain electroneutrality.

Calcium (Ca²⁺)

Ion vital for bones, muscle contraction, neurotransmission and clotting.

Parathyroid Hormone (PTH)

Hormone that raises blood calcium by acting on bone, kidneys and intestine.

Phosphate (PO₄³⁻)

Major ICF anion; component of bones, nucleic acids and ATP; urinary buffer.

Magnesium (Mg²⁺)

Second-most abundant ICF cation; cofactor for many enzymes and muscle relaxation.

Antidiuretic Hormone (ADH)

Posterior pituitary hormone that increases water reabsorption in collecting ducts.

Aldosterone

Adrenal cortex steroid that promotes renal Na⁺ (and water) reabsorption and K⁺ secretion.

Atrial Natriuretic Peptide (ANP)

Atrial hormone that lowers blood volume and pressure by increasing Na⁺ and water excretion.

Angiotensin II

Potent vasoconstrictor formed via renin-angiotensin system; stimulates thirst, ADH and aldosterone.

Renin-Angiotensin-Aldosterone System (RAAS)

Hormonal cascade regulating blood pressure, Na⁺ balance and fluid volume.

Bicarbonate Buffer System

Primary ECF chemical buffer composed of carbonic acid and bicarbonate ion.

Phosphate Buffer System

Urine and ICF buffer made of dihydrogen and monohydrogen phosphate ions.

Protein Buffer System

Most plentiful intracellular buffer; amino acids accept or donate H⁺ as needed.

Fixed Acid

Non-volatile metabolic acid (e.g., lactic, phosphoric) removed by kidneys.

Volatile Acid

Carbonic acid formed from CO₂; eliminated via lungs.

Acidosis

Arterial pH below 7.35; can depress central nervous system activity.

Alkalosis

Arterial pH above 7.45; leads to over-excitation of nervous system.

Respiratory Acidosis

Low pH, high PCO₂ (>45 mm Hg) due to hypoventilation or impaired gas exchange.

Respiratory Alkalosis

High pH, low PCO₂ (<35 mm Hg) caused by hyperventilation.

Metabolic Acidosis

Low pH, low HCO₃⁻ (<22 mEq/L) from acid gain or bicarbonate loss.

Metabolic Alkalosis

High pH, high HCO₃⁻ (>26 mEq/L) from acid loss or base ingestion.

Compensation

Physiologic response (respiratory or renal) that attempts to restore normal pH after disturbance.

Type A Intercalated Cells

Collecting duct cells that secrete H⁺ and reabsorb HCO₃⁻ during acidosis.

Type B Intercalated Cells

Collecting duct cells that secrete HCO₃⁻ and reabsorb H⁺ during alkalosis.

Hypercapnia

Elevated arterial CO₂; stimulates increased ventilation.

Hypocapnia

Low arterial CO₂; depresses respiratory drive.

Obligatory Urine Output

Minimum urine (≈500 mL/day) required to excrete metabolic wastes.

Alkaline Reserve

The pool of bicarbonate ions available to buffer strong acids.

Amphoteric

Molecule (e.g., protein) that can act as both acid and base depending on pH.

Osmotic Pressure Gradient

Difference in osmolality between two solutions that drives water movement.