Chapter 24: Fluid, Electrolyte, and Acid-Base Balance

Vocabulary practice flashcards covering water balance, fluid compartments, electrolyte regulation, buffer systems, and acid-base disorders.

Body Fluid Distribution

Body fluid constitutes approximately $60\%$ of total body weight (TBW) and is partitioned into three continually exchanging compartments: intracellular fluid ($40\%$ TBW), tissue fluid / interstitial fluid ($16\%$ TBW), and blood plasma ($4\%$ TBW).

Edema

The abnormal accumulation of water in tissue fluid, caused by increased capillary filtration, reduced capillary reabsorption, or obstructed lymphatic drainage.

Metabolic Water

Water gained internally by the body ($200\,\text{ml/day}$) that is generated as a byproduct of aerobic respiration.

Obligatory Water Loss

Unavoidable daily fluid loss through breath ($300\,\text{ml/day}$), cutaneous evaporation ($400\,\text{ml/day}$), feces ($200\,\text{ml/day}$), and a physiological minimum of urine excretion ($400\,\text{ml/day}$).

Thirst Mechanism

The physiological drive governing water intake, triggered when decreased blood volume and increased osmolarity stimulate peripheral volume sensors and central osmoreceptors, signaling the hypothalamus.

Antidiuretic Hormone (ADH)

A hormone released by the posterior pituitary in response to dehydration (decreased blood volume or increased osmolarity) that increases water reabsorption in the kidneys to promote water retention.

Atrial Natriuretic Factor (ANF)

A hormone released by cardiac atria in response to elevated blood volume that inhibits $\text{Na}^+$ and $\text{H}_2\text{O}$ reabsorption and suppresses renin and ADH secretion, thereby increasing water and sodium output.

Dehydration

A condition characterized by a decrease in body fluid, resulting from inadequate drinking water or excessive fluid loss due to overheat, diabetes, diuretic overuse, or diarrhea.

Electrolytes

Small, charged ions in body fluids comprising major cations ($\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, and $\text{H}^+$) and major anions ($\text{Cl}^-$, $\text{HCO}_3^-$, and $\text{PO}_4^{3-}$).

Sodium ($\text{Na}^+$)

The primary extracellular cation that plays an essential role in generating cell action membrane potentials and contributes predominantly to extracellular osmolarity.

Aldosterone

A adrenal hormone that reduces renal $\text{Na}^+$ excretion to elevate plasma sodium levels while simultaneously stimulating renal $\text{K}^+$ secretion to lower plasma potassium levels.

Potassium ($\text{K}^+$)

The predominant intracellular cation, serving as the greatest contributor to intracellular osmosis and cell volume, establishing resting membrane potentials, and acting as an essential cofactor for protein synthesis.

Chloride ($\text{Cl}^-$)

The most abundant extracellular anion that contributes to extracellular osmolarity and stomach acid ($\text{HCl}$) production; it has no direct endocrine regulation and passively follows sodium homeostasis.

Calcium ($\text{Ca}^{2+}$)

An extracellular cation that reinforces skeletal strength, activates muscle contraction, functions as a second messenger, triggers neurotransmitter exocytosis, and facilitates blood clotting.

Parathyroid Hormone (PTH)

A hormone that increases plasma $\text{Ca}^{2+}$ by dissolving bone matrix and reducing renal calcium excretion, while simultaneously decreasing plasma phosphate by increasing renal phosphate excretion.

Calcitonin

A hormone secreted by thyroid gland C cells that decreases plasma calcium levels by promoting the deposition of $\text{Ca}^{2+}$ into bone.

Calcitriol

A vitamin D derivative that raises plasma calcium levels by enhancing the intestinal absorption of dietary $\text{Ca}^{2+}$.

Phosphate ($\text{PO}_4^{3-}$)

A major intracellular anion required for the synthesis of ATP, GTP, DNA, RNA, and phospholipids; its excretion is accelerated by parathyroid hormone.

Protein Buffer System

The most concentrated chemical buffer system, accounting for about three-quarters of all chemical buffering capacity in body fluids; its carboxyl groups release $\text{H}^+$ when pH rises and its amino groups bind $\text{H}^+$ when pH falls.

Bicarbonate Buffer System

A reversible chemical buffer system governed by the equilibrium of carbonic acid, bicarbonate ($\text{HCO}_3^-$), and carbon dioxide ($\text{CO}_2$), which is continuously removed via the lungs.

Chemical Buffers

Substances that respond within a fraction of a second to bind or release $\text{H}^+$, stabilizing pH locally, though they cannot expel $\text{H}^+$ from the body and possess limited overall capacity.

Renal Control of pH

The body's most effective acid-base mechanism capable of neutralizing more acid or base than chemical buffers or lungs by having renal tubules secrete bound and free $\text{H}^+$ directly into urine for excretion.

Acidosis

An acid-base disturbance characterized by a blood pH below $7.35$; mild states depress the central nervous system (causing confusion, disorientation, and coma), while severe states are lethal.

Alkalosis

An acid-base disturbance characterized by a blood pH above $7.45$; mild states cause central nervous system hyperexcitability, spontaneous nerve firing, and muscle overstimulation, while severe states are lethal.

Metabolic Acidosis

A non-respiratory acid-base disorder where blood pH drops below $7.35$, commonly caused by conditions like diabetes that trigger the excessive production of organic acids.

Metabolic Alkalosis

A non-respiratory acid-base disorder where blood pH rises above $7.45$, typically caused by the excessive loss of stomach acid from chronic vomiting.