Fluid, Electrolyte, and Acid-Base Balance: ANP Study Notes

Fluid, Electrolyte, and Acid-Base Balance

Introduction

Fluid, electrolyte, and acid-base balance are essential for maintaining homeostasis in the human body. These processes regulate the distribution and composition of body fluids, the concentration of electrolytes, and the pH of body fluids, all of which are critical for normal cellular function and overall health.

Body Fluid Compartments

Body Water Content

• Infants: ~73% or more water (low body fat, low bone mass)

• Adult males: ~60% water

• Adult females: ~50% water (higher fat content, less skeletal muscle mass)

• Adipose tissue: least hydrated of all tissues

• Total body water in adults: ~40 L

• Water content declines to ~45% in old age

Main Fluid Compartments

• Intracellular fluid (ICF): Fluid inside cells; accounts for 2/3 of total body fluid (~25 L)

• Extracellular fluid (ECF): Fluid outside cells; accounts for 1/3 of total body fluid (~15 L)

  • Plasma: 3 L (20% of ECF)

  • Interstitial fluid (IF): 12 L (80% of ECF)

  • Other ECF: lymph, cerebrospinal fluid, synovial fluid, serous fluid, gastrointestinal secretions

Exchange Between Compartments

• Osmotic and hydrostatic pressures regulate continuous exchange and mixing of fluids.

• Water moves freely along osmotic gradients; all body fluid osmolality is almost always equal.

• Exchanges between plasma and IF occur across capillary walls; exchanges between IF and ICF occur across cell membranes.

Composition of Body Fluids

Electrolytes and Nonelectrolytes

• Nonelectrolytes: Most are organic molecules (e.g., glucose, lipids, creatinine, urea); do not dissociate in water; no charged particles created.

• Electrolytes: Dissociate into ions in water (e.g., inorganic salts, acids, bases, some proteins); ions conduct electrical current and have greater osmotic power than nonelectrolytes.

• Electrolyte concentrations are expressed in milliequivalents per liter (mEq/L).

Electrolyte Distribution

• ECF: Major cation is Na+; major anion is Cl–.

• ICF: Major cation is K+; major anion is HPO42–.

• Electrolytes are most abundant solutes in body fluids and determine most chemical and physical reactions.

• Bulk of dissolved solutes consists of proteins, phospholipids, cholesterol, and triglycerides.

Water Balance and ECF Osmolality

Water Intake and Output

• Water intake must equal water output (~2500 ml/day).

• Intake: Ingested foods and beverages, metabolic water (from cellular metabolism).

• Output: Urine (60%), insensible water loss (skin and lungs), perspiration, feces.

Regulation of Water Intake

• Thirst mechanism is the driving force for water intake, governed by the hypothalamic thirst center.

• Stimuli: Increased plasma osmolality (1–2%), dry mouth, decreased blood volume/pressure, angiotensin II, baroreceptor input.

• Drinking water inhibits the thirst center via relief of dry mouth and activation of stretch receptors in the stomach and intestine.

Regulation of Water Output

• Obligatory water losses: insensible water loss (lungs, skin), sensible water loss (urine, sweat, feces).

• Volume of urine excreted and solute concentration depend on fluid intake, diet, and water loss via other avenues.

Influence of Antidiuretic Hormone (ADH)

• Water reabsorption in collecting ducts is proportional to ADH release.

• Decreased ADH: dilute urine, drop in body fluid volume.

• Increased ADH: concentrated urine, increased body fluid volume.

• Other triggers: large changes in blood volume or pressure, intense sweating, vomiting, diarrhea, severe blood loss, burns, fever.

Disorders of Water Balance

Dehydration

• ECF water loss due to hemorrhage, burns, vomiting, diarrhea, sweating, water deprivation, diuretic abuse, endocrine disturbances.

• Symptoms: dry mouth, thirst, dry flushed skin, oliguria; may lead to weight loss, fever, confusion, hypovolemic shock, electrolyte loss.

Hypotonic Hydration (Water Intoxication)

• Occurs with renal insufficiency or rapid excess water ingestion.

• ECF osmolality decreases, causing hyponatremia; water enters tissue cells, causing swelling.

• Symptoms: nausea, vomiting, cramping, cerebral edema, possible death; treated with hypertonic saline.

Edema

• Atypical accumulation of IF, resulting in tissue swelling (not cell swelling).

• Can impair tissue function by increasing diffusion distance for oxygen and nutrients.

• Caused by increased fluid flow out of blood or decreased return of fluid to blood.

Electrolyte Balance

Overview

• Electrolyte balance usually refers to salt balance (mainly NaCl), though electrolytes include acids, bases, and some proteins.

• Salts control fluid movements, provide minerals for excitability, secretory activity, and membrane permeability.

• Salts enter by ingestion/metabolism and are lost via perspiration, feces, urine, vomit.

Causes and Consequences of Electrolyte Imbalances

The following tables summarize the main electrolyte imbalances, their causes, and consequences:

Ion	Abnormality	Possible Causes	Consequences
Sodium	Hypernatremia, Hyponatremia	Dehydration, excessive water loss, SIADH, etc.	Thirst, confusion, neuromuscular irritability, CNS symptoms
Potassium	Hyperkalemia, Hypokalemia	Renal failure, aldosterone deficit, GI disturbances	Bradycardia, arrhythmias, muscle weakness, paralysis
Phosphate	Hyperphosphatemia, Hypophosphatemia	Renal failure, hypoparathyroidism, malnutrition	Neuromuscular excitability, muscle weakness

Central Role of Sodium

• Sodium is the most abundant cation in ECF; only cation exerting significant osmotic pressure.

• Controls ECF volume and water distribution; changes in Na+ affect plasma volume, blood pressure, and ECF/IF volumes.

• Na+ concentration determines ECF osmolality and influences excitability of neurons and muscles.

	ECF Na+ Concentration	Body Na+ Content
Homeostatic Importance	ECF osmolality	Blood volume and blood pressure
Sensors	Osmoreceptors	Baroreceptors
Primary Regulators	ADH and thirst mechanisms	Renin-angiotensin-aldosterone and ANP hormone mechanisms

Regulation of Sodium Balance

• No known receptors monitor Na+ levels directly; regulation is linked to blood pressure and volume control mechanisms.

• Aldosterone plays the biggest role in regulation by the kidneys; renin-angiotensin-aldosterone mechanism is the main trigger for aldosterone release.

• Other hormones: Atrial natriuretic peptide (ANP) reduces blood pressure and blood volume by inhibiting Na+ and water retention.

Regulation of Potassium, Calcium, and Phosphate Balance

• Potassium: Affects resting membrane potential in neurons and muscle cells; regulated by kidneys, aldosterone, and dietary intake.

• Calcium: 99% in bones; regulated by parathyroid hormone (PTH), which increases blood Ca2+ by acting on bones, kidneys, and intestines.

• Phosphate: Reabsorbed in the kidneys; regulated by PTH, insulin, and glucagon.

Acid-Base Balance

pH Regulation

• Normal arterial blood pH: 7.4; venous blood and IF: 7.35; ICF: 7.0.

• Alkalosis: arterial pH > 7.45; Acidosis: arterial pH < 7.35.

• Most H+ is produced as a by-product of metabolism.

Buffer Systems

• Chemical buffer systems: First line of defense; include bicarbonate, phosphate, and protein buffer systems.

• Respiratory system: Eliminates CO2 (an acid); acts within minutes.

• Renal mechanisms: Most potent; require hours to days to effect pH changes.

Bicarbonate Buffer System

• Mixture of H2CO3 (weak acid) and salts of HCO3– (weak base).

• Main ECF buffer; also operates in ICF.

• Regulated by the kidneys.

Phosphate Buffer System

• Salts of H2PO4– (weak acid) and HPO42– (weak base).

• Important buffer in urine and ICF.

Protein Buffer System

• Most important buffer in ICF; also in blood plasma.

• Proteins are amphoteric (can act as acid or base).

Respiratory Regulation of H+

• Respiratory system eliminates CO2 (an acid); reversible equilibrium exists in blood.

• Increased PCO2 or H+ stimulates increased respiratory rate and depth, removing CO2 and reducing H+.

• Alkalosis depresses respiratory center, decreasing respiratory rate and increasing H+.

Renal Regulation of Acid-Base Balance

• Kidneys regulate acid-base balance by adjusting the amount of bicarbonate in blood.

• Conserve (reabsorb) or generate new HCO3–; excrete HCO3– as needed.

• Renal regulation depends on the kidney’s ability to secrete or retain H+.

Generating New Bicarbonate Ions

• Occurs via excretion of buffered H+ (mainly phosphate buffer system) and via NH4+ excretion (glutamine metabolism).

Abnormalities of Acid-Base Balance

Types of Imbalances

• Respiratory acidosis/alkalosis: Caused by failure of the respiratory system to regulate pH (main indicator: blood PCO2).

• Metabolic acidosis/alkalosis: All other abnormalities (main indicator: abnormal HCO3– levels).

Compensation Mechanisms

• If one physiological buffer system fails, the other compensates (respiratory system compensates for metabolic imbalances, kidneys for respiratory imbalances).

• Respiratory compensation: Lungs adjust rate and depth of breathing to correct metabolic pH problems.

• Renal compensation: Kidneys adjust bicarbonate levels to correct respiratory pH problems.

Developmental Aspects

• Infants have proportionately more ECF than adults until about 2 years of age; higher risk of dehydration and acidosis.

• At puberty, males develop greater muscle mass, leading to differences in body water content.

• In old age, total body water decreases; homeostatic mechanisms slow down, increasing risk for dehydration and acid-base disturbances.