Fluid and Electrolyte Balance

Human Anatomy & Physiology

Chapter 25 - Fluid and Electrolyte Balance

Part 1

Body Water Content

  • Varies with age and tissue type.

    • Skeletal muscle > adipose tissue.

    • Infants have a higher total body water content compared to the elderly.

Fluid Compartments

  • Intracellular Fluid (ICF): Represents 2/3 of total body fluids.

  • Extracellular Fluid (ECF): Comprises 1/3 of total body fluids and is subdivided into:

    • Interstitial Fluid: Forms 2/3 of the ECF.

    • Blood Plasma: Makes up 1/3 of the ECF.

  • Transcellular Fluid: A small portion of ECF, which includes several compartments such as:

    • Cerebrospinal Fluid.

    • Aqueous Humor.

    • Synovial Fluid (not directly involved in fluid balance).

Composition of Fluid Compartments

  • Intracellular Fluid:

    • Higher concentrations of:

    • Potassium ions [K+].

    • Magnesium ions [Mg+].

    • Proteins.

    • Phosphate ions [HPO4^2-].

  • Extracellular Fluid (Interstitial Fluid & Plasma):

    • Higher concentrations of:

    • Chloride ions [Cl-].

    • Sodium ions [Na+].

    • Bicarbonate ions [HCO3-].

Fluid Movement

  • Fluid movement occurs primarily due to osmosis of water.

Fluid Balance

Fluid Intake

  • Sources of fluid intake include:

    • Ingested water (H₂O).

    • Metabolic water produced by cellular respiration.

Fluid Output

  • Methods of fluid output include:

    • Breathing.

    • Sweating (cutaneous).

    • Defecation.

    • Urination.

  • Fluid output can be categorized as:

    • Sensible loss: Includes urine, defecation, and sweating.

    • Insensible loss.

    • Obligatory loss.

    • Facultative loss (variable).

Fluid Imbalance

  • Fluid imbalance can occur when:

    • Fluid intake does not equal fluid output.

    • Fluid is incorrectly distributed in compartments.

  • Changes in Osmolarity:

    • Dehydration: Water loss exceeds electrolyte loss, leading to hypertonic fluids.

    • Hypotonic hydration: Excessive water consumption without electrolytes.

  • Fluid Sequestration:

    • Accumulation of fluid in a specific area (e.g., edema).

    • Types of edema:

      • Systemic Edema.

      • Pulmonary Edema.

Regulation of Fluid Balance

  • Volume and concentration are indirectly monitored through:

    • Blood volume and pressure.

    • Blood osmolarity.

  • Regulation of Fluid Intake:

    • When fluid intake exceeds output:

    • Results in increased blood pressure and blood volume, possibly decreasing blood osmolarity.

    • Leads to decreased renin release (and subsequent RAA pathway activation).

    • Results in decreased aldosterone secretion and decreased thirst activation.

    • When fluid intake is less than output:

    • Results in decreased blood pressure and blood volume, possibly increasing blood osmolarity.

    • Stimulates increased renin release (and subsequent RAA pathway activation).

    • Leads to increased aldosterone secretion and increased thirst activation.

  • Thirst Center (Hypothalamus):

    • Stimulated by:

    • The RAA pathway (specifically angiotensin II).

    • Antidiuretic Hormone (ADH).

    • Decreased salivation.

    • Inhibited by:

    • The reversal of previous stimuli and stomach distention.

Regulation of Fluid Output

  • Controlled primarily through urine formation regulated by hormones, which include:

    • Renin.

    • Angiotensin II.

    • Antidiuretic Hormone (ADH).

    • Atrial Natriuretic Peptide (ANP).

Electrolyte Homeostasis

  • Electrolytes: Substances that dissociate into ions in solution, examples include:

    • Sodium ions [Na+].

    • Potassium ions [K+].

    • Chloride ions [Cl-].

    • Calcium ions [Ca2+].

    • Phosphate ions [PO4^3-].

    • Magnesium ions [Mg2+].

    • Hydrogen ions [H+] and bicarbonate ions [HCO3-].

Sodium Homeostasis

  • Sodium [Na+] is predominantly found in the ECF (99%).

  • Daily requirement: approximately 2 g/day; the average American consumes 3-7 g/day through table salt and processed foods.

  • Losses occur through urine, feces, and sweat, regulated by:

    • ADH.

    • Aldosterone.

    • ANP.

  • Importance of Sodium:

    • It is the most significant osmotic particle affecting blood pressure and volume.

  • Conditions Related to Sodium Levels:

    • Hypernatremia: Elevated blood sodium.

    • Hyponatremia: Low blood sodium.

Potassium Homeostasis

  • Potassium [K+] is primarily found in the ICF (98%) and is the principal cation involved in membrane potentials.

  • Losses occur mainly through urine.

  • K+ Imbalance: Potentially Lethal

    • Hyperkalemia: High plasma [K+] can lead to an irregular heartbeat.

    • Hypokalemia: Low plasma [K+] can cause paralysis of skeletal muscles.

  • K+ Shifts:

    • As blood pH changes, H+ ions move from ICF to ECF (or vice versa), prompting K+ ions to move in the opposite direction.

    • Insulin stimulates Na+/K+ pumps, influencing K+ levels in the bloodstream.

Chloride Homeostasis

  • Chloride [Cl-] is the most abundant anion found in ECF and is closely associated with Na+.

  • Functions include:

    • Contributing to hydrochloric acid (HCl) in the stomach.

    • Participating in ion shifts in red blood cells (RBCs).

  • Loss of chloride occurs through sweat, urine, and stomach secretions.

Calcium Homeostasis

  • Calcium [Ca2+] is 99% present in bone and teeth, playing crucial roles in:

    • Maintaining membrane potentials.

    • Neurotransmitter release.

    • Serving in second messenger systems.

    • Blood clotting and muscle contraction.

  • Losses of calcium occur through urine, feces, and sweat.

  • Calcium Imbalances:

    • Hypercalcemia: High calcium levels.

    • Hypocalcemia: Low calcium levels.

Phosphate Homeostasis

  • Phosphate [PO4^3-] is the most abundant anion found in ICF and is involved in:

    • Forming nucleotides and phospholipids.

    • 99% of phosphate is found in bone in the form of calcium phosphate (CaPO4).

  • Acts as an intracellular pH buffer through forms like HPO4^2- and H2PO4-.

  • Regulated by the same mechanisms applicable to calcium.

Magnesium Homeostasis

  • Magnesium [Mg2+] is the second most abundant cation in ICF and is essential for many enzymatic reactions, notably in:

    • ATP synthesis and protein synthesis.

Hormonal Regulation of Fluid and Electrolyte Balance

  • Key Hormones Involved in Fluid and Electrolyte Regulation:

    • Angiotensin I, Antidiuretic Hormone (ADH), Aldosterone, Atrial Natriuretic Peptide (ANP).

  • Angiotensin II:

    • Produced by the activation of the Renin-Angiotensin-Aldosterone (RAA) pathway.

    • Functions include:

    • Stimulating vasoconstriction of systemic blood vessels, thus increasing blood pressure.

    • Decreasing glomerular filtration rate (GFR) and urine formation.

    • Stimulating thirst center and the release of ADH and aldosterone.

  • Antidiuretic Hormone (ADH):

    • Released from the posterior pituitary gland in responses to low blood pressure or volume, high blood osmolarity, or angiotensin II presence.

    • Functions include:

      • Stimulating thirst center.

      • Increasing water reabsorption in the kidneys (via aquaporins in the collecting duct).

      • At high levels, can cause vasoconstriction of systemic blood vessels.

  • Aldosterone:

    • Secreted by the adrenal cortex in response to low blood pressure, low blood sodium, high blood potassium, or angiotensin II.

    • Stimulates:

      • Sodium and water reabsorption.

      • Excretion of potassium or hydrogen ions.

  • Atrial Natriuretic Peptide (ANP):

    • Released by atrial heart cells in response to high blood pressure or volume.

    • Stimulates:

      • Dilation of systemic blood vessels.

      • Dilation of the afferent arteriole and glomerulus, reducing reabsorption of sodium and water.

      • Inhibition of ADH, angiotensin II, and aldosterone.

Part 2 - pH Balance

pH Balance Overview

  • H+ concentration of arterial blood is approximately 7.4.

  • Influenced by:

    • Acids and bases entering or leaving the body.

    • Output through kidneys and respiration.

    • Buffers present in the body.

Types of Acids

  • Fixed Acids:

    • Produced from metabolic waste.

  • Volatile Acids:

    • Including carbonic acid (H2CO3).

Excess H+ Ions

  • Sources include dietary intake or metabolic processes.

  • Loss of bicarbonate ions (HCO3-) due to diarrhea leads to increased H+.

  • Kidneys respond by secreting H+ (into urine) or producing HCO3- to be reabsorbed.

Alkaline Conditions

  • Conditions characterized by low H+ ion concentration.

  • More common from:

    • Vegetarian diets.

    • Excessive antacid intake.

    • Vomiting leading to loss of H+.

  • Kidneys reabsorb H+ (back into blood) or produce HCO3- (that is secreted into urine).

Role of Respiration

  • Respiratory regulation helps remove excess carbon dioxide (CO2).

  • Increased CO2 leads to H+ concentration increase, lowering pH, which subsequently stimulates increased respiration.

Buffer Systems

  • Protein Buffers:

    • Located in plasma and ICF, accounting for roughly 3/4 of the buffering capacity.

    • Amine groups act as weak bases, while carboxylic groups act as weak acids.

  • Phosphate Buffering:

    • Primarily occurs in ICF.

    • Composed of HPO4²- (weak base) and H2PO4- (weak acid).

  • Bicarbonate Buffering:

    • Occurs in plasma.

    • Comprises HCO3- (weak base) and H2CO3 (weak acid).

Acid-Base Imbalance

  • Acidosis (Acidemia):

    • Blood pH falls below 7.35.

  • Alkalosis (Alkalemia):

    • Blood pH rises above 7.45, with levels beyond 7.0 or lower than 7.7 being life-threatening.

    • Indicates that buffering capacity has been exceeded.

Types of Acid-Base Imbalance

  • Respiratory Acidosis:

    • Caused by hypoventilation and decreased alveolar gas exchange.

    • The reaction: CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-.

  • Respiratory Alkalosis:

    • Caused by hyperventilation, which can result from anxiety, hypoxia, or aspirin overdose.

    • Diagnosis is based on low CO2 levels.

  • Metabolic Acidosis:

    • Due to increased lactic acid, ketoacidosis, or acetic acid or reduced kidney function leading to bicarbonate loss (HCO3-), often from diarrhea.

  • Metabolic Alkalosis:

    • Results from loss of H+ ions such as from vomiting or increased urine production (diuretics) and excessive intake of antacids.

K+ Shifts due to pH Changes

  • Increased blood H+ causes K+ to shift out of ICF into ECF (opposite movement of H+).

  • Decreased blood H+ causes K+ to shift into ICF, potentially leading to:

    • Hyperkalemia: High blood K+ levels.

    • Hypokalemia: Low blood K+ levels.

Compensation Mechanisms

Renal Compensation

  • In response to acidosis:

    • Increased H+ secretion.

    • Increased production and reabsorption of HCO3-.

  • In response to alkalosis:

    • Increased H+ reabsorption.

    • Increased secretion of HCO3-.

Respiratory Compensation

  • Adjustments made in respiration to compensate for acidosis or alkalosis, maintaining homeostasis of blood pH.