Fluid, Electrolyte, and Acid-Base Homeostasis Overview

Chapter 25: Fluid, Electrolyte, and Acid-Base Homeostasis Overview

I. Overview

A. Fluid Balance

  • Study of maintaining appropriate volume and concentration of body’s intracellular and extracellular fluids; largely focuses on water balance.

  • Factors influencing fluid balance:

    1. Amount of water ingested.

    2. Amount of physical activity.

    3. Kidney function.

    4. Medications.

    5. Digestive activities.

B. Electrolytes

  • Substances that dissociate into ions (charged particles) when placed in water, allowing them to conduct electricity.

  • Presence: Found in large quantities throughout the body.

C. Acids, Bases, and pH

  1. Acids: Chemicals that dissociate in water to release hydrogen ions (H+).

  2. Bases (Alkalis): Chemicals that accept hydrogen ions in a solution, resulting in a salt and water.

  3. Buffer Systems: Resist drastic changes in hydrogen ion concentration in body fluids.

  4. pH Scale: Measures hydrogen ion concentration of a solution.

    • pH < 7: Acidic

    • pH = 7: Neutral

    • pH > 7: Basic

II. Fluid Homeostasis

A. Total Body Water

  1. Reference Volume: Based on a “standard man” weighing 70 kg (154 lb); body water is about 60% of total body weight, approximately 42 kg (92.5 lb).

  2. Individual Variations: Total body water can vary based on gender, body mass, age, and amount of adipose tissue.

B. Body Fluid Compartments

  1. Intracellular Compartment (ICF):

    • Composed of trillions of cells and their cytosol.

    • Accounts for about 60% of body fluids (approximately 26 liters).

  2. Extracellular Compartment (ECF):

    • Composed of various body fluids.

    • Blood Plasma: About 8% (3 liters) of total body water.

    • Interstitial Fluid: About 32% (13 liters) of total body water.

III. Intracellular Solutes vs. Extracellular Solutes

A. Extracellular Solutes

  1. Concentrations of sodium, chloride, calcium, and bicarbonate ions are much higher in ECF than in cytosol (ICF).

  2. Plasma vs. Interstitial Fluid:

    • Plasma has a much higher protein content than interstitial fluid.

B. Intracellular (cytosol) Solutes

  1. Higher concentrations of proteins, potassium, magnesium, sulfate, and monohydrogen phosphate (HPO4^2–) ions in cytosol compared to ECF.

IV. Osmotic Movement of Water

A. Tonicity

  • Measurement of osmotic pressure gradient between two fluid components; commonly varies with fluid intake.

  • Osmotic pressure of ECF and cytosol is typically equal (isotonic), resulting in no net movement of water between compartments.

B. Osmotic Movement Between Compartments

  1. Hypotonic ECF:

    • Causes cell with higher osmotic pressure to pull water from the interstitial fluid and plasma, leading to cell swelling.

    • May occur after the consumption of large amounts of fluid.

  2. Hypertonic ECF:

    • Results in water being pulled out of the cell into plasma and interstitial fluid, causing cell shrinkage.

    • May occur from sweating without fluid replacement.

V. Water Losses and Gains

A. Water Loss

  1. Urine Production:

    • Majority of water lost daily via kidneys in urine; obligatory water loss is about 500 ml of urine daily, required to prevent toxic buildup of molecules and electrolyte imbalances.

  2. Feces:

    • About 100 ml of water lost in feces daily.

    • Both urine and feces loss are termed sensible water loss because it can be sensed.

  3. Insensible Water Loss:

    • Unnoticed (not sensed) daily water loss; typically around 600 ml from skin due to sweat and evaporation, and 300 ml lost in expired humidified air.

  4. Most individuals lose about 2.5 liters of water daily, subject to fluctuation based on water intake, physical activity, and food consumption.

B. Water Gain

  1. The body gains approximately 2.5 liters of water daily from three main sources:

    • Metabolic Water: Formed from all catabolic reactions; contributes around 250 ml daily (water of oxidation).

    • Food Intake: Adds about 750 ml daily.

    • Liquid Ingestion: Contributes approximately 1500 ml daily.

VI. Hormonal Regulation of Fluid Balance

A. Hormones Involved

  • Four hormones regulate mechanisms ensuring water input matches output:

  • ADH (Antidiuretic Hormone):

    • Triggers the insertion of aquaporins in the plasma membranes of kidney cells, facilitating water reabsorption from kidneys back to ECF.

    • Regulation:

      • Increased ADH → Increased water reabsorption.

      • Decreased ADH → Decreased water reabsorption.

  • Other Hormones:

    • Angiotensin-II, Aldosterone, and Atrial Natriuretic Peptide (ANP): also play roles in fluid balance.

Electrolyte Homeostasis

I. Sodium

A. Importance of Sodium Ions

  • Most abundant extracellular cation.

  • A steep ion gradient between cytosol and ECF is critical for all electrophysiological processes in the body.

B. Functions of Sodium

  1. Responsible for cell depolarization.

  2. Depolarization is a key event in all excitable cells (neurons and muscle types).

C. Regulation of Sodium Ions

  • Regulation is critical for Na+ balance and fluid homeostasis.

  • Water reabsorption in kidneys depends on a gradient consisting mainly of Na+ in interstitial fluid.

  • Hormones Influencing Regulation:

    1. Angiotensin-II and Aldosterone increase Na+ retention; RAAS activation is triggered by low extracellular Na+ concentrations.

    2. Atrial Natriuretic Peptide (ANP) decreases Na+ and water reabsorption.

II. Potassium

A. Role of Potassium Ions

  • Most abundant intracellular cation.

B. Maintaining Concentration Gradient

  • Steep concentration gradient maintained primarily by Na+/K+ ATPase pumps.

  • K+ moves out of the cell through plasma membrane channels during repolarization phase of an action potential, critical for returning excitable cells to resting membrane potential.

C. Regulation of Potassium Ions

  1. Various mechanisms within endocrine and urinary systems maintain normal K+ concentration.

  2. Hormones Involved:

    • Insulin, Aldosterone, and Epinephrine stimulate K+ uptake by cells, shifting K+ into intracellular compartments.

    • Excess K+ is secreted into urine.

III. Calcium and Phosphate Ions

A. Association in Bone

  • Ca2+ and PO4^3– are bound in hydroxyapatite crystalline structure in bone.

B. Physiological Functions of Calcium and Phosphate

  1. Calcium is essential for:

    • Muscle contraction.

    • Plateau phase of cardiac action potential.

    • Intracellular signaling and blood clotting.

    • Neuronal synaptic transmission.

  2. Phosphate anions (PO4^3–) are crucial for ATP formation.

C. Regulation

  1. Ca2+ levels are tightly maintained in plasma by bone tissue, kidneys, and small intestine.

    • When ECF Ca2+ levels drop, Ca2+ is released from bone by osteoclasts, reabsorbed in kidneys, and more readily absorbed from food.

    • When ECF Ca2+ levels increase, Ca2+ deposits in bone by osteoblasts, while less reabsorption occurs in kidneys and dietary absorption decreases.

  2. Hormones Involved:

    • Parathyroid Hormone (PTH) triggers osteoclast activity, Ca2+ reabsorption in kidneys, and inhibits PO4^3– reabsorption.

    • Vitamin D3 (calcitriol) stimulates Ca2+ absorption in the intestines and increases PO4^3– uptake.

IV. Chloride

A. Function

  • Chloride ions (Cl–) are abundant in ECF and are a critical osmotic particle alongside Na+.

  • Cl– forms HCl in stomach and assists in bicarbonate formation by erythrocytes.

B. Concentration Regulation

  • Cl– concentration in ECF is primarily determined by kidneys and is coupled with Na+ regulation.

V. Magnesium

A. Location

  • Mg2+ ions are located in bone, cytosol, and a small amount (~1%) in ECF.

B. Role in Cellular Processes

  • Critical in enzymatic activities, activating several enzymes involved in various biochemical pathways.

C. Regulation

  • Kidneys excrete most Mg2+ ingested; maintaining low plasma Mg2+ levels is vital because elevated Mg2+ blocks Ca2+ entry, potentially leading to hypercalcemia.

Acid-Base Homeostasis

I. pH Review

A. Normal pH Range

  • Normal H+ levels in body fluids are maintained within a pH range of 7.35–7.45, essential for preventing cellular damage.

B. Mechanisms for pH Maintenance

  • Blood pH is regulated by:

    1. Respiratory system.

    2. Urinary system.

    3. Chemical buffer systems.

II. Sources of Acids and Bases in the Body

A. Formation and Ingestion

  • Acids and bases are produced during metabolic processes or consumed through diet.

B. Metabolic Acids

  1. Carbon Dioxide (CO2):

    • A byproduct of glucose metabolism, serving as a volatile acid capable of converting between CO2 gas and carbonic acid (H2CO3).

    • Vital for acid elimination through kidneys and lungs.

  2. Fixed Acids:

    • Nonvolatile acids that must be eliminated by kidneys include:

      • Lactic acid (produced from glycolysis).

      • Uric acid (byproduct of nucleic acid catabolism).

      • Acidic ketone bodies (from fat metabolism).

      • Other acids as intermediates of citric acid cycle.

  3. Diet Contributions:

    • Dietary sources provide both acids and bases but do not significantly alter pH compared to metabolic reactions.

III. Chemical Buffer Systems

A. Function

  • Comprise a weak acid and its conjugate weak base, resist large swings in pH by binding or releasing H+.

B. Main Chemical Buffers

  1. Carbonic Acid–Bicarbonate Ion Buffer System:

    • Most important buffer system in blood: {CO2 + H2O
      ightleftharpoons H2CO3
      ightleftharpoons HCO_3^- + H^+}

  2. Phosphate Buffer System:

    • Composed of weak acid dihydrogen phosphate (H2PO4^–) and its conjugate base hydrogen phosphate (HPO4^2–).

    • Resists changes in pH in cytosol and kidney tubules.

  3. Protein Buffer System:

    • Consists of carboxylic acid groups (—COOH) in some amino acids, allowing ionization to release their weak conjugate base (—COO–).

    • These buffer systems account for 60–70% of the body's total buffering capacity (e.g., hemoglobin in erythrocytes).

IV. Physiological Buffers

A. Role of Organ Systems

  • Respiratory and urinary systems help regulate blood pH by ensuring a sufficient number of base ions.

  • Elimination of hydrogen ions is also part of their function.

B. Effects of the Respiratory System

  1. Controls CO2 levels, influencing the amounts of carbonic acid and hydrogen ions in blood.

  2. Affects the concentration of bicarbonate in plasma due to hemoglobin buffering H+, allowing bicarbonate to be released into plasma.

C. Effects of the Urinary System

  1. Works with lungs in the following ways:

    • Excretes fixed acids.

    • Regulates bicarbonate ion concentrations.

  2. Kidneys can produce new bicarbonate ions during blood acidosis (pH decreases) or secrete bicarbonate ions when blood is too alkaline (pH increases).

  3. Eliminates excess hydrogen ions in urine to maintain pH balance.

    • If filtrate pH is too acidic, hydrogen pumps become active.

    • This condition is known as limiting pH, which is around pH 4.5.

    • The distal tubular filtrate is buffered by the phosphate buffer system.

V. Acid-Base Imbalances

A. Acidosis

  1. Defined as body fluid pH less than 7.35.

  2. Develops when excess H+ is added beyond the capacity of buffer systems or when HCO3^- levels decrease.

  3. Causes decreased excitability of neurons, leading to signs and symptoms of nervous system depression.

B. Alkalosis

  1. Defined as body fluid pH greater than 7.45.

  2. Occurs when excess base ions are added beyond the buffer capacity or when H+ levels decrease.

  3. Increases neuron excitability, leading to inappropriate action potential firing.