chapter 24

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

Big Idea of Fluid Balance

  • Cell function necessitates a fluid medium with a carefully regulated composition.
  • Alterations in this balance can result in significant physiological issues.

Types of Homeostatic Balance

  • Fluid Balance: Achieved when daily gains equal losses of bodily fluids.
  • Electrolyte Balance: Maintained when electrolytes consumed match the electrolytes excreted from the body.
  • Acid-Base Balance: Occurs when hydrogen ions (H⁺) are expelled from the body at the same rate they are produced.

Expected Learning Outcomes

  • Identify and describe the major fluid compartments and the movement of water between them.
  • List various sources of water intake and output routes associated with water loss.
  • Explain the mechanisms that regulate water intake and output effectively.
  • Analyze conditions leading to water deficiency or excess in the body.
  • Understand the physiological functions of key electrolytes: sodium, potassium, calcium, magnesium, chloride, and phosphate.
  • Discuss the hormonal and renal processes controlling electrolyte concentrations.
  • Define the concept of a buffer and present the three main buffer systems.
  • Correlate pulmonary ventilation with pH levels of extracellular fluids and the bicarbonate buffer system.
  • Examine acidosis and alkalosis and their consequences on body pH imbalances.

Total Body Water Distribution

  • Body Water Percentage: Varies based on biological sex, age, and overall body composition.
  • In a 70-kg (150 lbs) young male, total body water is approximately 40 L or 55 - 60% of body weight.
  • **Fluid Compartments:
    • 65% intracellular fluid (ICF)
    • 35% extracellular fluid (ECF):
      • 25% in tissue (interstitial) fluid
      • 8% in blood plasma and lymphatic fluid
      • 2% in transcellular fluid (in diverse locations)

Water Movement Between Fluid Compartments

  • Water transitions across capillary walls through capillary filtration into interstitial fluid, followed by osmosis across plasma membranes.
  • The direction of osmotic movement (into or out of cells) depends on the relative solute concentrations in each compartment:
    • ICF: Dominated by potassium salts.
    • ECF: Dominated by sodium salts.

Water Gain and Loss Mechanisms

  • Water Gain Sources:
    • Cellular metabolism
    • Preformed water from food and drink.
  • Water Loss Routes:
    • Sensible Loss: Measurable losses such as urine.
    • Insensible Loss: Not directly measurable, including:
    • Cutaneous transpiration - water diffused through the skin.
    • Variability with physical activity and environmental conditions (e.g., respiratory loss in cold temperatures).
    • In hot weather, perspiration can increase loss up to 1200 mL/day.

Regulation of Water Intake

  • Water intake primarily controlled by thirst, which is regulated by several physiological mechanisms:
    • Thirst sensation decreases salivation due to hypothalamic stimulation inhibiting salivary glands and responding to low capillary blood pressure/high blood osmolarity.
    • Short-term mechanisms restrict excessive intake, lasting around 30-45 minutes.
    • Long-term effects include reduced blood osmolarity leading to hypothalamic osmoreceptor inhibition of thirst.

Regulation of Water Output

  • Water output correlated with urine volume, regulated through:
    1. Adjustments in sodium (Na⁺) reabsorption, where the water follows Na⁺ either being reabsorbed or excreted.
    2. Antidiuretic hormone (ADH) stimulating kidney collecting ducts to produce aquaporin proteins, which allow increased water flow into kidney extracellular fluid, reducing urine volume.
  • As urine concentration increases, the Na⁺: water ratio in urine rises while [Na⁺] in blood drops, providing negative feedback to the hypothalamus, inhibiting further declines in blood volume.

Disorders of Fluid Balance

  • Volume Depletion (Hypovolemia): Loss of proportional amounts of water and Na⁺ due to:
    • Blood loss
    • Severe burns
    • Chronic vomiting/diarrhea.
  • Dehydration: Occurs when water loss exceeds Na⁺ loss, primarily due to inadequate water intake (e.g., extreme temperatures), leading to dehydration impacts across fluid compartments.
  • Fluid Volume Excess: Both water and Na⁺ retention, often observed in renal failure.
  • Water Intoxication: More water retained than Na⁺, such as if excessive sweating is countered only by plain water intake.

Electrolyte Balance and Importance

  • Electrolyte Balance Defined: A state where electrolytes absorbed through the small intestine equal losses from the body.
  • Physiological Roles of Electrolytes:
    • Participate in chemical reactions and metabolism.
    • Determine membrane electrical potential across cells.
    • Strongly influence osmolarity and body fluid distribution.
  • Major Electrolyte Cations:
    • Sodium (Na⁺), Potassium (K⁺), Calcium (Ca²⁺), Magnesium (Mg²⁺), Hydrogen (H⁺).
  • Major Electrolyte Anions:
    • Chloride (Cl⁻), Bicarbonate (HCO₃⁻), Phosphate (PO₄³⁻).

Electrolyte Concentrations: ECF vs. ICF

  • Typical Concentrations in mEq/L:
    • Sodium (Na⁺): ECF: 145; ICF: 12
    • Potassium (K⁺): ECF: 4; ICF: 150
    • Chloride (Cl⁻): ECF: 103; ICF: 4
    • Calcium (Ca²⁺): ECF: 5; ICF: 0.0001
    • Magnesium (Mg²⁺): ECF: 2; ICF: 40
    • Phosphate (Pi): ECF: 4; ICF: 75
    • Total Osmolarity H (mOsm/L) for blood plasma ~300, ICF ~150.

Sodium Functions and Homeostasis

  • Functions of Na⁺:
    • Facilitates electrical signaling in nerves and muscles.
    • Maintains hydration of cartilage.
    • Key in determining total body water and distribution of water across compartments.
    • Na⁺ gradient serves as a potential energy source for co-transport of solutes (e.g., glucose, potassium, calcium).
  • Homeostatic Imbalances:
    • Adults generally need about 0.5 g/day of sodium; the typical American diet often comprises 3 to 7 g/day.
    • Primary concern centers on the excretion of excess sodium.

Mechanisms to Maintain Sodium Homeostasis

  • Aldosterone Function:
    • The “salt-retaining hormone” that primarily adjusts sodium excretion.
    • Stimulated by:
    • Low sodium (hyponatremia) and high potassium (hyperkalemia) levels.
    • Hypotension through the renin-angiotensin-aldosterone mechanism.
    • Inhibition of Aldosterone: Occurs with high blood pressure.
  • Effects of Aldosterone:
    • Decreases NaCl in urine while increasing potassium excretion and lowering urine pH.
  • Influence on ADH:
    • High Na⁺ promotes ADH secretion, enhancing water resorption; low Na⁺ has the opposite effect.
  • Natriuretic Peptides: Inhibit Na⁺ and water reabsorption.

Potassium Functions and Homeostasis

  • Functions of K⁺:
    • Most abundant intracellular cation influencing osmolarity and cell volume.
    • Accompanies Na⁺ in producing membrane potential and action potentials.
    • Essential for the Na⁺−K⁺ pump and protein synthesis processes.
  • Regulation by Aldosterone:
    • The more sodium excreted, the less potassium in urine and vice versa.

Calcium Functions and Homeostasis

  • Calcium Functions (Ca²⁺):
    • Provides structural strength to bones.
    • Activates sliding filament mechanism for muscle contractions.
    • Serves as a secondary messenger for hormones and neurotransmitters.
    • Key in exocytosis of neurotransmitters and cellular secretions.
    • Essential for blood clotting processes.
  • Homeostatic Regulation:
    • Calcitriol (Vitamin D) and Parathyroid Hormone (PTH) elevate blood calcium levels.
    • Calcitonin works to decrease blood calcium levels.

Other Electrolytes: Functions and Homeostasis

  • Chloride (Cl⁻):
    • Main anion in ECF, affecting its osmolarity and involved in stomach acid formation (HCl).
    • Plays a significant role in regulating body pH.
  • Magnesium (Mg²⁺):
    • Functions as a cofactor in various enzymatic reactions, transport processes, and nucleic acid stabilization.
    • Absorption from food regulated by Vitamin D, with losses occurring in feces and urine.
  • Phosphate (PO₄³⁻):
    • Essential for ATP-related processes and helps in pH stabilization.
    • Continuously lost via glomerular filtration, with renal tubules reabsorbing filtered phosphate as plasma concentration drops.

Acid-Base Balance

  • Importance of Acid-Base Balance:
    • Metabolism relies on enzyme function sensitive to pH.
    • Even slight deviations from normal pH (7.35 – 7.45) can disrupt metabolic pathways and disturb macromolecular structure and function.
    • Critical aspect of homeostasis, challenging due to constant acid production during metabolism.
  • Buffering Mechanisms: Stabilize internal pH and help maintain balance.

Acids, Bases, and Buffers: Chemistry Basics

  • pH Determination: Governed entirely by hydrogen ion (H⁺) concentration.
  • Acids:
    • Substances releasing H⁺ in solution (e.g., strong acids like hydrochloric acid (HCl) ionize freely, lowering pH).
  • Weak Acids: (e.g., carbonic acid (H₂CO₃)) ionize slightly, maintaining most H⁺ bound and minimally affecting pH.
  • Bases:
    • Compounds that accept H⁺ (e.g., strong bases like hydroxide ion (OH⁻)).
  • Weak Bases: (e.g., bicarbonate ion (HCO₃⁻)) have less effect on pH as they bind fewer H⁺ ions.
  • Buffer Defined: Mechanisms that resist pH changes by forming weak acids/bases from strong acids/bases to maintain pH within the normal range.

Categories of Buffers

  • Physiological Buffers: Control output of acids, bases, or CO₂ (e.g., urinary system manages acids/bases but requires several hours to days; respiratory buffers act faster but with less pH alteration potential).
  • Chemical Buffers: Bind H⁺ to reduce its concentration in solution quickly, working in fractions of a second.
  • Major Chemical Buffers: Include bicarbonate, phosphate, and protein systems. The effectiveness is determined by buffer concentration and the pH of the environment.

Bicarbonate Buffer System

  • Composition: A solution containing carbonic acid and bicarbonate ions.
  • Reversible Reaction:
    • CO_2 + H_2O
      ightleftharpoons H_2CO_3
      ightleftharpoons HCO_3⁻ + H^+
    • Determines pH changes based on reaction direction:
    • Left shift (producing HCO₃⁻) raises pH by binding H⁺.
    • Right shift (producing H⁺) lowers pH.
  • Kidneys and Lungs Role:
    • To lower pH, kidneys excrete HCO₃⁻.
    • To raise pH, kidneys excrete H⁺, and lungs exhale CO₂.

Phosphate Buffer System

  • Composition: Comprises monohydrogen phosphate (HPO₄²⁻) and dihydrogen phosphate (H₂PO₄⁻).
  • Reversible Reaction:
    • H_2PO_4^-
      ightleftharpoons HPO_4^{2-} + H^+
  • Buffering Role: Provides significant buffering in intracellular fluid and renal tubules, helping to balance metabolic pH variations (average pH = 7.0) in a range from 4.5 to 7.4.

Protein Buffer System

  • Role: Crucial in managing pH fluctuations across body fluids, constituting about 75% of chemical buffering.
  • Mechanism: Proteins contain side groups that help in buffering:
    • Carboxyl (−COOH): Acts as a weak acid releasing H⁺ when pH rises.
    • Amino (−NH₂): Functions as a weak base by binding H⁺ when pH falls, increasing pH.

Relationship Between Pulmonary Ventilation, pH, and Bicarbonate Buffer System

  • Respiratory Buffer System Role: Adjusts body fluid pH by modifying breathing depth/rate.
    • High CO₂ levels contribute to lower pH (more H⁺).
    • Conversely, removal of CO₂ increases pH (less H⁺).
  • Receptors: Increased CO₂ and lowered pH stimulate chemoreceptors enhancing pulmonary ventilation, while decreased CO₂ raises pH and reduces ventilation.

Effects of pH Imbalance

  • Acidosis Effects:
    • Leads to membrane hyperpolarization, causing nerve/muscle stimulation difficulty.
    • May result in CNS depression, confusion, disorientation, coma, and death.
  • Alkalosis Effects:
    • Causes cell depolarization, overstimulating nerves, resulting in muscle spasms, tetany (sustained muscle contraction), convulsions, and respiratory paralysis.
  • Criticality of pH Levels: Blood pH below 7.0 or above 7.7 is life-threatening.

Nervous System and pH Imbalance Relationship

  • In Acidosis: Elevated extracellular fluid (ECF) H⁺ leads to excess H⁺ entering intracellular fluid (ICF) where K⁺ diffuses out, making resting membrane potential more negative (hyperpolarized) - threshold more difficult to reach.
  • In Alkalosis: Low ECF H⁺ drives H⁺ into ECF while K⁺ enters ICF, enhancing positive charge in ICF and increasing action potential frequency, potentially resulting in spams and tetany.