chapter 26

Chapter 26: Fluid, Electrolyte, and Acid-Base Balance

26.1 Body Fluid Compartments

  • Body Water Content

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

    • Adult Males: Approximately 60% water

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

    • Adipose tissue has the least hydration among body tissues

    • Total Body Water in Adults: Averages about 40 L

    • Aging Effect: Water content declines to about 45% in older age

26.1.1 Fluid Compartments

  • Major Fluid Compartments

    • Total Body Water: Volume = 40 L (60% of body weight)

    • Intracellular Fluid (ICF): Volume = 25 L (40% of body weight)

    • Extracellular Fluid (ECF): Volume = 15 L (20% of body weight)

    • Interstitial Fluid (IF): Volume = 12 L (80% of ECF)

    • Plasma: Volume = 3 L (20% of ECF)

26.2 Composition of Body Fluids

  • Solvent: Water is considered the universal solvent.

  • Solutes: Substances dissolved in water, classified into:

    • Nonelectrolytes: Do not dissociate in water, usually organic molecules. Examples: glucose, lipids, creatinine, urea.

    • Electrolytes: Dissociate into ions in water, examples include inorganic salts, all acids and bases, and some proteins. They conduct electrical currents and have greater osmotic power than nonelectrolytes.

    • Example of dissociation:

      • Sodium Chloride: NaCl<br>ightarrowNa++Cl−NaCl <br>ightarrow Na^+ + Cl^- (electrolyte generates 2 particles)

      • Glucose: extglucose<br>ightarrowextglucoseext{glucose} <br>ightarrow ext{glucose} (nonelectrolyte generates 1 particle)

26.2 Fluid Movement among Compartments

  • Regulation of Fluid Exchange: Osmotic and hydrostatic pressures manage continuous fluid exchange between compartments.

    • Water moves freely along osmotic gradients, with all body fluid osmolality being almost always equal.

    • Net Water Movement:

    • Increased ECF osmolality leads to water leaving cells.

    • Decreased ECF osmolality results in water entering cells.

26.3 Water Balance and ECF Osmolality

  • Daily Fluid Balance:

    • Total average intake: 2500 mL/day consisting of:

    • Beverages: 1500 mL (60% of intake)

    • Foods: 750 mL (30% of intake)

    • Metabolism: 250 mL (10% of intake)

    • Total average output: 2500 mL/day consisting of:

    • Urine: 1500 mL (60% of output)

    • Insensible loss: 700 mL (28% via skin and lungs)

    • Feces: 100 mL (4% of output)

    • Sweat: 200 mL (8% of output)

  • Osmolality Regulation:

    • Maintained around 280–300 mOsm.

    • Changes in osmolality affect thirst and ADH release:

    • Increase in osmolality: Stimulates thirst and ADH release.

    • Decrease in osmolality: Inhibits thirst and ADH release.

26.4 Thirst Mechanism

  • Regulation Mechanism:

    • ECF osmolality detected by osmoreceptors in the hypothalamus.

    • Dry saliva and mouth also serve as stimuli for thirst.

    • Granular cells in the kidney are involved in the Renin-angiotensin-aldosterone mechanism triggered by blood pressure changes.

    • Result: Thirst mechanism triggers water intake, contributing to the regulation of ECF osmolality and plasma volume.

26.5 Regulation of Water Output

  • ADH Release Mechanism:

    • Stimulated by changes in ECF osmolality and plasma sodium concentration.

    • When plasma volume decreases (by 5–10%) or blood pressure drops, baroreceptors respond to increase ADH release.

    • ADH acts on collecting ducts in kidneys, enhancing water reabsorption:

      • Results in scant urine output.

26.6 Disorders of Water Balance

  • Dehydration: Caused by ECF water loss due to various factors like hemorrhage or severe burns. Symptoms include:

    • Thirst, dry skin, “cottony” oral mucosa, oliguria.

    • Possible consequences: weight loss, fever, mental confusion, hypovolemic shock, electrolyte loss.

  • Hypotonic Hydration: Also termed water intoxication, leads to cellular overhydration.

    • Symptoms include nausea, vomiting, muscular cramping, and cerebral edema.

    • Treatment typically involves hypertonic saline.

  • Edema: Refers to the atypical accumulation of interstitial fluid leading to tissue swelling.

    • Impairs tissue function by increasing diffusion distances for oxygen and nutrients.

26.8 Electrolyte Balance

  • Overview: Primarily focused on the balance of salts, though it also includes acids, bases, and some proteins.

    • Salts play key roles in controlling fluid movements and providing necessary minerals for excitability, secretory activity, membrane permeability.

    • Salts enter body through ingestion/metabolism and exit through perspiration, feces, urine, and vomit.

26.9 Sodium's Role in Fluid and Electrolyte Balance

  • Sodium: The most abundant cation in ECF, with sodium salts contributing to 280 mOsm of the total 300 mOsm ECF solute concentration.

    • Sodium primarily controls ECF volume and water distribution; thus, changes in Na+ levels influence blood pressure and plasma volume.

  • Regulation of Sodium Balance: Involves various mechanisms as follows:

    • No specific receptors monitor Na+ levels, but blood pressure and volume control indirectly influence Na+ balance.

    • Aldosterone plays a significant role in regulating Na+, encouraging its reabsorption in kidneys, which correlates with water reabsorption.

    • Renin-angiotensin-aldosterone mechanism triggers aldosterone release in response to low Na+ levels or low blood pressure.

26.10 Regulation of Potassium Balance

  • Importance of Potassium: Key factor affecting resting membrane potential in neurons and muscle cells.

    • Changes in extracellular [K+] lead to hyperkalemia or hypokalemia, which can disrupt normal cardiac function.

  • Aldosterone also stimulates K+ secretion, controlling its own ECF concentrations through feedback loops.

26.11 Regulation of Calcium Balance

  • Calcium (Ca²+): Essential for numerous biological functions including blood clotting, cell membrane permeability, secretory activities, and neuromuscular excitability.

    • Hypocalcemia leads to increased excitability, while hypercalcemia can cause neuronal inhibition and heart arrhythmias.

26.12 Acid-Base Balance

  • Importance of pH Regulation: pH affects functional proteins and biochemical reactions, with normal body fluid pH around:

    • Arterial blood: pH 7.4

    • Venous blood: pH 7.35

    • Intercellular fluid: pH 7.0

  • Acid-Base Disorders: Acidosis (pH < 7.35) and alkalosis (pH > 7.45) significantly impact physiological functions.

26.13 Mechanisms of Hydrogen Ion Regulation

  • Chemical Buffer Systems: Provide rapid defense against pH changes:

    • Strong acids/bases and weak acids/bases counterbalance changes in [H+].

    • Major buffering systems include bicarbonate, phosphate, and protein buffer systems.

  • Bicarbonate Buffer System: Maintains pH homeostasis by utilizing weak acids and bases to mitigate drastic pH fluctuations.

    • Regulated closely by kidneys, able to bind or release H+ based on need, thus impacting ECF pH levels.

  • Respiratory Regulation of H+: Involves elimination of CO2, the respiratory system’s primary avenue for managing pH levels. Increased CO2 results in respiratory drive for deeper, more rapid breathing to expel CO2 and thus reduce acidity.

26.14 Renal Regulation of Acid-Base Balance

  • Kidneys' Role: Control excess acids and bases, primarily by adjusting HCO3- levels.

  • Adjustments: Include reabsorption or excretion of bicarbonate, while also secreting or retaining hydrogen ions.

26.15 Abnormalities of Acid-Base Balance

  • Imbalances: Classifications include respiratory and metabolic based on the primary origin of the disturbance.

    • Respiratory issues usually correlate with changes in blood PCO2, whereas metabolic imbalances are identified by abnormal HCO3- levels.

  • Effects of pH Imbalances: Extreme deviations lead to CNS depression (acidosis) or overexcitation (alkalosis), with possible fatal outcomes based on severity.

Major Body Fluid Compartments

  • Intracellular Fluid (ICF): Major compartment containing about 25 L (40% of body weight).

  • Extracellular Fluid (ECF): Comprises about 15 L (20% of body weight) with:

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

    • Plasma: 3 L (20% of ECF).

  • Majority of Body Water: Stored in intracellular fluid, which accounts for approximately 25 L.

Electrolyte Composition

  • Most Common Cation:

    • Interstitial Fluid: Sodium (Na+).

    • Intracellular Fluid: Potassium (K+).

Fluid Loss Routes

  • Major Routes of Fluid Loss:

    • Urine: Significant route, averaging about 1500 mL/day.

    • Insensible loss: Approximately 700 mL/day (via skin and lungs).

    • Feces: About 100 mL/day.

    • Sweat: About 200 mL/day.

  • Most Fluid Lost: Predominantly through urine.

Osmolality and Thirst Regulation

  • Normal Osmolality Range: 280–300 mOsm.

  • Triggering Thirst Mechanism: Factors include increased ECF osmolality detected by osmoreceptors in the hypothalamus, dry saliva, and mouth stimuli. Granular cells in kidneys involved in the Renin-angiotensin-aldosterone mechanism also play a part in thirst regulation.

Cell Shape Changes

  • Hypotonic Hydration: Cells may swell leading to potential cellular damage.

  • Dehydration: Cells shrink due to fluid loss.

Hyponatremia

  • Definition: Low sodium levels in blood.

  • Causes: Can occur due to excess water intake, certain medications, or heart failure.

  • Symptoms: Include nausea, headache, confusion, seizures.

Sodium and Water Balance

  • Na+-Water Balance: Closely tied to sodium (Na+) levels.

  • Homeostatic Mechanisms: Include renin-angiotensin-aldosterone mechanism, which leads to Na+ reabsorption and subsequently water retention. Hormones like aldosterone lead to water retention, and estrogen is associated with cyclical changes affecting this balance.

Potassium Balance

  • Hyperkalemia: Elevated potassium levels, which can lead to cardiac issues including arrhythmias.

  • Hypokalemia: Low potassium levels, potentially causing muscle weakness and cardiac problems.

Calcium Levels

  • Hypercalcemia: High calcium levels can cause confusion, muscle weakness, and severe physiological disturbances.

  • Hypocalcemia: Low calcium levels can lead to increased excitability of neurons and muscle cramps.

Acid-Base Balance

  • Normal Blood pH: Arterial blood normal range is approximately 7.35-7.45.

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

  • Buffering Mechanisms: Chemical buffers (bicarbonate, phosphate, protein systems), with bicarbonate being the primary.

  • Bicarbonate Buffer System: Maintains pH through reversible reactions involving carbonic acid (H2CO3) and bicarbonate ions (HCO3−).

  • CO2, H+, and pH Connection: Increased CO2 leads to increased H+ (increased acidity) and decreased pH; similar patterns apply inversely.

  • Respiratory Acidosis/Alkalosis: Characterized by abnormal levels of CO2.

  • Metabolic Acidosis/Alkalosis: Characterized by abnormal levels of bicarbonate (HCO3−).

Renal Mechanism

  • Key Renal Mechanism: Adjusts bicarbonate (HCO3−) levels, with reabsorption or excretion of bicarbonate and secretion or retention of H+.

Compensation Mechanisms

  • Respiratory Compensation for Metabolic Alkalosis: Respiratory rate decreases to retain CO2 and increase acidity.

  • Respiratory Compensation for Metabolic Acidosis: Increased respiratory rate to expel CO2, reducing acidity.

  • Kidney Compensation for Respiratory Alkalosis: May excrete more bicarbonate to normalize pH.

  • Kidney Compensation for Respiratory Acidosis: May retain bicarbonate to raise pH back towards normal.