Fluid and Electrolyte Balance

Chapter 19: Fluid and Electrolyte Balance

Learning Objectives:
- List, describe, and compare body fluid compartments and their subdivisions.
- Discuss the avenues by which water enters and leaves the body and the forces that move fluids into and out of the blood.
- Explain the mechanisms used by the body to maintain fluid balance.
- Discuss the nature and importance of electrolytes in body fluids.
- Describe examples of common fluid and electrolyte imbalances.

Nature of Water:
- Water is the most abundant compound in the body.
- Average body water volume is considered based on a healthy, non-obese male weighing 70 kg (154 lbs).
- Composition:
- Males: Water is approximately 60% of body weight.
- Females: Water is approximately 50% of body weight.
- Total body water volume averages 40 L in a 70-kg male.

Relative Volumes of Body Fluids
- Extracellular Fluid (ECF):
- Comprises 3 L of plasma and 12 L of interstitial fluid.
- Intracellular Fluid (ICF):
- Contains about 25 L and serves as a solvent facilitating intracellular reactions.
Types of Body Fluids:
- Extracellular Fluid (ECF):
- Considered the internal environment of the body.
- Surrounds cells, facilitating transport of substances.
- Subtypes:
  - Plasma: Liquid portion of whole blood.
  - Interstitial Fluid: Fluid surrounding the cells.
  - Transcellular Fluid: Includes lymph, joint fluids, cerebrospinal fluids, and humors of the eye.
- Intracellular Fluid (ICF):
- The largest compartment, crucial for chemical reactions.

Sources of Fluid Intake:
- Liquids consumed, water contained in food, and metabolic water produced during metabolism.
Sources of Fluid Output:
- Loss via water vapor, perspiration, urine, and water content in feces.
Key Regulatory Factors:
- Maintain fluid balance through three factors:
- Regulating volume of fluid output.
- Regulating volume of fluid intake.
- Exchanging fluid across compartments and throughout the body.

Fluid Adjustment:
- Urine volume adjusts to match fluid intake.
- Antidiuretic Hormone (ADH):
- Released from the posterior pituitary gland.
- Increases reabsorption of sodium and water from the kidney tubules into the blood.
- Functions to increase ECF (and total body fluid) by decreasing urine volume, thereby maintaining fluid balance.

Influences on ECF Volume:
- Electrolyte concentration in ECF impacts its volume.
- Increase in ECF concentration results in more fluid movement out of ICF and stimulates ADH secretion, which subsequently decreases urine volume.

Capillary Dynamics:
- Capillary blood pressure promotes the movement of water from blood into interstitial fluid.
- Conversely, increased blood protein concentration encourages the movement of water back into the blood from interstitial fluid.
- These forces work together to regulate plasma and interstitial fluid volumes.

Function of Aldosterone:
- Increases ECF volume.
- If external factors (e.g., not eating for 24 hours) decrease ECF, urine volume drops, impacting arterial blood pressure.
- Increases kidney tubule reabsorption of water and sodium, ultimately triggering the renin-angiotensin-aldosterone system.

Dehydration:
- Characterized by total body fluid volume less than normal.
- Initial impact on interstitial fluid volume, followed by ICF and plasma volume if treatment is not initiated.
- Occurs when fluid output exceeds intake over a prolonged period.
Overhydration:
- Total body fluid volume greater than normal.
- Results from fluid intake exceeding output, often due to excessive IV fluid administration or rapid intake.
Water Intoxication:
- Can occur from rapidly consuming excessive amounts of water or administering hypotonic solutions to patients unable to excrete urine effectively.

Definitions:
- Nonelectrolytes:
- Organic substances that do not dissociate in water (e.g., glucose).
- Electrolytes:
- Compounds that dissociate into ions when dissolved in water (e.g., sodium chloride).
Electrical Charge of Ions:
- Ions:
- Dissociated particles of electrolytes that carry an electric charge.
- Cations:
- Positively charged ions (e.g., sodium [Na⁺] and potassium [K⁺]).
- Anions:
- Negatively charged ions (e.g., chloride [Cl⁻] and bicarbonate [HCO₃⁻]).

Electrolytes play crucial roles in various cellular functions including:
- Nerve Conduction: Essential for transmitting signals.
- Muscle Contraction: Necessary for muscle function, including the heart.
Sodium (Na⁺):
- Most abundant positively charged ion in plasma: 142 mEq/L.
- Average dietary intake: 100 mEq.
- Regulation is primarily through the kidneys, with aldosterone promoting sodium reabsorption.

Internal Secretions:
- Saliva: 1500 mL
- Gastric Secretions: 2500 mL
- Pancreatic Secretions: 500 mL
- Bile: 500 mL
- Intestinal Secretions: 3000 mL

Electrolyte Balance:
- Related to intake and output of specific electrolytes.
Common Sodium Imbalances:
- Hypernatremia: Plasma sodium level >145 mEq/L.
- Hyponatremia: Plasma sodium level <136 mEq/L.
Common Potassium Imbalances:
- Hyperkalemia: Blood potassium level >5.1 mEq/L.
- Hypokalemia: Blood potassium level <3.5 mEq/L.
Common Calcium Imbalances:
- Hypercalcemia: Elevated blood calcium levels.
- Hypocalcemia: Low blood calcium levels.

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