Chpt.25-Water, Electrolyte Body Fluids and Acid-Base Balance

Body Fluids

• Water Balance:

  • Regulation of body water through intake and output.

  • Intake should equal output under normal conditions.

Fluid Compartments

• Extracellular Compartment:

  • Consists of:

  • Interstitial fluid

  • Plasma

  • Lymph

  • Transcellular fluid

• Transcellular Fluids:

  • Includes:

  • Cerebrospinal fluid

  • Aqueous humor

  • Vitreous humor

  • Synovial fluids (in cavities)

  • Glandular secretions

Composition of Body Fluids

• Extracellular Fluid (ECF):

  • High ion concentrations of:

  • Sodium (Na⁺)

  • Chloride (Cl⁻)

  • Bicarbonate (HCO₃⁻)

  • Contains most proteins.

• Intracellular Fluid (ICF):

  • High ion concentrations of:

  • Potassium (K⁺)

  • Phosphate (PO₄³⁻)

  • Magnesium (Mg²⁺)

Water Intake and Output

• Water Intake:

  • Approximately 2,500 mL/day.

  • Sources:

  • 60% from liquids

  • 30% from food

  • 10% from metabolic breakdown of food.

• Water Output:

  • Primary means through kidneys (60%).

  • Other means:

  • Feces (6%)

  • Sweat (6%)

Fluid Shifts

• Causes of Fluid Shift:

  • Prevents hypovolemia due to water loss.

  • Responds to diseases and disorders.

• Forces Involved in Fluid Shift:

  • Capillary filtration pressure

  • Plasma oncotic pressure

  • Lymphatic drainage

  • Plasma protein trapped in tissue.

Fluid Spacing

• Terminology:

  • First Spacing: Normal distribution of body fluids.

  • Second Spacing: Accumulation of water in interstitial spaces (e.g., edema).

  • Third Spacing: Accumulation in spaces where absorption is difficult (e.g., ascites).

  • Examples: Pedal edema, paralytic ileus.

Daily Weights and Fluid Balance

• Assessing daily weights provides a slim estimate of fluid balance.

• Example: In heart failure, 4.4 lbs weight gain may indicate 2L fluid retention.

Electrolyte Balance

• Definition: When electrolytes gained equal electrolytes lost.

• Importance: Body fluids require electrolytes to maintain homeostasis.

  • Key electrolytes:

  • Sodium (Na⁺)

  • Potassium (K⁺)

  • Chloride (Cl⁻)

  • Bicarbonate (HCO₃⁻)

Acid-Base Balance

• Homeostasis: Body maintains pH through:

  • Buffers

  • Respiratory activity

  • Kidney function

• Buffer Systems:

  • First line of defense against pH changes.

  • Example: Bicarbonate buffer can either donate or absorb H⁺ ions.

The Role of Lungs and Kidneys in pH Regulation

• Lungs:

  • They control pH by exhaling CO₂.

  • Hypoventilation leads to acidosis, while hyperventilation leads to alkalosis.

• Kidneys:

  • They can reabsorb or excrete H₃O⁺ and HCO₃⁻ to regulate pH.

Compensation Mechanisms for Acid-Base Imbalances

• Types of Imbalances:

  • Acidosis (pH < 7.35)

  • Alkalosis (pH > 7.45)

• Examples include:

  • Respiratory Acidosis: Caused by hypoventilation.

  • Metabolic Acidosis: Caused by conditions like kidney disease or severe diarrhea.

  • Respiratory Alkalosis: Often due to hyperventilation (e.g., anxiety).

  • Metabolic Alkalosis: Caused by excessive loss of HCl (e.g., persistent vomiting).

Body Fluids

Water Balance:
Regulation of body water occurs through a delicate balance of intake and output, ensuring that the overall water volume within the body remains stable. Under normal physiological conditions, water intake should equal water output to maintain homeostasis. Both factors can be influenced by various parameters, including diet, physical activity, and environmental conditions.

Fluid Compartments
The body is organized into several fluid compartments, primarily classified as:

Extracellular Compartment:

• Consists of:

  • Interstitial fluid: The fluid that surrounds the cells but is not part of the bloodstream.

  • Plasma: The liquid component of blood that carries cells, nutrients, hormones, and waste products.

  • Lymph: A fluid that is part of the immune system, transporting white blood cells and filtering out toxins.

  • Transcellular fluid: This includes specialized fluids such as cerebrospinal fluid, synovial fluid, and various glandular secretions.

Transcellular Fluids:

• Includes:

  • Cerebrospinal fluid: Protects and cushions the brain and spinal cord.

  • Aqueous humor: Helps maintain intraocular pressure and provides nutrients to the eye.

  • Vitreous humor: A gel-like substance that fills the space between the lens and retina of the eye.

  • Synovial fluids: Provide lubrication to joints, facilitating smooth movements.

  • Glandular secretions: Include digestive juices, saliva, and mucus produced by various glands.

Composition of Body Fluids
Understanding the composition of body fluids is vital in assessing physiological and pathological conditions.

Extracellular Fluid (ECF):

• Characterized by high concentrations of:

  • Sodium (Na⁺): The most abundant electrolyte in ECF, crucial for maintaining fluid balance and transmitting nerve impulses.

  • Chloride (Cl⁻): Works with sodium to maintain osmotic pressure and fluid balance.

  • Bicarbonate (HCO₃⁻): Helps maintain the body's acid-base balance.

• The ECF contains most of the body’s proteins, which are essential for various physiological functions.

Intracellular Fluid (ICF):

• Characterized by high concentrations of:

  • Potassium (K⁺): Vital for cellular function, particularly in nerve and muscle cells.

  • Phosphate (PO₄³⁻): Plays a crucial role in energy transfer through ATP.

  • Magnesium (Mg²⁺): Involved in numerous enzymatic processes within cells.

Water Intake and Output
Water intake is critical for maintaining hydration and overall health:

• Water Intake:

  • Approximately 2,500 mL/day is recommended for adults.

  • Sources:

  • 60% from liquids such as water, tea, and juice.

  • 30% from food, particularly fruits and vegetables that have high water content.

  • 10% from metabolic processes, where water is produced during cellular respiration.

• Water Output:

  • The primary means of water output is through kidneys, which account for approximately 60% of fluid loss via urine.

  • Additional output occurs through:

  • Feces (6%): Water present in stool.

  • Sweat (6%): Evaporation of moisture from the skin aids in temperature regulation.

Fluid Shifts
Fluid shifts refer to the movement of fluid between compartments to maintain equilibrium:

• Causes of Fluid Shift:

  • Fluid shifts are crucial for preventing hypovolemia (a decrease in blood volume) due to water loss, such as in dehydration or hemorrhage.

  • They also respond to diseases and disorders, such as heart failure or renal impairment.

• Forces Involved in Fluid Shift:

  • Capillary filtration pressure: The force that pushes fluid out of the capillaries into the interstitial space.

  • Plasma oncotic pressure: The pull exerted by proteins in the plasma that retains fluid within the circulatory system.

  • Lymphatic drainage: Helps remove excess interstitial fluid and return it to the bloodstream.

  • Plasma proteins trapped in tissue can also affect fluid distribution by influencing osmotic pressure.

Fluid Spacing
Understanding fluid spacing is essential in clinical assessments:

• Terminology:

  • First Spacing: Normal distribution of body fluids within compartments.

  • Second Spacing: Represents an abnormal but not severe accumulation of fluid in interstitial spaces, often referred to as edema.

  • Third Spacing: Refers to significant accumulation of fluid in abnormal spaces where absorption is challenging, such as in ascites—a condition where fluid builds up in the abdominal cavity.

• Examples of Fluid Spacing Issues:

  • Pedal edema: Swelling in the feet and ankles often due to congestive heart failure or renal insufficiency.

  • Paralytic ileus: A condition where the intestines do not move food properly, leading to fluid accumulation.

Daily Weights and Fluid Balance
Monitoring daily weights is a practical method to estimate fluid balance:

• Importance of Assessing Daily Weights:

  • An increase or decrease in weight can indicate fluid retention or loss, which is particularly crucial in patients with conditions affecting fluid balance, such as heart failure or kidney diseases.

  • Example: A 4.4 lbs weight gain may indicate approximately 2L of fluid retention, necessitating further assessment and intervention.

Electrolyte Balance
Electrolyte balance is critical for physiological function and homeostasis:

• Definition:
  Electrolyte balance exists when the amount of electrolytes gained equals the amount lost.

• Importance:
  Body fluids require electrolytes to maintain physiological processes, such as nerve conduction, muscle contraction, and hydration.

• Key Electrolytes:

  • Sodium (Na⁺)

  • Potassium (K⁺)

  • Chloride (Cl⁻)

  • Bicarbonate (HCO₃⁻)

Acid-Base Balance
Homeostasis of body pH is vital for cellular functions:

• Mechanisms for pH Maintenance:

  • The body maintains proper pH levels through various mechanisms, including the use of buffers, respiratory activity, and kidney function.

• Buffer Systems:

  • Buffers act as the first line of defense against pH changes, helping to neutralize excess acids or bases.

  • Example: The bicarbonate buffer system can donate or absorb hydrogen ions (H⁺) to maintain pH stability.

The Role of Lungs and Kidneys in pH Regulation
Both the lungs and kidneys play integral roles in regulating pH:

• Lungs:

  • The lungs control pH by regulating the exhalation of carbon dioxide (CO₂); increased CO₂ levels lead to increased acidity.

  • Hypoventilation (not breathing enough) leads to acidosis (lower pH), while hyperventilation (breathing too fast) can induce alkalosis (higher pH).

• Kidneys:

  • The kidneys help regulate acid-base balance by reabsorbing or excreting hydrogen ions (H₃O⁺) and bicarbonate ions (HCO₃⁻), which helps modify blood pH.

Compensation Mechanisms for Acid-Base Imbalances
The body has several mechanisms to compensate for acid-base imbalances:

• Types of Imbalances:

  • Acidosis (pH < 7.35): Results in an excess of hydrogen ions or bases in the body.

  • Alkalosis (pH > 7.45): Results in a deficiency of hydrogen ions or excess of bases.

• Examples of Acid-Base Disturbances:

  • Respiratory Acidosis: Often caused by hypoventilation or impaired gas exchange, leading to increased CO₂ levels.

  • Metabolic Acidosis: May result from kidney disease, severe diarrhea, or lactic acidosis due to hypoxia.

  • Respiratory Alkalosis: Generally due to hyperventilation, often resulting from anxiety, pain, or altitude changes.

  • Metabolic Alkalosis: Can occur due to excessive loss of HCl, such as during persistent vomiting, affecting overall acid-base homeostasis.