Body Fluid Compartments and Water Balance

Learning Objectives

  • Role of Water: Understand how water functions in the gut lumen and throughout the body.
  • Fluid Compartments: Identify main fluid compartments, including volumes and predominant cations.
  • Regulation of Body Water and Sodium: Explain mechanisms that regulate total body water and sodium in response to plasma volume and plasma osmolality.
  • Failures of Body Fluid Balance: Know types of fluid balance failures and their underlying causes.

Body Water Content

  • Percentage of Total Body Weight:
    • Newborn: 80%
    • Adult Male: 60%
    • Adult Female: 50%

Water Balance

  • Water Gain:

    • Food: 30%
    • Drink: 60%
    • Metabolism: 10%
    • Total Gain: 90%

  • Water Loss:

    • Urine: 60%
    • Faeces: 4%
    • Insensible Losses: 28%
    • Sweat: 8%
    • Total Loss: 0% (Balanced)

Rates of Fluid Movement in the Digestive System

  • Ingested Water: 2 liters
  • Liver and Pancreatic Secretions: 2 liters
  • Salivary Gland Secretions: 1.5 liters
  • Secretions from Stomach & Small Intestines: 3.5 liters
  • Absorption:
    • Small Intestines: ~8.5 liters
    • Colon: 400ml
    • Feces: 100ml

Importance of Water in Digestion

  • Hydrolysis Reactions: Critical for digestion.
  • Facilitation of Absorption: Aids nutrient transfer to microvilli.
  • Gut Content Propulsion: Enhances movement of food through the digestive tract.
  • Mucus Viscosity Control: Combines with mucin to regulate viscosity.

Components and Distribution of Body Fluids

  • Total Body Fluid: 42L (60% of body weight in a 70kg male)
  • Extracellular Fluid (ECF): 14L
    • Plasma: 3L
    • Interstitial Fluid (ISF): 10.5L
    • Other ECF: 0.5L (includes lymph, cerebrospinal fluid, etc.)
  • Intracellular Fluid (ICF): 28L (40%)

Regulation of Body Fluid Balance

  • Key Regulators:
    1. Kidneys
    2. Hormones
    3. Central Nervous System (CNS)
Hormones Involved
  • Antidiuretic Hormone (ADH): Increases water permeability in kidney tubules.
  • Angiotensin II and Aldosterone: Promote Na+ and Cl- reabsorption, thus retaining water.
  • Atrial Natriuretic Peptide (ANP): Promotes excretion of Na+ and water to lower blood volume.

Thirst Mechanism

  • Stimulus for Thirst: 1-2% increase in plasma osmolality or 10-15% decrease in plasma volume.
  • Thirst Center: Located in the anterior hypothalamus, monitored by osmoreceptors.

Failures of Body Fluid Balance

  • Dehydration: Caused by insufficient water intake or excessive loss.
    • Risks include weight loss, fever, confusion, especially in the young and elderly.
    • Common Causes: Inadequate water intake, strenuous exercise, vomiting/diarrhea, fever, alcohol consumption, and certain medications.
Cholera as a Case Example
  • Description: Acute diarrheal disease caused by Vibrio cholerae.
  • Incidence: Approximately 1.3 - 4 million cases annually, leading to 21,000 - 143,000 deaths.
  • Treatment: Oral rehydration solutions (ORS), IV fluids for severe cases.

Water Intoxication

  • Occurs from rapid water intake or rehydration, leading to fluid dilution and reduced Na+ concentration in ECF.
  • Dangerous swellings in cells, particularly in the brain, can lead to convulsions or death.
Consequences of Oedema
  • Oedema Factors:
    1. Increased capillary hydrostatic pressure (e.g., liver disease).
    2. Loss of plasma proteins (e.g., malnutrition).
    3. Lymphatic obstruction.
    4. Increased capillary permeability due to inflammation.

Lactose Malabsorption and Consequences

  • Lactase deficiency leads to osmotic diarrhea, with lactose remaining undigested in the gut, drawing water and causing diarrhea.

High Blood Sugar (Hyperglycemia)

  • Impact on Sodium Levels: Hyperglycemia increases serum osmolality, causing water to leave cells, lowering Na+ concentration.
  • Renal Glucose Reabsorption: In diabetes, excess glucose results in osmotic diuresis, impacting fluid balance.