Water and Electrolyte Balance

Water and Electrolyte Balance

Introduction

  • Water is essential for all biological systems.
  • Functions of water:
    • Effective solvent: transports nutrients and waste.
    • Redistributes heat and reduces body temperature through sweat evaporation.
    • Lubricant: found in joint spaces and around tendons/muscles.
    • Medium for biochemical reactions.
  • Water constitutes 50-70% of body mass, varying with body fat content.
    • Fat tissue contains little water.
    • Fat-free tissue is 60-80% water.

Body Water Distribution

  • Total body water in a 70kg person is around 42 liters.
  • Distribution:
    • Intracellular water: 23 liters.
    • Extracellular water: 19 liters.
  • Extracellular water components:
    • Blood plasma.
    • Lymph.
    • Saliva.
    • Fluid around cells, nerves, and spinal cord.
    • Fluid in eyes.
    • Fluid secreted by glands, digestive tract, skin (sweat glands), and kidneys.

Hydration Definitions

  • Dehydration: Process of losing body water.
  • Euhydration: Normal body water content; absence of hyper- or hypohydration.
  • Hyperhydration: Excess body water content.
  • Hypohydration: Insufficient body water content.

Fluid Compartments

  • Two main compartments: intracellular (ICF) and extracellular (ECF).
    • ICF: About two-thirds of body fluid, inside cells.
    • ECF: About one-third of body fluid, outside cells.
      • 80% of ECF is interstitial fluid (between tissue cells).
      • 20% of ECF is plasma.
  • ICF and ECF differ in solute composition.
    • ICF: Higher concentration of potassium (K+K^+) salts.
    • ECF: Higher concentration of sodium (Na+Na^+) salts.
    • Concentration gradients maintained by active transport (ATP required).
    • Overall osmotic concentration of ICF and ECF is the same.

Water Balance

  • Daily water turnover is about 2.5 liters.
  • Water losses:
    • Evaporation from skin (sweat).
    • Evaporation from respiratory tract (breathing).
    • Excretion from kidneys (urine).
    • Excretion from large intestine (faeces).
  • Water intake:
    • Ingestion of food.
    • Drinking fluids.
    • Oxidation of substrates (metabolic water).
  • Water from food and fluid is absorbed via the large intestine.

Regulation of Water Balance

  • Hypothalamus, pituitary gland, and kidneys regulate water and electrolyte balance.
  • Antidiuretic hormone (ADH) acts on kidneys.
    • ADH causes kidneys to reabsorb more water, producing less urine, and helps maintain electrolyte balance.
    • Kidneys filter blood and remove waste products.
    • Adjust the amount of electrolytes reabsorbed into the bloodstream or excreted in urine.
  • Water loss leads to increased body fluid concentration, detected by the hypothalamus, which:
    • Activates thirst.
    • Triggers pituitary gland to secrete ADH, reducing urine production.
  • Increased water availability in ECF dilutes solutes, detected by the hypothalamus, which:
    • Switches off thirst.
    • Reduces ADH secretion.
  • Negative feedback maintains water balance.

The Kidneys

  • Kidneys control retention and loss of water.
  • Water and electrolytes are filtered from blood in the glomerulus.
  • Filtered fluid moves into the descending loop of Henle.
    • Descending loop is permeable to water but not electrolytes.
    • High osmotic concentration in the medulla causes water to be passively absorbed, concentrating the fluid in the tubule.
  • Ascending limb of the tubule actively transports sodium chloride but is impermeable to water.
    • Sodium chloride is transported out, rediluting the fluid.
  • Collecting duct: ADH regulates water reabsorption.
    • ADH increases permeability of the collecting duct wall, increasing passive water reabsorption and reducing urine volume.
    • This phase dictates the final urine volume and concentration.

Monitoring Hydration Status in Athletes

  • Thirst indicates suboptimal hydration status.
  • Precise hydration measures are needed for athletes.
  • Dehydration impairs performance.
  • Changes in body mass is a simple monitoring method.
Urine Analysis Techniques
  • Loss of body water results in smaller amounts of more concentrated urine due to ADH.
  • Concentrated urine indicates dehydration, evident by color.
    • Large amounts of pale urine indicate normal hydration.
    • Small amounts of darker-colored urine indicate risk of dehydration.
  • Subjective indicator - Urine color chart.
  • Hydrometer measures urine specific gravity.
  • Osmometer measures freezing point in urine; increased solute concentration reduces the freezing point.

Why Athletes Need More Fluid

  • Exercise and hot conditions disrupt water balance.
  • Metabolic heat from muscle contraction is released through sweat evaporation.
  • Sweat losses increase during exercise or in warm climates.
  • Increased fluid intake is needed to compensate for losses.
  • Factors determining sweat production: environmental temperature, humidity, air velocity, body size, metabolic rate.
  • Electrolytes are lost in sweat.
  • Athletes should aim to drop no more than 2% of their body mass due to fluid losses.

Electrolyte Regulation During Exercise

  • Water moves from plasma to interstitial and intracellular spaces at the onset of exercise, determined by active muscle mass and intensity.
  • Metabolic by-products increase osmotic pressure in muscles, causing water to move passively by diffusion.
  • Increased blood pressure forces water out of the blood as sweating increases, leading to plasma volume decrease.
  • The hypothalamus, pituitary gland, and kidneys monitor water levels and electrolyte balance during exercise.

Linking Question

  • Relationship between external environment and electrolyte balance.
    • Impact of environment: hot and dry, or cold at altitude, on the rate of water loss.
    • Functions of water in the body.
    • Electrolytes/ions and muscle contraction, nerve transmission, and fluid balance.
    • Electrolyte loss in sweat (and urine).
    • Individual variability in sweating rate and electrolyte composition of sweat.
    • Homeostasis maintains water and electrolyte balance via neural pathways and integrative centers in brain and peripheral effectors.

Hyponatremia and Hypernatremia

  • Hyponatremia: Low plasma concentration of sodium.
    • Sodium concentration below 135-145 mmol/L.
    • Early symptoms: bloating, nausea, vomiting, headache.
    • Severe symptoms: brain swelling, seizures, coma, potentially death.
    • Occurs during endurance sports.
    • Risk factors: duration of endurance performance, sports discipline, biological sex (females at higher risk), ambient temperature, and geographical location.
  • Hypernatremia: Deficit of total body water relative to total body sodium content.
    • Sodium concentration >145 mmol/L.
    • Caused by dehydration or excessive sodium in the diet.
    • Water moves from body cells into ECF, causing cell dehydration.
    • Symptoms similar to hyponatremia.
    • Occurs during or up to 24 hours after prolonged physical activity.

Global Impact of Science: Hyponatremia

  • Hyponatremia is a condition where sodium concentration in body fluid is too low (<135 mmol/L).
  • Causes water transport into cells, affecting brain cells.
  • Exercise-associated hyponatremia can be fatal.
  • Caused by excessive drinking of fluids during long-duration exercise, leading to weight gain.
  • Commercial sports drinks have low sodium concentrations due to sales-related reasons (salty drinks are less popular).
  • Conflicts of interest may exist with commercial sports drink manufacturers influencing scientific information on hyponatremia and drinking behavior.
  • Companies may downplay contradictory evidence or highlight supportive evidence.

Cardiovascular Drift

  • Cardiovascular drift is characterized by a rise in heart rate and a fall in stroke volume over time during prolonged aerobic exercise at a steady-state intensity.
  • Stroke volume decreases, and heart rate increases to maintain cardiac output.
  • Mean arterial blood pressure declines.