What is the physiology of fluid balance in the body and its compartments?

Physiology of Body Fluids and Compartments

Body Water Distribution Across the Lifespan

Water is the major component of the human body at any age, serving as the essential carrier for nutrients and metabolites, with total body water divided into two primary compartments: intracellular fluid (ICF) and extracellular fluid (ECF). 1

Age-Related Changes in Body Water Content

• Water contributes approximately 90% of body weight in the 24-week-old fetus, nearly 75% in term infants, and around 50% in adults, demonstrating a progressive decrease with age that correlates with increasing fat mass 1, 2
• Extremely low birth weight (ELBW, <1000 g) and very low birth weight (VLBW, <1500 g) infants have low body fat content and a higher percentage of lean body mass and body water than older infants, which relates to high water turnover 1
• The proportion of ECF (intra- and extravascular) also decreases during infancy up to adulthood 1

Blood Volume Variations

• Blood volume in neonates is 85-100 ml/kg body weight compared to 60-70 ml blood volume/kg body weight in adolescents and adults 1

Fluid Compartment Organization

Extracellular Fluid (ECF) Subdivisions

The ECF is subdivided into intravascular and extravascular components as well as a "third space" which characterizes free fluid in preformed body compartments under physiological conditions (urine, cerebrospinal fluid) and pathological conditions (ascites, pleural effusions). 1

Intracellular Fluid (ICF) Dynamics

• The total volume of ICF increases with the number and size of body cells during body growth 1
• Water turnover is related to lean body mass and has no close relationship to body fat mass 1

Water Turnover and Metabolic Production

Age-Dependent Water Turnover Rates

• Water turnover is high in neonates and decreases with increasing age and the concomitant decrease of metabolic rate and growth velocity 1
• Water and electrolyte requirements are usually proportional to growth rate, with needs per unit body mass very high in neonates that decrease with age until adulthood 1

Endogenous Water Production

• Metabolic water production equals 0.6 ml water per gram of carbohydrates oxidized, 1.0 ml per gram of fat oxidized, and 0.4 ml per gram of protein oxidized 1
• This endogenous water production may be of particular importance in pediatric patients because of their high metabolic rates 1

Insensible Water Loss

Respiratory and Cutaneous Losses

• Evaporation of water from upper respiratory passages accounts for approximately one-third of net insensible water loss 1
• Insensible water loss reaches the level of:
  • 0.8-0.9 ml/kg per hour in premature infants 1
  • 0.5 ml/kg per hour in term neonates 1
  • 0.4 ml/kg per hour in older children 1
  • 0.3 ml/kg per hour in adolescents 1

Electrolyte Distribution and Physiology

Sodium and Chloride (Extracellular Cations/Anions)

• Sodium (Na) is the principal cation of the ECF and Na concentrations influence intravascular and interstitial volumes 1
• Chloride (Cl) is the major anion of the ECF, with exchangeable Cl remaining relatively constant per unit of body weight at different ages 1
• Na excretion occurs primarily through urine, but also through sweat and feces 1
• Renal conservation occurs with tubular reabsorption of 60-70% of the filtered Cl 1

Potassium (Intracellular Cation)

• Potassium (K) is the major intracellular cation and the K pool correlates well with the lean body mass 1
• The intracellular K concentration is dependent on Na/K-ATPase activity which can be impaired if there are insufficient supplies of oxygen and energy 1
• Ten percent of the K body pools are not exchangeable (bone, connective tissue, cartilage) 1
• Extracellular K concentration is not always related to intracellular concentration, and intra- to extracellular K shifts can occur, e.g., in acidotic states through exchange with H ions 1

Strong Ion Difference (SID) and Acid-Base Balance

• Na and Cl are the major ions influencing the 'strong ion difference' (SID), one of the 4 systems acting on blood pH 1
• A decrease in the SID will result in an acidifying effect on plasma 1
• The SID is calculated as the charge difference between the sum of measured strong cations (Na+, K+, Ca2+, and Mg2+) and measured strong anions (Cl, lactate) 1
• An increase in the plasma Cl relative to Na+ decreases the plasma SID and lowers the pH 1

Developmental Limitations in Fluid Regulation

Neonatal Renal Immaturity

• Many of the regulatory processes involved in fluid and electrolyte balance have limitations in pediatric patients because of immaturity or limited efficacy 1
• The renal glomerular surface area available for filtration is small in preterm and term neonates compared to that in older infants and adults 1, 2
• Glomerular filtration rate increases significantly during the first week of life and continues to rise over the first two years of life 1
• The velocity of this increase is slower in premature infants and needs to be considered when estimating fluid and electrolyte physiology in these infants 1

Urinary Concentration Capacity

• Immaturity of the distal nephron with an anatomically shortened loop of Henle leads to reduced ability to concentrate urine 1
• Maximum urinary concentrations are up to 550 mosm/l in preterm infants, and 700 mosm/l in term infants, compared to 1200 mosm/l in adults 1, 2
• Neonates may be placed at risk for volume depletion when a high renal solute load cannot be compensated for by the ability to produce concentrated urine 1

Hormonal System Limitations

• Although hormonal factors (the renin-angiotensin-aldosterone system and the arginine-vasopressin-axis) are mature early in gestation, the effects are limited by renal immaturity 1
• In VLBW infants, urine output may frequently increase above 5 ml/kg/h 1

Capillary Permeability in Preterm Infants

• In preterm infants, a lower plasma oncotic pressure and higher permeability of the capillary wall enhance the shift of water from the intravascular to the interstitial compartment 1
• This puts preterm infants at an increased risk of edema, especially under pathologic conditions such as sepsis 1

Clinical Implications for Fluid Management

Individual Variability

• The needs of individual patients may deviate markedly from the ranges of recommended fluid intakes depending on clinical circumstances such as fluid retention, dehydration, or excessive water losses 1

Pathological Electrolyte Losses

• Incidental gastrointestinal and skin electrolyte losses are very low in neonates (Na gastrointestinal losses represent 0.1-0.2 mmol/kg/d in premature infants and around 0.01-0.02 mmol/kg/d in term infants) 1
• Electrolyte losses may be increased under pathological conditions like bowel obstruction, ileostomy, pleural effusions, peritoneal drainage, and external cerebrospinal fluid drainage 1