Water Homeostasis and its Implications in Health and Disease

• Water as an essential component:
  • Water is the body’s most abundant component and is vital for survival.
  • Water serves various functions in the human body, acting as a carrier of substances, coolant, lubricant, and as a reactant and product in metabolic reactions.
• Water homeostasis:
  • Water metabolism and balance present special challenges in critically ill patients admitted to the intensive care unit.
  • The body defends fluid volume and osmolarity within narrow ranges to maintain homeostasis.
• Water ingestion and regulation:
  • Water intake is regulated by thirst, and adequate intake for men is approximately 3 L/day and for women is around 2.2 L/day.
  • Thirst is vital for defending against hypovolemia and can be activated by various triggers such as salty food, hot weather, and exercise.
• Challenges in the elderly:
  • Elderly individuals are at increased risk of dehydration due to factors such as decreased kidney function, blunted thirst, and reduced renal sodium conservation.
  • The prevalence of water-loss dehydration in the elderly can be as high as 30% and is associated with concomitant morbidity.
• Metabolic water production:
  • Water is the principal end product of nutrient oxidation, with carbohydrates contributing to more water molecules than lipids.
  • An increased metabolic rate leads to increased metabolic water production.
• Water loss and regulation:
  • Water is mainly lost through the kidneys, with urine volume and composition depending on hydration status and osmole load.
  • Aging reduces the maximum urine-concentrating ability and increases insensible losses, particularly in febrile patients.
• Sweating and respiratory water losses:
  • Sweating is primarily a mechanism of thermoregulation and can lead to significant water losses, especially during intense exercise.
  • Factors such as cold exposure and respiratory rates can also impact respiratory water losses.
• Regulation of water balance:
  • Water balance is regulated through central and peripheral volume and osmolarity sensors, which activate a cascade of endocrine and local activity.
  • The production and secretion of antidiuretic hormone (ADH) are stimulated by various factors, including plasma osmolarity, blood volume, and blood pressure.
• ADH and Water Reabsorption:
  • ADH levels increase water permeability of distal renal tubules and collecting ducts, leading to greater water reabsorption and reduced renal water excretion.
  • ADH binds to vasopressin-2 receptors, activating adenylyl cyclase and increasing aquaporin-2, which allows water to move down an osmotic gradient into the nephron.
• Renin–Angiotensin–Aldosterone System (RAAS):
  • RAAS is stimulated by changes in renal blood flow, leading to the production of angiotensin I and II, which increase sodium and water reabsorption in the loop of Henle.
  • Angiotensin II also stimulates ADH release, causing water retention and increasing systemic blood pressure.
• RAAS with Intrarenal Activity:
  • Intrarenal RAAS activity involves local conversion of angiotensinogen to angiotensin I and II, which have effects on diuresis, natriuresis, and renal vasodilation.
  • Intrarenal angiotensin IV increases cortical blood flow and decreases sodium transport.
• Natriuretic Factors:
  • ANP and BNP decrease circulating water and sodium, reduce atrial distention, and lower blood pressure by increasing glomerular filtration rates and inhibiting renin release.
  • Natriuretic peptides induce diuresis and natriuresis, protecting the body from overhydration.
• Dopamine in Water Homeostasis:
  • Dopamine plays a role in water homeostasis, possibly by regulating sodium and water reabsorption in the kidney.
• Renal Dopamine:
  • Dopamine increases glomerular filtration rate and diminishes sodium reabsorption in renal proximal tubules and collecting ducts.
  • It decreases aldosterone secretion and inhibits the antinatriuretic effects of angiotensin II.
• Prostaglandins:
  • Renal prostaglandins counteract excessive sodium and water reabsorption by inhibiting the Na-K-2Cl cotransporter.
  • They lead to greater renal blood flow and natriuresis.
• Endothelin:
  • Endothelin-1 inhibits Na and water reabsorption through intrarenal ET receptors located in various parts of the nephron.
  • Activation of these receptors induces vasorelaxation and increased urinary water excretion.
• Evaluation of Water Balance:
  • In the ICU, water balance is generally assessed using body weight and fluid input-output, although both methods have limitations.
  • Abnormal water balance results in ICU admissions when the physiologic derangement endangers survival and/or requires close monitoring.
• Hypervolemic Disorders:
  • Caused by excessive water ingestion and/or inability to excrete excess body water.
  • Water intoxication leads to hyponatremia, hypochloridemia, and hypokalemia, resulting in brain edema and other symptoms.
• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH):
  • Causes hyponatremia and hypoosmolality due to impaired water excretion.
  • Includes inappropriate thirst sensation, leading to increased water intake and maintenance of hyponatremia.