Water Metabolism

1. Water Metabolism TUMS Azin Nowrouzi, PhD

2. Principal functions of water • Universal solvent and suspending medium • Helps to regulate body temperature • Participates in hydrolysis reactions • It is a medium where most cell’s metabolic reactions take place • Lubricates organs • Provides cellular turgidity • Helps to maintain body homeostasis • Ionization of water and its acid-base reactions important for the functions of proteins and nucleic acids • The shapes of proteins and nucleic acids and structure of biological membranes are a consequence of their interaction with water.

3. Permeability properties

4. Solution Equilibrium

5. Molar distributions

6. Donnan’s Equilibrium • According to Donnan’s equilibrium, the product of diffusible electrolytes in both compartments will be equal. [K+]L x [Cl-]L = [K+]R x [Cl-]R 9 x 4 = 6 x 6 b. The electrical neutrality in each compartment is maintained (the number of anions should equal the number of cations) In left: K+ = R- + Cl- 9 = 5 + 4 In right: K+ = Cl- 6 = 6 c. Total number of each type of ion is the same before and after equilibrium K+ = 9 + 6 = 15 Cl- = 4 + 6 = 10 d. When there is nondiffusible anion in one side of a semipermeable membrane, the diffusible cations are more and diffusible anions are less, in that side. Before equilibrium After equilibrium

7. Osmosis &Osmotic pressure Osmosis = diffusion of water across a semipermeable membrane (like a cell membrane) from an area of low solute concentration to an area of high solute concentration.

8. Cell membrane is a selectively permeable membrane

9. Osmotically effective solutes • The major extracellular solute is Na and its associated anions. • The major intracellularsolute is K and its associated anions. • These solutes are relatively restricted to their compartments Solutes that are relatively restricted to one particular body fluid compartment are able to exert an osmotic force for water movement from other compartments. Such solutes are called osmotically effective, or simply effective solutes.

10. Noneffective solutes • Some solutes, notably urea pass freely across cell membranes and do not exert a force for water movement between the two major body fluid compartments. • Ethanol, methanol and ethylene glycol are also noneffective solutes. • Such noneffective solutes contribute to body osmolality but not to tonicity.

11. Osmolarity, Osmolality Osmolarity: Number of particles dissolved in 1 L water = osm/L H2O • 1 osmol = 6x1023 solute molecules per liter • example: 150 mM NaCl = 300 mosmol solute (150 mMNa+ + 150 mM Cl-) • It is not an SI unit Volume depends on factors like temperature, thus osmolality is used: Number of particle in 1kg of water = osm/kg H2O For Glucose: molality = osmolality For NaCl: osmolality = molality x 2

12. Osmolality & osmolarity measurement • Osmolality can be measured using an osmometer. Osmometers are useful for determining the concentration of dissolved salts or sugars in blood or urine samples. • Osmolarity = 2 x [Na+] + [urea] + [glucose] (Concentrations are in mmol/L) • Difference between the two is osmolar gap. It occurs when abnormal species are present in plasma (such as poisons)

13. In medicine Relative to blood plasma, a solution can be: Isosmolal: Equal osmolality Hyposmolal: Lower osmolality Hyperosmolal: Higher osmolality Freezing point osmometer (cryoscope)

14. U-Tube Osmometer semipermeable membrane (only permeable to water) direction of net water movement?

15. Tonicity Tonicity is a unitless concept that can be expressed only in reference to a physiologic system: • A hypertonic solution is one that would shrink cells. • A hypotonic one would cause them to swell. • In an isotonic solution cells are not affected. • Loss of free water → The fluid becomes too concentrated (increased osmolarity) → Hypertonic • Gain in free water → The fluid becomes too dilute → It is called Hypotonic

16. Mean concentrations of the more important solutes in cell compartments All concentrations except those of glucose are in milliequivalents per liter. • The osmolality of ECF is in the range of 282-290 mOsm/kg of water (282-295 mmmol/Kg of water). • Intracellular fluid osmolality is the same (about 282-290 mOsm/kg water). • Water freely permeates across cell membranes. • Major extracellular and intracellular effective solutes do not. • Any loss or gain of water in the ECF will affect the water concentration in the ICF.

17. ECF osmolality Major contributors to ECF osmolality: • Sodium • Chloride • Bicarbonate • Glucose • Urea Osmotically noneffective solutes, such as urea, contribute to body osmolality but not to tonicity.

18. Liquid inside cells (Intracellular fluid) - 40 % • Extracellular fluids - 15 % • Liquid making the plasma of blood - 5 %

19. Intracellular fluid 35%-40% BW Total body water 50%-60% BW Blood plasma (intravascular fluid) 4%-5% BW Extracellular fluid 15%-20% BW Interstitial fluid (extravascular) 11%-15%BW Lymph BW = Body Weight Transcellular fluid: Cerebrospinal fluid Intraocular fluid Synovial fluid Pericardial fluid Pleural fluid Peritoneal fluid

21. Measurement of volumes of the fluid compartments by an indirect dilution technique Quantity of substance introduced (mg) Compartment V (ml) = Substance concentration in compartment (mg/ml) Interstitial fluid = Extracellular fluid – Plasma volume Intracellular fluid = Total body water – Extracellular fluid volume

22. How does total body water vary?

23. How much water can we lose?Infants compared to adults In adults: Percent body water is high compared to the skin surface Adults are not very prone to dehydration. Infants are at high risk of dehydration when febrile or lose fluids due to vomiting or diarrhea. It is critical to administer fluids to a febrile infant to maintain body homeostasis. Body can lose nearly all fat and over half of its protein and live. But 10 -15% loss of water will result in death. Loss of water through skin is increased 13% for each degree rise in centigrade in body temperature.

24. Sources of body water • Drinking water • In food • Metabolic water from nutrient oxidaton • Glucose + 6O2 6CO2 + 6H2O • 1g carbohydreate = 0.6ml water • Alanine + 3O2  2.5 CO2 + CO(NH2)2 + 2.5 H2O • 1g protein releases 0.4ml water • Palmitic acid + 23O2  16CO2 + 16H2O • 1g fat generates 1.1ml water • About 1000 kcal is equivalent to intake of 125ml of water

25. Water losses Urine Ambient temperature Digestible dry matter intake N intake, metabolism and excretion Feces Water intake Dry matter intake Fiber content of food Insensible loss through evaporation Ambient temperature Physiological state Lactation Pregnancy

26. Water balance Fluid intake = fluid output Hydration Positive water balance When body water intake exceeds water input Dehydration Negative water balance When water output exceeds intake

27. Water and salt disturbances • Depletion • Water depletion (Dehydration or hypovolemia) • Inadequate water intake • Excessive loss • Sodium depletion (Hyponatremia) • Inadequate oral intake • Inadequate parenteral input • Excessive sodium loss (isotonic loss for example in plasma, hypotonic loss (in sweat or urine) • Excess • Water excess (overhydration or hypervolemia) • Impaired excretion • Excessive intake (psychiatric disorders, organic brain disease for example trauma and following surgery) • Sodium excess (Hypernatremia) • Increased intake • Decreased excretion (renal disease, primary adrenal disease, secondary hyperaldosteronism).

28. Dehydration or hypovolemia(Water loss) • Causes: • Decreased intake (lack of water, psychogenic refusal to drink) • Increased output (vomiting, diarrhea, loss of blood, drainage from burns, diabetes mellitus, diuretic use, lack of ADH due to diabetes insipidus. • Symptoms: loss of weight, rise in body temperature, increase in heart rate and cardiac output, decrease in blood pressure, sunken eyeballs. • Response: • Decrease in salivary secretion and drying of the mouth and pharynx  thirst • Increased osmolality and low blood volume and pressure release of ADH from posterior pituitory, increased reabsorption of water in kidney tubules • Aldosterone secretion from adrenal gland

29. Overhydration or hypervolemia(Water gain) • Causes: • Excessive IV administration of fluids, psychogenic drinking episodes, decreased urinary output because of renal failure, congestive heart failure • Symptoms: Decrease in body temperature, increased blood pressure, edema, weight gain. • Response: • Decreased osmolarity of fluids in the hypothalamus inhibition of thirst, decreased release of ADH and decreased aldosterone secretion  increased urinary output

30. Water intoxication=consumption of too much water too quickly

31. Hyponatremia (sodium deficit) Hypernatremia (Sodium excess) • Normal [Na+] in blood plasma = 150mEq/L • Excess of water [Na+] below 120 mEq/L  lethargy, coma, or death. Sodium loss: Decreased ECF volume  Aldosterone secretion  Renal sodium reabsorption  decreased Na+ excretion. • [Na+] affects plasma & ECF osmolarity • [Na+] affects blood pressure & ECF volume

32. Too little aldosterone: Addison’s disease Hyperaldosteronism: 1- Primary (due to tumors of adrenal cortex) 2- Secondary: liver disease, heart failure, pregnancy, nephrosis…

34. The hormones interact when blood loss or dehydration occurs to maintain intravascular volume Factors that stimulate renin release: • Decreased blood pressure • Salt depletion • Prostaglandins • Beta-adrenergic drugs Inhibitors of renin release: • Increased blood pressure • Salt intake • Prostaglandin inhibitors • Beta-adrenergic antagonists • Angiotensin II

36. Disturbances in salt and water balance: 1, 2, and 3 result in hypovolemia 3 and 5 lead to intracellular edema (including cerebral swelling) 4, 5, and 6 result in extracellular edema (for example, pulmonary edema)

37. Clinical conditions regarding Na+ • Hyponatremia • Plasma [Na+] may be normal (if isotonic loss), high(if hypotonic loss), low(vasopressin secretion secondary to hypovolemia causes water retention). • Cause: • Depletion of sodium(hypovolemic hyponatremia) • Water excess(euvolemic hyponatremia) • Combined water and sodium excess(hypervolemic hyponatremia) • Hypernatremia

39. Water and sodium regulation Through the action of osmoreceptors in the hypothalamus, and baroreceptors (stretch receptors) in atria. Antidiuretic Hormone (ADH) (vasopressin) Increases the water permeability of the distal tubule and collecting duct, thus increasing the concentration of urine. Atrial Natriuretic Peptide (ANP) Released when atrial pressure is increased e.g. in heart failure or fluid overload. It promotesloss of sodium and chloride ions and water chiefly by increasing GFR. NaCl content of body determines the size of extracellular fluid. Renin Increases the production of angiotensin II Released when there is a fall in intravascular volume e.g. haemorrhage and dehydration Aldosterone Promotes sodium ion and water reabsorption in the distal tubule and collecting duct where Na+ is exchanged for potassium (K+) and hydrogen ions by a specific cellular pump

40. Role of the Kidney • GFR = Volume of filtrate formed by all the nephrons of both kidneys each minute. • In adult female GFR = 110 ml/min • In male, GFR = 125 ml/min • Thus a volume of 7.5 L/h, or 180 L/day is formed. • ~99% of the filtrate is reabsorbed from renal tubules and returned to the bloodstream. • ~1% is excreted in urine • Urine volume is regulated according to the needs of the body. • Most solutes are reabsorbed completely or almost completely according to the needs of the body.

41. Renal handling of different substances • Substances that are actively transported from peritubular capillaries to the tubule lumen: Hydrogen, potassium, penicillin, poisons, drugs, metabolic toxins, chemicals that are not normally present in the body.

42. Nephron segment Proximal tubule Glucose & Na+ Loop of Henle H2O, Na+, K+ & Cl- Distal tubule Na+ & H2O Collecting duct H2O, Na+ & urea Hormone regulated

44. Regulation of Urine Concentration by the kidneys

45. Countercurrent multiplier exchange

46. Diabetic Hyperosmolar state • In diabetics with serum glucose ~ 700 mg/dL, the hyperosmolar state caused efflux of cellular water  osmotic dilution of serum sodium. (For each 100 mg/dL increase in serum glucose, there is 1.6 mEq/L decrease in the serum [Na+]). • Transport of glucose into cells happens along with concurrent potassium transport into cells  insulin resistance causes high serum potassium. • Pattern: • Low serum sodium and high serum potassium (same as hypoaldosteronism), • High glucose levels.

47. Drugs affecting water absorption in kidney tubules: Diuretics & Antidiuretics • Diuretic drugs: • Act on nephronic tubules • Increase the excretion of salt (NaCl, NaHCO3) and water. • used in heart failure management • BP control • reduction of edema • correction of base-acid balance (pH) • Antidiuretic drugs: • Decrease the secretion of salt and water.

48. Free Water Passing Through Membrane Pores

49. Cellular Mechanisms of Water Transport • The precise pathways whereby water crosses membranes has only recently been discovered. • A family of membrane proteins has now been described, termed aquaporins. • To date three members of this family have been described in mammals and are termed CHIP28, WCH-CD and MIP26 and display about 45% homology.

50. Water channel molecules • CHIP28 (channel forming integral protein) was the first known water channel molecule. • A 28 kDa protein, and within the kidney is found exclusively in the proximal tubule. • CHIP28 protein is present in many tissues including lung, small intestine, and red blood cells, and therefore plays an important role in water transport in different organs • WCH-CD (water channel of the collecting duct) • Found exclusively in the collecting duct, on the luminal surface of the tubule, and is sensitive to ADH (whereas CHIP28 is not). • MIP26 (major intrinsic protein of the mammalian lens) has not been detected in renal tissue.