Fluid and electrolyte balance, imbalance

Fluid and electrolyte balance, imbalance

Body fluid The fluids are distributed throughout the body in various compartments. Body fluid is composed primarily of water Water is the solvent in which all solutes in the body are either dissolved or suspended Body fluids move constantly between compartments by passive and active transport mechanisms

Fluid compartments Intracellular compartment (all fluid contained within the cell membranes = ~63% of TBW) Extracellular compartment (all fluid not contained in cells = ~ 37% of TBW) Interstitial (tissue) fluid Blood plasma Lymph Transcellular compartment (cerebrospinal fluid, aqueous humor, vitreous humor, synovial fluid, glandular secretions, serous fluid within the body cavities)

Alimentary tract TCW=1% LBM Lungs Plasma 6% LBM Kidney Interstitial fluid 19% LBM Skin ICF 45% LBM Transcellular water 1% LBM Non-aqueous tissue 28% LBM

Milieu Interieur

Homeostasis Homeostasis is essential for optimal body function For homeostasis: fluids, electrolytes, acids, and bases must be balanced.

Balance = a set, desired level More than desired level--increasing excretion Below the desired level--increasing absorption Electrolytes = chemicals that can carry an electrical charge; dissolved in the body fluids; fluid and electrolyte levels are interdependent Electrolyte increases, water is added Electrolyte levels low, water is removed

Water Balance Total water intake = Total water loss (output)

Water balance The body gains and loses water each day The balance is maintained when water intake equals water output The primary source of body water are drinking fluids and eating foods; also generated from metabolism of carbohydrates, proteins, and fat Water loss from urin, sweat, perspiration and stools

Water balans

Electrolyte composition Extracellular fluid: More Na +, Cl -, HCO3 -, Less K +, Ca ++, Mg ++, PO4 ---, SO4 -- Intracellular fluid: More K +, PO4 ---, Mg ++, SO4 --, Less Na +, Cl -, HCO3 -

meq/l 200 180 Extracellular Fluid Intracellular Fluid 160 140 120 Plasma Interstitial Fluid 100 80 60 40 20 0 Gamblegram of plasma, ISF, and ICF (Winters RW, 1973) Na+ K+ Ca++ Mg++ HCO3- Cl- Org P-, Pr- UA Protein

neuromuscular fluid balance, osmotic pressure 10 142 Sodium Function Intracellular meq/liter Extracellular meq/liter Electrolyte neuromuscular fluid balance, osmotic pressure 10 142 Sodium Function Intracellular meq/liter Extracellular meq/liter Electrolyte Positive ions 205 154 Total enzymes 123 2 Magnesium bones, blood clotting - 5 Calcium neuromuscular excitability acid-base balance 100 5 Potassium 205 154 Total enzymes 123 2 Magnesium bones, blood clotting - 5 Calcium neuromuscular excitability acid-base balance 100 5 Potassium

Negative ions Electrolyte Extracellular meq/liter Intracellular meq/liter Function Chloride 105 2 fluid balance, osmotic pressure Bicarbonate 24 8 acid-base balance Proteins 16 55 osmotic pressure Phosphate 2 149 energy storage Sulfate 1 - protein metabolism Total 154 205

Normal levels of electrolytes Sodium Potassium Calcium Calcium unbound Magnesium Chloride Phosphate 135-145 3.5-5.5 8.8-10.4 4.7-5.2 1.4-2.1 100-108 2.5-4.5 meq/l (serum) meq/l (serum) mg/dl (serum) mg/dl (serum) meq/l (plasma) meq/l (serum) mg/dl (plasma)

General distribution of potassium in the body and its daily balance

Fluids movement Primarily by two forces: hydrostatic pressure (fluid) and osmotic pressure (substances) Plasma leaves bloodstream and becomes interstitial fluid The interstitial fluid, enters the lymphatic vessels (lymph) Lymph returned to the bloodstream to become plasma Transcellular fluids derived from the plasma and return to the bloodstream The osmotic pressure between the EC and IC compartments is at equilibrium Fluid exchange occurs between the two if the osmotic pressure in either compartment changes

Fluids movement Hydrostatic pressure (volume/pressure) Osmotic pressure (substances)

Solutes (electrolytes) movement Passive Movement Diffusion: Movement of a solute down a gradient, be it a concentration or electrical potential difference. Convection (Solvent Drag): The process of solute being dragged with H20, proportional to hydrostatic oncotic pressure or osmotic pressure

Solutes (electrolytes) movement Active Movement The movement of a solute against a gradient (concentration or electrical) Requires energy Unidirectional May be competitive May have limitations Primary Active Transport (Na+/K+ ATPase) Secondary Active Transport (Facilitated Transport): The action of a Primary Active Transport System creates energy for the movement of other solutes against a concentration or electrical gradient (Na + - glucose symport )

Solutes (electrolytes) movement Net Transport Determined by the relative contributions of active versus passive transport mechanisms; it can be calculated as active transport minus back diffusion.

Net sodium transport

Outside Primary Active Transport (Na+/K+ ATPase) Carbohydrates β α α α β Lipid Bilayer Protein Subunit ATP Inside

The sodium-potassium pump b a a ATP b 3 Na+ ATP Na+ Na+ Na+ ~ P ADP ATP K+ K+ +Pi K+ K+ Inside ~ P Na+ Na+ Na+ Outside Na+ Na+ Na+ 2 K+ Sweadner KJ, Goldin SM; N Engl J Med 1980; 302:777-783

Secondary Active (Facilitated Transport) (Na+-glucose symport)

Serum osmolality Normal cellular function requires normal serum osmolality Water homeostasis maintains serum osmolality The contributing factors to serum osmolality are Na, glucose and BUN Sodium is the major contributor (accounts for 90% of extracellular osmolality) Acute changes in serum osmolality will cause rapid changes in cell volume

Solute concentration Measurement of solute concentration (the number of dissolved particles per liter) in body fluid is based on the fluid s osmotic pressure, expressed as either osmolality or osmolarity Osmolality is the number of osmols (the standard unit of osmotic pressure) per kilogram of solution Osmolarity refers to the number of osmols per liter of solution

Osmotic pressure Osmotic pressure is defined as the pressure required to be placed on a solution separated from water by a membrane to prevent osmosis from taking place. If two solutions have identical osmotic pressures, they are isotonic. If one solution has a lower osmotic pressure (lower concentration of salts), it is hypotonic with respect to the other. In the opposite situation a solution of higher osmotic pressure is hypertonic with respect to the other.

The fluid exchange due to changes in osmotic pressure

Regulation of Sodium and Water Balance

Role of thirst Hypertonicity the most potent stimulus for thirst Arises with a 2 3 percent increase in serum tonicity Tonicity sensors residing anterior hypothalamus Additional control mechanism of thirst mediated by low-pressure baroreceptors in cardiac atria

Antidiuretic hormone (Vasopressin) Synthesized in hypothalamus Transported to the neural lobe/posterior pituitary Stored as secretory granules within the nerve terminals of neurohypophysis Depolarization of nerve terminal releases vasopressin into the circulation Hypertonicity/decreased ECF volume-arterial blood pressure stimulate secretion Vasopressin leads to water retention by the kidney

Vasopressin effects on the collecting duct principal cell Water channel (aquaporin-2, AQP2) insertion in the apical membrane. The basolateral membrane contains a different constitutive water channel (aquaporin-3, AQP3)

Renin-Angiotensin-Aldosteron System

Renin Synthesized by and released from the juxtaglomerular cells of the renal juxta glomerular apparatus Release controlled by renal arterial/ arteriolar hydrostatic pressure, renal sodium at the macula densa, and renal sympathetic activation Catalyze the conversion of Angiotensinogen to Angiotensin I

The renal juxta glomerular apparatus

Angiotensin Originates from Angiotensinogen produced in the liver and circulating in the blood Angiotensinogen is converted to Angiotensin I (biologically inactive), In the presence of Renin Angiotensin I converted to Angiotensin II in the presence of Angiotensin Converting Enzyme (ACE= present in the pulmonary capillary endothelium) Angiotensin II released Aldosterone from the adrenal cortex; high concentrations cause general vasoconstriction leading to systemic hypertension

Aldosterone Synthesized by and released from adrenal cortex Controlled by the renin-angiotensin-aldosterone (RAA) system Perfusion pressure activate the RAA system Release stimulated by Angiotensin II High plasma [K+] directly stimulate aldosterone release Increase active transport of Na-K-ATP-ase pump, leading to increased Na reabsorption and K excretion in distal segment of renal tubule

Atrial Natriuretic Peptide (ANP, atrin, auriculin, atriopeptin, cardiopeptin) Release from atrial cardiac cells Stimulating by increase of the right atrial pressure The biologically active of ANP produced by Proatrin Increases urinary excretion of Na+ and H20, Cl-, K-, PO4-, Ca++, Mg++ at distal tubule Smooth muscle relaxation (vascular) and decreases aldosterone/renin

Structure Proatrin Atrial Natriuretic Peptide

Nephron Function Filtration of plasma by the glomerulus Reabsorption of solute and water Secretion of solute Excretion of urine

Anatomy of the Nephron

Filtration (glomerulus) and final urine (excretion) Volume Na+ K+ Daily filtration 120 ml/min (GFR) = ~ 180 L 140 mmol/l (plasma) = 25.000 mmol 4.5 mmol/l (plasma) = 810 mmol Daily urine excretion 1-2 L ~150 mmol 100 mmol Conclusion: There must be massive reabsorption of solutes and water between the point of filtration (glomerulus) and final urine (excretion)

Summary of Na+ reabsorption in the early proximal tubule 70% of the filtered Na+ (i.e., 17.500 mmol per day) is reabsorbed by the end of the proximal tubule ("the work horse")

Summary of Na+ reabsorption in the distal tubule Aldosterone and Atrial Natriuretic Peptide (ANP) are the principal hormones that affect Na+ reabsorption in distal segments

The fluids and electrolytes balance

Summary of fluids and electrolytes balance Water and electrolyte balance are interrelated Water and electrolyte gains or losses affect solute concentration temporarily; the changes opposed by fluid shifts between the ECF and ICF, and by hormonal responses, to adjust the rates of water intake and excretion and the rates of ion absorption and secretion. Homeostatic mechanisms monitor ECF, not ICF Receptors can t monitor [ion] but monitor:plasma volume and osmotic concentration Cells cannot actively move H2O; water follows salt