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Fluid and electrolyte Fluid and electrolyte balance, imbalance balance, imbalance
• 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
Body fluid
• 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)
Interstitial (tissue) fluid
Blood plasma
Interstitial (tissue) fluid
Blood plasmaExtracellular compartment(all fluid not contained in cells= ~ 37% of TBW)
Transcellular compartment (cerebrospinal fluid, aqueous humor, vitreous humor, synovialfluid, glandular secretions, serous fluid within the body cavities)
Blood plasma
Lymph
Transcellular compartment (cerebrospinal fluid, aqueous humor, vitreous humor, synovialfluid, glandular secretions, serous fluid within the body cavities)
Blood plasma
Lymph
Plasma 6% LBM
Alimentary tractTCW=1% LBM
Lungs Kidney
Skin
ICF45% LBM
Transcellular water 1% LBM
Non-aqueous tissue28% LBM
Interstitial fluid19% LBM
Milieu Interieur
• Homeostasis is essential for optimal body function
Homeostasis
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 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)
• 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
Water balance
• The primary source of body water are drinking fluids and eating foods; also generated from metabolism of carbo-hydrates, proteins, and fat
• Water loss from urin, sweat, perspiration and stools
Water balans
Extracellular fluid: More Na+ , Cl- , HCO3-, Less K+, Ca++, Mg++, PO4---, SO4--
Electrolyte composition
K+, Ca++, Mg++, PO4---, SO4--
Intracellular fluid: More K+, PO4---, Mg++, SO4--, Less Na+ , Cl- , HCO3-
mEq/L200
180
160
140
120
100
Interstitial FluidPlasma
Intracellular Fluid
Na+Na+
Extracellular 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-UAProtein
Na+
K+
Ca++
Mg++HCO3-Cl-Org P-, Pr-UAProtein
neuromuscular
fluid balance, osmoticpressure
10142Sodium
FunctionIntracellularmeq/liter
Extracellularmeq/liter
Electrolyte
neuromuscular
fluid balance, osmoticpressure
10142Sodium
FunctionIntracellularmeq/liter
Extracellularmeq/liter
Electrolyte
Positive ions
205154Total
enzymes1232Magnesium
bones, blood clotting
-5Calcium
neuromuscular excitabilityacid-base balance
1005Potassium
205154Total
enzymes1232Magnesium
bones, blood clotting
-5Calcium
neuromuscular excitabilityacid-base balance
1005Potassium
acid-base 824Bicarbonate
fluid balance, osmotic pressure
2105Chloride
FunctionIntracellularmeq/liter
Extracellularmeq/liter
Electrolyte
acid-base 824Bicarbonate
fluid balance, osmotic pressure
2105Chloride
FunctionIntracellularmeq/liter
Extracellularmeq/liter
Electrolyte
Negative ions
205154Total
protein metabolism
-1Sulfate
energy storage1492Phosphate
osmotic pressure5516Proteins
acid-base balance
824Bicarbonate
205154Total
protein metabolism
-1Sulfate
energy storage1492Phosphate
osmotic pressure5516Proteins
acid-base balance
824Bicarbonate
Normal levels of electrolytes
mg/dL (serum)8.8-10.4 Calcium
mEq/L (serum)3.5-5.5Potassium
mEq/L (serum)135-145Sodium
mg/dL (serum)8.8-10.4 Calcium
mEq/L (serum)3.5-5.5Potassium
mEq/L (serum)135-145Sodium
mg/dL (plasma)2.5-4.5Phosphate
mEq/L (serum)100-108Chloride
mEq/L (plasma)1.4-2.1Magnesium
mg/dL (serum)4.7-5.2Calcium unbound
mg/dL (plasma)2.5-4.5Phosphate
mEq/L (serum)100-108Chloride
mEq/L (plasma)1.4-2.1Magnesium
mg/dL (serum)4.7-5.2Calcium unbound
General distribution of potassium in the body and its daily balance
• 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
Fluids movement
• 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 oncoticpressure or osmotic pressure
• The movement of a solute against a gradient (concentration or electrical)
• Requires energy • Unidirectional
Solutes (electrolytes) movement
Active Movement
• 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 passive transport mechanisms; it can be calculated as active transport minus back diffusion.
Net sodium transport
ααααααααββββββββ
ααααααααββββββββ
Carbohydrates
Outside
ααααααααββββββββ
ααααααααββββββββ
Carbohydrates
Outside
Primary Active Transport(Na+/K+ ATPase)
αααααααα αααααααα
ATP
Protein Subunit
Lipid Bilayer
Inside
αααααααα αααααααα
ATP
Protein Subunit
Lipid Bilayer
Inside
Inside
The sodium-potassium pump
ATP
3 Na+
ATPATP
3 Na+
Na+
Na+Na+~ P
ADP
Na+
Na+Na+~ P~ P
ADP
ATPATP
abb
ATP
aabb
ATPATP
a
Inside
Outside
Na+
Na+Na+~ P
2 K+
Na+
Na+Na+~ P~ P
2 K+Na+
Na+Na+
K+ K+
+Pi
Na+
Na+Na+
K+ K+
+PiK+ K+
ATP
K+ K+
ATP
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 • 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
• 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
Solute concentration
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 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
Osmotic pressure
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 tonicitytonicity
• Tonicity sensors residing anterior hypothalamus • Additional control mechanism of thirst mediated by low-pressure baroreceptors in cardiac atria
• 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
Antidiuretic hormone (Vasopressin)
• 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
Water channel (aquaporin-2, AQP2) insertion in the apical membrane.
Vasopressin effectson the collecting duct principal cell
insertion in the apical membrane. The basolateral membrane contains a different constitutive water channel (aquaporin-3, AQP3)
Renin-Angiotensin-Aldosteron System
• Synthesized by and released from the juxtaglomerular cells of the renal juxta glomerularapparatus
• Release controlled by renal arterial/ arteriolar
Renin
• 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
• 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
Angiotensin
• 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 • Release stimulated by Angiotensin II • High plasma [K+] directly stimulate aldosteronerelease
• 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 • 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
Atrial Natriuretic Peptide
• Structure Proatrin
Nephron Function
• Filtration of plasma by the glomerulus• Reabsorption of solute and water • Reabsorption of solute and water • Secretion of solute • Excretion of urine
Anatomy of the Nephron
Volume
Daily filtration
120 ml/min (GFR) = ~ 180 L
Daily urine excretion
1-2 L
Filtration (glomerulus)and final urine (excretion)
Volume
Na+
K+
120 ml/min (GFR) = ~ 180 L
140 mmol/L (plasma) = 25.000 mmol
4.5 mmol/L (plasma) = 810 mmol
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 intake and excretion and the rates of ion absorption and secretion.
• Homeostatic mechanisms monitor ECF, not ICF• Receptors can’t monitor [ion] but monitor:plasmavolume and osmotic concentration
• Cells cannot actively move H2O; “water follows salt”
