fluid and electrolyte balance,...

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”