Fluid electrolyte balance
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Fluid, Electrolyte Balance. Chapter26. Body Fluids & Fluid Compartments. Figure 26.1. Approximately 60% of body weight is H 2 O. Fluid Compartments Intracellular fluid (ICF): fluid within the cells Extracellular fluid (ECF): fluid outside the cells Plasma Interstitial fluid (IF). ICF.

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Body fluids fluid compartments
Body Fluids & Fluid Compartments

Figure 26.1

  • Approximately 60% of body weight is H2O.

  • Fluid Compartments

    • Intracellular fluid (ICF): fluid within the cells

    • Extracellular fluid (ECF): fluid outside the cells

      • Plasma

      • Interstitial fluid (IF)



What are electrolytes
What are electrolytes?????


Body fluids
Body Fluids

  • Composition (solutes):

    • Electrolytes

      • chemical compounds that dissociate in H2O to form ions – salts, acids, bases

      • anything with a charge

    • Nonelectrolytes: do not dissociate in H2O (glucose, lipids, creatinine, urea, etc.)

Body fluids1
Body Fluids

  • Osmosis: the diffusion of a solvent (such as water) across a semipermeable membrane

    • From a less concentrated solution (H2O moves out).

    • Toward a more concentrated solution (H2O moves in).

    • The solvent (H2O) moves down its concentration gradient.

  • Osmotic activity is based on the number of particles in solution.

Osmotic activity
Osmotic Activity

  • Electrolytes have a greater potential for osmotic activity than nonelectrolytes

    • NaCl  Na+ + Cl- 2 particles

    • MgCl2 Mg2+ + 2 Cl- 3 particles

    • Glucose  Glucose 1 particle

  • Electrolytes have the greatest ability to cause fluid shifts.

Electrolyte concentration
Electrolyte Concentration

  • Electrolyte concentration is an expression of the number of electrical charges in 1 liter [expressed as milliequivalents per liter (mEq/L)]

    • mEq/L = ion concentration (mg/L) x charge atomic weight

  • Normal plasma levels:

    • Na+ : 3300 mg/L x 1 = 143 mEq/L 23

    • Ca2+ : 100 mg/L x 2 = 5 mEq/L 40

Body fluids2
Body Fluids

  • Comparison of extracellular fluid (ECF) and intracellular fluid (ICF)

    • ECF: increased Na+ and increased Cl-

    • ICF: increased K+ and increased HPO42-

Figure 26.2

Fluid movement

Figure 26.1

Fluid movement



  • Movement between plasma and interstitial fluid (IF) across capillary membranes

    • Hydrostatic pressure in the capillaries pushes fluid into the IF

    • Oncotic pressure returns fluid to plasma

    • Lymphatic system returns the small remainder to the blood

  • Exchanges between IF and ICF occur across the selectively permeable cell membranes

    • H2O flow is conducted both ways

    • Ion movement is controlled and restricted

    • Ion transport is selective by active transport

Water balance and ecf osmolality
Water Balance and ECF Osmolality

Figure 26.4

  • H2O sources / losses

  • Sources:

    • Intake (~2500 ml/day)

    • Metabolic H2O : H2O produced by cell metabolism

  • Losses:

    • Insensible loss: vaporizes from lungs and skin

    • Losses in perspiration and feces

    • Urinary losses (~60%)

Regulation of intake output
Regulation of intake / output

Figure 26.5

  • Intake: Thirst is regulated by the hypothalamic thirst center

    • Sensory feedback from dry mouth stimulates the thirst center

    • Hypothalamic osmoreceptors lose H2O into hypertonic ECF and stimulate the thirst center

    • Angiotensin II stimulates the thirst center

  • Output:

    • Kidneys: make short term adjustments to compensate for low intake

    • Obligatory H2O loss

      • Insensible loss + Sensible loss in urine yields a daily minimum of 500ml

      • With a normal diet the kidneys must excrete 900-1200 mOsm of solute daily

Water balance conservation
Water Balance: Conservation

  • ADH

    • H2O reabsorption in collecting duct

    • Hypothalamic osmoreceptors sense ECF osmolality and regulate ADH release

    • Large decreases in BP trigger ADH release via signals from baroreceptors

    • ADH acts directly and via stimulation of Renin-Angiotensin system

Disorders of h 2 o balance
Disorders of H2O Balance

  • Dehydration: H2O loss and/or electrolyte imbalance

  • Hypotonic hydration: H2O intake with inadequate electrolytes; marked by hyponatremia

  • Edema: accumulation of fluid in the interstitial space

    • Hypoproteinemia: loss of colloid osmotic pressure H2O leaves plasma, enters IF

    • Any event that increases plasma  IF movement or hinders IF  plasma return

Electrolyte balance role of na
Electrolyte Balance: Role of Na+

  • Na+ is the most abundant cation in the ECF

  • Na+ is the only ECF ion with significant osmotic effect

  • Cell membranes are relatively impermeable to Na+

  • [Na+] across the cell membrane may be altered

  • Na+ has the primary role in control of ECF volume and H2O distribution

Electrolyte balance role of na1
Electrolyte Balance: Role of Na+

  • ECF total Na+ content may change but [Na+] remains stable because of shifts in water content

    • A change in the [Na+] in plasma will effect; plasma volume, BP, intracellular fluid volume and interstitial fluid volume.

Regulation of na balance
Regulation of Na+ balance

  • Regulation of Na+ balance is linked to BP and blood volume

  • 65% of Na+ is reabsorbed in the PCT

  • 25% of Na+ is reabsorbed in the ascending limb of the loop of Henle

  • 10% remains in DCT and collecting duct filtrate


  •  aldosterone :

    • Virtually all Na+ is actively reabsorbed in DCT & collecting duct (H2O follows Na+ if ADH is present)

  • Renin-Angiotensin system is the most important trigger of aldosterone release

  • Aldosterone effect occurs slowly (hours to days)

  • Changes in blood will feedback to modulate the effect of aldosterone.

Cardiovascular baroreceptors blood pressure homeostasis
Cardiovascular Baroreceptors: Blood Pressure Homeostasis

  • Decreased BP leads to:

    • Constriction of afferent arterioles

    • Activation of the renin angiotensin system

    • Release of aldosterone

    • Release of ADH

    • Conservation of Na+

    • Conservation of blood volume

    • Increased thirst