1. Fluid, Electrolyte Balance
Chapter26

2. Body Fluids & Fluid Compartments
Figure 26.1
ICF IF
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)

3. What are electrolytes?????

4. 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.)

5. 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.

6. Osmotic Activity
Electrolytes have a greater potential for osmotic activity than nonelectrolytes
NaCl  Na+ + Cl particles
MgCl2  Mg Cl particles
Glucose  Glucose particle
Electrolytes have the greatest ability to cause fluid shifts.

7. 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+ : mg/L x 1 = 143 mEq/L
Ca2+ : 100 mg/L x 2 = 5 mEq/L

8. 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

9. Figure 26.1
Fluid movement
ICF IF
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

10. 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%)

11. 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 mOsm of solute daily

12. Figure 26.5

13. 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

14. Figure 26.6

15. 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

16. 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

17. 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.

18. 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

19. Aldosterone  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.

20. 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

21. Maintenance of Blood Pressure Homeostasis