Fluid, Electrolyte Balance

Chapter26

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)

ICF IF

What are electrolytes?????

http://www.youtube.com/watch?v=e1fKzw05Q5A

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 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

Electrolytes have a greater potential for osmotic activity than nonelectrolytesNaCl Na+ + Cl- 2 particlesMgCl2 Mg2+ + 2 Cl- 3 particlesGlucose Glucose 1 particle

Electrolytes have the greatest ability to cause fluid shifts.

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 Fluids Comparison of extracellular

fluid (ECF) and intracellular fluid (ICF) ECF: increased Na+ and

increased Cl-

ICF: increased K+ and increased HPO4

2-

Figure 26.2

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

IFICF

Figure 26.1

Water Balance and ECF Osmolality

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%)

Figure 26.4

Regulation of intake / output

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

Figure 26.5

Figure 26.5

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

Figure 26.6

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+

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 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 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 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 Decreased BP leads to:

Constriction of afferent arteriolesActivation of the renin angiotensin system Release of aldosterone Release of ADHConservation of Na+ Conservation of blood volume Increased thirst

Maintenance of Blood Pressure Homeostasis