Fluid, Electrolyte Balance Chapter26

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)

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 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 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 HPO42- Figure 26.2

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

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 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

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 arterioles Activation of the renin angiotensin system Release of aldosterone Release of ADH Conservation of Na+ Conservation of blood volume Increased thirst

Maintenance of Blood Pressure Homeostasis