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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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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 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
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 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
