BIOL 221 Chapter 26 Fluids & Electrolytes 35 slides 1
Body Water Content Total Body Water is the percentage of a person s weight that is water. TBW can easily vary due to: gender males have higher TBW due to higher % of muscle mass. healthy, young, adult males have a TBW of about 60%. healthy, young, adult females have a TBW of about 50%. age infants have the highest TBW due to low bone mass and low fat volume. infants have a TBW at about 73%. elderly have a TBW of about 45%. adipose contains only 20% water, skeletal muscle is about 75% water. Vary them and you vary TBW. 2
Distribution of Water into Compartments Intracellular Fluid (ICF): is 66% of TBW. the major cations of ICF are K +, HPO4 - and Mg 2+. The major anions of ICF are protein and organic phosphates (ATP, ADP, AMP). Extracellular Fluid (ECF): is 33% of TBW. is composed of interstitial fluid and plasma. Plasma is 1/4 of ECF (thus it is 1/12 of TBW). Interstitial fluid is 3/4 of ECF (thus it is 1/4 of TBW). the composition of Interstitial fluid is the same as that of plasma except that it has little protein. Thus it is an ultrafiltrate of plasma. the major cation of ECF is Na +. the major anions of ECF are Cl - and HCO3 -. 3
Body Fluid Compartments 4
Composition of Body Fluids Water serves as the universal solvent in which both nonelectrolytes and electrolytes are dissolved. Nonelectrolytes have strong (usually covalent) bonds that prevent them from dissociating in solution. most are organic molecules like glucose, lipids, creatinine, urea. Electrolytes easily dissociate in solution into ions. because ions are charged particles, they can conduct an electrical current, hence the name electrolyte. have a greater osmotic power because they form at least 2 ions in solution, therefore contributing twice as many solute particles as glucose (which remains undissociated). most are inorganic salts, inorganic and organic acids and bases, and some proteins. Remember, water goes to the location where the highest number of solute particles are... this means the dissociated electrolytes will attract the most water! 5
Electrolyte Composition of the Blood Plasma, Interstitial fluid, and Intracellular Fluid 6
Number of Solute Particles Created: NaCl 2 particles electrolyte MgCl2 particles 3 particles electrolyte Glucose 1 particle nonelectrolyte 7
Fluid Movement Among Compartments the continuous movement and mixing of body fluids are regulated by: osmotic pressures hydrostatic pressures water moves freely between all compartments along osmotic gradients. however, solutes are unequally distributed because of: their size electrical charge binding to transport proteins. when there is a situation where more solutes are in a particular compartment for what ever reason... water will flow into that compartment along its osmotic gradient. 8
Fluid Movement Among Compartments Example: exchanges of fluid between the plasma of blood and the intracellular fluid occur across the capillary membrane. nearly protein-free plasma fluid is forced out of the blood into the interstitial space by hydrostatic pressures. this filtered fluid is then almost completely reabsorbed into the bloodstream in response to the colloid osmotic (oncotic) pressure of plasma proteins. the net leakage that remains is picked up by the lymphatic vessels and returned to the blood. 9
Fluid Movement Among Compartments Example: exchanges of fluid between the intracellular fluid and the interstitial fluid occur across the plasma membranes of cells this depends on the complex permeability proteins found imbedded within the cell s plasma membrane. two-way osmotic flow of water is substantial. ion don t move in great amounts unless some sort of channel protein is open. nutrients (oxygen, vitamins, glucose) and metabolic wastes travel down their concentration gradients... unidirectional. in areas such as the lungs, GI tract, and kidneys, exchanges of all types are occurring almost continually to maintain homeostasis. 10
Exchange of Gases, Nutrients, Water and Wastes Between the 3 Fluid Compartments 11
Major Sources of Water Intake & Output 12
Water Balance and ECF Osmolality healthy people can maintain the tonicity of their body fluids within very narrow limits: 280 to 300 mosm/kg a rise in plasma osmolality triggers: Thirst to increase water intake. ADH release to cause water reabsorption and therefore decreased urine output. a decrease in plasma osmolality triggers: decreased Thirst to lower water intake. blocks ADH release so more water can leave in the urine. 13
Regulation of Water Intake Thirst Mechanism driving force for water intake increased plasma osmolality of only 2-3% excites the hypothalamic thirst center. the osmoreceptor neurons located here are stimulated by: the loss of water by cells to the hypertonic ECF. angiotensin II stimulation. Baroreceptor input. this is the reason bars have free salty snacks... if you eat them, your need for a drink will increase due to increased plasma osmolality!!! this is also the reason why drinking salt water if stranded at sea is dangerous!!! as soon as you drink water, the hypothalamic thirst center becomes less stimulated to avoid over-drinking. 14
The Thirst Mechanism for Regulating Water Intake 15
Regulation of Water Output obligatory water loss insensible water loss evaporation moisture loss from breathing evaporation moisture loss from sweating water loss from feces a minimum daily sensible water loss of 500 ml in urine. this is because when we eat an adequate diet, our kidneys must excrete 900 to 1200 mosm of solutes to maintain homeostasis. these salts don t leave without water some desert mice can excrete a near solid urine!!! the kidneys normally begin to eliminate excess water about 30 minutes after it is ingested. this is why little kids should NOT drink a lot before bed! 16
Influence of ADH the amount of water reabsorbed in the renal collecting ducts is proportional to ADH release! low ADH... more water in urine high ADH... less water in urine more aquaporin channels are open for water to pass through ADH release is adjusted based on the ECF solute concentration. decrease ECF osmolality, ADH is not released. increase ECF osmolality, ADH is released. also, a large enough drop in blood pressure or volume will release ADH. other ADH release reasons: diarrhea, vomiting, burns. 17
Mechanisms & Consequences of ADH Release 18
Disorders of Water Balance Dehydration water output exceeds input (over a period of time). commonly due to: hemorrhage severe burns prolonged vomiting or diarrhea profuse sweating water deprivation diuretic abuse diabetes Early signs and symptoms sticky, dry cotton mouth thirsty decreased urine output (oliguria). In all causes, water is lost from the ECF. 19
Mechanism of Dehydration 20
Hypotonic Hydration the ECF osmolality drops (becomes more watery). ADH release is inhibited to encourage more water loss in urine. ADH is not enough to excrete the excess water in two situation: renal insufficiency water intoxication (drinking too much water). the hallmark of this condition is low ECF Na + (hyponatremia) which promotes water flow into cells causing them to swell (and possibly burst). if this happens to neuron cells you get disorientation, convulsions, coma and death. hypertonic saline IV can pull the water out of the cells. 21
Mechanism of Hypotonic Hydration 22
Edema an atypical accumulation of fluid in the interstitial space leading to tissue swelling. situations that increase fluid loss from blood increased capillary hydrostatic pressure: incompetent venous valves localized blood vessel blockage Congestive Heart Failure high blood volume increased capillary permeability: inflammation situations that decrease fluid return to blood hypoproteinemia (water need protein in blood plasma to return)»liver disease and glomerulonephritis (proteins lost in urine) blocked or removed lymphatic vessels. 23
Electrolyte Balance salts are important in controlling fluid movements and providing minerals essential for: excitability secretory activity membrane permeability the taste for very salty foods is learned, but some liking for salt may be innate to ensure adequate intake of sodium and chloride. salts are lost all the time in the fluids we lose each day. severe electrolyte deficiencies can be so low as to cause an intense urge for people to eat non-food substances like chalk, starch, burnt matches. (pica). 24
Electrolyte Balance while the sodium content of the body may change, its ECF concentration normally remains stable because of immediate adjustments in water volume. water quickly follows salt concentration increases so that ECF osmolarity and ICF osmolarity are equal at steady state conditions. 25
Regulation of Sodium Balance despite the crucial importance of sodium, receptors that specifically monitor Na + levels in body fluids have yet to be discovered! reabsorption of Na + does NOT exhibit a transport maximum, and in healthy people, nearly all Na + in the urinary filtrate can be reabsorbed. Aldosterone is a hormone with powerful control over renal regulation of Na + ion concentrations in the ECF. even without Aldosterone, Na + in the renal filtrate... is 65% reabsorbed in the proximal Convoluted Tubules plus another 25% reabsorbed in the Loop of Henle. this leaves 10% of Na+ in the renal filtrate that Aldosterone can have an effect over! this extra Na + reabsorption happens in the DCT and Collecting Duct. if ADH is also present, then water can also follow this extra Na +. 26
Regulation of Sodium Balance The most important trigger for Aldosterone release from the Adrenal Cortex (Zona Glomerulosa) is the renin-angiotensin mechanism mediated by the juxtaglomerular apparatus of the renal tubules. the JGA releases renin from granular cells if: there is sympathetic stimulation decreased filtrate NaCl concentration decreased stretch (low Blood Pressure) increased renin leads to increased levels of angiotensin II, which prompts aldosterone release from the adrenal cortex. this effect of the JGA and Aldosterone occur slowly, over days. People with Addison s disease (hypoaldosteronism) have a large amount of NaCl and water lost from the kidneys. See Chapter 16 notes. 27
Mechanisms & Consequences of Aldosterone Release 28
Baroreceptors & Atrial Natriuretic Peptide Cardiovascular Baroreceptors sense blood volume and pressure changes if low pressures or volume, Na + and water are reabsorbed in greater quantities. if high pressures or volume, Na + and water are released in the urine. Atrial Natriuretic Peptide is a hormone released by cells in the atria of the heart. increased stretch or pressure in atria triggers the release. ANP reduces blood pressure and blood volume by inhibiting nearly all events that promote vasoconstriction and Na + & Water retention. 29
Mechanisms & Consequences of ANP Release 30
Regulating Water & Sodium to Increase Blood Pressure 31
Estrogens and Water Balance Estrogens are chemically similar to aldosterone!!! Therefore, when estrogen amounts are higher during the menstrual cycle, more fluid will be retained due to aldosterone-like effects!!! 32
Regulation of Potassium Balance K + is the main intracellular cation. the relative ICF-ECF potassium concentration directly influences a cell s resting membrane potential. this is why very small changes (high or low) of K + can be life threatening. Death by Lethal Injection: First, inject Sodium Thiopental to render inmate unconscious. Second, inject Pancuronium to cause muscle paralysis. Third, inject Potassium Chloride to cause cardiac arrest! potassium is a buffer to excess hydrogen ions (H + ). in acidosis conditions, potassium leaves cells and H + enters. Hyperkalemia with Acidosis in basic conditions, potassium enters cells and H + leaves. Hypokalemia with Alkalosis 33
Regulation of Potassium Balance The Proximal Convoluted Tubules reabsorb about 60 to 80% of the filtered K +. The Thick ascending limb of the Loop of Henle absorbs another 10 to 20% of filtered K +. This leaves about 10% of filtered K + still in urine. if ECF K + is low: potassium leaves cells to the ECF renal principal cells conserve potassium by reducing its secretion and excretion. Type A intercalated cells in the collecting duct can reabsorb some of the potassium still in the urine. overall, the renal control of K + is done by controlling its excretion thus dietary intake is very important! 34
Regulation of Calcium & Phosphate 99% of body s calcium is found in the bones in the form of calcium phosphate salts. 98% of the filtered calcium is reabsorbed due to the actions of parathyroid hormone. PTH promotes an increase in calcium blood levels by: PTH activates osteoclasts in bones to break down matrix and release the Ca 2+ and HPO4 2- to the blood! PTH enhances intestinal absorption of Ca 2+ indirectly by stimulating the kidneys to transform the Vitamin D to its active form, which is necessary for Ca 2+ absorption by the small intestine. PTH increases Ca 2+ reabsorption mostly by the PCT, but it lets the phosphate ion stay in the urine. 35