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fluid balance.docx

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Biological Sciences
Kenneth Welch

Fluid, Electrolyte, and Acid-Base Balance Body Water Content • Infants have low body fat, low bone mass, and are 73% or more water • Total water content declines throughout life • Healthy males are about 60% water; healthy females are around 50% • This difference reflects females’: • Higher body fat • Smaller amount of skeletal muscle • In old age, only about 45% of body weight is water Fluid Compartments • Water occupies two main fluid compartments • Intracellular fluid (ICF) – about two thirds by volume, contained in cells • Extracellular fluid (ECF) – consists of two major subdivisions • Plasma – the fluid portion of the blood • Interstitial fluid (IF) – fluid in spaces between cells • Other ECF – lymph, cerebrospinal fluid, eye humors, synovial fluid, serous fluid, and gastrointestinal secretions Composition of Body Fluids • Water is the universal solvent • Solutes are broadly classified into: • Electrolytes – inorganic salts, all acids and bases, and some proteins • Nonelectrolytes – examples include glucose, lipids, creatinine, and urea • Electrolytes have greater osmotic power than nonelectrolytes • Water moves according to osmotic gradients Electrolyte Concentration • Expressed in milliequivalents per liter (mEq/L), a measure of the number of electrical charges in one liter of solution • mEq/L = (concentration of ion in [mg/L]/the atomic weight of ion)  number of electrical charges on one ion • For single charged ions, 1 mEq = 1 mOsm • For bivalent ions, 1 mEq = 1/2 mOsm Extracellular and Intracellular Fluids • Each fluid compartment of the body has a distinctive pattern of electrolytes • Extracellular fluids are similar (except for the high protein content of plasma) • Sodium is the chief cation • Chloride is the major anion • Intracellular fluids have low sodium and chloride • Potassium is the chief cation • Phosphate is the chief anion • Sodium and potassium concentrations in extra- and intracellular fluids are nearly opposites • This reflects the activity of cellular ATP-dependent sodium-potassium pumps • Electrolytes determine the chemical and physical reactions of fluids • Proteins, phospholipids, cholesterol, and neutral fats account for: • 90% of the mass of solutes in plasma • 60% of the mass of solutes in interstitial fluid • 97% of the mass of solutes in the intracellular compartment Fluid Movement Among Compartments • Compartmental exchange is regulated by osmotic and hydrostatic pressures • Net leakage of fluid from the blood is picked up by lymphatic vessels and returned to the bloodstream • Exchanges between interstitial and intracellular fluids are complex due to the selective permeability of the cellular membranes • Two-way water flow is substantial Extracellular and Intracellular Fluids • Ion fluxes are restricted and move selectively by active transport • Nutrients, respiratory gases, and wastes move unidirectionally • Plasma is the only fluid that circulates throughout the body and links external and internal environments • Osmolalities of all body fluids are equal; changes in solute concentrations are quickly followed by osmotic changes Water Balance • To remain properly hydrated, water intake must equal water output • Water intake sources • Ingested fluid (60%) and solid food (30%) • Metabolic water or water of oxidation (10%) • Water output: • Urine (60%) and feces (4%) • Insensible losses (28%), sweat (8%) • Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH) Regulation of Water Intake • The hypothalamic thirst center is stimulated by: • Decreases in plasma volume of 10% • Increases in plasma osmolality of 1-2% • Thirst is quenched as soon as we begin to drink water • Feedback signals that inhibit the thirst centers include: • Damping of mucosa of the mouth • Moistening of the throat • Activation of stomach and intestinal stretch receptors Regulation of Water Output • Obligatory water losses include: • Insensible water losses from lungs and skin • Water that accompanies undigested food residues in feces • Obligatory water loss reflects the facts that: • Kidneys excrete 900-1200 mOsm of solutes to maintain blood homeostasis • Urine solutes must be flushed out of the body in water Disorders of Water Balance: Dehydration • Water loss exceeds water intake and the body is in negative fluid balance • Causes include: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, and diuretic abuse • Signs and symptoms: cottonmouth, thirst, dry flushed skin, and oliguria • Prolonged dehydration may lead to weight loss, fever, and mental confusion • Other consequences include hypovolemic shock and loss of electrolytes Disorders of Water Balance: Hypotonic Hydration • Renal insufficiency or an extraordinary amount of water ingested quickly can lead to cellular overhydration, or water intoxication • ECF is diluted – sodium content is normal but excess water is present • The resulting hyponatremia promotes net osmosis into tissue cells, causing swelling • These events must be quickly reversed to prevent severe metabolic disturbances, particularly in neurons Disorders of Water Balance: Edema • Atypical accumulation of fluid in the interstitial space, leading to tissue swelling • Caused by anything that increases flow of fluids out of the bloodstream or hinders their return • Factors that accelerate fluid loss include: • Increased blood pressure, capillary permeability • Incompetent venous valves, localized blood vessel blockage • Congestive heart failure, hypertension, high blood volume Edema • Hindered fluid return usually reflects an imbalance in colloid osmotic pressures • Hypoproteinemia – low levels of plasma proteins • Forces fluids out of capillary beds at the arterial ends • Fluids fail to return at the venous ends • Results from protein malnutrition, liver disease, or glomerulonephritis • Blocked (or surgically removed) lymph vessels: • Cause leaked proteins to accumulate in interstitial fluid • Exert increasing colloid osmotic pressure, which draws fluid from the blood • Interstitial fluid accumulation results in low blood pressure and severely impaired circulation Electrolyte Balance • Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance • Salts are important for: • Neuromuscular excitability • Secretory activity • Membrane permeability • Controlling fluid movements • Salts enter the body by ingestion and are lost via perspiration, feces, and urine Sodium in Fluid and Electrolyte Balance • Sodium holds a central position in fluid and electrolyte balance • Sodium salts: • Account for 90-95% of all solutes in the ECF • Contribute 280 mOsm of the total 300 mOsm ECF solute concentration • Sodium is the single most abundant cation in the ECF • Sodium is the only cation exerting significant osmotic pressure • The role of sodium in controlling ECF volume and water distribution in the body is a result of: • Sodium being the only cation to exert significant osmotic pressure • Sodium ions leaking into cells and being pumped out against their electrochemical gradient • Sodium concentration in the ECF normally remains stable • Changes in plasma sodium levels affect: • Plasma volume, blood pressure • ICF and interstitial fluid volumes • Renal acid-base control mechanisms are coupled to sodium ion transport Regulation of Sodium Balance: Aldosterone • Sodium reabsorption • 65% of sodium in filtrate is reabsorbed in the proximal tubules • 25% is reclaimed in the loops of Henle + • When aldosterone levels are high, all remaining Na is actively reabsorbed • Water follows sodium if tubule permeability has been increased with ADH Regulation of Sodium Balance: Aldosterone • The renin-angiotensin mechanism triggers the release of aldosterone • This is mediated by the juxtaglomerular apparatus, which releases renin in response to: • Sympathetic nervous system stimulation • Decreased filtrate osmolality • Decreased stretch (due to decreased blood pressure) • Renin catalyzes the production of angiotensin II, which prompts aldosterone release + • Adrenal cortical cells are directly stimulated to release aldosterone by elevated K levels in the ECF • Aldosterone brings about its effects (diminished urine output and increased blood volume) slowly Cardiovascular System Baroreceptors • Baroreceptors alert the brain of increases in blood volume (hence increased blood pressure) • Sympathetic nervous system impulses to the kidneys decline • Afferent arterioles dilate • Glomerular filtration rate rises • Sodium and water output increase • This phenomenon, called pressure diuresis, decreases blood pressure • Drops in systemic blood pressure lead to opposite actions and systemic blood pressure increases • Since sodium ion concentration determines fluid volume, baroreceptors can be viewed as “sodium receptors” Influence and Regulation of ADH • Water reabsorption in collecting ducts is proportional to ADH release • Low ADH levels produce dilute urine and reduced volume of body fluids • High ADH levels produce concentrated urine • Hypothalamic osmoreceptors trigger or inhibit ADH release • Factors that specifically trigger ADH release include prolonged fever; excessive sweating, vomiting, or diarrhea; severe blood loss; and traumatic burns Atrial Natriuretic Peptide (ANP) • Reduces blood pressure and blood volume by inhibiting: • Event+ that promote vasoconstriction • Na and water retention • Is released in the heart atria as a response to stretch (elevated blood pressure) • Has potent diuretic and natriuretic effects • Promotes excretion of sodium and water • Inhibits angiotensin II production Influence of Other Hormones on Sodium Balance • Estrogens: • Enhance NaCl reabsorption by renal tubules • May cause water retention during menstrual cycles • Are responsible for edema during pregnancy • Progesterone: • Decreases sodium reabsorption • Acts as a diuretic, promoting sodium and water loss • Glucocorticoids – enhance reabsorption of sodium and promote edema Regulation of Potassium Balance • Relative ICF-ECF potassium ion concentration affects a cell’s resting membrane potential • Excessive ECF potassium decreases membrane potential • Too little K causes hyperpolarization and nonresponsiveness • Hyperkalemia and hypokalemia can: • Disrupt electrical conduction in the heart • Lead to sudden death • Hydrogen ions shift in and out of cells • Leads to corresponding shifts in potassium in the opposite direction • Interferes with activity of excitable cells Regulatory Site: Cortical Collecting Ducts • Less than 15% of filtered K is lost to urine regardless of need • K balance is controlled in the cortical collecting ducts by changing the amount of potassium secreted into filtrate + • Excessive+K is excreted over basal levels by cortical collecting ducts • When K levels are low, the amount of secretion and excretion is kept to a minimum • Type A intercalated cells can reabsorb some K left in the filtrate Influence of Plasma Potassium Concentration • High K content of ECF favors principal cells to secrete K + + + • Low K or accelerated K loss depresses its secretion by the collecting ducts Influence of Aldosterone • Aldosterone stimulates potassium ion secretion by principal cells • In cortical collecting ducts, for each Na reabsorbed, a K is secreted + • Increased K in the ECF around the adrenal cortex causes: • Release of aldosterone • Potassium secretion • Potassium controls its own ECF concentration via feedback regulation of aldosterone release Regulation of Calcium
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