Fluid Balance and Homeostasis PDF

Balance cellular function requires a fluid medium with a carefully controlled composition three types of homeostatic balance: 1. water balance 2. electrolyte balance 3. acid-base balance balances maintained by the collective action of the urinary, respiratory, digestive, integumentar...

Balance cellular function requires a fluid medium with a carefully controlled composition three types of homeostatic balance: 1. water balance 2. electrolyte balance 3. acid-base balance balances maintained by the collective action of the urinary, respiratory, digestive, integumentary, endocrine, nervous, cardiovascular, and lymphatic systems 24-1  Body Water newborn baby’s body weight is about 75% water young men average 55% - 60% women average slightly less obese & elderly people as little as 45% total body water (TBW) of a 70kg (150 lb) young man is about 40 liters 24-2  Fluid Compartments major fluid compartments of the body – 65% intracellular fluid (ICF) – 35% extracellular fluid (ECF) 25% tissue (interstitial) fluid 8% blood plasma and lymphatic fluid 2% transcellular fluid ‘catch-all’ category 24-3 Water Movement Between Fluid Compartments fluid continually exchanged btwn compartments – water moves by osmosis osmosis from one fluid compartment to another is determined by the relative concentrations of solutes in each compartment – electrolytes – the most abundant solute particles sodium salts in ECF potassium salts in ICF electrolytes are key to body’s water distribution 24-4 Water Movement Between Fluid Compartments Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intracellular fluid Digestive tract Bloodstream Tissue fluid Lymph Bloodstream 24-5  Water Gain fluid balance: when daily gains & losses are equal gains come from two sources: 1. preformed water (2,300 mL/day) ingested in food and drink 2. metabolic water (200 mL/day) by-product of aerobic metabolism and dehydration synthesis – C6H12O6 + 6O2 6CO2 + 6H2O 24-6  Fluid Balance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intake Output 2,500 mL/day 2,500 mL/day Individual values are FYI Metabolic water Feces 200 mL 200 mL Expired air 300 mL Food 700 mL Cutaneous transpiration 400 mL Sweat 100 mL Drink Urine 1,600 mL 1,500 mL 24-7 Regulation of Fluid Intake thirst mainly governs fluid intake dehydration – reduces blood volume and blood pressure – increases blood osmolarity osmoreceptors in hypothalamus: – respond to angiotensin II produced when BP or blood volume drops – communicate w/ hypothalamus & cerebral cortex – hypothalamus makes antidiuretic hormone (ADH) – cerebral cortex produces sense of thirst 24-8  Dehydration Increased Reduced blood osmolarity blood pressure Dehydration, Dehydration Renin Thirst, and Angiotensin II Stimulates Stimulates Rehydration hypothalamic osmoreceptors Reduced hypothalamic osmoreceptors salivation Dry mouth Thirst Sense of thirst Ingestion of water Cools and Distends Short-term moistens mouth stomach inhibition and intestines of thirst Long-term Rehydrates inhibition Rehydration blood of thirst 24-9 Regulation of Water Output only way to control water output: variation in urine volume – kidneys can’t replace water or electrolytes – can only slow rate of water & electrolyte loss until water & electrolytes can be ingested Two mechanisms: a) changes in urine volume linked to adjustments in Na+ reabsorption as Na+ is reabsorbed or excreted, water follows 24-10 Regulation of Water Output b) concentrate the urine through action of ADH ADH secretion stimulated by hypothalamic osmoreceptors in response to dehydration aquaporins synthesized in response to ADH – membrane proteins in renal collecting ducts whose job is to channel water back into renal medulla; Na+ is still excreted – concentrates urine – ADH release inhibited when blood volume and pressure is too high or blood osmolarity too low helps to compensate for hypertension 24-11 Secretion Dehydration and H 2O H2O Na+ Na+ Elevates blood osmolarity Effects of Negative feedback Stimulates hypothalamic ADH loop osmoreceptors Negative feedback Water Stimulates posterior pituitary loop ingestion to release antidiuretic hormone (ADH) Stimulates distal convoluted Thirst tubule and collecting duct Increases water reabsorption Reduces urine Increases ratio volume of Na+: H2O in urine 24-12 Disorders of Water Balance – volume depletion (hypovolemia) – water and Na both lost equally - osmolarity stays normal – hemorrhage, severe burns, chronic vomiting, or diarrhea – dehydration (negative water balance) – more water lost than sodium - osmolarity rises – lack of drinking water, diabetes, profuse sweating, overuse of diuretics – most serious effects: circulatory shock due to loss of blood volume, neurological dysfunction due to dehydration of brain cells, infant mortality from diarrhea 24-13  Fluid Excess fluid excess – not common; kidneys compensate for excessive intake by excreting more urine – renal failure can lead to fluid retention two types of fluid excesses: 1. volume excess both Na+ & water retained - ECF stays isotonic caused by aldosterone hypersecretion or renal failure (or salty diet) 2. hypotonic hydration (water intoxication) more water than Na+ retained or ingested ECF becomes hypotonic – cellular swelling, pulmonary & cerebral edema 24-14 Blood Volume & Fluid Intake Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5 4 Hypovolemia Blood volume (L) Death 3 2 1 Danger Normal Water diuresis 0 0 1 2 3 4 5 6 7 8 Fluid intake (L/day) kidneys compensate well for excessive fluid intake, but not for inadequate fluid intake 24-15  Fluid Sequestration fluid sequestration – excess fluid accumulates in a particular location total body water may be normal, but volume of blood may drop, causing circulatory shock 24-16  Fluid Sequestration There should be an equilibrium of fluid moving into and back out of the tissues (Starling’s Law of the Capillaries) – If equilibrium is lost: edema - build-up of fluid in the interstitial spaces, causes swelling of tissues hemorrhage - another cause of fluid sequestration blood that pools in the tissues is not circulating pleural effusion – several liters of fluid can accumulate in the pleural cavity caused by some lung infections 24-17 ? 24-18  Electrolyte Balance functions of electrolytes: – chemically reactive & participate in metabolism – determine electrical potential across cell membranes – strongly affect osmolarity of body fluids, thus affect body’s water content and distribution major cations: Na+, K+, Ca2+, and H+ major anions: Cl-, HCO3- (bicarbonate), and PO43- different concentrations in blood plasma and intracellular fluid (ICF) – but same osmolarity 24-19  145 300 103 Electrolyte Concentrations (know relative 4 5 4 proportions, not (a) Blood plasma 150 numbers) 300 75 12 4 145 mEq/L (FYI) Causes: loss of water faster than sodium (diarrhea), not drinking Results: water retention, hypertension & edema hyponatremia – plasma sodium concentration 7.45, Why is it bad? – H+ diffuses out of cells and K+ diffuses in, membranes depolarized, nerves and muscles overstimulated – a person cannot live for more than a few hours if the blood pH is below 7.0 or above 7.7 K+ H+ Protein leading to (b) Alkalosis Hypokalemia 24-52 Disorders of Acid-Base Balance two categories: respiratory and metabolic respiratory acidosis – when ventilation rate too slow for rate of CO2 production – CO2 accumulates in the ECF & lowers its pH – emphysema is a common cause – Arterial Blood Gas (ABG) measurement will show high PCO2 respiratory alkalosis – results from hyperventilation – CO2 eliminated faster than it is produced 24-53 Disorders of Acid-Base Balance metabolic acidosis – increased production of acids like lactic acid (anaerobic fermentation), or ketone bodies (alcoholism, diabetes mellitus) – ingestion of acidic drugs (aspirin) – loss of base due to chronic diarrhea, laxative overuse metabolic alkalosis – rare, but can result from: – overuse of antacids – loss of stomach acid (chronic vomiting) 24-54 Compensation for Acid-Base Imbalances compensated acidosis or alkalosis: – either the kidneys compensate for pH imbalances of respiratory origin, or – the respiratory system compensates for pH imbalances of metabolic origin uncompensated acidosis or alkalosis – a pH imbalance that the body cannot correct without clinical intervention 24-55 Compensation for Acid-Base Imbalances respiratory compensation – changes in pulmonary ventilation to correct changes in pH of body fluids – an acidic pH (like with ketoacidosis) stimulates pulmonary ventilation, eliminating CO2 and allowing pH to rise – an alkaline pH reduces ventilation and allows CO2 to build up, lowering pH 24-56 Compensation for Acid-Base Imbalances renal compensation – adjustment of pH by changing rate of H+ secretion by the renal tubules – slow, but better at restoring a fully normal pH – in acidosis, urine pH may fall as low as 4.5 renal tubules secrete more H+, elevating pH – in alkalosis, as high as 8.2 b/c of excess HCO3- renal tubules secrete less H+, lowering pH – kidneys cannot act quickly enough to compensate for short-term pH imbalances – better for imbalances that last > a few days 24-57