Fluid, Electrolyte, and Acid-Base Balance PDF

Fluid, Electrolyte, and Acid-Base Balance Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Video: Why This Matters Understanding the process of fluid, electrolyte and acid-base balance, and lab tests that evaluate these values, will help you to correctly interpret your p...

Fluid, Electrolyte, and Acid-Base Balance Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Video: Why This Matters Understanding the process of fluid, electrolyte and acid-base balance, and lab tests that evaluate these values, will help you to correctly interpret your patient’s test results. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Body Fluid Compartments Body Water Content Infants are 73% or more water (low body fat, low bone mass) Adult males: ~60% water Adult females: ~50% water (higher fat content, less skeletal muscle mass) – Adipose tissue is least hydrated of all – Total body water in adults averages ~40 L Water content declines to ~45% in old age Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Body Water Content Two main fluid compartments – Intracellular fluid (ICF) compartment: fluid inside cells accounts for 2/3 of total body fluid – Extracellular fluid (ECF) compartment: fluid in two main ECF compartments outside cells accounts for one-third of total body fluid:  Plasma: 3 L  Interstitial fluid (IF): 12 L in spaces between cells – Also considered part of IF: lymph, CSF, humors of the eye, synovial fluid, serous fluid, and gastrointestinal secretions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Composition of Body Fluids Water is the universal solvent Solutes are substances dissolved in water Solutes are classified as nonelectrolytes and electrolytes Electrolytes and nonelectrolytes – Nonelectrolytes: most are organic molecules  Do not dissociate in water  Examples: glucose, lipids, creatinine, and urea – No charged particles are created Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Composition of Body Fluids – Electrolytes  Dissociate into ions in water – Examples: inorganic salts, all acids and bases, some proteins  Ions conduct electrical current  Greater osmotic power than nonelectrolytes – Greater ability to cause fluid shifts due to ability to dissociate into two or more ions – NaCl Na+ + Cl- (electrolyte; 2 particles) – MgCl2 Mg2+ + 2Cl- (electrolyte; 3 particles) – glucose glucose (nonelectrolyte; 1 particle) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Composition of Body Fluids Comparison of extracellular and intracellular fluids – Each fluid compartment has a distinctive pattern of electrolytes – ECF: electrolyte contents are all similar except for higher protein, lower Cl– content of plasma  Major cation: Na+  Major anion: Cl– – ICF: contains more soluble proteins than plasma  Low Na+ and Cl–  Major cation: K+  Major anion HPO42– – Electrolytes are most abundant (largest number) solutes in body fluids  Determine most chemical and physical reactions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Fluid Movement among Compartments Osmotic and hydrostatic pressures regulate continuous exchange and mixing of fluids – Water moves freely along osmotic gradients – All body fluid osmolality is almost always equal – Change in solute concentration of any compartment leads to net water flow  ↑ ECF osmolality → water leaves cell  ↓ ECF osmolality → water enters cell Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Exchange of Gases, Nutrients, Water, and Wastes Between the Three Fluid Compartments of the Body Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Water Balance and ECF Osmolality Water intake must equal water output: ~2500 ml/day Water intake: most water is taken in via ingested foods and beverages, but small amount from metabolism – Metabolic water (water of oxidation): water produced by cellular metabolism Water output: urine (60%), insensible water loss (lost through skin and lungs), perspiration, and feces Osmolality: the concentration of a solution expressed as the total number of solute particles per kilogram. Osmolality is maintained around 280–300 mOsm Rise in osmolality – Stimulates thirst – Causes ADH release Decrease in osmolality – Causes ADH inhibition Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Major Sources of Water Intake and Output Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Water Intake Thirst mechanism is driving force for water intake Governed by hypothalamic thirst center – Hypothalamic osmoreceptors detect ECF osmolality and are activated by:  Increased plasma osmolality of 1–2%  Dry mouth  Decreased blood volume or pressure  Angiotensin II or baroreceptor input Drinking of water inhibits the thirst center Inhibitory feedback signals include: – Relief of dry mouth – Activation of stomach and intestinal stretch receptors Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Water Output Obligatory water losses: explain why we cannot live without water very long – Include:  Insensible water loss from lungs or skin  Sensible water loss from urine to excrete wastes (60%), obvious sweat (8%), and feces (4%) Volume of urine excreted and solute concentration also depend on fluid intake, diet, and water loss via other avenues Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Influence of Antidiuretic Hormone (ADH) Water reabsorption in collecting ducts is proportional to ADH release Decreased ADH leads to dilute urine and drop in volume of body fluids Increased ADH leads to concentrated urine, due to reabsorption of water, causing increased volume of body fluids Hypothalamic osmoreceptors sense ECF solute concentration and regulate ADH accordingly Other factors may trigger ADH release – “Large” changes in blood volume or pressure  Decreased BP causes increased ADH release due to blood vessel baroreceptors and renin-angiotensin- aldosterone mechanism  Decreased BP/volume can be caused by: – Intense sweating, vomiting, or diarrhea, severe blood loss, traumatic burns, and prolonged fever Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved  Electrolyte Balance Electrolyte balance usually refers only to salt balance even though electrolytes also include acids, bases, and some proteins Salts control fluid movements, provide minerals for excitability, secretory activity, and membrane permeability Salts enter body by ingestion and metabolism and are lost via perspiration, feces, urine, vomit Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Central Role of Sodium in Fluid and Electrolyte Balance Sodium is most abundant cation in ECF – Sodium salts in ECF contribute 280 mOsm of total 300 mOsm ECF solute concentration Only cation exerting significant osmotic pressure – Controls ECF volume and water distribution because water follows salt – Changes in Na+ levels affects plasma volume, blood pressure, and ECF and IF volumes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Sodium Balance There are no known receptors that monitor Na+ levels in body fluids Na+-water balance is linked to blood pressure and blood volume control mechanisms Changes in blood pressure or volume trigger neural and hormonal controls to regulate Na+ content Aldosterone plays biggest role in regulation of Na+ by kidneys Aldosterone brings about its effects slowly (hours to days) Renin-angiotensin-aldosterone mechanism is main trigger for aldosterone release Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Sodium Balance Influence of atrial natriuretic peptide (ANP) – Released by atrial cells in response to stretch caused by increased blood pressure – Effects  Decreases blood pressure and blood volume – Inhibits ADH, renin, and aldosterone production – Increases excretion of Na+and water – Promotes vasodilation directly and also by decreasing production of angiotensin II Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Sodium Balance Influence of other hormones – Female sex hormones  Estrogens: increase NaCl reabsorption (like aldosterone) – Leads to H2O retention during menstrual cycles and pregnancy  Progesterone: decreases Na+reabsorption (blocks aldosterone) – Promotes Na+and H2O loss – Glucocorticoids  Increase Na+ reabsorption and promote edema Cardiovascular baroreceptors – Baroreceptors alert brain to increases in blood volume and pressure  Sympathetic nervous system impulses to kidneys decline, causing: – Afferent arterioles to dilate – GFR increases – Increased Na+ and water output – Reduced blood volume and pressure Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Potassium Balance Importance of potassium – Affects resting membrane potential (RMP) in neurons and muscle cells (especially cardiac muscle)  Increases in ECF [K+] (hyperkalemia) cause decreased RMP, causing depolarization, followed by reduced excitability  Decreases in ECF [K+] (hypokalemia) cause hyperpolarization and nonresponsiveness – Disruption in [K+] (hyper- or hypokalemia) in heart can interfere with electrical conduction, leading to sudden death K+ is also part of body’s buffer system H+ shifts in and out of cells in opposite direction of K+ to maintain cation balance, so: – ECF K+ levels rise with acidosis – ECF K+ levels fall with alkalosis  Interferes with activity of excitable cells Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Regulation of Potassium Balance Influence of aldosterone – Aldosterone stimulates K+ secretion (and Na+ reabsorption) by principal cells – Increased K+ in adrenal cortex causes release of aldosterone, which increases K+ secretion Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Acid-Base Balance pH affects all functional proteins and biochemical reactions, so it is closely regulated by the body Normal pH of body fluids – Arterial blood: pH 7.4 – Venous blood and interstitial fluid: pH 7.35 – ICF: pH 7.0 Alkalosis or alkalemia: arterial pH >7.45 Acidosis or acidemia: arterial pH