Water, Electrolyte, and Acid-Base Balance Chapter 25

Body Fluids: Distribution An adult is composed of approximately 50-60% water; infants are up to 75% water Muscle tissue contains more water than adipose tissue Water and its dissolved electrolytes are distributed in 2 main compartments in the body: intracellular and extracellular

Fluid Compartments Intracellular Extracellular Includes water located in all the cells of the body Most water (63%) located intracellular Extracellular Includes the fluid located outside of the cells Can be found in different areas of the body

Fluid Compartments Extracellular Interstitial fluid (located between the cells) Plasma (water located in the blood vessels) Lymph (water located in the lymphatic vessels) Transcellular (includes CSF, aqueous and vitreous humors, synovial fluid, serous fluid and glandular secretions)

Composition of Body Fluids Intracellular High concentrations of potassium (K+), phosphate (PO43-) and magnesium (Mg2+) ions Extracellular High concentrations of sodium (Na+), chloride (Cl-) and bicarbonate (HCO3-) ions Plasma contains a higher level of proteins than other extracellular fluid

Water Balance The quantity of water taken in should equal the amount of water eliminated Intake = output Nurses are responsible for measuring intake and output, as well as calculating deficiencies and excess

Water Intake Average adult intake: 2500ml per 24 hours Primary regulator is thirst Thirst center is located in the hypothalmus As the body loses water, thirst center is activated and causes you to drink Drinking restores the water that has been lost The elderly have diminished thirst mechanism and are prone to dehydration

Water Output Average adult output: 2500ml per 24 hours Water leaves the body through Kidneys, skin, lungs and digestive tract Primary regulator: kidneys (eliminate about 60% of the water) When water levels in the body are low, ADH is released; stimulates the collecting ducts to reabsorb water (decreases urine output and increases blood volume)

Water Imbalances Dehydration Water output exceeds water intake Commonly seen with sweating, vomiting, diarrhea, use of diuretics Skin turgor will be poor (very poor turgor can be described as tenting) Dehydration can progress to hypovolemic shock

Water Imbalances Edema Excess water retained in the interstitial space Water can accumulate in in many regions of the body including the lungs (pulmonary edema), brain tissue (cerebral edema), or in the lower extremities (pedal edema) Can be mild or life threatening Goal of treatment is to remove the excess fluid and treat the underlying cause

An example of pedal edema in a patient with congestive heart failure (CHF)

Fluid Shifts Fluid “shifts” or changes compartments because of differences in pressure across the capillary membranes

Fluid Balance and Weight 1 Liter of water weighs 1 kg (2.2 lbs) If a patient gains weight quickly (overnight) suspect fluid retention If a patient loses weight quickly, the cause is probably diuresis Many patients may be on “daily weights” to monitor fluid status

Fluid Spacing Clinical term that refers to the distribution of body fluids First spacing Normal distribution of water throughout body Second spacing Accumulation of water in the interstitial spaces (interstitial edema) Can usually be reabsorbed and excreted

Fluid Spacing Third spacing Water accumulates in spaces where it is not easily absorbed Examples include ascites (from liver damage) and fluid accumulation from paralytic ileus or major burns

Electrolyte Balance Electrolyte balance Electrolyte imbalance Amount gained equals amount lost Electrolyte imbalance Common and can be life threatening Kidneys control the composition of body fluids by regulating the renal excretion of electrolytes

Electrolytes (review) Ion: an element or compound that carries an electrical charge Cation: a positively charged ion Anion: a negatively charged ion Electrolyte: substances that form ions when they dissolve in water Ionization: chemical reaction caused when an electrolyte splits into two ions Copyright © 2007 by Saunders, an imprint of Elsevier Inc. All rights reserved. 21 21

Major Ions, Plasma Levels, and Functions Sodium (Na+): 136 to 145 mEq/L Chief extracellular cation Regulates extracellular volume Participates in nerve-muscle function Aldosterone is the primary mechanism for regulating sodium concentration Hypernatremia (usually due to volume loss) Hyponatremia (usually dilutional) Copyright © 2007 by Saunders, an imprint of Elsevier Inc. All rights reserved. 22 22

Major Ions, Plasma Levels, and Functions Potassium (K+): 3.5 to 5.0 mEq/L Chief intracellular cation Participates in nerve-muscle function; especially important for cardiac muscle Aldosterone is the primary mechanism for regulating potassium concentration Hyperkalemia (usually due to kidney disease) Hypokalemia (usually due to diuretics, vomiting or diarrhea)

Major Ions, Plasma Levels, and Functions (cont’d.) Calcium (Ca2+): 4.5 to 5.8 mEq/L Strengthens bone and teeth Participates in muscle contraction Helps in blood clotting Regulated by parathyroid hormone Magnesium (Mg2+): 1.5 to 2.5 mEq/L Strengthens bone Participates in nerve-muscle function and cardiac function Copyright © 2007 by Saunders, an imprint of Elsevier Inc. All rights reserved. 24 24

Major Ions, Plasma Levels, and Functions (cont’d.) Chloride (Cl−): 95 to 108 mEq/L Chief extracellular anion (usually follows Na+) Involved in extracellular volume control Bicarbonate (HCO3−): 22 to 26 mEq/L Part of bicarbonate buffer system Participates in acid-base balance Phosphate (PO43−): 2.5 to 4.5 mEq/L Strengthens bone Copyright © 2007 by Saunders, an imprint of Elsevier Inc. All rights reserved. 25 25

Copyright © 2007 by Saunders, an imprint of Elsevier Inc. Acid-Base Balance Acid: substance that dissociates into H+ and an anion Base: substance that combines with H+ during a chemical reaction and removes H+ from solution pH: unit of measurement that indicates the number of H+ in solution Copyright © 2007 by Saunders, an imprint of Elsevier Inc. All rights reserved. 26 26

Acids Acids are produced in the body as a by-product of cellular metabolism Glucose breaks down into CO2, water and energy- CO2 then combines with water and forms carbonic acid; in the absence of O2, glucose forms lactic acid Metabolism of fatty acids yield ketoacids, and some proteins yield sulfuric acid Excess acid must be eliminated to maintain acid-base balance and homeostasis

Regulation of pH Three mechanisms in the body work together to regulate blood pH (maintain acid-base balance) Buffers Respiration Kidney function

Acid-Base Balance Buffer: chemical substance that prevents large changes in pH Works as a buffer pair; can either give up or take excess H+ most important buffers in the body include bicarbonate buffers, phosphate buffers, hemoglobin and plasma proteins

Acid-Base Balance Respiration breathing controls CO2 levels and affects blood pH (remember CO2 can combine with water to form an acid) Decreasing respiratory rate- body accumulates CO2, increases the H+ level and causes the pH to drop; respiratory acidosis Increasing respiratory rate- blows off CO2 which decreases the H+ level and causes pH to rise; respiratory alkalosis

Acid-Base Balance Kidneys help regulate pH by reabsorbing or excreting H+ as needed Can also secrete or reabsorb bicarbonate if needed

Acid-Base Imbalances Normal pH of the blood is 7.35 - 7.45

Acid-Base Imbalances Acidosis Plasma pH less than 7.35 Respiratory acidosis Caused by any condition that causes hypoventilation COPD, narcotics, chest injury, injury to medulla oblongata (depressed respirations)

Acid-Base Imbalances Acidosis Metabolic acidosis Caused by non-respiratory conditions Diabetes mellitus, prolonged vomiting or severe diarrhea (loss of bicarbonate) Body tries to correct by buffer system (remove excess H+), hyperventilation or Kussmaul respirations

Acid-Base Imbalances Alkalosis Plasma pH more than 7.45 Respiratory alkalosis Caused by hyperventilation (blow off CO2 and lose H+, therefore increasing pH) Corrected by buffers (donate H+ to lower pH) and kidneys (retain more H+ and increase excretion of bicarbonate)

Acid-Base Imbalances Alkalosis Metabolic alkalosis Increase in pH caused by non-respiratory disorders Can be caused by overuse of antacids or bicarbonate containing drugs, persistent vomiting or frequent suctioning (loss of HCl) Corrected with buffers (donate H+), kidneys (decrease excretion of H+) and hypoventilation (hold on to CO2 and H+ to decrease pH)

Arterial Blood Gas An arterial blood gas (ABG) test measures the acidity (pH) and the levels of oxygen and carbon dioxide in the blood. This test is used to check how well your lungs are able to move oxygen into the blood and remove carbon dioxide from the blood.

Arterial Blood Gas Partial pressure of oxygen (PaO2). This measures the pressure of oxygen dissolved in the blood and how well oxygen is able to move from the airspace of the lungs into the blood. Partial pressure of carbon dioxide (PaCO2). This measures how much carbon dioxide is dissolved in the blood and how well carbon dioxide is able to move out of the body. pH. The pH measures hydrogen ions (H+) in blood. The pH of blood is usually between 7.35 and 7.45. A pH of less than 7.35 is called acid and a pH greater than 7.45 is called basic (alkaline). Bicarbonate (HCO3). Bicarbonate is a chemical that keeps the pH of blood from becoming too acid. If the pH level drops, HCO3 is absorbed by the kidneys and returned to the blood instead of passing out of the body in the urine. Oxygen content (O2CT) and oxygen saturation (O2Sat) values. O2 content measures the amount of oxygen in the blood. Oxygen saturation measures how much of the hemoglobin in the red blood cells is carrying oxygen (O2). Taken from http://www.webmd.com/a-to-z-guides/arterial-blood-gases

Arterial Blood Gas (ABG) sample being taken from a patient

Normal Arterial Blood Gas Values pH 7.35-7.45 PaCO2 35-45 mm Hg PaO2  80-95 mm Hg HCO3  22-26 mEq/L O2 Saturation 95-99% BE  +/- 1

ABG calculations Respiratory Acidosis Respiratory Alkalosis pH <7.35, CO2 >45, HCO3 normal Respiratory Alkalosis pH >7.45, CO2 <35, HCO3 normal Metabolic Acidosis pH <7.35, CO2 normal, HCO3 <22 Metabolic Alkalosis pH >7.45, CO2 normal, HCO3 >26 *These are for uncompensated states

The End