1. Water, Electrolyte, and Acid-Base Balance
Chapter 25

2. 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

3. 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

4. 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)

7. 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

8. 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

9. 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

10. 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)

12. 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

14. 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

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

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

17. 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

18. 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

19. 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

20. 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

21. 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
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22. 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)
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23. 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)

24. 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
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25. 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
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26. 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
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28. 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

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

30. 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

31. 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

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

34. Acid-Base Imbalances
Normal pH of the blood is

35. 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)

36. 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

37. 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)

38. 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)

39. 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.

40. 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 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

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

42. Normal Arterial Blood Gas ValuespH
PaCO2
35-45 mm Hg
PaO2 
80-95 mm Hg
HCO3 
22-26 mEq/L
O2 Saturation
95-99%
BE 
+/- 1

43. 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

44. The End