Chapter 6
Disorders of Fluid, Electrolyte,
and Acid-Base Balance
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Fluid Distribution
• Intracellular compartment
• Extracellular compartment
– Interstitial spaces
– Plasma (vascular) compartment
– Transcellular compartment
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Osmosis: Which Way Will Water Move?
Blood:
Few solutes
Lots of water
Cell:
Many
solutes
Less water
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
“Water Follows Solutes”
Blood:
Few solutes
Lots of water
Cell:
Many
solutes
Less water
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Scenario:
• An athlete ran a marathon even though he felt ill…
• After the race he collapsed. He was pale with low blood pressure
and sunken eyes. One knee and ankle were badly swollen, and his
abdomen was distended with fluid. The doctor diagnosed
appendicitis and dehydration.
Question:
• What has happened to his:
– Blood osmolarity?
– Cell size?
– Transcellular fluid volume?
– Vascular compartment volume?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Forces Moving Fluid In and Out of
Capillaries
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
What forces work to keep blood in the capillary?
a. Capillary colloid osmotic pressure (COP) and tissue COP
b. Capillary hydrostatic pressure and tissue COP
c. Capillary hydrostatic pressure and tissue hydrostatic
pressure
d. Capillary COP and tissue hydrostatic pressure
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
d. Capillary COP and tissue hydrostatic pressure
Rationale: Hydrostatic pressure can be thought of as
“pushing pressure,” and osmotic pressure can be
thought of as “pulling” pressure. Pressure in the
capillary that pulled/kept fluid in (capillary COP) and
pressure pushing fluid out of the tissue (tissue
hydrostatic pressure) would result in more fluid in the
capillary.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Sodium
• Normal level is 135–145 mEq/L
• Regulates extracellular fluid volume and
osmolarity
Question:
• Why would “retaining sodium” cause high blood
pressure?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Scenario
It’s a very hot day and you fall down the stairs on the way to
see the doctor about your hepatitis and renal disease
• Explain why you have edema in your sprained ankle and
foot
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Controlling Blood Osmolarity
• High osmolarity causes:
– Thirst  increased water intake
– ADH release  water reabsorbed from urine
• Low osmolarity causes:
– Lack of thirst  decreased water intake
– Decreased ADH release  water lost in urine
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Tell whether the following statement is true or false.
Increased levels of ADH decrease urine output.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
True
Rationale: ADH prevents diuresis by causing more water
to be absorbed in the kidney tubules. If more water is
absorbed, there is less water left to eliminate as waste,
decreasing urine output.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Dehydration Due to Hypodipsia
• A common problem in elderly people
Scenario:
• Dr. Bob thinks it could be treated with ADH given
in a nasal spray
• Dr. Bill thinks renin injections would be better
Question:
• What is your evaluation of these two theories?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
ADH Imbalances
• Diabetes insipidus (DI)
– Neurogenic
– Nephrogenic
• Syndrome of inappropriate ADH (SIADH)
• Which will cause hyponatremia?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Sodium Imbalances
• Hyponatremia (<135 mEq/L)
– Hypertonic
– Hypotonic (dilutional)
• Hypernatremia (>145 mEq/L)
– Water deficit
– Na+ administration
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Scenario
• A man with hypernatremia was severely confused
Question:
• The doctor said this was due to a change in the
size of his brain cells. Why would this happen?
• A medical student suggested giving him a
hypotonic IV. Why?
• The doctor said that might worsen the change in
his brain cell size, and that his blood osmolarity
should be corrected very slowly. Why?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Potassium
• Normal level is 3.5–5.0 mEq/L
• Maintains intracellular osmolarity
• Controls cell resting potential
• Needed for Na+/K+ pump
• Exchanged for H+ to buffer changes in blood pH
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
What Will Happen to Blood K+ Levels
When the Client Has:
• Hyperaldosteronism?
• Alkalosis?
• An injection of epinephrine?
• Convulsions?
• Loop diuretics?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
The Basics of Cell Firing
• Cells begin with a
negative charge—
resting membrane
potential
• Stimulus causes
some Na+ channels to
open
• Na+ diffuses in,
making the cell more
positive
Threshold
potential
Resting
membrane
potential
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
stimulus
The Basics of Cell Firing (cont.)
• At threshold
potential, more Na+
channels open
• Na+ rushes in,
making the cell
very positive:
depolarization
• Action potential:
the cell responds
(e.g., by
contracting)
Action
potential
Threshold
potential
Resting
membrane
potential
stimulus
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
The Basics of Cell Firing (cont.)
• K+ channels open
• K+ diffuses out,
making the cell
negative again:
repolarization
• Na+/K+ ATPase
removes the Na+
from the cell and
pumps the K+ back
in
Action
potential
Threshold
potential
Resting
membrane
potential
stimulus
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Blood K+ Levels Control Resting Potential
• Hyperkalemia raises
resting potential
toward threshold
– Cells fire more
easily
– When resting
potential reaches
threshold, Na+
gates open and
won’t close
Threshold
potential
Hyperkalemia
Normal resting
membrane potential
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Blood K+ Levels Control Resting Potential
(cont.)
• Hypokalemia
lowers resting
potential away
from threshold
– Cells fire less
easily
Threshold
potential
Normal
resting
membrane
potential
Hypokalemia
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
What effect does a potassium level of 7.5 mEq/L have on
resting membrane potential (RMP)?
a. RMP becomes less negative, and it takes a greater
stimulus in order for cells to fire.
b. RMP becomes less negative, and it takes less of a
stimulus in order for cells to fire.
c. RMP becomes more negative, and it takes a greater
stimulus in order for cells to fire.
d. RMP becomes more negative, and it takes less of a
stimulus in order for cells to fire.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
b. RMP becomes less negative, and it takes less of a
stimulus in order for cells to fire.
Rationale: A potassium level of 7.5 mEq/L is considered
hyperkalemic. In hyperkalemia, RMP is moved closer to
the threshold (it becomes less negative). Because RMP
is nearer to the threshold, a weaker stimulus will cause
the cell to fire (a lesser distance must be overcome).
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Calcium
• Normal level is 8.5–10.5 mg/dL
• Extracellular: blocks Na+ gates in nerve and
muscle cells
• Clotting
• Leaks into cardiac muscle, causing it to fire
• Intracellular: needed for all muscle contraction
• Acts as second messenger in many hormone
and neurotransmitter pathways
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Scenario:
• A man with metastatic cancer complains of bone pain
and sudden weakness
Question:
• Why did the doctor measure:
– PTH?
– Calcium levels?
– Vitamin D levels?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Magnesium
• Normal level is 1.8–2.7 mg/dL
• Cofactor in enzymatic reactions
– Involving ATP
– DNA replication
– mRNA production
• Binds to Ca2+ receptors
• Can block Ca2+ channels
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Extracellular Calcium Controls Nerve
Firing
• Hypercalcemia
– Blocks more Na+ gates
– Nerves are less able to fire
• Hypocalcemia
– Blocks fewer Na+ gates
– Nerves fire more easily
• Which would cause Trousseau’s sign?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Tell whether the following statement is true or false.
Both hyperkalemia and hypercalcemia cause cells to fire
more easily.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
False
Rationale: Recall that hyperkalemia causes cells to fire
more easily by moving RMP closer to the threshold.
Hypercalcemia, on the other hand, blocks more sodium
gates. If less sodium enters the cell, it cannot depolarize
as quickly (it is less likely to fire). Hypocalcemia blocks
fewer sodium gates, so cells depolarize more quickly
(they are more likely to fire).
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Insert fig. 6-16
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Acid (H+)
• Normal value: pH = 7.35–7.45
• Blocks Na+ gates
• Controls respiratory rate
• Individual acids have different functions:
– Byproducts of energy metabolism
(carbonic acid, lactic acid)
– Digestion (hydrochloric acid)
– “Food” for brain (ketoacids)
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Respiratory or Volatile Acid
• CO2 + H2O   H2CO3 (carbonic acid)
• H2CO3   H+ + HCO3- (bicarbonate ion)
• An increase in CO2 will cause
– Increases in CO2 (increased PCO2)
– Increases in H+ (lower pH)
– Increases in bicarbonate ion
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Respiratory Acidosis and Alkalosis
• CO2 + H2O   H2CO3   H+ + HCO3- (bicarbonate ion)
Respiratory acidosis
Respiratory alkalosis
Increased PCO2
Decreased PCO2
Increased carbonic acid
Decreased carbonic acid
Increased H+ = low pH
(<7.35)
Decreased H+ = high pH
(>7.45)
Increased bicarbonate
Decreased bicarbonate
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Question
Tell whether the following statement is true or false.
Serum levels of pH and CO2 levels are directly proportional.
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Answer
False
Rationale: As blood levels of CO2 increase, pH becomes
more acidic (decreases).
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Respiration and Buffers Adjust Blood pH
Scenario:
• A woman was given an acidic IV. Soon she began to
breathe more heavily. Why?
• When her blood was tested, it had:
– Slightly lowered pH
– Low bicarbonate
– Low PCO2
– Slightly increased K+
• Her urine pH was slightly lowered
• Why?
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Buffer
Systems
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Metabolic Acid Imbalances
• Metabolic acidosis
– Increased levels of ketoacids, lactic acid, etc.
– Decreased bicarbonate levels
• Metabolic alkalosis
– Decreased H+ levels
– Increased bicarbonate levels
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Metabolic Acidosis and Alkalosis
• Increased metabolic acids raise H+ levels
• Some H+ combines with bicarbonate, decreasing it
• Breathing adjusts CO2 levels to bring pH back to normal
Metabolic acidosis
Metabolic alkalosis
Increased H+ = low pH
(<7.35)
Decreased H+ = high pH
(>7.45)
Decreased bicarbonate
Increased bicarbonate
Heavier breathing causes Lighter breathing causes
decreased PCO2
increased PCO2
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins