Chapter008

Chapter 8
Disorders of Fluid, Electrolyte,
and Acid-Base Balance

Fluid Distribution
• Intracellular compartment
• Extracellular compartment
– Interstitial spaces
– Plasma (vascular) compartment
– Transcellular compartment

Distribution of Water

Scenario
• An athlete ran a marathon even though he felt ill…
• After the race he collapsed. He was pale with a 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?

Forces Moving Fluid In and Out of
Capillaries

Question
What forces work to keep blood in the capillary?
a. Capillary colloid osmotic pressure (COP) & tissue COP
b. Capillary hydrostatic pressure & tissue COP
c. Capillary hydrostatic pressure & tissue hydrostatic
pressure
d. Capillary COP & tissue hydrostatic pressure

Answer
d. Capillary COP & tissue hydrostatic pressure
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.

Sodium
• Normal level is 135–145 mEq/L
• Regulates extracellular fluid volume and
osmolarity
Question:
• Why would “retaining sodium” cause high blood
pressure?

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.

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

Question
Tell whether the following statement is true or false:
Increased levels of ADH decrease urine output.

Answer
True
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.

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?

ADH Imbalances
• Diabetes insipidus (DI)
– Neurogenic
– Nephrogenic
• Syndrome of inappropriate ADH (SIADH)
• Which will cause hyponatremia?

Sodium Imbalances
• Hyponatremia (<135 mEq/L)
– Hypertonic
– Hypotonic (dilutional)
• Hypernatremia (>145 mEq/L)
– Water deficit
– Na+ administration

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?

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

What Will Happen to Blood K+ Levels
When the Client Has:
• Hyperaldosteronism?
• Alkalosis?
• An injection of epinephrine?
• Convulsions?
• Loop diuretics?

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 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)
Threshold
potential
Resting
membrane
potential stimulus
Action
potential

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
Threshold
potential
Resting
membrane
potential stimulus
Action
potential

Blood K+ Levels Control Resting Potential
• Hyperkalemia raises
resting potential
toward threshold
– Cells fire more
Hyperkalemia
easily
– When resting
potential reaches
threshold, Na+
gates open and
won’t close
Threshold
potential
Normal resting
membrane potential

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

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
c. RMP becomes more negative, and it takes a greater
d. RMP becomes more negative, and it takes less of a

Answer
b. RMP becomes less negative, and it takes less of a
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).

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

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?

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

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 sign?

Question
Both hyperkalemia and hypercalcemia cause cells to fire
more easily.

Answer
False
Recall that hyperkalemia cause 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–cells depolarize more quickly (they are more likely
to fire).

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)

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

Respiratory Acidosis and Alkalosis
• CO2 + H2O   H2CO3   H+ + HCO3
- (bicarbonate ion)
Respiratory acidosis Respiratory alkalosis
Increased PCO2
Increased carbonic acid
Increased H+ = low pH
(<7.35)
Increased bicarbonate
Decreased PCO2
Decreased carbonic acid
Decreased H+ = high pH
(>7.45)
Decreased bicarbonate

Question
Serum levels of pH and CO2 levels are directly proportional.

Answer
False
As blood levels of CO2 increase, pH becomes more acidic
(decreases).

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?

Buffer Systems

Metabolic Acid Imbalances
• Metabolic acidosis
– Increased levels of ketoacids, lactic acid, etc.
– Decreased bicarbonate levels
• Metabolic alkalosis
– Decreased H+ levels
– Increased bicarbonate levels

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 bicarbonate
Heavier breathing causes
decreased PCO2
Decreased H+ = high pH
(>7.45)
Increased bicarbonate
Lighter breathing causes
increased PCO2

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