Essentials of Pathophysiology
CHAPTER 8
DISORDERS OF FLUID, ELECTROLYTE,
AND ACID-BASE BALANCE
PRE LECTURE QUIZ (TRUE/FALSE)
F

The extracellular compartment contains
approximately two thirds of the body water in healthy
adults, and is the larger of the two compartments.
F

Potassium is the most abundant cation in the body.
F

Edema of the brain, larynx, or lungs is not an acute,
life-threatening condition.
T

Polydipsia is a term that means “excessive thirst.”
F

Decreased vascular volume would yield
manifestations such as full and bounding pulses,
venous distention, and signs and symptoms of
pulmonary edema, such as shortness of breath,
crackles, dyspnea, and cough.
PRE LECTURE QUIZ
ADH

Diabetes insipidus is a condition that is caused by a
deficiency or a decreased response to __________, also
known as vasopressin.

The most serious side effect of hyperkalemia is on
______________ function.

Serum _______________ is directly or indirectly
regulated by parathyroid hormone (PTH) and vitamin D.

Respiratory acidosis is characterized by a decrease in
__________, reflecting a decrease in ventilation and an
increase in PCO2.

Vomiting, removal of gastric secretions through the use
of nasogastric suction, and low potassium levels
resulting from diuretic therapy are the most common
causes of _________________ alkalosis in hospitalized
patients.
Calcium
Cardiac
Metabolic
pH
FLUID DISTRIBUTION

Intracellular compartment

Extracellular compartment

Interstitial spaces

Plasma (vascular) compartment

Transcellular compartment
DISTRIBUTION OF WATER
•Intracellular
•Extracellular
•Interstitial
•Plasma
•3rd Space
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 and Cells
OSMOSIS: WHICH WAY WILL WATER MOVE?
Blood: (ECF)
Few solutes
Lots of water
Cell: (ICF)
Many
solutes
Less water
Water Follows Solutes
FORCES MOVING FLUID IN AND OUT OF
CAPILLARIES
Hydrostatic pressure
 Pressure created by the Heart
 i.e. Blood Presssure


Forces water out of capillaries
FORCES MOVING FLUID IN AND OUT OF
CAPILLARIES
Hydrostatic
pressure
generated by
the heart
Greatly reduced
Hydrostatic
pressure due to
capillary resistance
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
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.
d.
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: (Hypothalamic detection)
 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 the urine
Negative
_________________
Feedback
QUESTION
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 less negative, i.e.
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 positive:
depolarization
Action potential: the
cell responds (e.g., by
contracting)
Action
potential
Threshold
potential
Resting
membrane
potential
stimulus
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
BLOOD K+ LEVELS CONTROL RESTING
POTENTIAL

Hyperkalemia raises resting
potential toward threshold
 Cells fire more easily
 Increased K+ can move
resting potential to
threshold, Na+ gates
open and won’t close
Threshold
potential
Hyperkalemia
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 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.
ANSWER
b.
RMP becomes less negative, and it takes less
of a stimulus in order for cells to fire.
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
Calcium
Regulation
Blood Ca2+ can be
increased by:
1. Increased intestinal
absorption
2. Release from bones
CALCIUM REGULATION
Blood Ca2+ can be
increased by:
1. Increased intestinal
absorption
2. Release from bones
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’s & Chvostek’s
sign?
Hypocalcemia because they indicate easy
nerve firing
CHVOSTEK’S SIGN &TROUSSEAU’S SIGN














CHVOSTEK’S SIGN
Elicitation: Tapping on the face at a point just anterior
to the ear and just below the zygomatic bone
Postitive response: Twitching of the ipsilateral facial
muscles, suggestive of neuromuscular excitability
caused by hypocalcemia
TROUSSEAU’S SIGN
Elicitation: Inflating a sphygmomanometer cuff above
systolic blood pressure for several minutes
Postitive response: Muscular contraction including flexion
of the wrist and metacarpophalangeal joints,
hyperextension of the fingers, and flexion of the thumb
on the palm, suggestive of neuromuscular excitability
caused by hypocalcemia
QUESTION
True or False:
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).
Balance between
blood pH, HCO3=
and H2CO3
Normal Balance
Metabolic
Acidosis
Respiratory
Alkalosis
Important
Relationships
Insert fig. 6-16
H2CO3 = .03 * PaCO2
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-

(carbonic acid)  (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
Respiratory distress may be an attempt to compensate for low
serum pH
QUESTION
Tell whether the following statement is true or
false:
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 H+ = high pH
(>7.45)
Decreased bicarbonate
Increased bicarbonate
Heavier breathing causes Lighter breathing causes
decreased PCO2
increased PCO2