Fluids & Electrolytes

Intracellular fluid (ICF)

about two thirds by volume,
cytosol of cells

Extracellular fluid

consists of two major
subdivisions

Interstitial fluid

fluid in spaces between cells

Plasma

the fluid portion of the blood

Interstitial fluid

80% of ECF is found in localized
areas such as lymph, cerebrospinal fluid, synovial fluid,
aqueous and vitreous humors of eyes, serous fluid, and
glomerular filtrate of kidneys

Blood plasma

20% of ECF found in circulatory
system. Plasma is the only fluid that circulates
throughout the body linking the external and internal
environments

water

… is the main component of all body fluids, making
up 45-75% of the total body weight
… moves by osmosis due to concentration
differences between compartments and by filtration due
to differences in pressure between compartments.
Sources of water include ingested foods and liquids
(preformed water) and metabolic water produced
during dehydration/synthesis of anabolism.
… is rid from our systems in the form of urine,
sweat, and feces.

Water intake sources:

Ingested fluid (60%) and
solid food (30%)
Metabolic water or water
of oxidation (10%)

Water output:

Urine (60%), sensible
and insensible
perspiration (36%) and
feces (4%)

Short term

drops in blood pressure (often caused by a drop
in blood volume) cause an increase in cardiac output by
increasing heart rate/force of contraction and an increase in
total peripheral resistance via vasoconstriction. Both of these
help the blood pressure to rise. Short term mechanisms will
also kick in to increase reabsorption of fluid at the
capillaries. This will also help to increase blood pressure by
increasing blood volume

Long term

long term regulation relies on decreased water
output by the kidneys and increased water ingestion. Let’s
look at this long term regulation further

Diameter of the afferent arteriole (WATER OUTPUT

when the salt
level drops in the body (usually accompanied by a
drop in blood volume and blood pressure), the
afferent arteriole reflexively decreases. This
decreases the GFR and excretion of salt. Conserving
salt in this manner will help to conserve water,
restoring blood volume (and pressure).

Renin-angiotensin-
aldosterone system (WATER OUTPUT)

when the
salt levels drop in the body (again
usually accompanied by a drop in
volume and pressure), renin is
released. Renin eventually results
in release of both ADH and
aldosterone. Aldosterone
increases salt conservation at the
kidneys. As salt is conserved,
water is also conserved.
Remember: water follows salt.
Because both water and salt are
being reabsorbed, the plasma
volume is increased without
changing its osmolarity.

ECF hypertonicity

associated with dehydration
Causes: Insufficient water intake
Excessive water loss
Diabetes insipidus (insufficient production of ADH)

ECF hypertonicity

Results: water moves into the ECF and the cell shrinks
Symptoms: mental confusion, delirium, and coma can result
from the shrinkage of brain cells
Drop in blood pressure and circulatory shock can
result from circulatory disturbance
Parched skin, dry skin, sunken eyeballs, and decreased
salivation
Correction: increase ADH release to increase water reabsorption.
Water alone is reabsorbed decreasing the osmolarity or
concentration of the ECF.

ECF hypotonicity

associated with overhydration
Causes: Renal failure
Excessive water ingestion
Excessive ADH production

ECF hypotonicity

Results: water moves into the cell and the cell swells
Symptoms: confusion, headache, vomiting, and coma
can result from the swelling of brain cells
Weakness from swelling of muscle cells
Hypertension and edema from ECF overhydration
Correction: decrease in ADH release to decrease water
reabsorption. This will increase water release,
increasing the concentration of the fluid in the ECF.

7.4

Normal pH of body fluids of arterial blood is….

acidosis

When
the pH drops below 7.35, we call this. ..

alkalosis

When
the pH rises above 7.45, we call this…

Where do hydrogens come from in our body?

Breakdown of phosphorus-containing proteins
releases phosphoric acid into the ECF
Anaerobic respiration of glucose produces lactic
acid
Fat metabolism yields organic acids and ketone
bodies
Transporting carbon dioxide as bicarbonate
releases hydrogen ions from carbonic acid

Bicarbonate buffer system

carbonic acid and bicarbonate
ions in the ECF resist pH changes
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-

Protein buffer system

proteins contain both acidic and basic
groups that resist pH changes in the ICF

Phosphate buffer system

acidic phosphate salt (NaH2PO4)
and basic phosphate salt (Na2HPO4) in the kidneys resist ECF pH
changes

H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-

This system works within the ECF to prevent changes in pH. If
a strong acid is added, hydrogen ions released in solution will
combine with the bicarbonate ions in the ECF and be
neutralized. The pH will, therefore, only decrease slightly. If a
strong base is added to solution, it will react with the carbonic
acid to form sodium bicarbonate & the pH will only rise slightly.
(I have circled carbonic acid to remind you of which it is in the
equation below

When plasma H+ levels rise:

Deeper and more rapid breathing expels more carbon dioxide
Hydrogen ion concentration is reduced to compensate

When plasma H+ levels falls:

Slower, more shallow breathing conserves carbon dioxide
Hydrogen ion concentration increases to compensate

The most important renal mechanisms for regulating
acid-base balance are:

Controlling H+ excretion
Controlling bicarbonate excretion
Controlling ammonia excretion

secreted

Hydrogen ions can be … into the urine along the
proximal and distal convoluted tubules

Proximal convoluted tubules

along apical surface
Na+-H+ move hydrogen ions by secondary
active transport (antiporter)
H+ ATPase pumps move hydrogen ions by
primary active transport

Distal convoluted tubules

pumps found on the
apical membrane
H+-K+ move hydrogen ions by secondary
active transport (antiporter)
H+ ATPase pumps move hydrogen ions by
primary active transport

reabsorbed

Hydrogen ions can be … along the distal
convoluted tubules

Distal convoluted tubules

along basal surface
H+-K+ move hydrogen ions by secondary
active transport (antiporter)
H+ ATPase pumps move hydrogen ions by
primary active transport

opposite directions

Kidney cells move hydrogen ions and bicarbonate ions in…

“reabsorbed”

When hydrogen ions are secreted, bicarbonate ions are generated
and …

secreted

When hydrogen ions are reabsorbed, bicarbonate ions are…

Ammonium ion excretion

ammonium and bicarbonate
ions are produced by the
metabolism of amino acids

one ammonia

is secreted and
one is reabsorbed
Both ammonia combine
with hydrogen
Bicarbonate ions are also
reabsorbed

Respiratory acidosis and respiratory alkalosis

are
usually indicated by changes in CO2 levels. Normal
PCO2 fluctuates between 35 and 45 mmHg. Values
above 45 mmHg signal respiratory acidosis while
values below 35 mmHg indicate respiratory alkalosis.
To understand acidosis and alkalosis, we must revisit
our favorite formula:
CO2 + H2O → H2CO3 → H+ + HCO3-

Respiratory acidosis

is the most common cause of acid-base
imbalance, occurring when a person breathes shallowly or gas
exchange is hampered by disease. Regardless of the cause,
respiratory acidosis results from carbon dioxide retention
(elevated pCO2) that drives the equation towards formation of
hydrogen and bicarbonate ions. The pH goes down as a result.
Compensation for respiratory acidosis occurs through renal
excretion of H+ and reabsorption of bicarbonate ions

Respiratory alkalosis

is a common result of hyperventilation.
Increased breathing rates result in decreased carbon dioxide
levels that drive the equation towards the reactants side. This
decreases the hydrogen and bicarbonate ion levels. The pH goes
up. Compensation will occur via conservation of hydrogen ions
by the kidneys.

Metabolic acidosis

is the second most common cause of acid-base
imbalance.
    Typical causes include ingestion of too much alcohol and excessive loss of
bicarbonate ions. Other causes include accumulation of lactic acid, shock,
ketosis in diabetic crisis, starvation, and kidney failure.
Increased H+ levels drives the equation to the reactants side, decreasing the
concentration of bicarbonate levels.
     If the kidneys can compensate, they will do so by secreting hydrogen ions
and reabsorbing bicarbonate ions. The lungs will compensate by increasing
the breathing rate to rid the body of excess carbon dioxide

Metabolic alkalosis

due to a rise in blood pH and bicarbonate
levels.
Typical causes are vomiting of the acid contents of the stomach and intake of
excess base (e.g., from antacids).
Decreased H+ levels drives the equation to the product side, increasing the
amount of bicarbonate and decreasing CO2 levels.
If the kidneys can compensate, they will do so by reabsorbing hydrogen
ions. The lungs will compensate by slowing the breathing rate to conserve
carbon dioxide.

Compensation

returns the proportion of hydrogen
ions to normal but does NOT fix the underlying
cause. Compensation is usually achieved by the
system NOT associated with the dysfunction. In
other words, the respiratory system will attempt to
correct metabolic acid-base imbalances, while the
kidneys will work to correct imbalances caused by
respiratory disease AND metabolic disturbances
(UNLESS the kidneys are the source of the
disturbance).