Transcript A&P Chapter 21

Fluid & Electrolyte
Balance
Part 4: Regulation &
Maintenance
Fluid

In a lean adult…..
 Females
have 55% body mass as fluids
 Males have 60% body mass as fluids.

That’s a lot of fluid!
Fluid Compartments

Two fluid compartments where fluids are present!

Intracellular Fluid Compartment: Contains the fluid within the
cells – composes 2/3 of the body fluid!
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Intracellular fluid (ICF) aka Cytosol: The bodily fluid actually
located inside cells.
Extracellular Fluid Compartment: Contains the fluid not inside
body cells – the other 1/3 of body fluid!
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Interstitial Compartment: Contains the fluid between cells.
Intravascular Compartments: Contains the fluid within the blood
vessels.
Extracellular Fluid (ECF): The fluid outside of body cells, including
interstitial fluid, plasma, etc.


Interstitial Fluid: Occupies the microscopic spaces between tissue –
80% of ECF.
Intravascular Fluid aka Plasma: The liquid portion of the blood – 20%
of the ECF.
Fluid Compartments

Barriers exist between the intracellular
fluid, interstitial fluid, & blood plasma.
 Plasma
Membranes of the Body Cells
 Blood Vessel Walls (except for capillaries
with thin walls designed to allow leakage)
Fluid Balance

Fluid Balance: The presence of the required
amount of water & solutes, in correct
proportions, in the all compartments.
 Water:
45-75% of body mass, dependent on age &
gender.
 Electrolytes: The solid inorganic compounds within
the solute that dissociate into ions.

Fluid Balance is maintained by the kidneys via
filtration, reabsorption, diffusion, & osmosis.
Water Gain
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Preformed Water: Water ingested via food & liquids –
roughly 2300 mL.
Metabolic Water: Water produced as a by-product of
cellular respiration, depending on the level of aerobic
cellular respiration. The more ATP used, the more water
is produced – roughly 200 mL per day.
Thirst Center: The area in the hypothalamus that
controls the urge to drink – triggered by an increase in
osmolarity, Angiotensin II, and a drop in blood pressure.

Thirst also triggered by neurons in the mouth detecting dryness
due to decreased salivary production, or baroreceptors detecting
low blood pressure in the heart and blood vessels.
Water Loss

Water Loss is controlled via variations in urine
volume.
 Sodium
reabsorption is adjusted proportionately to
the amount of water excreted.
 ADH is used to minimize water loss by stimulating the
collecting tubules to retain more water, causing the
sodium levels to become imbalanced.
 ADH is inhibited if the blood volume & pressure are
too high or osmolarity is too low to encourage water
output.
Water Loss

Dehydration: Occurs when the rate of water
loss is greater than the rate of water gain.
 Causes
reduced blood volume, lowered blood
pressure, and increased osmolarity of bodily fluids.
 Can be triggered by thirst not occurring quickly
enough, fluid access being restricted, excessive heat,
etc.
 The body compensates by decreasing saliva
production, decreasing the amount of water excreted
by the kidneys, and the triggering of ADH to control
water output.
Water Loss
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Total body fluid volume is determined mainly by the
extent of urinary salt excretion.
Extracellular fluid contains two main ions – Sodium ions
(Na+) and Chloride ions (Cl-).
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Fluid osmolarity is regulated via water loss.
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Excretion rates must be varied to maintain homeostasis.
Increases in NaCl would lead to increase in plasma Na+ & Clwhich increases osmolarity.
Water moves from intercellular fluid to interstitial fluid then into
blood plasma to increase blood volume.
Renal sodiom & chloride reabsorption and loss via urine
is regulated via Angiotensin II, Aldosterone, & Atrial
Natriuetic Peptide (ANP).
Factors Regulating Body Water
Balance

Thirst Center in Hypothalamus:
 Mechanism:
Triggers sensation of thirst & desire to
drink.
 Results: Water gained if thirst is quenched.

Angiotensin II:
 Mechanism:
Stimulates secretion of aldosterone.
 Results: Reduces the loss of water in urine.

Aldosterone:
 Mechanism:
Increase water reabsorption via
osmosis – promotes urinary reabsorption of sodium &
chloride.
 Results: Reduces the loss of water in urine.
Factors Regulating Body Water
Balance

Atrial Natriuretic Peptide:
 Mechanism:
Promotes natriuresis (elevated
urinary excretion of sodium, chloride, &
water).
 Results: Increases the loss of water in urine.

Antidiuretic Hormone:
 Mechanism:
Promotes reabsorption of water
in collecting ducts of kidneys.
 Results: Reduces the loss of water in urine.
Movement of Water
Increased Osmolarity of Interstitial
Fluid: Causes fluid to draw out of cells &
into interstitial fluid – causes cells to
shrink.
 Decreased Osmolarity of Interstitial
Fluid: Causes the cells to swell &
potentially lyse.

Movement of Water

Water Intoxication: Occurs when the
person consumes water faster than the
kidneys can excrete it.
 Causes
excess levels of body water, which
causes the cells to swell to dangerous sizes.
 Can cause cell lysing and tissue death.
Movement of Water

Loss of Na+ and body water is dangerous!
 Can
be caused by blood loss, excessive sweating,
vomiting, or diarrhea leading to body water loss – if
this is replaced by plain water it can cause problems!
 Hyponatremia: Na+ concentration in plasma &
interstitial fluid to fall below normal, leading to
osmolarity falling. Water moves from the interstitial
fluid to the intercellular fluid to correct this, causing
cells to swell & lyse.
 Can lead to convulsions, coma, & possibly death.
 Simply add electrolytes – even just a small amount of
table salt. Electrolyte drinks are critical when these
symptoms are present!
Electrolytes & Body Fluids

Ions form when electrolytes dissolve &
dissociate – these serve 4 main functions.
 Control
the osmosis of water between fluid
compartments.
 Maintain the acid-base balance necessary for
normal cellular activity.
 Ions carry electrical currents that allow the
production of action potentials & graded
potentials.
 Several ions are cofactors needed for
optimal activity of enzymes.
Electrolytes & Body Fluids

Milliequivalents: The unit for measuring
concentration of ions.
 Measured
as milliequivalents per liter (mEq/liter).
 Tells us the concentration of cations & anions in a
given volume of solution.
 For ions with a single positive or negative charge, the
mEq is equal to one thousandth of its molecular
weight.

E.g. Na+, K+, Cl-.
 For
ions with two positive or negative charges, the
mEq/liter is twice the number of mml/liter.

E.g. Ca+2 (calcium) & HPO4-2 (phosphate).
Electrolytes & Body Fluids
Major Cations: Sodium, Potassium,
Calcium, Magnesium.
 Major Anions: Chloride, Bicarbonate,
Phosphate.
 Intracellular Fluid: K+ is the most
abundant cation, while HPO4-2 is the most
abundant anion.
 Extracellular Fluid: Na+ is the most
abundant cation, while Cl- is the most
abundant anion.

Electrolytes & Body Fluids

Why are electrolytes important?
 They
are chemically active & participate in all
metabolism.
 They determine the electrical potential across
cell membranes.
 They strongly affect the osmolarity of body
fluids & the body’s water content &
distribution.
Ions

Sodium: The main ion responsible for resting membrane
potential in cells – normal range for blood plasma
concentration Is 136-148 mEq/liter.
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Principle cation of the ECF.
Important in determining total body water & its concentration
among fluid compartments.
Accounts for half the osmolarity of extracellular fluids.
Primary homeostatic concern is adequate renal excretion of the
excess.
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Aldosterone increases renal reabsorption of sodium.
Hyponatremia: Blood plasma sodium concentration below 135
mEq/liter – stops the release of ADH, permitting greater
excretion of water in urine to restore the sodium in ECF.
Hypernatremia: Blood plasma sodium concentration above 149
mEq/liter – triggers Atrial Natriuretic Peptide release to increase
sodium excretion by the kidneys.
Ions

Potassium: Greatest contributor to intracellular osmosis & cell
volume – normal concentration is 3.5 – 5.0 mEq/liter.
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Most abundant cation in the ICF.
Promotes resting membrane potentials & action potentials of nerve &
muscle cells.
Acts as a cofactor for protein synthesis & some other metabolic
processes.
Helps regulate pH of body fluid by “exchanging” itself for hydrogen when
moving through cells.
90% of potasssium ions are filtered by the glomerulus & reabsorbed by
the PCT – the rest is secreted in urine.
Aldosterone is used to control potassium levels in the blood plasma.
Hyperkalemia: K+ concentrations in the blood plasma above normal –
triggers aldosterone to be secreted, which causes increase excretion of
K+ in the urine. Can cause death via ventricular fibrillation.
Hypokalemia: K+ concentrations in the blood plasma below normal –
prevents aldosterone secretion to minimize K+ excretion in the urine.
Ions
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Chloride: Provide a major contribution to the osmolarity
of the ECF – normal concentration is 95 – 105 mEq/liter.
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Most abundant anion in the ECF.
Needed for the formation of stomach acid H+CL-.
Involved in chloride shift mechanism that accompanies carbon
dioxide loading & unloading in the red blood cells & plasma.
Homeostasis achieved via sodium balance.
Chloride secretion follows sodium’s excretion.
Chloride ions easily move between the fluid compartments due
to plasma membranes typically containing Cl- leakage channels
& antiporters.
Hyperchloremia: Blood plasma concentration of chloride above
normal.
Hypochloremia aka Hypochloraemia: Blood plasma
concentration of chloride below normal ranges – rarely occurs as
the only problem. Often due to vomiting if accompanied by
metabolic alkalosis (decreased blood pH).
Ions

Calcium: Plays countless roles in the body – normal blood plasma
concentration is 4.5 – 5.5 mEq/liter.
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Most abundant mineral in the body.
98% of calcium in the body stored in bone.
Combines with phosphates to form mineral salts.
Mainly an extracellular cation.
Makes bones & teeth hard.
Plays important roles in blood clotting, neurotransmitter release, maintenance of
muscle tone, & the excitability of nervous & muscle tissue.
 Calcium concentration in blood plasma regulated via parathyroid hormone (PTH)
& calcitriol (vitamin D – to help absorb calcium from food).


If concentrations drop, PTH secretion is stimulated, allowing bone to be broken down
and its calcium reabsorbed.
Hypercalcemia: An elevated calcium level in the blood, causing increased
urinary excretion – can be asymptomatic, but often is a sign of serious disease.
 Hypocalcemia: An abnormally low concentration of calcium in the blood plasma
– typically triggers the kidneys to reabsorb calcium instead of excreting it.
Ions

Phosphates: A critical component to several systems –
normal blood plasma concentration is 1.7 – 2.5 mEq/liter.
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Needed to synthesize ATP, other nucleotide phosphates, nucleic
acids, & phospholipids.
85% of phosphates in adults are calcium phosphate salts – a
structural component of bones & teeth.
15% of phosphates are ionized.
3 Important Phosphate Ions: Dihydrogen phosphate (H2PO4-),
orthophosphate (PO4-3 ), & monohydrogen phosphate (HPO4-2).
HPO4-2 Is the most prevalent form – acts as an important buffer
of H+ in body fluids & urine.
Regulated by PTH - stimulates the release of phosphates &
calcium into the blood stream – inhibits reabsorption of
phosphate & stimulates the reabsorption of calcium in the
kidneys to lower blood phosphate levels.
Regulated by calcitriol (promotes the absorption of phosphates &
calcium in the digestive tract).
Ions
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Magnesium: One of those components needed for everything –
normal blood plasma concentration is 1.3 – 2.1 mEq/liter.
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Second-most common intracellular cation.
54% of magnesium in adults is part of the bone matrix as magnesium
salts.
46% if magnesium in adults is ions in the ECF & ICF.
Cofactor for enzymes that metabolize proteins & carbohydrates & the
sodium-potassium pumps.
Needed for normal neuromuscular activity, synapse transmission &
myocardial functioning.
Controls the secretion of the parathyroid hormones.
Blood plasma levels regulated by the rate at which it is excreted in the
urine.
Hypermagnesemia: An increase in magnesium concentration due to
renal failure, increased in take of Mg+, increases in extracellular fluid
volume, decreases
Hypomagnesemia: A lowered amount of Mg+, typically due to either an
inadequate intake or an excessive loss through urine or feces.
Ions
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Bicarbonate: An important metabolic
component – normal concentrations from 2.2 –
2.6 mEq/liter.
 Second most prevalent extracelular ion.
 HCO3- concentration increases as blood
flows through
the systemic capillaries.
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CO2 combines with H2O, forming carbonic acid.
Carbonic acid dissociates into hydrogen & bicarbonate ions.
Bicarbonate decreases as CO2 is exhaled.
 Kidneys
mainly responsible for regulating bicarbonate
concentration.
 Intercalated cells in renal tubules form bicarbonate &
release it into the blood stream when levels are low,
or excrete the excess if levels are high.
Acid-Base Balance

Acid-Base Balance: The major homeostatic challenge responsible
for keeping pH (H+ concentration) of body fluids at the correct level.
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This is vital for normal cellular function.
Remember: Blood pH is normally 7.35-7.45!
Any excess H+, which is a normal metabolic waste, must be removed
from the body.
Buffer Systems: Act quickly to temporarily bind H+ to raise pH, but
they don’t remove H+ from the body.
Exhalation of CO2: Increases the rate & depth of breathing to
cause more CO2 to be exhaled – this reduces the carbonic acid in
the blood to raise pH and reduce H+ levels.
Kidney Excretion of H+: Urination eliminates acids other than
carbonic acid – slowest mechanism for pH change, but only method
of removing acids.
Buffer Systems
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Buffer System: Consists of a weak acid & its salt (that
functions as a weak base).
Acid: A chemical that releases H+ into a solution.
Base: Chemical that accepts H+.
Buffer: Any mechanism that resists changes in pH by
rapidly converting a strong acid or base into a weak acid
or base.
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Strong acids lower pH more than weak acids due to their readily
releasing H+.
Strong bases raise pH more than weak bases do.
3 Main Buffer Systems:
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Protein Buffer System
Carbonic Acid-Bicarbonate Bugger System
Phosphate Buffer System
Protein Buffer Systems

Protein Buffer System: Most abundant buffer in
intracellular fluid & blood plasma – can buffer
acids or bases.
 Proteins
are composed of amino acids with one
carboxyl group (-COOH), which is a functional
component of the buffer, & one amino group (NH2).
 The free carboxyl group at one end of a protein acts
like an acid by releasing H+ when pH rises.
 When it dissociates, the H+ can react with excess OHin the solution to form water.
 NH2 groups can act as a base by combining with H+
when pH falls.
Carbonic Acid-Bicarbonate Buffer
System

Carbonic Acid-Bicarbonate Buffer System:
Includes bicarbonate ions acting as a weak
base, and carbonic acid acting as a weak acid.
H+ would cause bicarbonate ions to
function as a weak base, removing excess H+.
 The carbonic acid dissociates into water & carbon
dioxide, and the CO2 is exhaled from the lungs.
 If there is a deficiency of H+ then carbonic acid can
function as a weak base to relase more H+.
 Excessive
Phosphate Buffer System

Phosphate Buffer System: Works similarly to the
carbonic acid-bicarbonate buffer system.
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Dihydrogen phosphate (H2PO4-) and monohydrogen phosphate
(HPO42-) work as the major components.
Phosphates are major anions in intracellular fluid & minor anions
in extracellular fluid.
Combining a strong base, such as OH- with a weak acid such as
dihydrogen phosphate yields monohydrogen phosphate (a weak
base).
Monohydrogen phosphate ions can act as a weak base to buffer
H+ released by strong acids (such as hydrochloric acid (HCL)).
Dihydrogen phosphate forms in the presence of excess H+ in the
kidney tubules that then combines with monohydrogen
phosphate – the H+ then passes into the urine.
The concentration of phosphates is higher in intracellular fluid,
causing the buffer system to regulate within the cells.
Exhalation of Carbon Dioxide


Breathing helps maintain the pH of body fluid by
exhaling CO2.
Exhaling CO2 removes carbonic acid from the
system, which raises the blood pH.
 This
is why carbonic acid is called a volatile acid (an
acid produced from carbon dioxide).


An increase in ventilation causes more CO2 to
be exhaled, resulting in H+ concentration falling
& blood pH raising.
Decreases in ventilation causes less CO2 to be
exhaled, resulting in H+ concentration rising &
blood pH falling.
Renal Regulation
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Nonvolatile Acids: An acid produced from a source
other than carbon dioxide, such as metabolic reactions.
H+ secretion in the urine is the only way to get rid of
these acids.
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Cells in the PTC & collecting ducts secrete H+ ions into the
tubular fluid.
Intercalated cells have apical membranes containing proton
pumps (H+ ATPases) that secrete H+ into the tubular fluid.
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Bicarbonate ions inside the reneal intercalated cells cross the
basolateral membrane by Cl-/HCO3- antiporters, then diffuse into
peritubular capillaries.
Some intercallated cells have proton pumps in the basolateral
memrbanes and Cl-/HCO3- antiporters in the apical membranes –
these secrete bicarbonate ions & reabsorb hydrogen.
Intercallated cells regulate pH by excreting excess bicarbonate
ions when pH is too high and H+ when pH is too low.
Mechanisms Maintaining Fluid pH
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Buffer Systems: Mostly consist of a weak acid & that
acid’s salt, which functions as a weak base – prevents
drastic changes in pH.
Proteins: Most abundant buffers in body cells & blood –
histidine & cysteine (amino acids) contribute most of the
buttering along with hemoglobin.
Carbonic Acid-Bicarbonate Phosphates: Important
regulator of blood pH – most abundant buffer in ECF –
important ion intracellular fluid & urine.
Exhalation of CO2: Increase exhalation raises pH while
decreased exhalation lowers pH.
Kidneys: Renal tubules secrete H+ into the urine &
reabsorb HCO3 so it is not lost.
Acid-Base Imbalances

Acidosis (Acidemia): Occurs when the blood pH falls
below 7.35, resulting in H+ diffusing into the cells and
driving out potassium.
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Depresses the central nervous system by inhibiting synaptic
transmission.
Symptoms include confusion, disorientation, & coma (if pH falls
below 7).
Alkalosis (Alkalemia): Occurs when the blood pH rises
higher than 7.45, resulting in H+ diffusing out of the cells
while potassium diffuses in.
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Lowers the potassium concentration in the ECF.
Over-excites the central & peripheral nervous systems, causing
nervousness, muscle spasms, convulsions, and sometimes even
death.
Acid-Base Imbalances
Respiratory Acidosis & Alkalosis:
Result from changes in the partial
pressure of CO2 in the blood –
compensated for by the kidneys.
 Metabolic Acidosis & Alkalosis: Result
from changes in the concentration of
HCO3 in the blood – compensated for by
the lungs.

Acid-Base Imbalances

Compensation: The response to an acid-base
imbalance that seeks to normalize arterial blood pH.

Respiratory Compensation: Seeks to correct altered pH dye to
metabolic causes via hyperventillation & hypoventillation.

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Renal Compensation: A change in the secretion of hydrogen &
reabsorption of bicarbonate ions by the kidney tubules that
counters altered pH due to respiratory causes.
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Used to counter the effects of metabolic acidosis & metabolic
alkalosis by elevating the bicarbonate concentration & pH of the
urine.
Used to counter the effects of respiratory acidosis & respiratory
alkalosis.
Simple Version: If the lungs caused it, the kidneys try to
fix it. If the kidneys caused it, the lungs try to fix it.
Acid-Base Imbalances

Respiratory Acidosis: Increased PCO2 (above
45 mmHg) and decreased pH (below 7.35) if no
compensation occurs.
 Common
Causes: Hypoventilation due to
emphysema, pulmonary edema, respiratory trauma,
airway obstruction, or dysfunction of the respiratory
muscles.
 Compensatory Mechanisms: Renal – kidneys
increase the excretion of H+ & increase the
reabsorption of HCO-3.

If compensation completes, pH will be normal but PCO2 will
be high.
Acid-Base Imbalances

Respiratory Alkalosis: Decreased PCO2
(below 35 mmHg) & increased pH (above 7.45)
if no compensation occurs.
 Common
Causes: Hyperventilation due to oxygen
deficiency, pulmonary disease, cerebrovascular
accident (CVA), or severe anxiety.
 Compensatory Mechanisms: Renal – kidneys
decrease the excretion of H+ & decrease the
reabsorption of HCO-3.

If compensation is complete, pH will be normal but PCO2 will
be low.
Acid-Base Imbalances

Metabolic Acidosis: Decreased HCO3
(below 22 mEq/liter) & decreased pH
(below 7.35) if no compensation occurs.
 Common
Causes: Loss of bicarbonate in the
ions due to diarrhea, accumulation of acid
(ketosis) or renal dysfunction.
 Compensatory Mechanisms: Respiratory –
hyperventilation occurs to increase the loss of
CO2.

If compensation completes, pH will be normal but
HCO-3 will be low.
Acid-Base Imbalances

Metabolic Alkalosis: Increased HCO3
(above 26 mEq/liter) & increased pH
(above 7.45) if no compensation occurs.
 Common
Causes: Loss of acid due to
vomiting, gastric suctioning, use of certain
diuretics or excessive intake of alkaline drugs.
 Compensatory Mechanisms: Respiratory –
hypoventillation, which slows down the loss of
CO2.

If compensation completes, pH will be normal but
HCO-3 will be high.