Physiology 31 Lecture
Chapter 20 – Fluid & Electrolyte Balance
I. Overview
A. Cellular function requires a fluid medium with a homeostatic balance of the following
1. _______, in which daily water intake and loss are equal
2. _______________, in which the amount of electrolytes absorbed by the small intestine
balance the amount lost from the body, mainly through the urine
3. ______-_____, in which the body rids itself of acid (H+ ions) at a rate that balances its
metabolic production, maintaining a stable pH
B. These balances are maintained by the all of the bodily systems, except for the reproductive
system
II. __________ Balance & the regulation of ECF Volume
A. The average person’s body is about ___-__% water
B. _________________ in which water is found include
1. 65% in intracellular fluid (___) within cells
2. 35% in extracellular fluid (___), which includes
a. 25% ______________ fluid around cells
b. 8% blood __________ and lymph
c. 2% ___________ fluid, such as cerebrospinal, synovial, serous, vitreous & aqueous
humors, bile, and fluids in the GI, urinary, and respiratory tracts
C. Water moves by __________ from one compartment to another, so the ECF and ICF
osmolarities rarely differ
D. A person is in a state of water __________ when daily water intake and losses are equal
1. Water gains come mainly from __________ water, in addition to a small amount of
water from cellular respiration
2. Water is lost mainly in ________, as well as from feces, expired breath, and sweat
through the skin
3. ______________ can cause a significant water loss
4. Intravenous (___) fluids may be needed to replace water loss
E. Fluid intake is regulated by the thirst center in the _____________
1. The hypothalamus responds to signs of ______________, such as
a. __________ II, produced in response to falling blood pressure
b. Decreased ___________ stretch, due to low blood volume
c. Signals from ____________ neurons in the hypothalamus that monitor blood
osmolarity
d. _____ (vassopressin), is synthesized and released in response to rising blood
osmolarity
e. ADH stimulates water _____________ from kidney tubules
2. __________ is stimulated and satiated by
a. The thirst center sends sympathetic signals to the salivary glands to inhibit
____________
b. Short-term satiation of thirst is accomplished by cooling and ____________ the
mouth and inflating the stomach
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b. Long-term satiation of thirst depends on ____________ water from the small
intestine and lowering blood osmolarity
F. Fluid output and retention is regulated by factors that control _______ output
1. ADH is secreted by the posterior pituitary gland in response to ______________, which
causes the kidneys to reabsorb water. ADH release is ____________ if
a. Blood __________ or pressure are too high, or
b. Blood ____________ is too low
2. Aldosterone, released by the adrenal cortex, causes _________ (and water) to be
reabsorbed from the kidneys. Adosterone secretion is influenced by
a. Increased plasma ___ concentration causes aldosterone release; results in K+
___________ from the blood to the kidney tubules (prevents hyper_________)
b. Angiotensin II, formed in response to ______ release when BP is low, causes
secretion of ____________
c. Increased ECF osmolarity inhibits aldosterone secretion, thus more ____ is excreted in
the urine
3. Atrial Naturetic Peptide (_____), released during high BP, promotes Na+ and water
___________, and inhibits the release of _____, renin, and aldosterone
G. ___________ of Water Balance occur if there is an abnormality of total fluid volume,
concentration, or distribution among the compartments
1. Fluid deficiency arises when output exceeds intake over a period of time. Two kinds of
fluid ____________ are
a. Volume depletion (hypo__________), in which proportionate amounts of water and
_________ are lost; may be due to hemorrhage, burns, vomiting, diarrhea, or
hyposecretion of ____________
b. Dehydration (negative water balance), in which volume is reduced and osmolarity is
elevated because the body has lost more ________ than sodium; caused by lack of
drinking water, diabetes mellitus, _____ hyposecretion, profuse sweating
c. Severe fluid deficiency can result in circulatory _____ and death
2. Fluid __________ can occur in two forms
a. Volume excess - the retention of excess fluid with normal ___________; can result
from aldosterone hypersecretion or _________ failure
b. _____________ hydration (water intoxication) – the retention of more water than
sodium, reducing osmolarity, as when one loses water and sodium in ________ and
drinks plain water; can cause pulmonary and cerebral edema
3. Fluid sequestration – total body water may be normal, but fluid may accumulate in a
particular location; occurs in _________, hemorrhage, and pleural effusion where fluids
accumulate in the pleural cavity
III. Electrolyte Balance
A. Electrolyte (_____) functions include
1. Provide enzyme ______________ (e.g., Zn2+, Mg2+)
2. Allow action potentials in neurons and muscles (____, K+, Cl-)
3. Stimulate the secretion and action of hormones and neurotransmitters (e.g., _____)
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4. Allow muscle contraction (____)
5. Maintain acid-_____ balance (H+, HCO3-, and phosphates)
6. Allow secondary active _________ across membranes (Na+, K+)
7. Stimulate __________ across cell membranes
B. Major ________ include Na+, K+, Ca2+, and H+
C. Major _________ are Cl-, HCO3- , phosphates (HPO42- and H2PO4-), and proteins
D. Sodium (____) is the main cation in the _____
1. Na+ ____________ include
a. __________ and fluid balance
b. _________ and muscle activity
c. __________ of molecules (e.g., glucose, amino acids) across cell membranes
d. Acid-base ___________ (NaHCO3)
e. _______ generation via the Na+/K+ pump
2. Sodium homeostasis is maintained by
a. ______________ promotes Na+ reabsorption
b. ______ reduces Na+ concentration by promoting water reabsorption independently
of Na+
c. Atrial naturietic __________ inhibits Na+ and water reabsorption, lowering blood
pressure
3. Imbalances of sodium include
a. Hyper__________ is an excess of Na+, which causes water retention,
hyper__________, and edema
b. _______natremia is a deficiency of Na+, often a result of hypotonic
___________; salt appetite is stimulated in the hypothalamus
E. Potassium (___) is the major cation in the _____. It has similar functions as Na+, and is a
cofactor for some enzymes
1.Potassium homeostasis is maintained mainly by ____________, which promotes excess
K+ excretion by the kidneys
2. Imbalances of potassium include
a. Hyper________ causes serious nerve and muscle dysfunction, and can cause cardiac
arrest!
1) If plasma and ECF K+ increases, the concentration ________ decreases, and more
K+ remains inside cells, ____________ them initially
2) Cells are unable to ____________ fully, causing cells to become less excitable
b. ______kalemia inhibits nerve and muscle function
1) Increase in K+ gradient causes more K+ to ________ cells, ________polarizing
them
2) Hyperpolarized cells are more difficult to depolarize to ____________ for action
potentials
F. Chloride (___) is the major anion of the _____
1. Chloride __________ include
a. Regulation of osmotic balance (with ____)
b. Formation of stomach acid (____)
c. The chloride shift mechanism in respiratory and renal function
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2. Chloride homeostasis follows Na+ and other cations, and is regulated as a side effect
of ____ homeostasis
3. The primary effect of chloride imbalances is a ___ imbalance
G. Calcium (_____) has low intracellular concentrations, but is often sequestered in smooth
___. __., then released when needed
1. Calcium is necessary for
a. _________ contraction
b. _________ transmission and exocytosis of neurotransmitters
c. Blood ___________
d. A second _____________ for some hormone actions
e. ________ and tooth formation
2. Calcium homeostasis is ____________ by
a. Parathyroid hormone (____) – ____________ serum Ca2+ levels by _______
reabsorption and intestinal uptake
b. Calcitriol (vit. ___) is required for intestinal uptake of Ca2+
c. Calcitonin – ________ serum Ca2+ levels and increases bone deposition
3. Hyper________ can result from alkalosis, hyperparathyroidism, or hypo________;
causes muscle weakness, depressed reflexes, and cardiac _____________
4. ______calcemia can result from acidosis, vit. ___ deficiency, diarrhea, pregnancy,
lactation, hypoparathyroidism, or hyperthyroidism; causes potentially fatal muscle
________
H. Phosphates (PO43-, HPO42-, H2PO4-) are relatively concentrated in the ____, where they
are generated by _____ hydrolysis
1. Phosphate __________ include
a. Phosphates are a component of nucleic acids, phospholipids, ______, GTP, cAMP,
and related compounds
b. Phosphates activate many metabolic pathways by _______________ substances
such as glucose and enzymes
c. Phosphates are important acid-base __________
2. Phosphate levels are regulated by parathyroid hormone, which increases phosphate
___________ and minimizes the formation of CaPO4
3.Phosphate imbalances are not as __________ as other electrolyte imbalances
IV. Acid-Base Balance
A. The pH of the ECF is normally maintained between ____-____, despite constant
production of _______ products (e.g., lactic acid, phosphoric acids, fatty acids, carbonic
acid)
B. Acids, Bases, & Buffers
1. An ______ is any chemical that releases H+ ions in solution
a. _______ acids (HCl) give up most of their ____ ions and can lower pH
significantly
b. _______ acids (H2CO3) do not give up many ___ ions, thus affect pH only slightly
2. A _______ is any chemical that takes up ____ ions in solution
a. _________ bases (-OH) have a strong tendency to bind H+ ions and ________ pH
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b. _________ bases (HCO3-) bind less H+, thus have less of an effect on pH
3. A ___________ is any mechanism that resists changes in pH by converting strong
acids or bases to ______ ones. The body has both physiological and chemical buffers
a. Physiological buffers, such as the respiratory and _________ systems, stabilize
pH by controlling the body’s output of acids, bases, or _____
b. Chemical buffers bind ____ and remove it from solution as its concentration
begins to rise, or releases H+ into solution as its concentration falls.
4. Three chemical __________ systems in the body are the
a. _____________ buffer system, represented by the eqn.:
CO2 + H2O  H2CO3  HCO3- + H+
(has an optimal pH of 7.4)
1) The lungs and kidneys remove ____, which keeps the rxn. moving to the
______, reducing H+ ions
2) If there is a need to lower pH, the kidneys excrete ______, which moves the
rxn. to the _________, increasing the H+ concentration
b. ___________ buffer system has an optimal pH of 6.8, and is important for
buffering the renal tubules & ICF. The rxn. is
H2PO4-  HPO42- + H+
c. ____________ buffer system accounts for ¾ of all chemical buffering in body
fluids, due to side groups of amino acids
1) Carboxylic ______ groups (-COOH) release H+ when ph begins to rise
2) _________ groups (-NH2) bind H+ when pH falls too low
C. Respiratory control of pH - the respiratory system buffers pH by adjusting pulmonary
______________
1. Reduced ventilation allows _____ to accumulate in the blood and ________ its pH
by the rxn.
CO2 + H2O  H2CO3  HCO3- + H+
2. Increased ventilation expels ____, reversing the above rxn, lowering H+, and
___________ the pH
D. Renal control of pH – the _________ neutralize more acid or base than any other buffer
system in the body
1. They secrete H+ into the tubular fluid, where it binds to chemical buffers and is
__________ in the urine
2. The above H+ normally _________ all the HCO3- in the tubular fluid, making urine
bicarbonate free
3. Excess H+ in the tubular fluid can be __________ by phosphate and ammonia (NH3+)
E. Disorders of acid-base balance
1. _____________ is a pH of  7.35
a. ___________ acidosis occurs when pulmonary gas exchange is insufficient to
expel ____ as fast as the body produces it
b. _____________ acidosis is the result of lactic acid or ________ accumulation,
ingestion of acidic drugs, such as aspirin, or loss of base, as in diarrhea
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2. _____________ is a pH of  7.45
a. Respiratory alkalosis results from _______ventilation
b. Metabolic alkalosis is rare, but can be caused by overuse of antacids or loss of
stomach acid through __________
3. ________________ acidosis or alkalosis is a pH imbalance that the body cannot
correct on its own; it requires clinical intervention (i.e., fluid replacement therapy)
4. _____________ acidosis or alkalosis is an imbalance that the body’s homeostatic
mechanisms can correct
a. Respiratory compensation is correction of the pH through changes in pulmonary
____________
b. Renal compensation is correction of pH by changes in ___ excretion by the
kidneys
5. _______, electrolyte, and acid-base imbalances are intimately entwined; an imbalance
in one area can cause or result from an imbalance in another