0-1 Chapter 24 - water, electrolytes, acid-base balance Flashcards
Balance
cellular function requires a fluid medium with a carefully controlled composition
•balances maintained by the collective action of the urinary, respiratory, digestive, integumentary, endocrine, nervous, cardiovascular, and lymphatic systems
three types of homeostatic balance
water balance
electrolyte balance
acid-base balance
water balance
•average daily water intake and loss are equal
electrolyte balance
the amount of electrolytes absorbed by the small intestine balance with the amount lost from the body, usually in urine
acid-base balance
•the body rids itself of acid (hydrogen ion –H+) at a rate that balances metabolic production
Body Water
- newborn baby‟s body weight is about 75% water
- young men average 55% -60%
- women average slightly less
- obese and elderly people as little as 45% by weight
total body water (TBW)
of a 70kg (150 lb) young male make is about 40 liters
major fluid compartments of the body
–65% intracellular fluid (ICF)
–35% extracellular fluid (ECF)
–35% extracellular fluid (ECF)
- 25% tissue (interstitial) fluid
- 8% blood plasma and lymphatic fluid
- 2% transcellular fluid „catch-all‟ category
transcellular fluid „catch-all‟ category
–cerebrospinal, synovial, peritoneal, pleural, and pericardial fluids
–vitreous and aqueous humors of the eye
–bile, and fluids of the digestive, urinary, and reproductive tracts
Water Movement Between Fluid Compartments
•fluid continually exchanged between compartments
•water moves by osmosis
•because water moves so easily through plasma membranes, osmotic gradientsnever last for very long
•if imbalance arises, osmosis restores balance within seconds so the intracellular and extracellular osmolarity are equal
–if osmolarity of the tissue fluid rises, water moves out of the cell
–if it falls, water moves in
osmosis from one fluid compartment to another is determined by
the relative concentrations of solutes in each compartment
–electrolytes–the most abundant solute particles, by far
–sodium salts in ECF
–potassium salts inICF
electrolytes
electrolytesplay the principal role in governing the body‟s water distribution and total water content
fluid balance
when daily gains and losses are equal (about 2,500 mL/day)
Water gains come from two sources:
–preformed water (2,300 mL/day)
•ingested in food (700 mL/day) and drink (1600 mL/day)
–metabolic water (200 mL/day)
•by-product of aerobic metabolism and dehydration synthesis
sensible water loss
is observable
–1,500 mL/ day is in urine
–200 mL/day is in feces
–100 mL/day is sweat in resting adult
insensible water loss
is unnoticed
–300 mL/day in expired breath
–400 mL/day is cutaneous transpiration
•diffuses through epidermis and evaporates
–does not come from sweat glands
–loss varies greatly with environment and activity
obligatory water loss
output that is relatively unavoidable
•expired air, cutaneous transpiration, sweat, fecal moisture, and urine output
thirst
mainly governs fluid intake
dehydration
–reduces blood volume and blood pressure
–increases blood osmolarity
osmoreceptors in hypothalamus
–respond to angiotensin II produced when BP drops and to rise in osmolarity of ECF with drop in blood volume
–osmoreceptors communicate with the hypothalamus and cerebral cortex
hypothalamus produces
antidiuretic hormone
•promotes water conservation
cerebral cortex produces
conscious sense of thirst
intense sense of thirst
with 2-3% increase in plasma osmolarity or 10-15% blood loss
salivation
is inhibited with thirst
•sympathetic signals from thirst center to salivary glands
long term inhibition of thirst
–absorption of water from small intestine reduces osmolarity of blood
•stops the osmoreceptor response, promotes capillary filtration, and makes the saliva more abundant and watery
•changes require 30 minutes or longer to take effect
short term inhibition of thirst
–cooling and moistening of mouth quenches thirst
–distension of stomach and small intestine
–30 to 45 min of satisfaction
•must be followed by water being absorbed into the bloodstream or thirst returns
–short term response designed to prevent overdrinking
Regulation of Water Output
only way to control water output significantly, is through variation in urine volume
–kidneys can‟t replace water or electrolytes
–only slow rate of water and electrolyte loss until water and electrolytes can be ingested
mechanisms:
–changes in urine volume linked to adjustments in Na+ reabsorption
•as Na+is reabsorbed or excreted, water follows
–concentrate the urine through action of ADH
ADH secretion stimulated by
hypothalamic osmoreceptors in response to dehydration
aquaporins synthesized in response to
ADH
–membrane proteins in renal collecting ducts whose job is to channel water back into renal medulla, Na+is still excreted
–slows decrease in water volume and increased osmolarity –concentrates urine
ADH release inhibited when
blood volume and pressure is too high or blood osmolarity too low
•effective way to compensate for hypertension
Disorders of Water Balance
the body is in a state of fluid imbalance if there is an abnormality of total volume, concentration, or distribution of fluid among the compartments
fluid deficiency
fluid output exceeds intake over long period of time
volume depletion
(hypovolemia)
–occurs when proportionate amounts of water and sodium are lost without replacement
–total body water declines, but osmolarity remains normal
–hemorrhage, severe burns, chronic vomiting, or diarrhea
hypovolemia
volume depletion
dehydration
dehydration(negative water balance)
–body eliminates significantly more water than sodium
–total body water declines, osmolarity rises
–lack of drinking water, diabetes, ADH hyposecretion (diabetes insipidus), profuse sweating, overuse of diuretics
infants more vulnerable to dehydration than adults due to
high metabolic rate that demands high urine excretion, immature kidneys cannot concentrate urine effectively, greater ratio of body surface to mass
most serious effects
circulatory shock due to loss of blood volume, neurological dysfunction due to dehydration of brain cells, infant mortality from diarrhea
Fluid Balance in Cold Weather
the body conserves heat by constricting blood vessels of the skin forcing blood to deeper circulation
–raises blood pressure which inhibits secretion of ADH
–increases secretion of atrial natriuretic peptide
–urine output is increased and blood volume reduced
cold air is drier and
increases respiratory water loss also reducing blood volume
cold weather respiratory and urinary loses cause
a state of reduced blood volume (hypovolemia)
–exercise will dilate vessels in skeletal muscles
–insufficient blood for rest of the body can bring on weakness, fatigue, or fainting (hypovolemic shock)
Dehydration from Excessive Sweating
1) water loss from sweating
2) sweat produced by capillary filtration
3) blood volume and pressure drop, osmolarity rises
4) blood absorbs tissue fluid to replace loss
5) tissue fluid pulled from ICF
6) all three compartments lose water
7) 300 mL from tissue fluid and 700 mL from ICF
fluid excess
less common than fluid deficiency because the kidneys are highly effective in compensating for excessive intake by excreting more urine
–renal failure can lead to fluid retention
two types of fluid excesses
volume excess
hypotonic hydration
volume excess
- both Na+ and water retained
- ECF remains isotonic
- caused by aldosterone hypersecretion or renal failure
hypotonic hydration
(water intoxication) (positive water balance)
•more water than Na+ retained or ingested
•ECF becomes hypotonic
–can cause cellular swelling
–pulmonary and cerebral edema
fluid sequestration
a condition in which excess fluid accumulates in a particular location
•total body water may be normal, but volume of circulating blood may drop to a point causing circulatory shock
most common form
edema -abnormal accumulation of fluid in the interstitial spaces, causing swelling of the tissues
hemorrhage
another cause of fluid sequestration
•blood that pools in the tissues is lost to circulation
pleural effusion
several liters of fluid can accumulate in the pleural cavity
•caused by some lung infections
physiological functions of electrolytes
–chemically reactive and participate in metabolism
–determine electrical potential (charge difference) across cell membranes
–strongly affect osmolarity of body fluids
–affect body‟s water content and distribution
major cations
–Na+, K+, Ca2+, and H+
major anions
–Cl-, HCO3-(bicarbonate), and PO43-
great differences between electrolyte concentrations of
blood plasma and intracellular fluid (ICF)
–have the same osmolarity (300 mOsm/L)
concentrations in tissue fluid (ECF) differ only
slightly from those in the plasma
sodium
principal ions responsible for the resting membrane potentials
–inflow of sodium through membrane gates is an essential event in the depolarization that underlies nerve and muscle function
principal cation in ECF
–sodium salts accounts for 90 -95% of osmolarity of ECF
–most significant solute in determining total body water and distribution of water among the fluid compartments
Na+-K+pump
–exchanges intracellular Na+ for extracellular K+
–generates body heat
Homeostasis
•adult needs about 0.5 g of sodium per day
–typical American diet contains 3 –7 g/day
•primary concern -excretion of excess dietary sodium
sodium concentration coordinated by:
aldosterone
ADH
ANP
others:
aldosterone
aldosterone-“salt retaining hormone”
•primary role in adjusting sodium excretion
•hyponatremia and hyperkalemia directly stimulate the adrenal cortex to secrete aldosterone
•hypertension stimulates its secretion by way of the renin-angiotensin-aldosterone mechanism
•aldosterone receptors in ascending limb of nephron loop, the distal convoluted tubule, and cortical part of collecting duct
aldosterone, a steroid, binds to nuclear receptors
–activates transcription of a gene for the Na+ -K+ pumps
–in 10 –30 minutes enough Na+ -K+ pumps are inserted in the plasma membrane to make a noticeable effect
–tubules reabsorb more sodium and secrete more hydrogen and potassium
–water and chloride passively follow sodium
primary effects of aldosterone are that
the urine contains less NaCl and more potassium and a lower pH
ADH
modifies water excretion independently of sodium excretion
•high sodium concentration in the blood stimulate the posterior lobe of the pituitary to release ADH
•kidneys reabsorbs more water
•slows down any further increase in blood sodium concentration
•drop in sodium inhibits ADH release
•more water is excreted, raising the sodium level in the blood
ANP
(atrial natriuretic peptide)
•inhibit sodium and water reabsorption, and the secretion of renin and ADH
•kidneys eliminate more sodium and water lowering blood pressure
estrogen
mimics aldosterone and women retain water during pregnancy
progesterone
reduces sodium reabsorption and has a diuretic effect
sodium homeostasis is achieved by regulating salt intake
salt cravings in humans and other animals
hypernatremia
plasma sodium concentration greater than 145 mEq/L
•from administration of IV saline
•water pretension, hypertension and edema
hyponatremia
plasma sodium concentration less than 130 mEq/L
•person loses large volumes of sweat or urine, replacing it with drinking plain water
•result of excess body water, quickly corrected by excretion of excess water
Potassium -Functions
most abundant cation of ICF
•greatest determinant of intracellular osmolarity and cell volume
•produces (with sodium) the resting membrane potentials and action potentials of nerve and muscle cells
•Na+-K+pump
–co-transport and thermogenesis
•essential cofactor for protein synthesis and other metabolic processes
Homeostasis
•potassium homeostasis is closely linked to that of sodium
•90% of K+in glomerular filtrate is reabsorbed by the PCT
–rest excreted in urine
•DCT and cortical portion of collecting duct secrete K+in response to blood levels
•Aldosterone stimulates renal secretion of K+
Potassium -Imbalances
most dangerous imbalances of electrolytes
hyperkalemia
effects depend on whether the potassium concentration rises quickly or slowly
–greater than 5.5 mEq/L
–if concentration rises quickly, (crush injury) the sudden increase in extracellular K+makes nerve and muscle cells abnormally excitable
–slow onset, inactivates voltage-regulated Na+channels, nerve and muscle cells become less excitable
•can produce cardiac arrest
hypokalemia
–less than 3.5 mEq/L
–rarely results from dietary deficiency
–from sweating, chronic vomiting or diarrhea
–nerve and muscle cells less excitable
•muscle weakness, loss of muscle tone, decreased reflexes, and arrhythmias from irregular electrical activity in the heart
Chloride -Functions
•most abundant anions in ECF
–major contribution to ECF osmolarity
•required for the formation of stomach acid
–hydrochloric acid (HCl)
•chloride shift that accompanies CO2loading and unloading in RBCs
•major role in regulating body pH
Chloride -Homeostasis
•strong attraction to Na+, K+and Ca2+, which chloride passively follows
•primary homeostasis achieved as an effect of Na+homeostasis
–as sodium is retained, chloride ions passively follow
hyperchloremia
result of dietary excess or administration of IV saline
hypochloremia
side effect of hyponatremia
–sometimes from hyperkalemia or acidosis
•primary effects:
–disturbances in acid-base balance
Calcium -Functions
- lends strength to the skeleton
- activates sliding filament mechanism of muscle contraction
- serves as a second messenger for some hormones and neurotransmitters
- activates exocytosis of neurotransmitters and other cellular secretions
- essential factor in blood clotting
calcium homeostasis is chiefly regulated by
PTH, calcitriol(vitamin D), and calcitonin(in children)
–these hormones affect bone deposition and resorption
–intestinal absorption and urinary excretion
calsequestrin
proteins that bind Ca2+ and keep it unreactive in Ca2+ storage cells
hypercalcemia
greater than 5.8 mEq/L
–caused by alkalosis, hyperparathyroidism, hypothyroidism
–reduces membrane Na+permeability, inhibits depolarization of nerve and muscle cells
–concentrations greater than 12 mEq/L causes muscular weakness, depressed reflexes, cardiac arrhythmias
hypocalcemia
less than 4.5 mEq/L
–caused by vitamin D deficiency, diarrhea, pregnancy, acidosis, lactation, hypoparathyroidism, hyperthyroidism
–increases membrane Na+permeability, causing nervous and muscular systems to be abnormally excitable
–very low levels result in tetanus, laryngospasm, death
Phosphates -Functions
•relatively concentrated in ICF due to hydrolysis of ATP and other phosphate compounds
•inorganic phosphates (Pi) of the body fluids are an equilibrium mixture of phosphate (PO43-), monohydrogen phosphate (HPO42-), and dihydrogen phosphate (H2PO4-)
•components of:
–nucleic acids, phospholipids, ATP, GTP, cAMP, hydroxyapatite, and creatine phosphate
•activates many metabolic pathways by phosphorylating enzymes and substrates such as glucose
•buffers that help stabilize the pH of body fluids
renal control
–normally phosphate is continually lost by glomerular filtration
–if plasma concentration drops, renal tubules reabsorb all filtered phosphate
parathyroid hormone
–increases excretion of phosphate which increases concentration of free calcium in the ECF
–lowering the ECF concentration of phosphate minimizes the formation of calcium phosphate and helps support plasma calcium concentration
•imbalances not as critical
–body can tolerate broad variations in concentration of phosphate
Acid-Base Balance
one of the most important aspects of homeostasis
–metabolism depends on enzymes, and enzymes are sensitive to pH
–slight deviation from the normal pH can shut down entire metabolic pathways
–slight deviation from normal pH can alter the structure and function of macromolecules
•7.35 to 7.45 is the normal pH range of blood and tissue fluid
challenges to acid-base balance:
metabolism constantly produces acid
lactic acids
from anaerobic fermentation
phosphoric acid
from nucleic acid cataboli
fatty acids and ketones
from fat catabolism
carbonic acid
from carbon dioxide
pH of a solution is determined solely by
its hydrogen ions (H+)
acids
any chemical that releases H+ in solution
strong acids
like hydrochloric acid (HCl) ionize freely
•gives up most of its H+
•markedly lower pH of a solution
weak acids
like carbonic acid (H2CO3) ionize only slightly
•keeps most H+ chemically bound
•does not affect pH as much
bases
any chemical that accepts H+
strong bases
like the hydroxide ion (OH-), has a strong tendency to bind H+, markedly raising pH
weak bases
such as the bicarbonate ion (HCO3-) bind less available H+ and has less effect on pH
buffer
any mechanism that resists changes in pH
–convert strong acids or bases to weak ones
physiological buffer
system that controls output of acids, bases, or CO2
urinary system
buffers greatest quantity of acid or base
•takes several hours to days to exert its effect
respiratory system
buffers within minutes
•cannot alter pH as much as the urinary system
chemical buffer
a substance that binds H+ and removes it from solution as its concentration begins to rise, or releases H+ into solution as its concentration falls
–restore normal pH in fractions of a second
–function as mixtures called buffer systems composed of weak acids and weak bases
three major chemical buffers
bicarbonate, phosphate, and protein systems
bicarbonate buffer system
a solution of carbonic acid and bicarbonate ions.
•functions best in the lungs and kidneys to constantly remove CO2
–to lower pH, kidneys excrete HCO3-
–to raise pH, kidneys excrete H+and lungs excrete CO2
functions best in the lungs and kidneys to
constantly remove CO2
–to lower pH, kidneys excrete HCO3-
–to raise pH, kidneys excrete H+and lungs excrete CO2
phosphate buffer system
-a solution of HPO42-and H2PO4-
H2PO4-HPO42-+ H+
–as in the bicarbonate system, reactions that proceed to the right liberating H+ and decreasing pH, and those to the left increase pH
more important buffering the ICF and renal tubules
–where phosphates are more concentrated and function closer to their optimum pH of 6.8
•constant production of metabolic acids creates pH values from 4.5 to 7.4 in the ICF, avg. 7.0
proteins
are more concentrated than bicarbonate or phosphate systems, especially in the ICF
protein buffer system
accounts for about three-quarters of all chemical buffering in the body fluids
•protein buffering ability is due to certain side groups of their amino acid residues
carboxyl (-COOH) side groups
which releases H+ when pH begins to rise
•others have amino (-NH2) side groups that bind H+ when pH gets too low
Respiratory Control of pH
basis for the strong buffering capacity of the respiratory system
–the addition of CO2to the body fluids raises the H+ concentration and lowers pH
–the removal of CO2has the opposite effects
•neutralizes 2 to 3 times as much acid as chemical buffers
CO2 is constantly produced by aerobic metabolism
–normally eliminated by the lungs at an equivalent rate
H2CO3HCO3-+ H+
•raises pH by binding H+
•increased CO2and decreased pH stimulate pulmonary ventilation, while an increased pH inhibits pulmonary ventilation
Renal Control of pH
- the kidneys can neutralize more acid or base than either the respiratory system or chemical buffers
- renal tubules secrete H+ into the tubular fluid
tubular secretion of H+
–continues only with a steep concentration gradient of H+ between tubule cells and tubular fluid
–if H+ concentration increased in tubular fluid, lowering pH to 4.5, secretion of H+ stops –limiting pH
bicarbonate system
all bicarbonate ions in tubular fluid are consumed neutralizing H+
•so there is no HCO3-in the urine
•the more acid the kidneys secrete, less sodium is in the urine
phosphate system
dibasic sodium phosphate is contained in glomerular filtrate
•reacts with some of the H+ replacing a Na+in the buffer which passes into the urine
•Na2HPO4+ H+ NaH2PO4 + Na+
ammonia
(NH3) -from amino acid catabolism acts as a base to neutralize acid
•NH3+ H+ and Cl-NH4Cl (ammonium chloride –a weak acid)
acid –base balance
depends on bicarbonate buffer system
acidosis
pH below 7.35
–H+ diffuses into cells and drives out K+, elevating K+ concentration in ECF
•H+ buffered by protein in ICF, causes membrane hyperpolarization, nerve and muscle cells are hard to stimulate; CNS depression may lead to confusion, disorientation, coma, and possibly death
alkalosis
pH above 7.45
–H+diffuses out of cells and K+ diffuses in, membranes depolarized, nerves overstimulated, muscles causing spasms, tetany, convulsions, respiratory paralysis
–a person cannot live for more than a few hours if the blood pH is below 7.0 or above 7.7
acid-base imbalances fall into two categories:
respiratory and metabolic
respiratory acidosis
–occurs when rate of alveolar ventilation fails to keep pace with the body‟s rate of CO2production
–carbon dioxide accumulates in the ECF and lowers its pH
–occurs in emphysema where there is a severe reduction of functional alveoli
respiratory alkalosis
–results from hyperventilation
–CO2 eliminated faster than it is produced
metabolic acidosis
–increased production of organic acids such as lactic acid in anaerobic fermentation, and ketone bodies seen in alcoholism, and diabetes mellitus
–ingestion of acidic drugs (aspirin)
–loss of base due to chronic diarrhea, laxative overuse
metabolic alkalosis
–rare, but can result from:
–overuse of bicarbonates (antacids and IV bicarbonate solutions)
–loss of stomach acid (chronic vomiting)
compensated acidosis or alkalosis
–either the kidneyscompensate for pH imbalances of respiratory origin, or
–the respiratory system compensates for pH imbalances of metabolic origin
uncompensated acidosis or alkalosis
–a pH imbalance that the body cannot correct without clinical intervention
respiratory compensation
changes in pulmonary ventilation to correct changes in pH of body fluids by expelling or retainingCO2
hypercapnia
(excess CO2) -stimulates pulmonary ventilation eliminating CO2and allowing pH to rise
hypocapnia
(deficiency of CO2) reduces ventilation and allows CO2 accumulate lowering pH
renal compensation
an adjustment of pH by changing the rate of H+ secretion by the renal tubules
–slow, but better at restoring a fully normal pH
in acidosis
urine pH may fall as low as 4.5 due to excess H+
•renal tubules increase rate of H+ secretion elevating pH
in alkalosis
as high as 8.2 because of excess HCO3-
•renal tubules decrease rate of H+ secretion, and allows neutralization of bicarbonate, lowering pH
renal compensation
speed
–kidneys cannot act quickly enough to compensate for short-term pH imbalances
–effective at compensating for pH imbalances that lasts for a few days or longer
Fluid Replacement Therapy
one of the most significant problems in the treatment of seriously ill patients is the restoration and maintenance of proper fluid volume, composition, and distribution among fluid compartments
fluids may be administered to:
–replenish total body water
–restore blood volume and pressure
–shift water from one fluid compartment to another
–restore and maintain electrolyte and acid-base balance
drinking water is the simplest method
–does not replace electrolytes
–broths, juices, and sports drinks replace water, carbohydrates, and electrolytes
patients who cannot take fluids by mouth
–enema –fluid absorbed through the colon
–parenteral routes –fluid administration other than digestive tract
•intravenous (I.V.) route is the most common
•subcutaneous (sub-Q) route
•intramuscular (I.M.) route
•other parenteral routes
excessive blood loss
–normal saline (isotonic, 0.9% NaCl)
–raises blood volume while maintaining normal osmolarity
•takes 3 to 5 times the normal saline to rebuild normal blood volume because much of the saline escapes blood and enters interstitial fluid compartment
•can induce hypernatremia or hyperchloremia
correct pH imbalances
–acidosis treated with Ringer‟s lactate
–alkalosis treated with potassium chloride
plasma volume expanders
hypertonic solutions or colloids that are retained in the bloodstream and draw interstitial water into it by osmosis
–used to combat hypotonic hydration by drawing water out of swollen cells
–can draw several liters of water out of the intracellular compartment within a few minutes
patients who cannot eat
–isotonic 5% dextrose (glucose) solution
–has protein sparing effect –fasting patients lose as much as 70 to 85 grams of protein per day
•I.V. glucose reduces this by half
patients with renal insufficiency
–given slowly through I.V. drip
