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Fluid, Electrolyte, and Acid-Base Homeostasis
Terms in this set (39)
What are body fluids?
- the water and dissolved solutes throughout the body
- constitute about 55-60% of total body mass
What are the two main "compartments" where body fluids are contained?
- inside cells (INTRACELLULAR FLUID)
- outside cells (EXTRACELLULAR FLUID)
What is intracellular fluid (ICF)?
- makes up about 2/3 of body fluid
What is extracellular fluid (ECF)?
- makes about the other 1/3 of body fluid
- includes all other body fluids, i.e. interstitial fluid, plasma, etc.
What is interstitial fluid?
- fluid which occupies the microscopic spaces b/w tissue cells
- makes up about 80% of ECF
What is plasma?
- the liquid portion of blood
- makes up the other 20% of ECF
What are the other body fluids that make up ECF?
- lymph, cerebrospinal fluid, synovial fluid, aqueous humor and vitreous body, pleural, pericardial, and peritoneal fluids, etc
What is fluid balance?
- the body is in fluid balance when the required amounts of water and solutes are present and are correctly proportioned among the various compartments
- water is by far the largest single component of body, making up 45-75% of total body mass
What are electrolytes and how do they affect fluid balance?
- inorganic compounds that dissociate into ions
- b/c most solutes in body fluids are electrolytes, fluid balance is closely related to electrolyte balance (b/c osmosis is the primary means of water movement)
What are the sources of body water gain?
- by ingestion (liquids=~1600mL and solids=~700mL) and by metabolic synthesis (when electrons are accepted by oxygen during aerobic cellular respiration; ~200mL)
What are the sources of body water loss?
- four ways:
=> kidney excretion (1500mL in urine)
=> evaporation on skin (600mL in sweat)
=> exhalation via lungs (300mL as water vapour)
=> GI elimination (100mL in feces)
*additional water loss in menstruating women
How is body water gain regulated?
- regulated mainly by the volume of water intake
- thirst center in the hypothalamus
How are water and solute loss regulated?
- elimination of excess occurs mainly by control of their loss in urine
- hormonal changes regulate the urinary loss of ions, which in turn affects blood volume; hormones include angiotensin II, aldosterone, and atrial natriuretic peptide (ANP); the major hormone that regulates water loss is ADH
How does water move b/w body fluid compartments?
- an increase in the osmolarity of interstitial fluid dwars water out of cells, and they shrink slightly
- a decrease in the osmolarity of interstitial fluid causes cells to swell
- changes in osmolarity most often result from changes in the concentration of Na+
Name the four general functions of electrolytes in the body.
1. certain ions control the osmosis of water b/w fluid compartments
2. ions help maintain the acid-base balance required for normal cellular activities
3. ions carry electrical current, which allows production of action potentials and graded potentials
4. several ions serve as cofactors needed for optimal activity of enzymes
How do concentrations of electrolytes compare b/w plasma, interstitial fluid, and intracellular fluid?
- chief difference b/w plasma and interstitial fluid is that plasma contains more protein anions; interstitial fluid has few b/c normal capillary membranes are virtually impermeable to proteins so rarely leak out of vessels into interstitial fluid; otherwise, the two fluids are similar
- intracellular differs considerably from extracellular fluid; in ECT, the most abundant cations is Na+ and most abundant anion is Cl-; in ICF, most abundant cation is K+, and most abundant anions are proteins and phosphates (HPO42-)
Discuss the role of sodium in fluid balance.
- most abundant ions in ECT
- plays a pivotal role in fluid and electrolyte balance b/c it accounts for almost half of the osmolarity of ECT
- flow of Na+ is also necessary for the generation and conduction of action potentials in neurons and muscle fibers
- Na+ level in blood is controlled by aldosterone (increases renal reabsorption of Na+), ADH (lack ADH permits greater water excretion and restores Na+ levels), and ANP (increases Na+ excretion by kidneys when Na+ level is above normal)
Discuss the role of chloride in fluid balance.
- most prevalent anions in ECT
- moves easily b/w extracellular and intracellular compartment so Cl- can help balance the level of anions in different fluid compartments
Discuss the role of potassium in fluid balance.
- most abundant cations in ICF
- plays a key role in establishing the resting membrane potential and in the repolarization phase of action potentials in neurons and muscle fibers
- also helps maintain normal intracellular fluid volume
- also helps regulate pH of body fluids since when K+ moves into or out of cells, it is often exchanged for H+
- controlled mainly by aldosterone
Discuss the role of bicarbonate in fluid balance.
- the second most prevalent extracellular anions
- concentrations increase as blood flows through systemic capillaries b/c CO2 released by metabolically active cells combines w/ water to form carbonic acid, which then dissociates into H+ and HCO3-
- as blood flows through pulmonary capillaries, concentrations decrease as CO2 is exhaled
- kidneys are the main regulators of HCO3- concentration
Discuss the role of calcium in fluid balance.
- most abundant mineral in the body (stored in bone)
- mainly an extracellular cation
- plays an important role in blood clotting, neurotransmitter release, maintenance of muscle tone, and excitability of nervous and muscle tissue
- most important regulator is parathyroid hormone, which is released when levels of Ca2+ are low and stimulates osteoclasts; also enhances reabsorption of Ca2+ from glomerular filtrate through renal tubule cells back into blood
Discuss the role of phosphate in fluid balance.
- mostly present as calcium phosphate salts in bone and teeth
- three phosphate ions are important intracellular anions (H2PO4-, HPO42-, and PO43-)
- at normal pH, HPO42- is most prevalent and is an important buffer of H+ in body fluids and in urine
- governed by parathyroid hormone (phosphate is released along with calcium; however PTH inhibits reabsorption of phosphate ions) and calcitriol (promotes absorption of both phosphates and calcium from the GI tract)
What is a reason for maintaining pH balance in the body?
- the three-dimensional shape of proteins is very sensitive to pH changes
What are the 3 major mechanisms for the removal of H+ from body fluids?
1. Buffer Systems
- quickly bind H+ temporarily, thus removing the highly reactive, excess H+ from solution, though they do not remove the H+;
2. Exhalation of CO2
- increasing the rate and depth of breathing removes more CO2 and within minutes, the level of carbonic acid lowers in the blood, raising blood pH;
3. Kidney Excretion of H+
- the slowest mechanism, but the only way to eliminate acids other than carbonic acid
What do most buffer systems consist of?
- a weak acid and the salt of that acid, which functions as a weak base
How do buffer systems function?
- they prevent rapid, drastic changes in pH of body fluids by converting strong acids and bases into weak acids and bases within a fraction of a second
- strong acids lower pH more than weak acids, because strong acids release H+ more readily
What are the 3 principal buffer systems of the body?
- the protein buffer system
- the carbonic acid-bicarbonate buffer system
- the phosphate buffer system
What is the protein buffer system?
- the most abundant buffer in intracellular fluid and blood plasma, i.e. hemoglobin, albumin, etc
- functional components of the buffer system are the carboxyl group and the amino group on proteins
- the free carboxyl group acts like an acid by releasing H+ when pH rises
- the free amino group can act as a base by combining with H+ when pH falls
What is the carbonic acid-bicarbonate buffer system?
- bicarbonate ion acts as a weak base, and carbonic acid can act as a weak acid
H+ + HCO3- -> H2CO3
- H2CO3 then dissociates into water and CO2, and the CO2 is exhaled from the lungs
- conversely: H2CO3 -> HCO3- + H+
- b/c CO2 and H2O combine to form H2CO3, this buffer system cannot protect against pH changes due to respiratory problems in which there is an excess or shortage of CO2
What is the phosphate buffer system?
- acts via a mechanism similar to the carbonic acid-bicarbonate buffer system
- components are H2PO4-, which acts as a weak acid, and HPO42-, which acts as a weak base
- b/c the concentration of phosphates is highest in ICF, this buffer system is an important regulator of pH in the cytosol
How does exhalation of CO2 help in regulating pH?
- an increase in the CO2 concentration in body fluids increases H+ concentration and thus lowers the pH, and vice versa
CO2 + H2O <-> H2CO3 <-> H+ + HCO3-
- when CO2 decreases, the reaction is driven to the left; and vice versa
- changes in the rate and depth of breathing can alter the pH of body fluids in minutes; works via a negative feedback loop through central and peripheral chemoreceptors
How does kidney excretion help in regulating pH?
- metabolic reactions produce large amounts of acids and the only way to eliminate this acid load is to excrete H+ in the urine
- kidneys (esp intercalated cells of the collecting duct) can excrete excess H+ when pH is too low, and excrete excess HCO3- when pH is too high
What is acidosis?
- a condition in which blood pH is below 7.35
- major physiological effect is depression of the CNS through depression of synaptic transmission
What is alkalosis?
- a condition in which blood pH is higher than 7.45
- major physiological effect is overexcitability in both the CNS and PNS
What is the body's reaction to acidosis or alkalosis?
- if a person has altered blood pH due to metabolic causes, hyperventilation or hypoventilation can help bring blood pH back toward the normal range (RESPIRATORY COMPENSATION)
- if a person has altered blood pH due to respiratory causes, RENAL COMPENSATION, changes in secretion of H+ and reabsorption of HCO3- by the kidney tubules, can help reverse the change (begins in minutes, but takes days to achieve maximal effectiveness)
What is respiratory acidosis?
- marked by an abnormally high PCO2 in systemic arterial blood caused by inadequate exhalation of CO2, causing blood pH to drop
- caused by any condition that decreases movement of CO2 from the blood to the alveoli of the lungs, causing a buildup of CO2, HCO3-, and H+ (i.e. emphysema, pulmonary edema, airway obstruction, etc.)
- renal compensation = increasing excretion of H+ and reabsorption of HCO3-
What is respiratory alkalosis?
- systemic arterial blood PCO2 falls; the cause of the drop and the resulting increase in pH is hyperventilation, which occurs in conditions that stimulate the inspiratory area in the brain stem (i.e. O2 deficiency due to high altitude or pulmonary disease, stroke, severe anxiety, etc)
- renal compensation = decrease excretion of H+ and reabsorption of HCO3-
What is metabolic acidosis?
- the systemic arterial blood HCO3- level drops, causing the blood pH to decrease
- can be caused by 3 situations: 1) actual loss of HCO3-, i.e. severe diarrhea or renal dysfunction; 2) accumulation of an acid other than carbonic acid, i.e. ketosis; or 3) failure of the kidneys to excrete H+ from metabolism or dietary proteins
- resp compensation = hyperventilation
What is metabolic alkalosis?
- systemic arterial blood HCO3- rises above normal
- a nonrespiratory loss of acid or excessive intake of alkaline drugs causes blood pH to increase above 7.45
- most frequent cause is excessive vomiting of gastric contents (substantial loss of hydrochloric acid); other causes include gastric suctioning, use of certain diuretics, endocrine disordes, severe dehydration, etc.
- resp compensation = hypoventilation
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