In regards to fluid and electrolyte balance, what is the body's main task?
maintaining mass balance: what comes in must be excreted if the body does not need it
What is the primary route for water loss and for removal of many ions?
What is lost via the lungs in regards to fluids and electrolytes?
lose water and help remove H+ and HCO3- by excreting CO2
What is lost via the feces and sweat?
small amounts of both water and ions
Why is thirst critical?
drinking is the only normal way to replace lost water
What is a salt appetite?
a behavior that leads people and animals to seek and ingest salt
Why are we concerned with homeostasis of water and Na+?
they are associated with ECF volume and osmolarity
Why are we concerned with homeostasis of K+?
disturbance can cause serious problems with cardiac and muscle function by disrupting the membrane potential of excitable cells
Why are we concerned with homeostasis of Ca2+?
it is involved in a variety of body processes, from exocytosis and muscle contraction to bone formation and blood clotting
Why are we concerned with homeostasis of H+ and HCO3-?
they are the ions whose balance determines body pH
Why is maintaining osmolarity so important to the body?
because of water movement in and out of cells
What happens to the cells if ECF osmolarity decreases as a result of excess water intake?
water moves into cells and they swell (can lyse)
What happens to the cells if ECF osmolarity increases as a result of salt intake?
water moves out of cells and they shrink
How do renal tubule cells in the medulla of the kidneys, which are constantly exposed to high ECF osmolarity, maintain normal cell volume? What are some examples of how they do this?
by synthesizing organic solutes as needed to make their intracellular osmolarity match that of the medullary interstitial fluid; sugar alcohols, certain AAs
Why is the process of fluid and electrolyte balance considered truly integrated?
it involves the respiratory, cardiovascular, renal and behavioral systems
Why does homeostatic compensation by the kidneys occur more slowly than say adjustments made by the lungs and CV system?
because the kidneys are primarily under endocrine and neuroendocrine control, whereas the lungs and CV system are under neural control, which occurs rapidly
A decrease in blood volume occurs that decreases blood pressure. Starting with the correct receptor, what adjustments are made by the CV system? Result?
increase in cardiac output triggered by a decrease in parasympathetic firing to the SA node, vasoconstriction triggered by an increase in sympathetic neuron firing to smooth muscle; increased BP
A decrease in blood volume occurs that decreases blood pressure. Starting with the correct receptor, what adjustments are made by the behavioral system? Result?
thirst causes increased water intake, which increases ECF and ICF volume, which increases BP
A decrease in blood volume occurs that decreases blood pressure. Starting with the correct receptor, what adjustments are made by the renal system? Result?
kidneys conserve water to minimize further volume loss
A decrease in blood volume occurs that decreases blood pressure. What receptors intercept this change? (3)
volume receptors in atria; carotid and aortic baroreceptors
An increase in blood volume occurs that increases blood pressure. Starting with the correct receptor, what adjustments are made by the CV system? Result?
decreased cardiac output triggered by an increase in parasympathetic firing to the SA node, vasodilation triggered by a decrease in sympathetic firing to the smooth muscle; decreased BP
An increase in blood volume occurs that increases blood pressure. Starting with the correct receptor, what adjustments are made by the renal system? Result?
kidneys excrete salts and water in urine; decreases ECF and ICF volume; decreases BP
How do the renal and CV system overkap in terms of hormones?
endocrine oathways initiated by the kidneys have direct effects on the CV system; hormones released by myocardial cells act on the kidneys
How do sympathetic pathways from the CV control center affect not only cardiac output and vasoconstrction, but also the renal system?
the sympathetic output affects glomerular filtration (afferent and efferent arterioles) and hormone release by the kidneys
What percentage of water is the average man? woman? Where is this water located in the body of the average 70kg man?
60% (42 L); 50% (30 L); 2/3 inside cells, ~3L in plasma, ~11 in interstitial fluid
To maintain a constant volume of water in the body, what must occur?
intake must equal output
What is the only means by which water normally enters the body from the external environment?
by absorption through the GI tract
If fluids must be rapidly replaced or an individual is unable to eat or drink, how can fluid be replaced?
it can be added directly to the plasma by means of intravenous injection (IV)
What is the average daily value of urine excretion?
What is insensible water loss?
water loss we are unaware of, usually occurs across the skin surface and during exhalation of humidified air
Where is the only place that water loss can be regulated?
in the urine
What are two examples besides urine that could pose a major threat to the maintenance of water balance?
excessive sweating, diarrhea
How does pathological water loss disrupt homeostasis? (disease) (2 ways)
volume depletion of the ECF decreases BP, if BP cannot be maintained through homeostatic compensation, the tissues can't get enough O2; if fluid lost is hyposmotic to body (sweating), the solutes left behind raise osmolarity, potentially disrupting cell function
What is the fluid and electrolyte mass balance equation?
intake + metabolic production - output = 0 (theoretically)
If the blood volume falls too low, what stops in the kidneys?
Why must an extreme fluid loss be replenished regardless of homeostatic compensations and other fluid and electrolyte balance means?
because the kidneys can only conserve volume over long term, they cannot replenish it
What is a measure of how much water is excreted by the kidneys?
the concentration, or osmolarity, or urine
When the maintenance of homeostasis requires eliminating excess water, the kidneys do what?
produce copious amoutns of dilute urine with an osmolarity as low as 50 mOsM
What is removal of excess water in urine?
What are drugs that promote the excretion of urine?
What happens to the urine when kidneys are conserving water?
urine becomes concentrated, up to four times as concentrated as the blood (1200 mOsM)
How do the kidneys control urine concentration?
varying the amounts of water and Na+ reabsorbed in the distal nephron (distal tubule and collecting duct)
in order to produce dilute urine, what must the apical tubule cell membrane be impermeable to?
water (solutes have to be able to move across without water)
In order to produce concentrated urine, what must the nephron be impermeable to?
solutes (water must be able to move across without solutes)
What is the mechanism for absorbing water without solute?
collecting duct cells and interstitial fluid is more concentrated than the fluid flowing into the tubule; if the cells have water pores, water can be absorbed from the lumen without reabsorbing solute
What is maintained in the renal medulla in order to ensure water, but not solute, movement?
high osmotic concentration in the cells and interstitial fluid
What does high medullary interstitial osmolarity allow in the kidneys?
allows urine to be concentrated as it flows through the collecting duct
What are the reabsorbing water pores in the kidney tubules called?
What happens in the descending portion of the loop of Henle (i.e. the thin portion)?
isosmotic fluid leaving the proximal tubule becomes progressively more concentrated as water moves out
What happens in the ascending portion of the loop of Henle (i.e. the thick portion)?
removal of solute but not water creates hyposmotic fluid
What controls movement in the distal nephron?
What does urine osmolarity in the collecting duct depend on?
Describe the surrounding osmolarity of the renal cortex as fluid and solutes flows through the proximal tubule.
isosmotic (300 mOsM)
Describe the surrounding osmolarity of the renal medulla as fluid and solutes flow through the descending portion of the loop of Henle.
as the loop of Henle descends into the medulla, the osmolarity increases and water flows out of the tubule because of its osmotic gradient through aquaporins
Describe the surrounding osmolarity of the renal medulla as fluid and solutes flow through the ascending portion of the loop of Henle.
as the loop of Henle ascends towards the cortex, the osmolarity increases to 300 mOsM, ions as being reabsorbed out of the tubule, but water remains (reaches 100 mOsM)
What ions are transported out of the tubule lumen in the ascending portion of the loop of Henle?
Na+, K+, Cl-
What is the primary site where the kidney creates hyposmotic fluid?
loop of Henle
In the distal nephron, _____ ______ is variable and under ______ control.
water permeability; hormonal
When does the permeability of the distal nephron change?
when the body senses a need to conserve or expel water
How do the distal tubule and collecting duct cells alter their permeability to water?
by adding or removing water pores int he apical membrane under the direction of the posterior pituitary hormone vasopressin
Because vasopressin causes the body to retain water, it is also known as?
What happens when vasopressin acts on target cells?
water pores are inserted into the apical membrane, allowing water to move out of the lumen by osmosis
Why does water move out of the lumen after vasopressin acts on the cells?
because solute concentration in the cells and interstitial fluid of the renal medulla is higher than that of fluid in the tubule
Describe the permeability to water in the collecting duct in the absence of vasopressin.
Is water permeability of the collecting duct an all-or-none phenomenon? Describe permeability with varying levels of vasopressin?
No; permeability varies as the amount of vasopressin varies (graded effect)
What does the graded effect of vasopressin allow the body to do?
match urine concentration closely to the body's needs
What is the specific water channel regulated by vasopressin?
Where are the two places in a collecting duct cell where AQP-2 can be found?
on the apical membrane facing the tubule lumen and in the membrane of cytoplasmic storage vesicles
When vasopressin is low in the collecting duct, where are the AQP-2?
in cytoplasmic storage vesicles
When vasopressin arrives at target cells, what does it bind? Where is this? What does the binding activate (2)? What does this cause?
V2 receptors; basolateral side of the cell; activates a G-protein/cAMP second messenger system and phosphorylation of intracellular proteins which causes the AQP-2 vesicles to move to the apical membrane and fuse with it through exocytosis
What is membrane recycling?
process in which cell membrane is withdrawn by endocytosis and stored in vesicles in the cytoplasm until needed, when exocytosis reinserts the vesicle into the membrane
Does the apical membrane of collecting duct cell have more water pores when vasopressin is present or when it is absent?
If vasopressin is suppressed, will urine be dilute or concentrated?
What are the three stimuli for vasopressin secretion?
plasma osmolarity, blood volume, BP
What is the most potent stimulus for vasopressin release?
an increase in plasma osmolarity
What is osmolarity monitored by? What are they?
osmoreceptors; stretch-sensitive neurons that increase their firing rate as osmolarity increases
What happens to non-specific cation channels when osmoreceptor shrink?
the channels linked to actin filaments open, depolarizing the cell
Where are the primary osmoreceptors for vasopressin release?
in the hypothalamus
What value of plasma osmolarity is the trigger for osmoreceptor ceasefire? What happens?
<280 mOsM; osmoreceptors do not fire, vasopressin release from pituitary ceases and urine remains dilute
What happens if plasma osmolarity rises above the trigger value?
>280 mOsM, osmoreceptors fire and stimulate release of vasopressin to allow water flow out of the collecting duct
What are the four steps to AQP-2 insertion into the apical membrane?
vasopressin binds to membrane receptor; receptor activates cAMP second messenger system; cell inserts AQP-2 water pores into apical membrane; water is absorbed by osmosis into blood
Where are the primary receptors for BP and blood volume?
stretch-sensitive receptors in the atria
When BP or blood volume is low, what do the receptors in the atria do in regards to fluid/electrolyte balance?
receptors signal hypothalamus to secrete vasopressin and conserve fluid (take it out of urine)
Describe how vasopressin secretion shows a circadian rhythm.
increased secretion during overnight hours, less urine is produced, the first urine secreted in the morning is more concentrated
What is nocturnal enuresis? What is the theory regarding this and vasopressin?
bed-wetting; a developmental delay in the normal pattern of increased vasopressin at night is present
With the stimulus of osmolarity greater than 280 mOsM in order to increase water reabsorption to conserve water, what is the: receptor? afferent pathway? integrating center? efferent pathway? tissue response?
hypothalamic osmoreceptors; interneurons to hypothalamus; hypothalamic neurons that synthesize vasopressin; vasopressin (released from Post.Pit.); collecting duct epithelium; insertion of water pores in apical membrane
With the stimulus of decreased atrial stretch due to low blood volume in order to increase water reabsorption to conserve water, what is the: receptor? afferent pathway? integrating center? efferent pathway? tissue response?
atrial stretch receptor; sensory neuron to hypothalamus; hypothalamic neurons that synthesize vasopressin; vasopressin (released from Post.Pit.); collecting duct epithelium; insertion of water pores in apical membrane
With the stimulus of decreased BP in order to increase water reabsorption to conserve water, what is the: receptor? afferent pathway? integrating center? efferent pathway? tissue response?
carotid and aortic baroreceptors; sensory neuron to hypothalamus; hypothalamic neurons that synthesize vasopressin; vasopressin (released from Post.Pit.); collecting duct epithelium; insertion of water pores in apical membrane
A scientist monitoring the activity of osmoreceptors notices that infusino of hyperosmotic saline causes increased firing of osmoreceptors. Infusion of hyperosmotic urea (a penetratin solute) had no effect on firing rate. If osmoreceptors fire only when cell volume decreases, explain why hyperosmotic urea did not affect them.
hyperosmotic NaCl is hypertonic and causes osmoreceptors to shrink, but hyperosmotic urea is hypotonic and causes them to swell. Only cell shrinkage causes firing
If vasopressin increases water reabsorption by the blood vessels of a nephron, would vasopressin secretion be increased or decreased with dehydration?
Experiments suggest that there are peripheral osmoreceptors in the lumen of the upper GI tract and in the hepatic portal vein. What is the adaptive significance of osmoreceptors in these locations?
osmoreceptors there would sense high-osmolarity food or drink that has been ingested and absorbed, before it is in the general circulation. This would allow an anticipatory secretion of vasopressin to conserve body water
What is the key to the kidney's ability to produce concentrated urine?
high osmolarity of the meduallary interstitium
What is the medullary interstitium?
interstitial fluid compartment of the kidney
Why is the medullary interstitium's high osmolarity important?
without it, there would be no concentration gradient for osmotic movement of water out of the collecting duct
What is a countercurrent exchange system? What does it allow?
anatomical arrangement of vessels so that flow in one vessel is in the opposite direction from flow in the adjacent vessel; allows the passive transfer of heat or molecules from one vessel to the other
What is a countercurrent multiplier?
anatomical arrangement of the loop of Henle that concentrates solute in the renal medulla
Because the kidney forms a closed system, where do solutes go that leave the lumen? What is this process aided by?
concentrate in the interstitium; active transport of solutes out of the ascending limb, which makes ECF osmolarity even greater
What are the two components of the countercurrent multiplier system in the renal medulla?
loops of Henle that leave the cortex, dip down into the more concentrated environment of the medulla and then ascend up again; the peritubular capillaries known as the vasa recta, which dip down into the medulla and then go back up to the cortex
Three characteristics of the countercurrent exchange in the medulla of the kidney. (hint: more like steps)
filtrate entering the descending limb becomes progressively more concentrated as it loses water; blood in the vasa recta removes water leaving the loop of Henle (it is high concentrated because it is coming from the ascending side); the ascending limb pumps out Na+, K+, Cl-; filtrate becomes hyposmotic
What are the four steps of active reabsorption of ions in the thick ascending limb? What is created?
1200 mOsM entering the ascending loop of Henle, salt reabsorption, water cannot follow solute, 100 mOsM leaving the loop; a dilute filtrate in the lumen
What is the net result of the countercurrent multiplier in the kidney?
to produce hyperosmotic interstitial fluid in the medulla and hyposmotic fluid leaving the loop of Henle
Where is the NKCC symporter? How does it work?
the apical side of the cells of ascending loop of Henle; uses energy stored in the Na+ concentration gradient to transport Na+, K+, and 2 Cl- from the lumen into the epithelial cells of the ascending limb
What kinds of transporters are on the basolateral side of the cells of the ascending loop of Henle?
Cl-, K+ open channels (out); K+/Cl- symporter (out); Na+/K+ ATPase (Na+ out, K+ in)
Explain why patients taking a loop diuretic that inhibits solute reabsorption excrete greater than normal volumes of urine
solutes that remain in the lumen when the NKCC symporter is inhibited force water to reamin in the lumen b/c urine can be concentrated only to 1200 mOsM
Loop diuretics that inhibit the NKCC symporter are sometimes called "potassium-wasting" diuretics. Explain why people who are on loop diuretics must increase their dietary intake of K+.
diuretics that inhibit the NKCC symporter leave K+ in the lumen, where it is likely to be excreted