Fluid and Electrolyte Balance

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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?

the kidneys

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?

1.5 L/day

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?

GFR

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?

diuresis

What are drugs that promote the excretion of urine?

diuretics

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?

aquaporins

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?

hormones

What does urine osmolarity in the collecting duct depend on?

reabsorption

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?

antidiuretic hormone

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.

impermeable

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?

aquaporin-2 (AQP-2)

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?

present

If vasopressin is suppressed, will urine be dilute or concentrated?

dilute

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?

increased

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

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