Block 6 Renal Physio Dr Shams W Xpl

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Block 6 Renal Physio Dr Shams W Xpl - Quiz

The purpose of the urinary system is to eliminate waste from the body, regulate blood volume and blood pressure, control levels of electrolytes and metabolites, and regulate blood Ph. This is made possible through the various parts of the system. Take the Block 6 renal physio dr shams w xpl and see how well you know them.


Questions and Answers
  • 1. 

    In which of the following parts of the renal tubule does the lowest concentration of potassium in the tubular fluid occur?

    • A.

      Proximal tubule

    • B.

      Thick ascending limb of Henle's loop

    • C.

      Collecting duct

    • D.

      Distal tubule

    Correct Answer
    B. Thick ascending limb of Henle's loop
    Explanation
    The highest amounts of potassium is absorbed in the proximal tubule ‐ but the concentration
    remains essentially the same as that in plasma. The thick ascending limb of Henle's loop absorbs
    potassium from the tubular fluid and the concentration falls to something like 2 mmol/L by the
    time the fluid reaches the macula densa. The distal tubule and collecting duct both secrete
    potassium and the concentration in the tubular fluid in these segments is higher than in plasma.
    For further reading about tubular handling of K+ see Vander's renal Physiology

    Rate this question:

  • 2. 

    In which of the following parts of the renal tubule does the lowest concentration of sodium (Na+) in the tubular fluid occur?

    • A.

      Proximal tubule

    • B.

      Thick ascending limb of Henle's loop

    • C.

      Collecting duct

    • D.

      Distal tubule

    Correct Answer
    C. Collecting duct
    Explanation
    Na+ is absorbed out of the lumen in most parts of the nephron. In the proximal tubule, the
    concentration remains the same as in plasma. Na+ concentration in the tubular fluid is reduced
    throughout the distal nephron, because it is absorbed. But the lowest concentration is reached
    in the papillary collecting duct due to amiloride ‐ sensitive absorption through ENaC.
    For further reading around tubular handling of Na+ see Vander's Renal Physiology.

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  • 3. 

    Which of the following parts of the renal tubule is associated with regulated changes in water permeability?

    • A.

      Proximal tubule

    • B.

      Thick ascending limb of Henle's loop

    • C.

      Collecting duct

    • D.

      Distal tubule

    Correct Answer
    C. Collecting duct
    Explanation
    The water permeability of the nephron is relatively invariant except in the collecting duct. Only
    in the collecting duct are there receptors (V2 subtype) for ADH (antidiuretic hormone) which are
    coupled to adenylate cyclase ‐ sensitive membrane expression of aquaporin 2 (AQP2). Insertion
    of AQP2 into the apical plasma membrane of principal cells allows equilibration of water with
    the medullary interstitiium and therefore concentration of the tubular fluid. As cAMP is
    degraded by intracellular phosphodiesterases, AQP2 is retrieved into the cell and the water
    permeability falls. Water equilibration with the interstitium is reduced and the urine becomes
    more dilute.
    For further reading see Vander's Renal Physiology and/or Mollina's Endocrine Physiology

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  • 4. 

    Which of the following parts of the renal tubule is associated with the highest permeability to urea?

    • A.

      Proximal tubule

    • B.

      Thick ascending limb of Henle's loop

    • C.

      Collecting duct

    • D.

      Distal tubule

    Correct Answer
    A. Proximal tubule
    Explanation
    Around 50% of the filtered urea is reabsorbed in the proximal tubule by passive absorption. The
    concentration of urea rises along the proximal tubule due to water absorption ‐ it is the
    concentration gradient from lumen to plasma that provides the driving force for mostly
    paracellular urea absorption in the proximal tubule which delivers fluid to the loop of Henle.
    Medullary interstitial urea concentration is higher than in the lumen of the thin loops of Henle,
    so urea is secreted into the lumen by facilitated diffusion ‐ (the thin loops express urea
    transporters for this purpose) and the concentration in the tubular fluid rises further. From the
    thick ascending limb onwards, the urea permeability of the distal tubule and cortical collecting
    duct is close to zero. As water is reabsorbed in the collecting duct system, the concentration of
    urea continues to rise and may become 40 ‐ 50 times that in plasma! In the inner medullary
    collecting duct, urea is reabsorbed by facilitated diffusion driven by the high urea concentration
    in the lumen. It is this process that provides the driving force for diffusion of urea into the loop
    of Henle. In the presence of high circulating levels of ADH water absorption in the cortical and
    outer medullary portions of the collecting duct is increased ‐ which further increases the luminal
    concentration of urea. ADH also increases the urea permeability of the inner medullary
    collecting duct ‐ allowing more urea to enter the inner medullary interstitium. This increases the
    osmotic pressure of the interstitium and therefore the concentrating ability of the kidney as a
    whole.
    For further reading on the role of urea and its renal handling see Vander's Renal Physiology

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  • 5. 

    Which of the following parts of the renal tubule is associated with most potassium reabsorption?

    • A.

      Proximal tubule

    • B.

      Thick ascending limb of Henle's loop

    • C.

      Collecting duct

    • D.

      Distal tubule

    Correct Answer
    A. Proximal tubule
    Explanation
    Most potassium reabsorption occurs in the proximal tubule, mostly in the late proximal tubule
    and is driven by passive diffusion due to the small positive luminal potential difference (which is
    positive), and solvent drag.
    For further reading on renal handling of potassium see Vander's Renal Physiology

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  • 6. 

    Which of the following parts of the renal tubule is associated with most sodium reabsorption?

    • A.

      Proximal tubule

    • B.

      Thick ascending limb of Henle's loop

    • C.

      Collecting duct

    • D.

      Distal tubule

    Correct Answer
    A. Proximal tubule
    Explanation
    Most sodium (65 ‐ 70% of filtered load) reabsorption occurs in the proximal tubule. This
    reabsorption is coupled to the reabsorption of filtered bicarbonate, glucose and amino acids in
    the early proximal tubule and chloride in the later parts of the proximal tubule.
    For further reading around tubular handling of Na+ see Vander's Renal Physiology

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  • 7. 

    A male volunteer aged 48 years and weight 154 pounds (70 kg) is given an intravenous infusion of PAH (para‐amino‐ hippuric acid) and inulin that achieved constant plasma levels of both. 100 mL of urine was collected over a period of 2 hours. His hematocrit was 40%. His laboratory values for urine and plasma are shown in the table. Which of the following rates of glomerular filtration (GFR) most closely approximate his true rate of filtration?   Plasma conc [mmol/L] Urine conc [mmol/L] PAH 0.05 30 Inulin 0.05 6

    • A.

      GFR = 90 mL/min

    • B.

      GFR = 50 mL/min

    • C.

      GFR = 100 mL/min

    • D.

      GFR = 500 mL/min

    Correct Answer
    C. GFR = 100 mL/min
    Explanation
    The clearance (volume of plasma cleared of a substance in a given time) is calculated by the
    equation GFR = U X V / P, where U = urine concentration; V = urine flow (volume/unit time) and
    P = plasma concentration. Clearance is usually expressed as mL/min, so his urine flow is
    calculated as 100/2/60 = 0.8 mL/min; Once filtered, inulin is effectively inert, so it's clearance is
    equal to the rate of filtration. His plasma inulin concentration was 0.05 mmol/L and his urine
    inulin concentration was 6 mmol/L. This gives 6 X 0.8 / 0.05 = 96 mL/min ‐ the closest value to
    100 mL/min.

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  • 8. 

    In the same volunteer described in the last question, with a 100 mL of urine being collected over a period of 2 hours and the lab values presented in the table ‐ what is the percentage of filtered water being reabsorbed?   Plasma conc [mmol/L] Urine conc [mmol/L] PAH 0.05 30 Inulin 0.05 6

    • A.

      90%

    • B.

      95%

    • C.

      99%

    • D.

      99.2%

    • E.

      99.5%

    Correct Answer
    D. 99.2%
    Explanation
    If GFR (from the clearance of inulin) is approximately 100 mL/min and he is producing 100/120 =
    0.825 mL of urine, then 99.2 mL or 99.2% of the filtered water is reabsorbed.
    Feedback when incorrect: You did not select the correct response. If GFR (from the clearance of
    inulin) is approximately 100 mL/min and he is producing 100/120 = 0.825 mL of urine, then 99.2
    mL or 99.2% of the filtered water is reabsorbed.

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  • 9. 

    A 48‐year‐old woman presents to her doctor with persistent cough. She reports feeling constantly tired and weak and complains of loss of appetite. She reveals a life‐long smoking habit of 1 ‐ 2 packs per week. Her blood pressure is at the high end of normal, other vital signs are normal, but a chest X‐ray reveals a lower left lobe mass. Her laboratory results show the following results (normal ranges in parentheses): Plasma Na+              = 125 mEq/L (136 ‐ 145 mEq/L) Plasma K+                 = 3.9 mEq/L (3.5 ‐ 5.0 mEq/L) Plasma HCO3‐          = 24 mEq/L (22‐28 mEq/L) Plasma osmolality    = 253 mOsmol /Kg H2O (275 ‐ 295 mOsmol /Kg H2O) Urine Na+                   = 48 (>25 mEq/L) Urine osmolality        = 280 mOsm/Kg H2O ( 50 ‐ 1400 mOsmol/Kg H2O)   What hormonal mechanism is most likely responsible for her abnormal laboratory values?

    • A.

      Elevation of adrenal cortical cAMP levels leading to increased production and secretion of corticosteroids

    • B.

      Elevated hypothalamic cAMP levels resulting in inhibition of the thirst mechanism

    • C.

      Activation of both V2 receptors causing vasoconstriction and V1 receptors leading to urine concentration

    • D.

      Activation of V2 receptors leading to increased water absorption in the collecting duct and activation of V1 receptors causing decreased peripheral resistance

    • E.

      Activation of V2 receptors leading to increased water absorption in the collecting duct and activation of V1 receptors causing increased peripheral resistance

    Correct Answer
    E. Activation of V2 receptors leading to increased water absorption in the collecting duct and activation of V1 receptors causing increased peripheral resistance
    Explanation
    This patient's symptoms are consistent with her having an ADH‐secreting small cell carcinoma
    leading to inappropriate ADH secretion (SIADH). This condition is most often seen with
    neurologic disease, malignancy and after major surgery. The persistent secretion of ADH results
    in a gradual reduction in plasma Na+ level. You should realize that the reduction in plasma Na+
    depends on both the severity of the concentrating defect and the level of water intake.
    Increased ADH does not cause hyponatremia in the absence of a source of water for intake. In
    patients suffering from malignancy, reduced appetite may limit the amount of fluid intake and
    their plasma Na+ may be in the normal range on initial presentation. Only when fluids are
    administered does hyponatremia manifest itself.
    ADH acts through V2 receptors in the renal collecting duct to promote cAMP ‐ dependent
    insertion of Aquaporin 2 water channels into the apical membrane of principal cells. The
    increased water permeability of the collecting duct promotes water absorption and excretion of
    a concentrated urine. This retention of free water leads to hyponatremia and decreased plasma
    osmolality without alterations in acid ‐ base status or changes in plasma potassium. Activation of
    vascular V1 receptors activates the Gq ‐ protein signaling cascade causing contraction of
    vascular smooth muscle and increased total peripheral resistance.
    ACTH may also be secreted by small cell carcinomas but would produce symptoms more like
    Cushing's syndrome (hypertension, weight gain, hyperglycemia, buffalo hump, etc) rather than
    hyponatremia.
    Release of ADH is associated with an increase not a decrease in the thirst mechanism.
    More information on SIADH can be found here in "Quick Answers"
    A useful summary and comparision of high and low ADH levels can be found in Greenspan's
    Basic & Clinical Endocrinology

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  • 10. 

    Calcium undergoes absorption in more than one part of the nephron. What is the primary driving force for its absorption in the thick ascending limb of Henle's loop?

    • A.

      Activity of the loop diuretic ‐ sensitive Na+‐K+‐2Cl‐ triporter, NKCC2

    • B.

      Basal action of parathyroid hormone PTH

    • C.

      The maintenance of a lumen negative electrical potential difference

    • D.

      Na+/Ca+ exchange

    • E.

      Activity of the thiazide‐sensitive Na+ ‐ Cl‐ cotransporter NCCT

    Correct Answer
    A. Activity of the loop diuretic ‐ sensitive Na+‐K+‐2Cl‐ triporter, NKCC2
    Explanation
    Ca2+ ‐ions are absorbed in three regions of the nephron. The proximal tubule, the thick
    ascending limb and the distal tubule. In the thick ascending limb, Ca2+ reabsorption is
    dependent on the activity of NKCC2. The reason for this is that the coordinated activity of
    NKCC2 with K+ secretion through ROMK channels and Cl‐ exit through the basolateral
    membrane results in the generation of a lumen positive potential which is the driving force for
    paracellular absorption of cations ‐ including Ca2+. NCCT is not expressed in the thick ascending
    limb but in the distal tubule. Here, absorption occurs through Ca2+ channels. Absorption
    through these channels is indirectly coupled to Na+ absorption and is stimulated by PTH, in
    contrast to the proximal tubule where PTH inhibits Ca2+ reabsorption. PTH has no direct action
    in the thick ascending limb.
    Further information about calcium handling in the DCT is available in Vander's Renal Physiology

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  • 11. 

    In a laboratory study, a number of novel drug compounds are being investigated for their cardiovascular actions. One compound is identified that when injected intravenously causes increased renal blood flow. Through which of the following mechanisms is the drug likely to be acting?

    • A.

      Stimulation of only alpha adrenergic receptors in the afferent arteriole

    • B.

      Stimulation of only angiotensin II receptors in the afferent arteriole

    • C.

      Stimulation of angiotensin II receptors and activation of stretch activated Ca2+ channels in vascular smooth muscle

    • D.

      Stimulation of both renal alpha adrenergic receptors as well as angiotensin II receptors

    • E.

      Stimulation of renal dopamine and bradykinin receptors

    Correct Answer
    E. Stimulation of renal dopamine and bradykinin receptors
    Explanation
    Renal blood flow (Q) like that of other vascular beds is determined by the change in pressure
    and resistance (Q = ΔP.R). In the kidney resistance is provided by the afferent and efferent
    arterioles. The resistance of both of the resistance vessels is modulated by a variety of
    vasoactive substances. Dopamine and bradykinin are both vasodilator peptides. Dilation of the
    afferent and efferent arterioles reduces R and therefore blood flow increases.
    Stimulation of α ‐ adrenergic receptors in the afferent arteriole would constrict these vessels.
    This constriction would increase R, and reduce blood flow not increase it. These receptors are
    expressed in both the afferent and efferent arterioles, though the afferent arteriole has the
    highest levels of expression. Similarly, stimulation of AT1 receptors in the afferent arteriole (as
    well as in the efferent arteriole) would also cause constriction of both vessels and reduce blood
    flow.
    Increased activity of stretch activated Ca2+ channels (by whatever mechanism) causes an
    increase in intracellular Ca2+ ions and a contraction of vascular smooth muscle. This contraction
    would lead to vasoconstriction, an increase in R and a reduction in blood flow.
    A brief description of the control of renal blood flow is available in Clinical Anesthesiology and
    discussion of the renal effects of dopamine can be found in UpToDate

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  • 12. 

    Diuretics are drugs that are used to increase urine flow. Their effectiveness and effects of individual ion excretion depends on their renal site of action. Which of the following types of diuretic can cause a reduction in renal excretion of Ca2+ and therefore may be effective in reducing renal stone formation?

    • A.

      A carbonic anhydrase inhibitor

    • B.

      An inhibitor of ENaC

    • C.

      An inhibitor of Na+‐K+‐2Cl‐ cotransport (NKCC2)

    • D.

      An inhibitor of Na+‐Cl‐ cotransport (NCCT)

    • E.

      An inhibitor of the mineralocorticoid receptor

    Correct Answer
    D. An inhibitor of Na+‐Cl‐ cotransport (NCCT)
    Explanation
    Inhibition of Na+‐Cl‐ cotransport (by NCCT) by thiazide diuretics in the distal convoluted tubule
    inhibits reabsorption of NaCl which is the basis of its diuretic effect, but also reduces the
    reabsorption of Ca2+ . A reduction in intracellular Na+ increases the driving force for Ca2+ exit
    across the basolateral membrane via a Na+/Ca2+ exchanger in distal tubule cells, thereby
    increasing Ca2+ absorption into the blood.
    Inhibition of carbonic anhydrase by drugs such as acetazolamide inhibits reabsorption of
    bicarbonate, but this has little effect on calcium excretion.
    Inhibition of salt transport through NKCC2 in the loop of Henle by compounds such as
    Furosemide (Lasix®) also inhibits calcium reabsorption. This is because transcellular movement
    of salt generates a lumen positive potential difference that drives the paracellular absorption of
    cations. Reduction of this potential difference by loop diuretics leads to loss of divalent cations
    in the urine, and this may lead to hypocalcemia (and hypomagnesemia).
    Drugs such as Amiloride inhibit sodium reabsorption through ENaC, in the late distal tubule and
    collecting duct system. This causes a reduction in excretion of potassium (hence the name
    potassium‐sparing) but has little effect on calcium excretion. Likewise, mineralocorticoid
    antagonists such as spironolactone act as potassium sparing diuretics because the activity of
    ENaC is dependent on circulating levels of aldosterone.
    For further reading around tubular transport mechanisms see Vander's renal physiology
    For further reading about diuretics and their actions see Goodman & Gillman

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  • 13. 

    A group of healthy volunteers embark on a study in which their dietary intake of sodium chloride is doubled over a period of one month. At the end of this period the average blood pressure of the group was found to be raised when compared to the control period before they started increasing their salt intake. What is the most likely physiological response to the raised salt intake?

    • A.

      Decreased release of Atrial Natriuretic Peptide (ANP) and dilation of afferent arterioles

    • B.

      Afferent arteriolar dilation caused by decreased sympathetic nerve activity

    • C.

      Increased ANP release and constriction of afferent arterioles

    • D.

      Increased release of aldosterone

    • E.

      Increased plasma oncotic pressure

    Correct Answer
    B. Afferent arteriolar dilation caused by decreased sympathetic nerve activity
    Explanation
    Increased salt intake leads to volume expansion of the extracellular fluid and resultant increased
    stretch in the walls of the afferent arteriole (as well as other baroreceptors elsewhere in the
    body). The baroreceptor function of the afferent arteriole results in decreased sympathetic
    nerve activity which produces vasodilation of glomerular afferent arterioles. This increases
    glomerular filtration rate by increasing glomerular capillary hydrostatic pressure. The proximal
    tubule receives direct innervation from the sympathetic nerves, which normally stimulate Na+
    reabsorption. The decreased sympathetic nerve activity resulting from the baroreceptor reflex
    reduces reabsorption of NaCl and water by the proximal tubule.
    ANP is release is increased (not decreased) by volume expansion due to the increased venous
    return that occurs on volume expansion. However, ANP causes dilatation of the afferent
    arteriole. Not constriction.
    In the volume expanded state, delivery of NaCl and water to the macula densa is increased. Via
    the tubuloglomerular feedback system, this results in suppression of renin release by the
    juxtaglomerular apparatus and reduced circulating aldosterone.
    Expansion of the plasma volume will dilute plasma proteins, so oncotic pressure will fall not
    increase.
    Further reading on the response to salt loading and the control of body fluid volume can be
    found in Vander's Renal Physiology

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  • 14. 

    A 70‐year‐old male recovering from hip‐replacement surgery is given an NSAID for pain management. 48 hours later, his urine production has decreased markedly and his serum blood urea nitrogen and creatinine levels have risen to 64 mg/dL and 2.5 mg/dL respectively. Through which of the following mechanisms is the NSAID causing acute renal failure?

    • A.

      Inhibition of renal prostaglandin production causing afferent arteriolar constriction

    • B.

      Inhibition of renal prostaglandin production causing efferent arteriolar constriction

    • C.

      Inhibition of renal prostaglandin production causing dilation of efferent arterioles

    • D.

      Stimulation of renal prostaglandin production and reflex arteriolar vasoconstriction

    • E.

      Stimulation of renal prostaglandin production and reflex arteriolar dilatation

    Correct Answer
    A. Inhibition of renal prostaglandin production causing afferent arteriolar constriction
    Explanation
    This patient is in acute renal failure brought about by the NSAID. Synthesis of renal
    prostaglandins produces a vasodilatory effect on the afferent arterioles that counteract the
    vasoconstrictor effects of angiotensin II. Inhibition of prostaglandin synthesis compromises the
    ability of the afferent arteriole to dilate, inadequate vasodilation of the afferent arteriole results
    in a reduction of glomerular capillary hydrostatic pressure, thus reducing GFR, which causes a
    reduction in urine production and consequently raised serum levels of creatinine and BUN.
    Constriction of the efferent arteriole would increase GFR not decrease it and inhibition of
    synthesis would not dilate the efferent arteriole but constrict the efferent arteriole. The action
    of prostaglandins is preferentially on the afferent arteriole. NSAIDs inhibit the production of
    prostaglandins not stimulate it.
    For more information on the role of prostaglandins in the kidney and NSAIDS and acute renal
    failure See Current Diagnosis & Treatment: Nephrology & Hypertension

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  • 15. 

    As part of an annual health check on a 48‐year‐old woman, a 24‐hour urine test revealed an albumin excretion of 100 mg (normally <20 mg). Which cell type is likely to be damaged or is malfunctioning in this individual.

    • A.

      Glomerular endothelial cell

    • B.

      Juxtaglomerular cell

    • C.

      Mesangial cell

    • D.

      Podocyte

    Correct Answer
    D. Podocyte
    Explanation
    The major barrier to filtration of proteins resides partly in the basement membrane and the
    space between the foot processes of podocytes.

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  • 16. 

    A 38‐year‐old male complains of fatigue and dizziness on standing up. Lab results show that his urine is positive for glucose and there is excessive renal loss of bicarbonate and phosphate. Abnormal function in which part of the nephron is likely?

    • A.

      Proximal tubule

    • B.

      Loop of Henle

    • C.

      Distal tubule

    • D.

      Collecting duct

    Correct Answer
    A. Proximal tubule
    Explanation
    The presence of high urinary glucose, bicarbonate and phosphate suggest dysfunction of the
    proximal tubule. This segment is responsible for reabsorption of all filtered glucose, 95% of
    filtered phosphate and 80% of filtered bicarbonate.
    Feedback when incorrect: You did not select the correct response. The presence of high urinary
    glucose, bicarbonate and phosphate suggest dysfunction of the proximal tubule. This segment is
    responsible for reabsorption of all filtered glucose, 95% of filtered phosphate and 80% of filtered
    bicarbonate.

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  • 17. 

    A novel drug is infused into healthy volunteers and is found to decrease creatinine clearance, increase the clearance of p‐aminohippurate, but not alter mean blood pressure. Calculated filtration fraction fell. By what mechanism was creatinine clearance likely reduced?

    • A.

      Constriction of the afferent arteriole

    • B.

      Dilation of the afferent arteriole

    • C.

      Constriction of the efferent arteriole

    • D.

      Dilation of the efferent arteriole

    Correct Answer
    D. Dilation of the efferent arteriole
    Explanation
    Increased renal plasma flow (clearance of PAH) without a change in blood pressure indicates a
    decrease in renal vascular resistance. Dilation of the efferent arteriole would increase
    glomerular capillary outflow, thus reducing capillary hydrostatic pressure which causes GFR
    (creatinine clearance) to decrease. A reduced GFR with an increase in plasma flow causes the fall
    in filtration fraction, since FF=GFR/RPF.
    • Dilatation of the afferent arteriole, would increase capillary hydrostatic pressure and
    therefore GFR and RPF.
    • Constriction of the afferent arteriole would reduce hydrostatic pressure, GFR and RPF.
    • Constriction of the efferent arteriole would reduce plasma flow, but increase capillary
    hydrostatic pressure and therefore GFR.

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  • 18. 

    In a patient suffering from pulmonary edema, there is a need to increase urine flow as rapidly as possible to remove fluids and relieve the edema. This is accomplished by the use of a diuretic ‐ a drug that inhibits the reabsorptive function in different parts of the nephron depending on the type of diuretic administered. Inhibition of the function of which nephron segment would bring about the most powerful diuretic effect?

    • A.

      Proximal tubule

    • B.

      Loop of Henle

    • C.

      Distal tubule

    • D.

      Collecting duct

    Correct Answer
    B. Loop of Henle
    Explanation
    The major classes of diuretics act in the proximal tubule (carbonic anhydrase inhibitors); thick
    ascending limb (loop diuretics); the distal tubule (thiazides) or collecting duct (potassium sparing
    diuretics). They all reversibly inhibit sodium entry mechanisms ‐ specific to that particular
    nephron segment.
    The relative potency of different classes of diuretics depends on the quantity of filtered sodium
    reabsorbed at a particular site as well as the capacity of more distal sites to deal with the
    increased delivery of salt and water.
    The proximal tubule absorbs as much as 67% of the filtered load of NaCl. The loop of Henle 30 ‐
    35%; the distal tubule around 5% and the collecting duct about 4%.
    Intuitively, since the proximal tubule is the site of the greatest sodium reabsorption, it might be
    expected that inhibition of sodium bicarbonate ‐ dependent fluid absorption would be the most
    potent. However, this is not the case. The reason for this is that the more distal segments
    (particularly the loop of Henle, but also the distal tubule and collecting duct absorb more
    sodium in response to increased flow. As a result of flow‐ dependent increased reabsorption in
    these distal segments, the diuretic action of carbonic anhydrase inhibitors is relatively weak.
    The most potent diuretics are those that inhibit salt transport from the loop of Henle, which
    reabsorbs the second greatest fraction of the filtered load of Na and water. The ability of the
    distal tubule and collecting duct to cope with the increased load resulting from loop inhibition is
    limited since these segments have only limited sodium‐absorbing capacity. Inhibition of salt
    transport in the thick ascending limb ‐ also leads to dissipation of the medullary concentration
    gradient, vital to the urinary concentrating mechanism.

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  • 19. 

    An increase in blood volume brings about a natriuresis and diuresis, mediated in part by alterations in proximal tubule reabsorption. What are the mechanisms that produce this response?

    • A.

      Increased baroreceptor firing : decreased sympathetic tone: decreased peripheral resistance: decreased proximal tubule absorption

    • B.

      Increased baroreceptor firing : Increased sympathetic tone: decreased peripheral resistance: decreased proximal tubule absorption

    • C.

      Decreased baroreceptor firing : decreased sympathetic tone: decreased peripheral resistance: decreased proximal tubule absorption

    • D.

      Decreased baroreceptor firing : decreased sympathetic tone: increased peripheral resistance: increased proximal tubule absorption

    Correct Answer
    A. Increased baroreceptor firing : decreased sympathetic tone: decreased peripheral resistance: decreased proximal tubule absorption
    Explanation
    An increased blood volume will increase the activity of both high and low pressure
    baroreceptors, that brings about a decrease in sympathetic tone. Decreased activity of the renal
    nerve supply to the kidney decreases arterial resistance and therefore GFR. Simultaneously,
    reduced catecholamine release reduces proximal tubule reabsorption of sodium, chloride,
    bicarbonate and water. This in combination with increased release of natriuretic peptides brings
    about a rise in excretion of salt and water.

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  • 20. 

    Infants have a renal concentrating ability of about 600 mosmol/kg/H2O ‐ only about half that of an adult. In a three‐month‐old infant with this concentrating ability, and a typical 24 hour solute load of 150 mosmol, what is the minimum urine volume per 24 hours that is required to excrete this solute load?

    • A.

      25

    • B.

      50

    • C.

      100

    • D.

      125

    • E.

      250

    • F.

      500

    Correct Answer
    E. 250
    Explanation
    The calculation is straightforward. The solute load being 150 mosmol and the maximum
    concentrating ability 600 mosmol/kg H2O. This solute output will require a minimum urine
    volume of 150/600 = 0.25 kg H2O per 24 hour = 250 mL. This is about 50% of the daily water
    intake of an infant of this age, so only modest reductions in fluid intake or increases in solute
    excretion can lead to dehydration in infants.

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  • 21. 

    A consequence of inhibition of salt transport in the thick ascending limb of Henle's loop is increased potassium excretion ‐ which can result in hypokalemia. What is the main mechanism underlying this potassium wasting?

    • A.

      Increased secretion of potassium in the thick acending limb

    • B.

      Decreased potassium absorption in the proximal tubule

    • C.

      Decreased aldosterone ‐ stimulated potassium secretion in the distal tubule

    • D.

      Increased flow ‐ dependent potassium secretion in the distal tubule and collecting duct

    • E.

      Decreased ADH ‐ stimulated potassium secretion

    Correct Answer
    D. Increased flow ‐ dependent potassium secretion in the distal tubule and collecting duct
    Explanation
    Potassium secretion in the distal tubule and collecting duct is flow dependent. Inhibition of salt
    transport in the thick ascending limb reduces medullary osmolarity and increases fluid delivery
    to the distal tubule and collecting duct. This reduces luminal K+ concentration, thus increasing
    the driving force for potassium secretion from Principal cells. The increased delivery of sodium
    to these nephron segments stimulates sodium reabsorption which raises the driving force for
    potassium secretion increasing potassium loss.
    Inhibition of salt transport in by whatever mechanism decreases not increases secretion in the
    thick ascending limb, since the function of the transporters and channels are functionally linked
    through the respective ionic driving forces.
    Because inhibition of salt transport in the thick ascending limb leads to a decrease in
    extracellular fluid volume, proximal tubule reabsorption increases. Aldosterone and ADH levels
    will also be increased. Aldosterone and ADH stimulate potassium secretion in the distal tubule
    and collecting duct which exacerbates the potassium loss.

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  • 22. 

    As part of a class experiment, you ingest 1L of a salt solution which has an osmolarity of 290 mOsmol/L H2O. What changes in Starling forces occurs at the renal peritubular capillaries in response to this ingestion?

    • A.

      Increased hydrostatic pressure and reduced oncotic pressure

    • B.

      Increased hydrostatic pressure and increased oncotic pressure

    • C.

      Decreased hydrostatic pressure and decreased oncotic pressure

    • D.

      Decreased hydrostatic pressure and increased oncotic pressure

    Correct Answer
    A. Increased hydrostatic pressure and reduced oncotic pressure
    Explanation
    Ingesting 1L of isotonic saline increases the volume of the ECF which includes the plasma
    volume. This results in an increase in plasma volume. The largest fraction of the plasma volume
    is in the systemic veins, so increased venous pressure raises capillary hydrostatic pressure
    including that in the peritubular capillaries. This dilution of the plasma proteins reduces the
    concentration of plasma proteins, thereby decreasing oncotic pressure. These changes in
    Starling's forces suppress proximal tubular fluid reabsorption and promote renal salt and water
    excretion.

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  • 23. 

    A patient being evaluated after an episode of renal colic is found to have kidney stones. He is diagnosed as suffering from primary hyperparathyroidism. Compared to a normal individual what is his fractional excretion of calcium likely to be? Correct Choice

    • A.

      Higher

    • B.

      Lower

    • C.

      Unchanged

    Correct Answer
    B. Lower
    Explanation
    The majority of the filtered load of calcium is reabsorbed in the proximal tubule. Parathyroid
    hormone inhibits calcium reabsorption in the proximal tubule, leading to increased delivery to
    more distal nephron segments. These more distal segments then increase their reabsorption of
    calcium via the glomerulotubular balance mechanism. Also, in contrast to its effects in the
    proximal tubule, PTH stimulates calcium reabsorption in the distal tubule. This effect, combined
    with the glomerular tubular balance mechanism reduces fractional excretion of calcium.

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  • 24. 

    Preferential activation of afferent arteriolar alpha adrenoceptors will have which of the effects on renal blood flow (RBF), GFR (GFR) and filtration fraction (FF)? ..........(RBF)______________(GFR)_________________(FF)  

    • A.

      ..NO CHANGE.............DEC .............DEC

    • B.

      ..DEC .............. DEC ............. NO CHANGE

    • C.

      ..DEC .............. NO CHANGE ........ DEC

    • D.

      ..INC ............. NO CHANGE .........NO CHANGE

    • E.

      ..INC ............. INC .............. NO CHANGE

    Correct Answer
    B. ..DEC .............. DEC ............. NO CHANGE
    Explanation
    Constriction of the afferent arteriole increases vascular resistance and reduces blood flow. Flow
    into the glomerular capillaries therefore decreases resulting in a reduction of capillary
    hydrostatic pressure, and therefore reduced GFR. However since both GFR and RBF change
    proportionately ‐ FF remains unchanged.

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  • 25. 

    • A.

      A

    • B.

      B

    • C.

      C

    • D.

      D

    • E.

      E

    Correct Answer
    E. E
  • 26. 

    • A.

      A

    • B.

      B

    • C.

      C

    • D.

      D

    • E.

      E

    • F.

      F

    • G.

      G

    • H.

      H

    • I.

      I

    • J.

      J

    Correct Answer
    B. B

Quiz Review Timeline +

Our quizzes are rigorously reviewed, monitored and continuously updated by our expert board to maintain accuracy, relevance, and timeliness.

  • Current Version
  • Mar 21, 2023
    Quiz Edited by
    ProProfs Editorial Team
  • Mar 18, 2012
    Quiz Created by
    Chachelly
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