ECF Volume and Plasma K+ Quiz Flashcards
Select the TRUE statement regarding the renin-angiotensin-aldosterone system.
Angiotensin II directly stimulates proximal tubular sodium reabsorption.
Angiotensin II has two direct actions on the kidneys, to increase proximal tubular sodium reabsorption and to constrict renal afferent and efferent arterioles. These actions increase sodium and water reabsorption. Renin is secreted from the juxtaglomerular cells in response to low tubular fluid flow rate in the distal tubule. Aldosterone stimulates sodium reabsorption in the late distal tubules and collecting ducts.
A severely dehydrated patient is admitted to the emergency department. Which of the following would NOT be elevated in this patient
Plasma atrial natriuretic peptide (ANP)
The reduction in vascular volume will stimulate sympathetic vasoconstriction and elevate sodium and fluid-retaining systems. Arterial natriuretic peptide is released from cardiac myocytes in response to increased arterial stretch during volume expansion. Thus, during dehydration, circulating ANP will be low.
For which substance is it possible to excrete more than is filtered?
Potassium
Aldosterone secretion by the adrenal gland is stimulated by:
ANG II
Dehydration increases the plasma concentration of all the following hormones except:
Answer
ANP
Renin is secreted by:
granular cells in the juxtaglomerular apparatus.
Potassium handling by the kidneys is affected by all of the following EXCEPT:
plasma antidiuretic hormone.
Plasma antidiuretic hormone has no direct effect on renal potassium handling, while the other conditions cause either secretion (high dietary or plasma potassium and aldosterone) or enhanced reabsorption (low dietary or plasma potassium and acidosis)
Which of the following changes would you expect to find in a patient consuming a high-sodium diet (200 mEq/day) compared with the same patient on a normal-sodium diet (100 mEq/day), assuming steady-state conditions?
Decreased plasma renin activity
Increasing sodium intake would decrease renin secretion and plasma renin activity, as well as reduce plasma aldosterone concentration and increase plasma atrial natriuretic peptide owing to a modest expansion of extracellular fluid volume. Although a high sodium intake would initially increase distal sodium chloride delivery, which would tend to increase potassium excretion, the decrease in aldosterone concentration would offset this effect, resulting in no change in potassium excretion under steady-state conditions. Even very large increases in sodium intake cause only minimal changes in plasma sodium concentration as long as the antidiuretic hormone-thirst mechanisms are fully operative
The most serious hypokalemia would occur in which of the following conditions?
Fourfold increase in aldosterone secretion plus high sodium intake
A large increase in aldosterone secretion combined with a high sodium intake would cause severe hypokalemia. Aldosterone stimulates potassium secretion and causes a shift of potassium from the extracellular fluid into the cells, and a high sodium intake increases the collecting tubular flow rate, which also enhances potassium secretion. In normal persons, potassium intake can be reduced to as low as one-fourth of normal with only a mild decrease in plasma potassium concentration (for further information, see TMP11 Figure 29-7). A low sodium intake would tend to oppose aldosterone’s hypokalemic effect, because a low sodium intake would reduce the collecting tubular flow rate and thus tend to reduce potassium secretion. Patients with Addison’s disease have a deficiency of aldosterone secretion and therefore tend to have hyperkalemia.
When the dietary intake of K+ increases, body K+ balance is maintained by an increase in K+ excretion primarily by which of the following?
Increased K+ secretion by the late distal collecting tubules
Most of the daily variation in potassium excretion is caused by changes in potassium secretion in the late distal tubules and collecting tubules. Therefore, when the dietary intake of potassium increases, the total body balance of potassium is maintained primarily by an increase in potassium secretion in these tubular segments. Increased potassium intake has little effect on glomerular filtration rate or on reabsorption of potassium in the proximal tubule and loop of Henle. Although high potassium intake may cause a slight shift of potassium into the intracellular compartment, a balance between intake and output must be achieved by increasing the excretion of potassium during high potassium intake.
A 46-year-old woman visited her family physician because she was urinating many times a day and was constantly thirsty. She was evaluated in the hospital to find out the cause of her severe polydipsia and polyuria. She was not given any fluids for 6 hours before testing, and no change in urine osmolarity was measured during this period. A nonpressor ADH agonist was then given, which produced a rapid increase in urine osmolarity. Which diagnosis is most likely to account for this patient’s polydipsia and polyuria?
Central diabetes insipidus
Removal of drinking water for 6 hours would increase urine osmolarity in cases of compulsive water drinking or in patients with diabetes mellitus. Restoration of urine concentration with exogenous ADH demonstrates a failure of endogenous ADH secretion and a diagnosis of central diabetes insipidus.
Diabetes insipidus:
results in excretion of large quantities of hypotonic urine.
Diabetes insipidus (DI) is usually central origin (nephrogenic DI is rare), following trauma, disease, or surgery affecting the pituitary gland. Central DI involves the loss of ADH, so water channels are not present in the apical membranes of the collecting ducts, and urine cannot be concentrated. This leads to massive excretion (3 to 18 L/day) of hypotonic urine.
CHALLENGE QUESTION: Which of the following tends to increase potassium secretion by the cortical collecting tubule?
A diuretic that decreases loop of Henle sodium reabsorption (e.g., furosemide)
Potassium secretion by the cortical collecting ducts is stimulated by (1) aldosterone, (2) increased plasma potassium concentration, (3) increased flow rate in the cortical collecting tubules, and (4) alkalosis. Therefore, a diuretic that inhibits aldosterone, decreased plasma potassium concentration, acute acidosis, and low sodium intake would all tend to decrease potassium secretion by the cortical collecting tubules. A diuretic that decreases loop of Henle sodium reabsorption, however, would tend to increase the flow rate in the cortical collecting tubule and therefore stimulate potassium secretion.
