Tubular Secretion & Excretion Flashcards
what is the 4th step of urine production?
tubular secretion
where does tubular secretion occur?
late DCT and cortical region of collecting duct
what is the osmolarity of the late DCT and collecting duct?
300 mmol
what 5 substances are secreted from peritubular capillaries into the filtrate?
H+ ions
ammonium ions
phosphate ions (buffer system)
drugs
toxins
by the end of the DCT, 99% of ____ are reabsorbed
water & dissolved solutes
what is reabsorption of water and dissolved solutes determined by?
combined action of aldosterone and ADH (vasopressin)
where does aldosterone act?
principal cells
describe the mechanism of action of aldosterone
- binds to nucleus
-> increases transcription of Na/K ATP pumps
-> increases Na reabsorption -> increases [Na] in blood, decreasing in urine
-> increases K secretion -> decreases [K] in blood, increasing in urine
what is the other role of aldosterone?
increases bicarbonate buffer system in intercalated cells which increases H+ secretion and HCO3 reabsorption
what effect does aldosterone have on pH of the blood and urine?
decreased urine pH
increased blood pH
where does ADH act?
principal cells
where is ADH made?
supraoptic nuclei of hypothalamus
what stimulates production of ADH?
increased Na concentration in blood
where is ADH released from?
posterior pituitary
where does ADH bind?
anti-luminal surface of cell
what is the mechanism of action of ADH?
- by intercellular signaling, increases transcription of aquaporins which are inserted into membrane
- aquaporins allow reabsorption of water in DCT and collecting duct
what effect does ADH have on sodium concentration in the urine and blood?
decreases in blood (diluted)
increased in urine (less water)
what is the final step of urine production?
excretion
where does excretion occur?
collecting duct
what is the collecting duct permeable to?
water with ADH only
when does the filtrate become urine?
once in the renal pelvis
there is no reabsorption of ____
creatinine
when ADH is present, the collecting duct holds the most ____ filtrate
concentrated (water reabsorbed)
without ADH, the collecting duct holds the most ____ filtrate
diluted (water remains)
the collecting duct is also permeable to small amounts of ____
urea
what is the osmolarity at the end of the collecting duct?
1200
what is the equation for excretion?
filtration - reabsorption (filtered load) - transport max
what would a diuretic that inhibits the action of aldosterone do to potassium secretion by the cortical collecting tubule?
decrease
in normal kidneys, what is the osmolarity of renal tubular fluid that flows through the early DCT in the region of the macula densa?
usually hypotonic compared to plasma
what changes to the following would be expected in a patient with diabetes insipidus due to lack of ADH secretion?
- plasma osmolarity []
- plasma [Na]
- plasma renin
- urine volume
all would increase
when the dietary intake of K+ increases, body K+ balance is maintained by an increase in K+ excretion primarily by…
increased K+ secretion by later DCT and collecting tubule
what are the 2 mechanisms by which inhibition of aldosterone causes hyperkalemia?
- shifts K+ out of cells into ECF
- decreases cortical collecting tubular secretion of K+
what changes would occur to the following with an adrenal tumor that causes excess aldosterone secretion:
- plasma [K+]
- plasma pH
- renin secretion
- BP
- decrease plasma K+
- increase plasma pH
- decrease renin secretion
- increase BP
what effect would excessive ADH secretion have on plasma sodium concentration?
decrease (hyponatremia)
in a patient with severe central diabetes insipidus caused by lack of ADH secretion, which part of the tubule would have the lowest tubular fluid osmolarity?
late DCT and collecting duct
how do intracellular and interstitial body fluids compare in terms of total osmolarity?
same total osmolarity
what is the most likely diagnosis:
- severe polyuria (0.5 L urine/hr)
- polydipsia (drinking 2-3 cups water /hr)
- no glucose in urine
- [Na] = 160mEq/L
- urine osmol = 80mOsm/L
diabetes insipidus (can’t reabsorb water)
with a high potassium diet, which part of the nephron would secrete the most potassium?
collecting tubules
what changes to the collecting duct would you expect when dehydrated for 24 hours?
increased water permeability
what would the osmolarity of fluid in the early distal tubule be in a dehydrated person with normal kidneys and increased ADH levels?
less than 300mOsm/L
what changes to ADH, renin, angiotensin II, and aldosterone would you expect in a severely dehydrated person?
release of ADH & renin stimulates formation of angiotensin II and aldosterone secretion
at which renal tubular sites would the concentration of creatinine be expected to be highest in a normally hydrated person?
collecting duct
