10 - Tubular Reabsorption and Secretion PPT (QUIZ 3) Flashcards
How is urine formed?
Via glomerular filtration and tubular reabsorption/secretion
How many times is our entire plasma volume filtered a day?
60 times!
Define reabsorption.
The retention of substances contained in filtrate back into peritubular capillary blood.
What is Filtration Only?
- NO reabsorption
- Metabolism products like urea and creatinine get filtered out
- Foreign substances like drugs (crack) get filtered out
What is Filtration with Partial Reabsorption?
- Electrolytes like sodium and bicarbonate get reabsorbed easily
- Some partial reabsorption and some secretion (I love that the ppt says “maybe”)
What is Filtration with Complete Reabsorption?
- Nutritional substances like glucose and AAs get completely reabsorbed
What is Secretion?
- NO reabsorption
- Stuff like organic acids gets secreted into tubular acid to become urine
What is the filtration rate of glucose through the kidneys dependent on?
Plasma glucose concentration
Where does glucose reabsorption mainly occur?
Proximal convoluted tubule (PCT)
What are the two steps of glucose reabsorption?
- Glucose moves across the apical membrane into the brush border cells
- Glucose diffuses across the basolateral membrane into the peritubular capillaries
How is glucose moved across the apical membrane?
Sodium/glucose transporters use energy from the existing sodium conc. gradient to move glucose against its conc. gradient
What type of diffusion does glucose utilize to enter the peritubular capillaries?
FACILITATED diffusion with GLUT1 and GLUT2
What happens during normal plasma glucose levels (less than 200mg/dl)?
Glucose reabsorption matches with filtration
What happens during high plasma glucose levels (more than 200mg/dl)?
The limited number of glucose transporter proteins prevents reabsorption from keeping up filtration
What happens during higher (highest) plasma glucose levels (more than 350mg/dl)?
Glucose transport proteins are fully saturated and reabsorption cannot go any faster, thus reaching the TRANSPORT MAXIMUM!
Where is excess glucose excreted?
Da urine
What is the threshold concerning the glucose titration curve?
Threshold: the plasma glucose level at which glucose excretion starts
What is splay concerning the glucose titration curve?
Splay is the initial, nonlinear increase in urine excretion
What’s glycosuria?
Glucose excreted in urine due to:
- Diabetes (decreased insulin, increased plasma glucose)
- Pregnancy (hormonal changes resulting in increased renal blood flow and filtration
What’s PAH?
Para-aminohippuric acid
PAH is an organic acid (destined for urine). How much of PAH gets cleared from the plasma?
90%
(TRUE/FALSE): PAH can be used for diagnostics due to its high clearance rate.
(TRUE)
(TRUE/FALSE): There’s always renal absorption of PAH.
(FALSE): There is NO renal absorption of PAH
Where does PAH secretion occur?
PCT
How does PAH secretion relate with PAH concentration during low plasma PAH?
It’s linear, so if PAH concentration increases so will its secretion.
During higher plasma PAH, what prevents all the PAH from being secreted despite its increasing concentration?
Carrier proteins prevent the increase of secretion, so some PAH will be left behind.
Whether filtered or secreted, the body is unbiased against PAH- where is it excreted?
URINE!
What can PAH be used as a diagnostic for?
Calculating renal plasma flow ([PAH entering]=[PAH leaving])
If you know the renal plasma flow, what can you calculate for?
Renal blood flow (thanks PAH!)
What is urea?
A waste byproduct of AAs
Is urea freely filtered or partially filtered across the glomerular capillaries?
Freely filtered
What does urea recycling help establish?
The corticopapillary gradient, which reabsorbs water from the kidney back into blood
Here we go again… what’re the four steps to urea recycling?
- 50% of urea is reabsorbed by simple diff. in PCT, the rest gets left with water
- Urea from the medullary interstitium gets secreted back into the DCT tubule for a whopping 110% of initial urea present at the bottom of the loop of Henle (because of the higher urea concentration in the medullary intersitium)
- The thick ascending limb of the loop of Henle and early DCT are impermeable to urea/water so urea lvls stay the same
- 70% of initial urea gets reabsorbed into the late DCT interstitium and collecting duct, 40% gets excreted (ADH induces water reabsorption through aquaporin channels)
Sorry for that whopper of a flashcard about urea recycling. Not much left!
Urea contributes to how much of the renal medulla’s osmolarity?
40-50%
ADH being present makes a more ___________ urine because of more ____________________.
- concentrated
- water reabsorption
ADH causes the body to reabsorb more of what in the collecting duct?
UREA
More urea reabsorbed in the collecting duct increases the…
… osmolarity of the medulla.
What does higher medullary osmolarity cause the nephron to do?
Reabsorb more water
