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89 Cards in this Set

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4 basic process that determine the composition of urine:
1. Filtration
2. Reabsorption
3. Secretion
4. Excretion
Excretion equals:
-Filtered
plus
-Secreted
minus
-Reabsorbed
What happens to most substances at nephrons?
They are freely filtered and extensively reabsorbed by the renal tubules.
How is the daily turnover of water and electrolytes regulated?
By hormones which adjust the rate of tubular reabsorption.
How much glucose is daily
-filtered
-reabsorbed
-excreted
180 g filtered
180 g reabsorbed
0 excreted
What percent of the filtered load of bicarbonate is reabsorbed daily?
99.9%
What percent of the filtered load of sodium is reabsorbed daily?
99.4%
What percent of the filtered load of chloride is reabsorbed daily?
99.1%
What percent of the filtered load of potassium is reabsorbed daily?
87.8%
What percent of the filtered load of urea is reabsorbed daily?
50%
What percent of the filtered load of creatinine is reabsorbed daily?
0
How much creatinine is daily
-filtered
-reabsorbed
-excreted
Filtered = 1.8 g/day
Reabsorbed = none
Excreted = 1.8 g/day
In the beercan model for renal tubule epithelial organization, how are cells connected?
By tight junctions at their apical sides.
What are the 2 alternative routes across the renal tubule epithelium?
1. Across the cells
2. Across the tight junctions
2 things that must occur for a substance to be reabsorbed:
1. Transport across tubular epithelial membranes into renal interstitial fluid
2. Transport through the peritubular capillary membrane into the blood.
What mechanisms allow for transport of water and solutes across tubular epithelial cells?
Passive and active, via transcellular or paracellular routes.
What process allows substances to be transported from the renal interstitium to the peritubular capillary?
Ultrafiltration mediated by starling's forces!
What are the net forces like at the peritubular capillaries?
Like the venous ends of systemic capillaries - net force is for movement of stuff INTO the capillaries.
What sets up the driving force for passive and active transport of solutes across the tubule cells?
Na/K ATPase
By what process is water reabsorbed?
By osmosis - it follows the solutes reabsorbed.
What type of processes occur at
-The transcellular pathway?
-The paracellular pathway?
Transcellular = AT and PT
Paracellular = Passive diffusion only
What is achieved by the Na/K ATPase pump in the basolateral membranes of tubule cells?
1. Sets up a concentration gradient for sodium to enter
2. Sets up a -70 mV electrical gradient that pulls pos charges on sodium in.
What are 3 types of 2ndary active transporters in tubule cells?
1. Na/Glucose transporter
2. Na/Amino acid transporter
3. Sodium/H transporter
Which 2ndary active transporter is counter?
Sodium/H
How are the few proteins that do get into ultrafiltrate get reabsorbed?
Via pinocytosis
What is a Transport Maximum?
The maximum rate at which a substance can be reabsorbed from renal tubules because its transporters are finite in number.
What is the Threshold for excretion?
The amt of a substance in the plasma above which excretion will begin to climb.
What is the overall renal transport max for glucose?
320 mg/min
What gets reabsorbed in the Proximal tubule?
-65% of the filtered load of Na, Cl, HCO3, K, and H2O.
-Almost all glucose and amino acids
What gets secreted in the Proximal tubule?
H+, Organic acids, and bases
What is the tonicity of the proximal tubule lumen?
Isosmotic
How many mechanisms are there for Sodium reabsorption in the proximal tubule?
Three
What are the three mechanisms of Sodium reabsorption in the Proximal Tubule?
1. Na/H antiport
2. Na/Glu and Na/AA symport
3. Passive diffusion with Cl-
How does chloride drive its passive reabsorption w/ Na?
As sodium gets reabsorbed by all its other mechanisms, Cl ends up being left by itself; as its concentration increases then its driving force for diffusion does.
Where exactly do Cl- and Na diffuse into tubule cells?
Across tight junctions in the LATE proximal tubule.
How much Na/Cl and H2O get reabsorbed in the proximal tubule in total?
65%
How much of that 65% is transported by each mechanism?
1/3 by each of the 3 mechanisms.
Is chloride the only thing that gets passively reabsorbed?
No; urea does too.
What happens to the concentration of creatinine and urea as you go along the length of the proximal tubule?
They increase rapidly because they aren't reabsorbed.
What conc increases more; urea or creatinine?
Creatinine - some urea is passively reabsorbed w/ Cl-
What is reabsorbed most completely?
Glu / Amino acids
What is reabsorbed largely but less than Glu / AA?
Bicarb
How is water reabsorbed in the prox tubule?
Via isosomtic reabsorption.
What generates the force for Chloride passive reabsorption?
The reabsorption of sodium bicarb and water, increasing [Cl] in their wake; also increases the neg potential in the lumen, retarding Cl-.
What feature of peritubular capillaries allows bulk flow or reabsorbed substances?
Their hydrostatic and oncotic pressures favor reabsorption.
What is the oncotic pressure of the peritubular capillaries?
What is the hydrostatic pressure of the interstitial space?
32 mm hg
6 mm Hg
Net = 38 mm Hg pulling in.
What is the oncotic pressure of ISF around the peritubular capillaries?
Hydrostatic pressure?
Net?
15 mm Hg
13 mm hg
Net = 28 mm Hg pulling out
What is the sum of Starling's forces at peritub capillaries?
38 - 28 = 10 favoring reabsorption
So what is the net filtration pressure at the peritubular capillaries?
-10 mm Hg -> meaning it favors reabsorption from the renal interstitium!
How are organic anions secreted at the proximal tubule?
How do the organic anions get into the tubule cells at all?
-Via active transporters at the apical membrane of tubule cells.
-Via 2nd ATs in exchange for Na, DCA, and DCA in exch for Na.
6 endogenous organic anions:
CHUBPO
-cAMP
-hippurates
-urate
-bile salts
-prostaglandins
-oxalate
What is an important exogenous organic anion secreted in the proximal tubule?
PAH
How does the kidney handle PAH?
-Freely filtered
-Secreted
-Not reabsorbed
-totally excreted
How are secreted substances similar to reabsorbed substances?
Both have a transport maximum for their transporters
What happens when a secretion transporter reaches its transport maximum?
-Secretion plateaus
-Excretion rate rises less rapidly
-Filtration continues to increase
What is an important endogenous cation secreted in the proximal tubule?
Creatinine
List 4 endogenous cations secreted in the prox tubule:
-Creatinine
-Dopamine
-Epinephrine
-Norepinephrine
Exogenous cations secreted in the prox tubule:
-Atropine
-Isoproterenol
-Cimetidine
-Morphine
-Quinine
-Amiloride
What do we use creatinine as an index for? Why?
GFR - because it is freely filtered almost completely.
Why is it bad that creatinine is slightly secreted?
Measuring creatinine in the urine gives a bit of an overestimate of the GFR.
What part of the renal tubule comes after the proximal tubule?
The thin loop of Henle.
What are the 3 functionally distinct segments of the Loop of Henle?
-Thin descending
-Thin ascending
-Thick ascending
What makes the thin desc and ascending limbs THIN?
-Thin epithelial membranes
-No brush borders
-Few mitochondria
-Minimal levels of metabolic activity
What happens in the thin descending loop of Henle?
Not much - reabsorption of water
What causes the reabsorption of water in the thin descending loop of henle?
The cortical-medullary osmotic gradient, which sets up the gradient for passive osmosis.
What happens in the thin ascending limb of Henle's loop?
It becomes impermeable to water!
What gets reabsorbed at the thin ascending limb of Henle's loop? How?
What else happens here?
-Na and Cl
-via passive diffusion
-Urea gets secreted from blood to lumen to pass to late distal tubule for its cycling.
What happens to the tonicity of the lumen fluid in the ascending thin limb of henle's loop?
It becomes DILUTE as Na/Cl are taken out but water is left behind.
What is reabsorbed in the Thick Ascending limb of Henle's loop?
25% of filtered Sodium
-Na/K/Ca/Mg
How is the 25% of sodium reabsorbed?
Via Na/K/2Cl transport
How are Na/K/Mg/Ca reabsorbed? What is the driving force?
Via paracellular passive diffusion due to the positive potential in the lumen.
What is secreted at the Thick ascending limb of henle's loop?
Via what transporter?
Acid via the Na/H transporter.
What happens to water at the thick ascending limb?
Nothing - it is impermeable so the solution becomes more hyposmotic.
What is the Na/K/2Cl transporter a target of?
Loop diuretics like Lasix and furosemide
What is reabsorbed in the early distal tubule?
Na, Cl, Ca, and Mg
What happens to water at the early distal tubule?

How does Na/Cl reabsorption change?
Nothing; it is impermeable

Na/Cl transporter is now active
What is the Na/Cl transporter at the early distal tubule sensitive to?
Thiazide diuretics
What is the lumen electric potential at:
-Thick ascending limb of henle
-Early distal tubule
Thick asc limb = +8 mV

Early dist tubule = -10 mV
What is the late distal tubule similar to functionally?
The cortical collecting tubule
What are the 2 cell types that compose the late distal tubule and cortical collecting duct?
-Principle cells
-Intercalated cells
What is reabsorbed in the Late distal tubule?
(either always or potentially)
Sodium
Chloride
Water
Bicarb
What is secreted in the late distal tubule?
K+ and H+
What do Principle cells do?
under what control?
Reabsorb sodium
Secrete potassium
-Under the control of Aldosterone
What do Intercalated cells do?
-Reabsorb K (when needed) and Bicarb
-Secrete Acid
What regulates the water permeability of the late distal tubule and cortical collecting duct?
ADH
What blocks the reabsorption of sodium and potassium secretion here?
Aldosterone antagonists
(spironolactones)
What specific transporters do Aldosterone antagonists act on?
The Na/K ATPase pumps on the basolateral membrane.
What blocks the apical Na channels in late distal tubule principle cells?
Amilioride and Triamterene
What happens at the Medullary collecting duct?
-Sodium reabsorption
-ADH-stimulated H2O reabsorption
-Urea reabsorption