Tubular Absorption and Secretion Flashcards

1
Q

What parts of excretion is tightly controlled? What has no control?

A

Filtration has no regulation.

Absorption and secretion are tightly regulated

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2
Q

Which molecules have 100 or nearlt 100% filtered load reabsorbed?

A

Glucose

Bicarb

Sodium

Chloride

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3
Q

What molecule has 50% filtered load reabsorbed?

A

Urea

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4
Q

What molecule has 0% filtered load reabsorbed?

A

Creatinine

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5
Q

Where does the majority of reabsorption happen?

A
  • Majority reabsorption in proximal tubule (60%)
  • Little in loop of henle and distal tubule
  • In general, of the total filtrate coming into the nephrons, the proximal tubule reabsorbs:
  • ▪ 65% to 70% of the Na + and H 2 O

▪ 80% to 85% of the K +

▪ 65% of the Cl −

▪ 75% to 80% of the phosphate

▪ 100% of the glucose

▪ 100% of the amino acids

After this bulk reabsorption, “fine-tuning” of the reabsorption of the remaining tubular fluid occurs in subsequent segments of the nephron.

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6
Q

What are the 2 different pathways that the nephron absorbs substances?

A

Transcellular- most common way.

Paracellular- pulling from between cells. Junctions in nephron very tight, so not a lot of paracellular movement

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7
Q

What is active transport?

A

Using ATP to move molecules against their concentration gradient

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8
Q

What is secondary active transport

A
  • Secondary active transport is how we absorb everything in kidney
  • Everything uses secondary active transport except reabsorbing bicarb
  • Uses energy of created concentration gradient to move other ion (sometimes the ion being brought in is going against concentration gradient)
    • i.e. Na/K ATPase creates gradient. Decreases Na inside cell
    • Now sodium can come into cell, bringing glucose, which glucose can be coming in against its concentration gradient
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9
Q

Where is countertransport used in kidney?

A

To secrete substances into nephron

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10
Q

What is exception to rule for absorption in kidney?

A

Bring in bicarb with counter transport (typically we secrete with counter transport)

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11
Q

What is the general location for transporters in cellular membrane of kidney?

A
  • High [Na] and [Glucose] in tubule,
  • Apical membrane
    • Bunch of Na/Glucose symporter channels
  • Create [Na] gradient by Na/K ATPase pump on basolateral membrane
  • Allows us to move Na and glucose down created concentration gradient, into cell
  • GLUT-2 transporter on basolateral membrane to facilitate with diffusion into capillary
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12
Q

What is Tmax?

A

Transport max.

  • Maximum amount of solute transport can handle. Property of the channel
  • After Tmax, will begin to see solute excreted in urine
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13
Q

What is splay?

A
  • reference to heterogeneity of nephron, some shorter, some do not have a lot of tubule. That’s why there’s a curved line at filter and excretion
  • Why we see glucose in urine before we expect it
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14
Q

Which substrates are absorbed in proximal tubule?

A
  • Glucose
  • Amino acids
  • Acetate
  • Krebs cycle intermediates
  • Water-soluble vitamins
  • Lactate
  • Acetoacetate
  • Many others
  • all are 100% reabsorbed attached to Na
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15
Q

What are some characteristics of typical transport process?

A
  • They are (secondary) active
  • They manifest a Tmax
    • Well above what is normally filtered
  • They manifest specificity
  • They are inhibitable by drugs and disease
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16
Q

What is filtered load?

A

GFRx Px

  • Filtered load= how much moves to blood into nephron
  • Na has highest filtered load of any ion
    • Under hormonal control
    • For excretion, we can go as low as 5% or as high as 99% based on bodies needs
  • Na is reabsorbed in 3 areas:
  • 1) Proximal tubule (65-75%)
  • 2) Part of loop of henle (15-20%)
  • 3) Distal tubule and top of collecting duct (5-7%)
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17
Q

What is ion movement in Na/K ATP ase pump?

A

2 K in for 3 Na out using ATP

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18
Q

What is the only antiport in proximal tubule?

A

Na/H antiport for acid/base regulation

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19
Q

What does movement of Cl in proximal tubule look like?

A
  • As Na absorbed, H2O follow
  • Moving lots of positives into intracellular space
  • Cl- then comes in because of positive gradient created
  • Cl- is pulled in via paracellular pathway
  • Only allowed because strong electrochemical gradient created
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20
Q

Describe this graph.

A
  • [Na] stays the same because taking H2O with it (line remains at 1)
    • As going down tubule, reabsorbing mass Na and water follows
    • Lost Na down tubule, but [Na] does not change!
    • So much Na is reabsorbed!!
  • Amino acids, HCO3, glucose curve down because concentration is so much lower than Na.
    • These are absorbed at beginning of tubule
  • Inulin’s line, not reabsorbed or excreted!
  • Cl- line is delayed increase because it follows electrochemical gradient created by Na
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21
Q

Describe the loop of henle, purpose and what is absorbed there.

A
  • Shaped like horseshoe, essential to function of creating osmotic gradient
  • Descending limb- completely impermeable to solute, no reabsorption/secretion
    • ONLY permeable to H2O
    • AKA Concentrating segment
  • Ascending limb- impermeable to H2O permeable to solute
    • Aka-Diluting limb
22
Q

What types of transporters are found in thick ascending loop of henle?

A
  • Basolateral membrane= Na/K ATPase pump
  • Inside lumen, sodium will move down concentration gradient
  • 4 things moving in via transporter on apical membrane=
    • 1 Na
    • 1 K
    • 2 Cl
  • Other transporters to allow K/Cl to be reabsorbed/ Not focusing on that right now
23
Q

What are the 2 types of cells in distal tubule?

A

Principal Cell

Intercalated cells

24
Q

What is transported on principal cells?

A
  • Na/K ATPase on basolateral membrane
    • Na out, K in, low concentration Na gradient in cell
  • Na/Cl symporter on apical side
  • Cl molecule has chloride channel on basolateral membrane, Cl able to move out on own via channel
25
Q

What is role of alodsteron on distal tubule?

A
  • Aldosterone is lipophilic, can cross cell membrane easily, goes to nucleus and increases protein
  • Makes 3 protein:
    • Make Na-K ATPase to go on basolateral membrane
    • Make Na-channel on apical membrane
    • Also opens K channel on apical membraneà secretion
  • This causes use to absorb more Na, therefore more H2O and therefore increasing blood volume.
  • Aldosterone secretes K and absorbs Na/H2o
26
Q

How do diuretics work?

A
  • Diuretics- make you urinate
  • Block reabsorption of Na in tubule (so h2o doesn’t get reabsorbed)
  • Either turn off Na/Cl symptomer on apical membrane, or turn off aldosterone mediated Na channel on apical membrane
27
Q

What is the osmolarity in proximal tubule?

A

Isoosmotic (absorbing Na, other solutes, and H2O)

Losing mass, not concentration!

28
Q

What is osmolarity like in descending loop of henle?

A

Hyperosmotic

absorbing only H2O, impermeable to solutes

29
Q

What is osmlarity like in ascending loop of henle?

A

Hypoosmotic

Impermeable to h2o, reabsorbing solutes

30
Q

What is osmolarity like in distal tubule?

A

Hypoosmotic

Reabsorbing H2O and solute

Remains same concentration as from ascending loop of henle

31
Q

What is osmolarity in collecting duct?

A

Hypoosmotic

  • concentration is going to be dilute; hyposmotic as low as 100mOsm/L. This is where it decides how concentrated we want it with ADH
32
Q

Potassium handling by kidney?

A
  • Potassium is interesting ion because potassium is low in ECF, but a lot of K in bodies
  • K is high in ICF
    • we have more ICF volume in general, so we have more K than Na, but K not as relevant because it’s staying inside the cell
  • K is regulated by hormones and things that happen inside cell (unlike Na which is regulated by kidney)
  • Lots of tissue stores of potassium, but every time you use insulin/epi/aldo, will move K in/out
  • K is not 100% regulated by kidney like Na
  • K absorbed in GI tract in the colon
    • Excrete 5-10 mEq K/day via feces
  • Excrete 90-95 mEq/K in urine
33
Q

How do we usually handle K on a typical diet (normal-High K diet)

A

We have net secretion of K

34
Q

In a low K diet, how do we alter K transport?

A

In low K diet, we change distal tubule configuration so it reabsorbs K

35
Q

How do we reabsorb K in proximal tubule of nephron

A

Bulk Flow

  • don’t need to know specifics, just know that as Na/H2O/Solutes are brought in membrane, K gets caught up in paracellular pathway and brought into cell
36
Q

What is absorption of K like in thick ascending limb?

A

Comes in via Na/2Cl/K symporter transporter on apical membrane

37
Q

How does aldosterone affect K?

A
  • How K secreted in distal tubule comes down to aldosterone
  • Aldosterone:
    • Open Na channel
    • Create more Na/k ATP ase on basolateral
    • Open K channel on apical membrane- secrete K
  • As we pump more Na into cell, K is going to be secreted into lumen
  • Increases basolateral Na/K ATPase activity, pumping more K into the cells. K is then secreted into lumen through apical aldosterone sensitive channels.
38
Q

How does K affect aldosterone?

A
  • Aldosterone also mediated by K level
  • If increase K level, causes increase aldosterone secretion
  • This causes an increase in K secretion
  • Increase in plasma excretion
39
Q

What does decrease in calcium levels stimulate?

A
  • A decrease in calcium causes: release of PTH
    • To kidney to increase Ca reabsorption
    • Bone- breakdown Ca stores
  • Increase amount Vitamin D from kidney causes increase absorption of Ca in intestine
40
Q

What does increase in Ca levels trigger?

A
  • Reduces Ca uptake in kidney
  • Stimulate Ca deposition in bone
41
Q

What 3 areas play role in Ca levels?

A

Gi tract

Kidney

Bone

42
Q

Where is Ca reabsorbed? What is interesting about its interaction with phosphate?

A
  • Reabsorbed at same places as Na
    • Proximal tubule
    • Loop henle
    • Distal tubule
  • Ca with phosphate- when bound, NOT active
  • If you want to increase ­Ca, need to decrease phosphate at same time in order to ­ increase bioactive Ca (and vice versa)
43
Q

Where are most PTH receptors located?

A

Distal nephron has most dense area of receptors

44
Q

How is Ca absorbed in proximal tubule?

TALH?

A
  • For Ca reabsorption in proximal tubule, we don’t know HOW it happens, but it DOES happen
  • We do know how it’s absorbed in loop henle and distal tubule
  • In Loop of Henle:
    • When PTH is bound:
    • On apical membrane- opens up Ca channelCa is normally VERY low inside cell
      • Ca floods inside cell
    • On basolateral: Ca-ATPase pumps all Ca all out, reabsorbing Ca into blood
      • Both Ca channel and ATPase are regulated by PTH. PTH increases both
  • Don’t worry about Ca paracellular transport
45
Q

Distal Tubuel absorption of Ca?

A
  • Distal limb has same mechanism as TALH. Ca channel on apical, basolateral has Ca/ATP ase
  • Also has Ca/Na antiport transporter on basolateral
46
Q

Aldosterone’s site of action? Effect?

A

Site of action: Distal tubule/CD

Effect: increase NaCl, water reabsorption, increase K secretion

47
Q

ADH site of action? Effect?

A

Site of action:

  • Proximal tuuble- effect= increase NaCl, water reabsorption. Increase H secretion
  • Distal tubule/CD- effect= increase water reabsorption
48
Q

What is ANP’s site of action, effect?

A

Site of action: distal tubule/CD

Effect: decrease NaCl reabsorption

49
Q

What is PTH’s site of action, effect?

A

Site of action: PT, TALH, DT

Effect: decrease phosphate reabsorption, increase Ca reabsorption

50
Q
A