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Urinary Tract Physiology > Basic Epithelial Transport Mechanisms > Flashcards

Basic Epithelial Transport Mechanisms Flashcards

1
Q

Reabsorption and secretion involves crossing which 2 barriers?

A
  1. tubule epithelium
  2. endothelial cells lining the peritubular capillaries
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2
Q

How are the following handled in the tubules?

Sodium

Phosphate

PAH

Potassium

A

Sodium - filtration + reabsorption

Phosphate - filtration + reabsorption

PAH - filtration + secretion

Potassium - reabsorption + secretion

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3
Q

How can the filtrate be modified?

A
  • reabsorption
  • secretion
  • combinations of the two
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4
Q

What are the 2 possible routes of crossing the epithelial barrier?

A
  1. paracellular route
  2. transcellular route
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5
Q

Paracellular route

A
  • single step
  • substance goes around cells through the matrix of the tight junctions
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6
Q

Transcellular route

A
  • two steps
  • step 1: across apical membrane facing lumen
  • step 2: across basolateral membrane
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7
Q

How do substances move across membranes and cells? (4)

A
  • diffusion
  • transporters
  • channels
  • active transport
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8
Q

What are the driving forces for movement by diffusion?

A
  • chemical gradient
  • electrical gradient (potential)
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9
Q

Diffusion applies to what type of movement?

A
  • across capillary walls (endothelial barrier)
  • paracellular route (some transcellular)
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10
Q

Characteristics of transporters

A
  • binds substance more strongly (specificity)
  • carrier proteins undergo more elaborate conformational change
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11
Q

Name 3 types of transporters

A
  1. uniporters
  2. symporters or antiporters
  3. primary active transport
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12
Q

Characteristics of uniporters

A
  • facilitated diffusion
  • follows electrochemical gradient
  • transporter protein facilitates movement
  • permit movement of a single solute
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13
Q

Characteristics of symporters and antiporters

A
  • move 2 (or more) substances across membranes
  • at least one of the solutes moves down its electrochemical gradient
  • provides energy to move 1 or more solutes up its electrochemical barrier
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14
Q

What type of transporter is sodium glucose linked transporter (SGLT)?

A

symporter - moves 1 glucose and 1 or 2 sodium molecules into cells

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15
Q

What type of transporter is sodium hydrogen exchange transporter (NHE)?

A

antiporter - moves Na+ into cells and H+ out of cells

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16
Q

Characteristics of channels

A
  • small pores allowing passage by diffusion with movement down electrochemical gradient
  • usually specific for water or substance (examples: sodium channel, potassium channel, aquaporins)
17
Q

Characteristics of secondary active transport

A
  • ATP not hydrolyzed
  • energy from transport of another solute, often sodium
  • energetics of Na+ distribution always favors entrance into the cell
18
Q

Characteristics of primary active transport

A
  • moves 1 or more solutes up electrochemical gradient using energy from hydrolysis of ATP
  • example: sodium pump (Na/K-ATPase)
19
Q

How can proteins be moved across membranes?

A
  • receptor-mediated endocytosis
  • receptor-mediated transcytosis
20
Q
A

C. Right hand column will increase

21
Q

What is osmolality?

A

ability of solutes to lower water concentration

(ability: proteins > sugars > small ions)

22
Q

Which one of the following provides the energy required for glucose reabsorption in the proximal tubules?

A. Solute drag associated with water flux

B. The Na-K ATPase pump in the basolateral membrane

C. The Na-glucose symporter in the apical cell membrane

D. The glucose uniporter in the basolateral cell membrane

A

B. The Na-K ATPase pump in the basolateral membrane

23
Q

What is the main transport mechanism in the kidneys?

A
  • reabsorption
  • most reabsorption occurs in the proximal tubules
  • most movement is essentialy iso-osmotic
24
Q

What is mostly reabsorbed in the proximal tubules?

A
  • sodium
  • anions to maintain elecroneutrality (Cl and bicarbonate)

(PT cells highly permeable to water, water follows the solute)

25
Q

How might overzealous administration of 0.9% (normal) saline impact PT reabsorption of filtrate in a well hydrated dog with normal kidneys?

A. Decrease reabsorption by increasing hydraulic pressure in peritubular capillaries

B. Increase reabsorption by increasing interstitial hydraulic pressure

C. Decrease reabsorption by diluting oncotic pressure in the peritubular capillaries

D. Increase reabsorption by increasing GFR

A

C. Decrease reabsorption by diluting oncotic pressure in the peritubular capillaries

(explanation: if you overload the capillaries with fluid, you will dilute oncotic pressure)

26
Q

What is edema?

A

fluid in interstitial spaces

excess fluid administration that overwhelms ability to move through the system

27
Q

What are the effects of renal interstitial edema?

A
  • reduced RBF
  • increased interstitial pressure
  • reduced GFR
  • uremia
  • salt and water retention
28
Q

Steps of transcellular epithelial transport with sodium

A

Step 1 - Na/K-ATPase pumps sodium out of cell

Step 2 - sodium from lumen moves down concentration gradient into cell to replace lost sodium

Step 3 - water follows sodium out of cell from basolateral membrane

Step 4 - sodium, anions, and water move into peritubular capillary

29
Q

What type of transports can be involved with passive reabsorption? (3)

A
  1. simple diffusion
  2. paracellular transport
  3. non-ionic transport
30
Q

Characteristics of passive reabsorption

A
  • occurs by simple diffusion
  • determined by concentration gradient and permability of tight junctions
  • example: urea
31
Q

Characteristics of paracellular transport

A
  • need a chemical gradient across tight junctions
  • tight junctions must be permeable to substance
  • voltage gradients also play a role (early PT lumen slightly -, late PT lumen slightly +)
  • facilitates paracellular transport of anions in early proximal tubules
32
Q

Is glucose reabsorbed by paracellular transport?

A

No, tight junctions are impermeable to glucose

33
Q

Characteristics of non-ionic diffusion

A
  • passive process by which impermeant ions derived from dissociation of weak acids or bases can cross cell membranes
  • only uncharged species can diffuse across cell membrane
34
Q

Are transport systems quantitatively limited in their ability to move substances across membranes?

A

yes

35
Q

What are the two limits to transport mechanisms?

A
  1. tubular maximum-limited system (Tm)
  2. gradient-limited system
36
Q

Explain tubular maximum-limited transport

A
  • in tubule, symporter transports glucose into cells with sodium
  • if the plasma [glucose] > Tm, the transporter is saturated
  • increased plasma [glucose] will not increase the rate of absorption
37
Q

Explain gradient-limited transport

A
  • epithelium has a finite passive permeability
  • limit is reached when transcellular and paracellular transport are equal
  • the leakier the tight junctions, the lower the gradient limit

Urinary Tract Physiology (9 decks)

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