Lecture 17: Tubular Reabsorption And Secretion

Question 1

Describe paracellular and transcellular routes for reabsorption

Answer 1

Para: movement btw tubular cells

Trans: movement through tubular cells

Question 2

List the 3 kinds of aquaporins and their specific locations in the renal tubules.

Answer 2

Aquaporin-1: widespread, renal tubules.

Aquaporin-2: present in apical membranes of collecting tubule cells. Controlled by ADH

Aquaporin-3: present in basolateral membranes of collecting tubule cells

Question 3

List and compare types of transporters:

Answer 3

Symporter: contransport- moves w/ Na+ gradient

Antiport: countertransport . Moves against Na+ gradient

Question 4

List and describe major types of ATPases and channels found in the renal tubules and indicate their specific locations

Answer 4

ENaC channel: found in apical membrane. Closed by Rx amiloride. Opened by some hormones.

CFTR (Cl-) channels and K+ channels: found in apical membranes of some segments of nephron- driven by [ ] gradient of substance concerned.

Na+K ATPase
H+ ATPase
H+K+ ATPase
Ca++ATPase

Question 5

Discriminate among facilitated, active, and secondary active transport and know examples of specific transporters

Answer 5

Facilitated transport: transport occurring through channels or uniporters, ex: glucose transport.

Active Transport: transport directly coupled to an energy source-Na+K+ATPase. Ca++ ATPase

Secondary active transport COTRANSPORT: transport coupled indirectly to an energy source. SGLT2: reabsorbs 90% glu. SGLT1 reabsorbs 10% glu

Question 6

List the substances thatre actively secreted into renal tubules

Answer 6

Creatinine

Para-ampinohippuric acid

Question 7

Define transport maximum and the limiting factor and explain how the r/t glucose reabsorption

Answer 7

Transport maximum: limit to the rate at which the solute can be transported

Limiting factor: # of transporters available.

Maximum transport is the amt. that will be reabsorbed by the tubule. Anything past that will be excreted.

Question 8

List reasons as to why some passively reabsorbed substances dont have a transport maximum to

Answer 8

1. Rate of diffusion is determined by electrochemical gradient of the substance.
2. Permeability of membrane for the substance
3. Time that the fluid containing the substance remains w/in the tubule.

Question 9

How do electrochemical gradients and time the substance is in the tubule affect rate of transport?

Answer 9

More time = more reabsorption

More difference in gradients will cause more reabsorption

Question 10

Define solvent drag

Answer 10

osmotic movement of water reabsorption can also carry some solutes [mostly Na+]

Question 11

Describe the transport characteristics of the proximal tubule and the specific kinds of molecules/ions and direction transported.

Answer 11

Highly metabolic=lots of mito.
Reabsorbs 65% of filtered Na+, Cl-, HCO3, and K+
Reabsorbs all filtered glucose and AAs

Secretes H+, organic acids, bases

First 1/2: cotransport w/ glu, AAs, and other solutes

Second 1/2: reabsorption is mainly w/ Cl-

ACID-BASE here -carbonic acid -> HCO3

Question 12

Describe the transport characteristics of the thin descending loop of henle and the specific kinds of molecules/ ions and direction transported.

Answer 12

Highly permeable to water and moderately permeable to most solutes, including urea and Na+

Reabsorbs 20% of filtered water

Question 13

Describe the transport characteristics of the ascending [thin and thick] loop of henle and the specific kinds of molecules/ions and direction transported

Answer 13

Thin ascending loop: impermeable to water

Thick ascending segment: Na+K+ATPase pump, Na+, K+, and Cl- co-transporter, slight back leak of K+ into lumen.
IMPERMEABLE TO WATER
Site of action for lop diuretics

Question 14

Describe the transport characteristics of the early and late distal tubule and the specific kinds of molecules/ions and direction transported

Answer 14

Early distal tubule: IMPERMEABLE to water and urea.
Na+, Cl-, Ca++, and Mg++ reabsorbed

Late distal tubule:
Reabsorbs Na+, Cl-, HCO3,
secrete K+ into lumen
Primary site of K+ sparing diuretics

Question 15

Describe the specific functions and locations of the principal cells and the intercalated cells

Answer 15

Late distal tubule and collecting duct:
Principal cells:
reabsorb Na+ and secrete K+

Intercalated cells:
Reabsorb K+ and HCO3 ions from lumen and secrete H+ ions

Question 16

Describe the transport characteristics of the medullary collecting duct and the specific kinds of molecules/ions and direction transported

Answer 16

Epithelial cells are cuboidal. Smooth surfaces. Few mito.

Permeable to water CONTROLLED BY ADH.
Permeable to urea.

Capable of secreting H+ against a [ ] gradient.

Can also reabsorb Na+,Cl-

Question 17

Review forces [starling forces] that determine fluid reabsorption by peritubular capillaries

Answer 17

Pc=13 ->interstitial
Pif=6 interstitial

38-28= 10

Bulk flow= net reabsorption 10 mm Hg

*pushed to the capillaries more than the lumen for secrection/excretion

Question 18

For aldosterone, describe the source, function, site of action, and stimulus for secretion.

Answer 18

Aldosterone Source: adrenal cortex

Function: increases Na+ reabsorption and stimulates K+ secretion.

Site of action: major-on the principal cells of the cortical collecting ducts

Stimulus for secretion: increased extracellular K+ and levels of angiotensin II

Question 19

Relate addison’s disease to lack of aldosterone and conn’s syndrome to aldosterone hyerpsecretion.

Answer 19

Addisons disease is asso. W/ absence of aldosterone= marked loss of Na+ and accumulation of K+

Conn’s syndrome asso. W/ hypersecretion of aldosterone

Question 20

Describe function and effects of angiotensin II and r/t its effects on the proximal tubule.

Answer 20

Angiotensin II function: increased Na+ and water reabsorption.
Returns BP and extracellular volume toward normal.

Effects angiotensin II:
Stimulates aldosterone secretion.
Constricts efferent arterioles.
Directly stimulates Na+ reabsorption in proximal tubules, loops of henle, distal tubules, and collecting tubules.

Question 21

Describe source, function, and effects of ADH, ANP, PTH

Answer 21

ADH:
source: posterior pituitary.
Function: increases water reabsorption.
Effects: binds to V2 receptors in late distal tubules, collecting tubules, and collecting ducts. Increases formation of cAMP.

ANP:
Source: atrial cells in response to distension. Function: inhibits reabsorption of Na+ and water

PTH: source parathyroid glands. Function: increases Ca++ reabsorption

Question 22

Explain mathematically how renal clearance is calc and the role of insulin in calc GFR

Answer 22

Renal clearance=volume of plasma that is completely cleared of the substance by the kidneys per unit time.

Clearance rate = (urine [ ] of substance s X urine flow) x plasma [ ] of substance s

Inulin: polysaccharide (5200) not produced in body. Completely filtered but not reabsorbed or secreted.

GFR= (urine [ ] of substance X urine flow) / plasma [ ] substance = Cs

GFR= (Us x V)/Ps = Cs