Lecture 16: Tubular Reabsorption and Secretion

Question 1

Paracellular route of reabsorption

Answer 1

Transport of filtrate between cells; diffusion

Question 2

Transcellular route of reabsorption

Answer 2

Transport of filtrate through cells; transporters and channels involved

Question 3

Aquaporins

Answer 3

Specific channel for water transport which can be removed or inserted onto membrane, regulating H2O reabsorption

Question 4

Aquaporin-1

Answer 4

Widespread, including in renal tubules

Question 5

Aquaporin-2

Answer 5

Present in apical membranes of collecting tubule cells, CONTROLLED BY ADH

Question 6

Aquaporin-3

Answer 6

Present in basolateral membranes of collecting tubule cells

Question 7

ATPases

Answer 7

Establish ionic gradients across nephron cell membranes which drive reabsorption or secretion of many solutes; SECONDARY active transport; Na+ gradient powers movement (symporters, antipoers)

Question 8

ENaC channel

Answer 8

Found in apical membrane of nephron cells, closed by amiloride, opened by several hormones

Question 9

Facilitated transport

Answer 9

Transport occurring through channels or uniporters; example: glucose transport

Question 10

Active transport

Answer 10

Transport directly coupled to an energy source; example: Na+/K+ ATPase pump, Ca2+ ATPase pump, H+ATPase pump, etc.

Question 11

Secondary active transport

Answer 11

Transport coupled indirectly to an energy source; example: Ion gradient, Na+/glucose cotransporters

Question 12

SGLT2

Answer 12

Reabsorbs 90% of glucose in early proximal tubule (sodium glucose cotransporter on brush border of PCT cells)

Question 13

SGLT1

Answer 13

Reabsorbs 10% of glucose in late proximal tubule (sodium glucose cotransporter on brush border of PCT cells)

Question 14

What substances are actively secreted into the renal tubules?

Answer 14

Creatinie, para-aminohippuric acid

Question 15

Transport maximum

Answer 15

There is a limit to the rate at which the solute can be transported due to saturation of a specific transport system

Question 16

Transport maximum for glucose

Answer 16

375 mg/min; if we exceed this amount, all glucose will be excreted in urine

Question 17

Solvent drag

Answer 17

Solutes dragged along with solvent regardless of gradient due to osmotic movement of water

Question 18

Which of the following in L/day represents the GFR in the average adult human?

A. 3
B. 125
C. 180
D. 360

Answer 18

C. 180

Question 19

An increase in which of the following factors will result in a decrease in glomerular hydrostatic pressure?

A. Arterial pressure
B. Afferent arteriolar resistance
C. Efferent arteriolar resistance
D. Both B and C
E. All of the above

Answer 19

B. Afferent arteriolar resistance

Question 20

Almost 90% of glucose reabsorption occurs in the early proximal tubule. Which of the following mechanisms is responsible for moving glucose from the tubular lumen into the cytoplasmic compartment?

A. Primary active transport via glucose ATPase pump

B. Secondary active transport via Na+/glucose cotransporter

C. Secondary active transport via Na+/glucose antiporter

D. Diffusion due to concentration differences between tubular fluid and cytoplasm

Answer 20

B. Secondary Active Transport via a sodium/glucose cotransporter

Question 21

Transport characteristics of proximal tubule

Answer 21

highly metabolic, many mitochondria, extensive brush borders (lumen), extensive channels (interstitial)

Question 22

What substances are reabsorbed in proximal tubule?

Answer 22

65% filtered sodium, chloride, bicarbonate, and potassium; all filtered glucose and amino acids

Question 23

What substances are secreted into lumen by proximal tubule?

Answer 23

H+, organic acids, bases

Question 24

Transport characteristics of thin descending limb of Loop of Henle

Answer 24

Highly permeable to H2O, moderately permeable to most solutes (urea, sodium)

Question 25

What substances are reabsorbed by the thin descending segment of the Loop of Henle?

Answer 25

20% of filtered H2O

Question 26

Thin ascending segment of Loop of Henle

Answer 26

IMPERMEABLE TO WATER

Question 27

Thick ascending segment of Loop of Henle

Answer 27

Na+/K+ ATPase pump in basolateral membranes, Na+/K+/Cl- cotransporter, slight leak of K+ into lumen; IMPERMEABLE TO WATER

Question 28

Distal tubule

Answer 28

Contains macula densa, reabsorbs most ions but is IMPERMEABLE to WATER and UREA (diluting segment)

Question 29

Principal cells

Answer 29

Reabsorb Na+, H2O from lumen; secrete K+ into lumen using Na+/K+ ATPase pump (late distal/collecting ducts)

Question 30

Intercalated cells

Answer 30

Reabsorb K+ (HCO3-?) from lumen; secrete H+ into lumen via H_ ATPase transporter

Question 31

Medullary collecting duct

Answer 31

Cuboidal epithelium, smooth surface with few mitochondria; ADH controls permeability to H2O; permeable to urea via transporters; can secrete H+ against large concentration gradient (acid/base control)

Question 32

What substances are reabsorbed in medullary collecting duct?

Answer 32

Sodium, Chloride, H2O in presence of ADH, Urea, Bicarbonate

Question 33

Aldosterone

Answer 33

Source: adrenal cortex Function: increases Na+ reabsorption, stimulates K+ secretion Site of action: principal cells of cortical collecting ducts Stimulus for secretion: increased extracellular K+, increased levels angiotensin II

Question 34

Addison's Disease

Answer 34

Occurs due to the absence of aldosterone, results in marked loss of sodium and accumulation of potassium

Question 35

Conn's Syndrome

Answer 35

Hypersecretion of aldosterone (few symptoms)

Question 36

Angiotensin II

Answer 36

Function: increased Na+, H2O reabsorption, returns blood pressure and extracellular volume toward normal Effects: stimulates aldosterone secretion, constricts efferent arterioles, stimulates Na+ reabsorption in proximal tubules

Question 37

ADH

Answer 37

Source: posterior pituitary Function: increases H2O reabsorption Effects: binds to V2 receptors in late distal tubules, collecting tubules/ducts; increases formation of cAMP stimulating movement of aquaporin-2 proteins

Question 38

Atrial Natriuretic Peptide (ANP)

Answer 38

Source: cardiac atrial cells in response to distension Function: inhibits reabsorption of Na+, H2O

Question 39

Parathyroid Hormone (PTH)

Answer 39

Source: Parathyroid glands Function: increases Ca2+ reabsorption

Question 40

Renal clearance

Answer 40

Volume of plasma that is completely cleared of the substance by the kidneys per unit time Cs = (Us x V)/Ps

Question 41

Inulin

Answer 41

Polysaccharide that is not produced in body; used to measure GFR via renal clearance (125 ml/min = normal)