Module 4: Section 4

Question 1

How many steps is the tubular reabsorption process

Answer 1

two steps

Question 2

step 1 of reabsorption

Answer 2

begins with either active or passive movement of substances from tubule into interstitial

Question 3

step 2 of reabsorption

Answer 3

continues with passive movement of substances from the interstitial space back into blood stream

Question 4

how is glomerular filtration and tubular reabsorption different?

Answer 4

tubular reabsorption is highly selective and variable

Question 5

what is the reabsorption capacity like in tubules for substances needed in the body?

Answer 5

high (with water and sodium most because they are most important)

Question 6

transepithelial transport (or transcellular transport)

Answer 6

the movements of solutes across an epithelial cell layer through the cell

Question 6

luminal membrane

Answer 6

area of epithelial cells in contact with tubule of lumen

Question 7

basolateral membrane

Answer 7

area of epithelial cells in contact with interstitial fluid

Question 8

transepithelial transport (or transcellular transport)

Answer 8

the movement of solutes across an epithelial cell layer through the cell

Question 9

are the membranes of neighbouring epithelial cells in contact?

Answer 9

no, other than where there are tight junctions connecting them
- therefore substances that enter one cell can’t enter the neighbouring cell
– it has to move through interstitial space

Question 10

how many steps much occur during transepithelial transport?

Answer 10

5

Question 11

what are the steps of transepithelial transport?

Answer 11

1. substance must cross luminal membrane
2. substance must pass through cytosol
3. substance must cross the basolateral membrane
4. it must diffuse through interstitial fluid
5. must cross the capillary wall to enter the plasma

Question 12

why is Na+ reabsorbed in so many places?

Answer 12

because it is critical in the reabsorption of many other substances

Question 13

locations of Na+ reabsorption

Answer 13

• proximal tubule
• ascending limb of loop of Henle
• distal and collecting tubules

Question 14

Na+ reabsorption in proximal tubule

Answer 14

• 76% is reabsorbed
• needed for reabsorption of glucose, amino acids, water, Cl- and urea

Question 15

Na+ reabsorption in ascending limb of loop of Henle

Answer 15

• 25% is reabsorbed
• Na+ and Cl- are essential here to either concentrate, or dilute urine depending on what the body needs

Question 16

Na+ reabsorption in distal and collecting tubules

Answer 16

• collectively reabsorb 8%
• here, Na+ reabsorption is under hormonal control
• plays key role in regulating ECF volume and secretion of both K+ and H+

Question 17

is reabsorption of Na+ active or passive

Answer 17

BOTH

Question 18

where is Na+ reabsorption active vs passive

Answer 18

passive= when moving across lumen membrane

active= moving across basolateral membrane
– involves Na+K+ ATPase pump

Question 19

what does acitve transport of Na+ into interstitial fluid help?

Answer 19

helps keep cytosol Na+ conc. low to allow passive diffusion across luminal membrane

Question 20

passive transport of Na+ in proximal tubule

Answer 20

• crossed by cotransport carrier
• simultaneously moves organic nutrients (glucose and amino acids) by secondary active transport
  – use conc. gradient

Question 21

passive transport of Na+ in collecting duct

Answer 21

Na+ passively enters the epithelial cells through a Na+ channel

Question 22

what is the most important and well-known hormonal system involved in Na+ regulation?

Answer 22

renin-angiotensin-aldosterone-systems (RAAS)

Question 23

what are the 3 primary triggers of renin secretion?

Answer 23

1. when granular cells detect a drop in BP
2. when increase in sympathetic activity
3. when a decrease in luminal Na+, macula densa will release

Question 24

what are granular cells innervated by?

Answer 24

sympathetic NS

Question 25

what are the macula densa cells sensitive to?

Answer 25

Na+ and when theres a decrease in luminal Na+

Question 26

what happens when renin is secreted?

Answer 26

it acts like an enzyme to convert aniotensinogen into ingiotensin I

Question 27

angiotensinogen

Answer 27

a protein made in the liver that is present at high conc. in the plasma

Question 28

what happens when angiotensin I passes through the lungs?

Answer 28

it is converted to angiotensin II by the enzyme angiotensin converting enzyme (ACE)

Question 29

what does angiotensin II stimulate?

Answer 29

the adrenal cortex to release aldosterone

Question 30

what does the aldosterone do?

Answer 30

increase in Na+ reabsorption in the distal and collecting tubules - tubular epithelial cells increase insertion of Na+ in luminal membrane and Na+K+ ATPase carriers in the basolateral membrane

Question 31

what does ANP stand for

Answer 31

atrial natriuretic peptide

Question 32

what is ANP

Answer 32

- a hormone involved in the regulation of Na+ and water - opposite actions of aldosterone - its release reduces Na+ load and BP

Question 33

what happens to ANP if blood volume increases or there's an increase in venous return?

Answer 33

- stretch receptors in the left atrium, aortic arch, and carotid sinus stimulate the release of ANP

Question 34

ANP 3 main actions

Answer 34

1. inhibits Na+ reabsorption in the distal tubules so there's more Na+ excreted in the urine 2. inhibits renin and aldosterone secretion 3. it dilates the afferent arterioles and increases GFR (Glomerular Filtration Rate) - as more salt and water are filtered, more salt and water is excreted

Question 35

tubular or transport maximum (Tm)

Answer 35

how there is a limited number of proteins in the membrane therefore a limit to the amount of substances that can be reabsorbed

Question 36

what happens if the conc of tubular fluid exceeds the Tm?

Answer 36

the excess will be excreted in the urine

Question 37

renal threshold

Answer 37

the plasma conc. at which the Tm is exceeded

Question 38

what's the difference between reabsorption of organic nutrients (like glucose and amino acids) and the reabsorption of phosphate or calcium

Answer 38

the reabsorption of phosphate or calcium are under hormonal control

Question 39

what is the normal plasma conc. of glucose per 100 ml of plasma

Answer 39

100 mg per 100 ml of plasma

Question 40

filtered load of a substance calculation

Answer 40

filtered load= plasma conc. x GFR

Question 41

what happened to filtered load with an increase in GFR?

Answer 41

it increases

Question 42

at what level of filtered load of glucose will have glucose appear in the urine?

Answer 42

375mg/min

Question 43

what type of channels does water flow through?

Answer 43

aquaporins

Question 44

aquaporin channels in the proximal tubule

Answer 44

always open

Question 45

aquaporin channels in the distal tubule

Answer 45

under control of vasopressin - therefore not always open

Question 46

how does chloride leave tubular fluid?

Answer 46

by moving between epithelial cells - goes down electrochemical gradient - follows amount of Na+ reabsorption

Question 46

do the kidneys directly regulate chloride?

Answer 46

no

Question 47

what is the amount of chloride reabsorbed dependent on?

Answer 47

how much sodium is being reabsorbed

Question 48

what is the conc. of urea at the beginning of the proximal tubule the same as? and what does this cause?

Answer 48

the plasma conc. of urea - causes no net diffusion

Question 49

what happens to urea conc. as it moves down the proximal tubule

Answer 49

- reduced 2/3 as water is reabsorbed, so the tubular cinc. of urea increases 3x - therefore passively reabsorbed

Question 50

what is blood urea measured as?

Answer 50

blood urea nitrogen (BUN)

Question 51

what has blood urea nitrogen (BUN) been a measure of historically?

Answer 51

renal failure

Question 52

what happens to urea during renal failure?

Answer 52

less urea is excreted so it accumulates in plasma and can be measured