Proximal Convoluted Tubules

Question 1

In paracellular pathway, substance passes through……., it occurs by……..

Answer 1

Tight junctions
Simple diffusion & osmosis

Question 2

In 2ry active transport given an example for:
1. Co-transport
2. Counter-transport

Answer 2

1. Na+ & glucose
2. Na+ & H+

Question 3

How does the passage of water & solutes from interstitial soace PTC take place?

Answer 3

It is a passive process depends on pressure gradient and is called bulk flow where water creates hydrostatic pressure in interstitial space pushing solutes with its movement to inside peritubular capillaries

Question 4

What is the proportion & mechanism of Na+ reabsorption?

Answer 4

65-67%
1. Na+ passively diffuses into tubular epithelial cells down its concentration gradient.
2. They are actively pumped out of these cells at basolateral border by Na+/K+ ATPase carrier; then K+ diffuses back into cells
This results in:
1. Na+ conc inside the tubular cells dec & a conc gradient favors movement of Na+ from the lumen to tubular cells passively
2. The electrical potential inside the cell becomes more -ve, so there is an electrical gradient. Na+ diffuses passively from lumen to tubular cells.

Question 5

What is the proportion & mechanism of water reabsorption?

Answer 5

65-57%
Whn Na+ reabsorption occurs osmolarity dec in tubular lumen & inc in lateral intercellular spaces this forces passive water diffusion down osmotic gradient.

Question 6

GR: Water absorption in proximal convoluted tubule is called obligatory reabsorption?

Answer 6

It is not dependent of H2O load or hormonal regulation
1. The walls if PCT are highly permeable to water even in tight junctions
2. As the solutes are reabsorbed an osmotic gradient for water is created & water follows the solute

Question 7

GR: Fluid reabsorbed in PCT is iso-osmotic with filtrate

Answer 7

Because the same proportions for water & sodium are reabsorbed

Question 8

What is the proportion & mechanism of urea reabsorption?

Answer 8

50%
As 50% of urea is passively reabsorbed in PCT 2ry to Na & water down a conc gradient. The wall of PCT is partially permeable to urea but highly permeable to water, this creates a conc gradient between tubular lumen & interstitial fluid.

Question 9

What is the proportion & mechanism of Cl- reabsorption?

Answer 9

65-67%
In early part of PCT, it occurs through paracellular pathway following Na reabsorption & creation of +ve voltage in interstitial space
In late part, it occurs through paracellular pathway after water reabsorption as its conc gradient inc

Question 10

Describe mechansim of HCO3- reabsorption & proportion

Answer 10

90%
1. From Na+/H+ antiport, the secreted H+ reacts with HCO3- in the filtrate, to form carbonic acid which is acted upen by brush corder carbonic anhydrase forming water & CO2
2. CO2 is absorbed inside the cell and reacts with water forming carbonic acid by intracellular carbonic anhydrase
3. Thus bicarbonate is regenerated intracellularly
4. The intracellular HCO3- is transported back into blood via HCO3-/Na+ symport in 3:1 ratio

Question 11

What is the sugnificance of HCO3- absorption?

Answer 11

It has a role in buffering hydrogen secreted by PCT & making the pH of the filtrate change only from 7.4 to 6.8

Question 12

GR: Reabsorption of bicarbonate is active?

Answer 12

It requires H+ secretion by 2ry active transport
It requires enzymatic activity of carbonic anhydrase

Question 13

Describe the effect of carbonic anhydrase inhibition

Answer 13

1. Will prevent HCO3- reabsorption leading to its loss in urine taking water with it (diuresis by diamox)
2. Affects intracellular one & dec H+ secretion & affecting to some extent Na+ reabsorption & loss of Na in urine adding to diuresis

Question 14

What is the proportion & mechanism of K+ reabsorption?

Answer 14

65-67%
In early part, tightly linked to water & Na+, reabsorption of Na drives water which may carry some K with it it, the potassium gradient peoduced by reabsorption of water from tubular lumen drives paracellular reabsorption of K may be enhanced by removal of Ka from paracellular space by Na/K ATPase pump
In late part, positive potential in lumen drives K reabsorption by paracellular route

Question 15

Amnio acids are reabsorbed by…….

Answer 15

2ry active transport with Na (100%)

Question 16

What is the proportion & mechanism of glucose reabsorption?

Answer 16

100%
By 2ry active transport as glucose & Na attach to symport carrier SGLT-2 at luminal cell border & enter the cell, the glucose is transported to interstitial fluid by facilitated diffsion via GLUT2 & Na is pumped by Na/K ATPase into lateral intercellular space.

Question 17

SGLT-2 is inhibited by……

Answer 17

Phlorizin

Question 18

Define transport max & why is it reached

Answer 18

It is the maximum amount of a substance that the tubular cells can transport within a given period of time, it may refer to secretion or reabsorption
Because a limited number of each specific carrier is present on cells of tubules, it is reached when all the carriers specific for a particular substance are fully occupied or saturated so they can’t handle additional amounts at that time

Question 19

GR: Na+ has no Tm

Answer 19

As the carrier never saturated

Question 20

TmG in males is…..& females is……

Answer 20

375 mg/min
300 mg/min

Question 21

Renal threshold for glucose is……

Answer 21

180 mg/dl

Question 22

What is the filtered load of glucose?

Answer 22

It is the anount of glucose (mg) filtered per unit time (min)
=plasma glucose conc (/1ml)*GFR

Question 23

At blood glucose level 200mg%, glucose appears in urine although it Tm is 300 mg/min

Answer 23

This is because TmG of some nephrons is reached i.e. glucose transport mechanism in some nephrons is reached & they can no more absorb glucose, so their exceeded amount starts to appear in urine. On the other hand, the rest of the nephrons still reabsorb glucose completely.

Question 24

GR: Above 300 mg% plasma glucose any excess glucose is excreted in urine.

Answer 24

Because ALL the tubules become saturated & are reabsorbing at the maximum capacity.

Question 25

TmG=………

Answer 25

Amount filtered/min - amount excreted in urine/min

Question 26

List causes of glucosuria

Answer 26

1. Diabetes mellitus
2. Renal glucosuria
3. Phlorizin
4. Pregnancy (inc GFR)

Question 27

Proportion & mechanism of phosphate reabsorption

Answer 27

80%
By 2ry active transport with Na+ (it has low Tm, 0.1 mM/min)

Question 28

How does PTH affect PO4 reabsorption?

Answer 28

Dec it by dec Tm i.e. carrier

Question 29

The form of phosphate that is NOT reabsorbed is…….

Answer 29

H2PO4-

Question 30

Mention the role of mono-hydrogen phosphate in tubules

Answer 30

Little is used to buffer H+ secreted by PCT but as it is concentrated along the nephron, it becomes an effective buffer for H+ in DCT & CD

Question 31

Secretion of H+ occurs by…..

Answer 31

2ry active transport with Na+ (antiport)

Question 32

Describe the secretion of PAHA

Answer 32

Tm is 80mg/min
At low plasma PAH conc, the rate of secretion inc linearly with PAH conc in plasma
At high plasma PAH conc, the secretory carriers are saturated & secretion stabilizes at a max constant value (Tm)

Question 33

What is the significance of Tm(PAH)?

Answer 33

It is directly related to the number of functioning proximal tubules & therefore provides a measure of the mass of proximal secretory tissue.

Question 34

What are sources & fate of glutamine in tubular cells?

Answer 34

1. In filtrate as AA which enters the cell by 2ry active transport with Na
2. From the blood in peritubular capillaries
   Inside the mitochondria of renal cell: 2 molecules of ammonia & 2 molecules of bicarbonate are formed

Question 35

Describe secretion of ammonia

Answer 35

As a gas it is easily diffusible from inside the cell to lumen, but if it binds to H+ intracellular it will be converted into ammonium (NH4+) for which cell membrane is impermeable and so to go out to the lumen it uses Na+/H+ antiporter in competition with H+.

Question 36

What is the ammonia cycle?

Answer 36

Ammonium is secreted into PCT then reabsorbed in loop of Henle by replacing K in K/Na/2Cl cotransporterand converted back to NH3 in interstitial space to diffuse to CD tubular cell to the lumen to be used in buffering H+ in the lumen.