Reabsorption/secretion in different segments of the nephron

Question 1

What is reabsorbed in proximal tubules?

Answer 1

-Na, Water, Cl, HCO3-, K (around 65%)

-glucose, amino acids

Question 2

Epithelial cells of proximal tubules adaptation

Answer 2

-adapted for high reabsorption capacity
1.Large number of mitochondria (energy for active mechanisms)
2.Enhanced SA (extensive brush borders)
3.Abundance of carrier molecules

Question 3

Proximal tubule reabsorption

Answer 3

-First half: glucose and amino acids are co-transported with Na
-Second half: more Cl is reabsorbed due to increased Cl concentration through paracellular diffusion

Question 4

Secretion in proximal tubules

Answer 4

-organic acids (bile salts, oxalate, urate, catecholamines) are secreted and can be removed quite quickly
-many drugs and toxins can also be secreted

Question 5

Proximal tubule concentration

Answer 5

-Glucose and amino acids, and bicarbonate removed quickly

-Na and Cl are reabsorbed too but water is also absorbed, resulting in Na and Cl concentration remaining the same

-waste products (creatinine and urea) get more concentrated in tubule because of low to no reabsorption and the reduction of water volume

Question 6

What is a characteristic of reabsorption in the proximal tubules?

Answer 6

-High glucose and bicarbonate reabsorption

Question 7

3 segments of loop of henle

Answer 7

1.thin descending
2.thin ascending
3.Thick ascending

Question 8

Thin segments of loop of henle histology

Answer 8

-thin epithelium
-no brush borders
-few mitochondria

Question 9

Thin descending loop of henle reabsorption

Answer 9

-Highly permeable to water (20%)
-medium permeability to solutes (including Na)
-No calcium, magnesium and bicarbonate reabsorption

Question 10

Thin ascending loop of henle reabsorption

Answer 10

-no permeability to water
-Low Ca, Mg, HCO3- reabsorption

Question 11

Thick ascending loop of henle

Answer 11

-thick epithelium with high metabolic activity
-not permeable to water
-Na,K,Cl reabsorption
-high Ca, Mg, HCO3- reabsorption
**not permeable, but solute reabsorption makes this section good for diluting urine

Question 12

Reabsorption capacity of loop of henle

Answer 12

Thick ascending > thin ascending > thin descending

Question 13

Thick ascending Reabsorption steps

Answer 13

1.Co transporter for Na, K, Cl brough inside (1 Na, 2 chloride, 1 K)
2.Na/K ATPase
3.H+ secretion via counter transport of H+ and Na+
4.Leak of K into lumen
**Everything above results in positive charge on tubular side compared to interstitial fluid facilitating diffusion of Ca and Mg through paracellular space.
5.Tubular content becomes diluted because Segment is impermeable to water and lots of solutes are reabsorbed here, so urine is diluted in this segment

Question 14

Loop diuretics

Answer 14

-eg. Furosemide
-inhibit the Na, K, Cl transporters at the thick ascending segment of the loop of Henle
-results in more solutes being kept in the lumen which reduces water reabsorption in the next segments which means more water is excreted

Question 15

Early distal tubules

Answer 15

-initial portion has macula densa (regulating GFR)
-similar to thick ascending loop of henle
>water impermeable
>reabsorption of Na, K, Cl, Mg occurs
-Na/Cl cotransport important: 5% of filtered load of NaCl is reabsorbed in this section

Question 16

Two cell types of late distal tubule and collecting tubule

Answer 16

1.principle cells
2. intercalated cells

Question 17

Reabsorption of late distal tubule and collecting tubules

Answer 17

-Principle cells reabsorb Na
-intercalated cells reabsorb K and HCO3-

Question 18

Secretion of late distal tubule and collecting tubules

Answer 18

-Principle cells secrete K+
-intercalated cells secrete H+ which helps with acid-base balance

Question 19

Control of principle cells

Answer 19

-controlled by aldosterone and K concentration in body fluids
>aldosterone is stimulated by hyperkalemia, and has receptors on principal cells which increase Na/K pumps

Question 20

Aldosterone antagonists

Answer 20

-interfere with Na reabsorption in late distal tubules and cortical collecting tubules
Means that more Na and water is excreted because Na/K pumps altered

Question 21

Anti-diuretic hormone (ADH/vasopressin)

Answer 21

-controls the permeability of the late distal and cortical collecting tubules to water
*important for controlling dilution and/or concentration of urine

Question 22

Reabsorption in collecting duct

Answer 22

-Less metabolically active cells
>less than 10% Na and water reabsorption occurs, but it is important since it is the final stage of reabsorption

Question 23

How is water permeability regulated in collecting duct?

Answer 23

ADH

Question 24

Permeability of collecting duct

Answer 24

-cells permeable to urea, water, HCO3,Na

Question 25

Collecting duct secretion

Answer 25

-secretes H+ which makes this area important for regulating acid-base