Tubular Transport of NaCl and Water

Question 1

Do carnivores have acidic or alkaline urine? How about herbivores?

Answer 1

Carnivores produce acidic urine.

Herbivores produce alkaline urine.

Question 2

What are the 4 steps of the symplified scheme of excretion?

Answer 2

Filtration, reabsorption, secretion, excretion

Question 3

Renal secretion plays an important role in ____

Answer 3

potassium handling and acid-base balance

Question 4

What’s the difference between transcellular and paracellular transport? Give examples of each

Answer 4

Transcellular transport is transport through the apical and basolateral membranes. Examples of this include reabsorption (tubular fluid to interstitium to blood) and secretion (blood to interstitium to tubular fluid).

Paracellular transport is transport between the tubular cells. This is regulated by junctional complexes and tight junctions.

Question 5

The proximal tubule cell has a large brush border. What does this help with?

Answer 5

This greatly increases surface area to help with reabsorption.

Distal tubule cells have less microvilli and mitochondria because less reabsorption happens there.

Question 6

What are the 3 forms of passive transport?

Answer 6

1. Simple diffusion
2. Facilitated diffusion (carrier-mediated)
3. Diffusion through a membrane channel (pore)

Question 7

What is simple diffusion?

Answer 7

Simple diffusion is a form of passive transport (does not require ATP) that happens due to electrochemical gradients. The movement is influenced by concentration (move from areas of higher concentration to lower concentration) and charge (difference in charge).

Question 8

What is facilitated diffusion?

Answer 8

It is a type of passive transportation that is carrier-mediated. Carrier proteins facilitate movement across lipid bilayer. Two or more ions or molecules share the carrier.

Question 9

What is active transport?

Answer 9

Movement of solute against an electrochemical gradient. Energy is required

Question 10

True or False: Energy is required to form a urine that differs in solute (S) composition from that of blood/plasma.

Answer 10

True

For reabsorption, energy is required when [S_urine] is less than [S_blood].

For secretion, energy is required when [S_urine] is greater than [S_blood].

Question 11

What is the most important pump/transporter/channel in tubules? Where is it located and what does it do?

Answer 11

The Na⁺/K⁺ ATP-ase.

It is located in the basolateral membrane of tubules and it establishes an electrochemical gradient with a net negative intracellular charge and low intracellular sodium concentration. This electrochemical gradient drives most other passive tubular transport processes in the kidney.

It pumps 3 Na⁺ out per 2 K⁺ in.

Question 12

How does the Na/K ATPase drive Na reabsorption?

Answer 12

The Na/K ATPase pumps Na across the basolateral membrane of the tubular cells into the serosa. The decreased Na concentration and net negative charge inside the cell allows Na to diffuse passively from the lumen into the cell where it can be pumped out by the Na/K ATPase.

Question 13

How does Na reabsorption help in transport of other solutes?

Answer 13

When Na goes from the lumen into the cell (due to the electrochemical gradient formed by the Na/K ATPase), a negative luminal charge is established which drives Cl^- between the cells (paracellular transport) to go from the lumen to the serosa.

Also, the reabsorption of glucose and amino acids is helped by Na reabsorption as well. Glucose and amino acids “ride with” Na through secondary active transport.

Question 14

How does reabsorption of water happen?

Answer 14

Water follows the movement of solutes. So, when solutes are reabsorbed, water follows. This happens through transcellular water pores (aquaporins) and paracellular tight junctions.

Question 15

Where does most of reabsorption happen?

Answer 15

Proximal convoluted tubule

Question 16

What primarily happens in the loop of henle?

Answer 16

Urinary dilution.

In the thick, ascending limb of the loop of henle, about 25% of the NaCl is reabsorbed but water cannot follow it because the thick, ascending limb is impermeable to water. This causes a dilution of the urine concentration.

Question 17

What primarily happens in the distal convoluted tubule?

Answer 17

Fine-tuning of Na balance

Question 18

What primarily happens in the collecting duct?

Answer 18

Fine-tuning of potassium, sodium, acid-base, and water.

Question 19

What is special about the thick, ascending limb of the loop of henle?

Answer 19

Impermeable to water

Question 20

About what percent of water and NaCl is reabsorbed at the different tubular segments?

• proximal tubule
• loop of henle (Ascending and descending)
• distal tubule
• collecting duct

Answer 20

Proximal tubule: 65% NaCl and water

Descending loop of henle: 15% water

Ascending loop of henle: 25% NaCl

Distal tubule: 8-10% NaCl and water

Collecting duct: 6-20% NaCl and water

Note: the collecting duct has the widest range and is based on the physiologic needs of the body.

Question 21

In the proximal tubule, what % of the following are reabsorbed?

• Na
• Water
• Glucose
• Amino Acids
• Calcium
• Phosphate
• Citrate

Answer 21

• Na = 65%
• Water = 65%
• Glucose = 100%
• Amino Acids = 100%
• Calcium = 60%
• Phosphate = 80%
• Citrate = variable

Question 22

In the proximal tubule, is H⁺ secreted or reabsorbed? How about HCO₃^- ?

Answer 22

In the proximal tubule,

H⁺ is secreted and HCO₃^- is reabsorbed.

Question 23

Are organic anions secreted or reabsorbed in the proximal tubule?

Answer 23

Secreted

Question 24

True or False: Macromolecules like polypeptides and proteins are reabsorbed in the proximal tubule.

Answer 24

True. Most of these macromolecules remain in the blood and are not filtered but the ones that are are reabsorbed in the proximal tubule.

Question 25

What can block the Na-K-2Cl co-transporters in the ascending loop of henle?

Answer 25

Loop diuretics (e.g. furosemide, torsemide, etc)

Question 26

The thick ascending limb of the loop of henle is impermeable to water. What happens because of this?

Answer 26

This results in a dilution of the urine/filtrate because sodium is being reabsorbed but water cannot follow.

This also results in an increase in medullary osmolarity because sodium is being moved there but not water.

Question 27

What pump/transporter is responsible for the transcellular Na reabsorption in the thick, ascending loop of henle?

Answer 27

Na-K-2Cl co-transporter

Question 28

True or False: The reabsorption of Na in the thick ascending limb of the loop of henle is primarily driven by transcellular transport.

Answer 28

False. There is equal transcellular and paracellular reabsorption of Na in the ascending limb of the loop of henle.

Question 29

The thick ascending limb of the loop of henle has Na reabsorption. What are two other ions that are reabsorbed?

Answer 29

Calcium and magnesium

Question 30

In the thick ascending limb of the loop of henle, are calcium and magnesium reabsorbed paracellularly or transcellularly?

Answer 30

Paracellularly

Question 31

Which transporter facilitates Na reabsorption in the distal tubules?

What can block this transporter?

Answer 31

Na-Cl cotransporter.

Can be blocked by thiazide diuretics.

Question 32

In addition to Na reabsorption, what are two other ions that are reabsorbed at the distal tubule? What transporter does this?

Answer 32

Calcium and magnesium.

Ca-Mg-ATPase.

Question 33

Where in the nephron is the highest activity of Ca-Mg-ATPase?

Answer 33

Distal convoluted tubles have the highest activity of Ca-Mg-ATPase (for reabsorption of Ca and Mg)

Question 34

What are the major cell types in the collecting tubules? (3)

Answer 34

Principal cells

Intercalated cells (alpha and beta)

Question 35

What do principal cells in the collecting duct do? (3)

Answer 35

Sodium reabsorption

Potassium secretion

Water reabsorption

Question 36

Which segment of the nephron is the “fine-tuning” segment?

Answer 36

Collecting duct

Question 37

What do the intercalated cells of the collecting duct do?

Answer 37

Acid-base balance

Question 38

What is ENaC?

Answer 38

ENaC = epithelial sodium channel.

It is the aldosterone mediated channel in the principal cells of the collecting duct that causes Na reabsorption (Na moves from the lumen into the principal cell by itself)

Question 39

What are ROMK and Maxi-K?

Answer 39

The ROMK and Maxi-K channels are aldosterone mediated channels in the principal cells of the collecting duct that secrete potassium (K)

Question 40

Aldosterone mediates Na reabsorption and K secretion in the principal cells of the collecting duct. What mediates water reabsorption via aquaporins?

Answer 40

Vasopressin (ADH) mediates the H₂O reabsorption via aquaporins in the principal cells of the collecting tubule.

Question 41

True or False: alpha and beta intercalated cells have opposite functions

Answer 41

True

Question 42

What do alpha intercalated cells do? (3)

What about beta intercalated cells? (2)

Answer 42

Alpha intercalated:

H⁺ secretion

HCO₃^- synthesis

K⁺ absorption

Beta intercalated:

HCO₃^- secretion

Cl^- absorption

Question 43

What’s the effect of aldosterone on principal cells of the collecting duct?

Answer 43

In the presence of aldosterone, there are more ENaC channels on the cell apical membrane which increases the reabsorption of Na⁺ into the principal cells. An increase of Na reabsorption causes a greater negative charge in the lumen which causes an increase in paracellular transport of Cl from the lumen into the serosa.

Question 44

True or False: In absence of vasopressin, the collecting duct is relatively impermeable to water.

Answer 44

True

Question 45

What does vasopressin (ADH) do to principal cells in the collecting duct?

Answer 45

The principal cells in the collecting duct typically only have aquaporins on the basolateral side but not the apical side. When there is vasopressin (ADH) present, aquaporins are formed on the apical side so that water can pass from the lumen through the cell into the interstitium.

Without vasopressin, the collecting duct is impermeable to water.

Question 46

What is countercurrent multiplication?

Answer 46

Energy is spent in the thick, ascending limb of the loop of henle to reabsorb ions into the medulla. This increases the osmolarity of the medulla which can multiply the amount of water that can be passively reabsorbed in the descending loop of henle.

Question 47

What is the vasa recta?

Answer 47

Vasa recta are loop shaped capillaries that maintain the osmotic gradient in the medulla with passive movement of water and solute.

In the descending capillaries, water moves out of the blood into the interstitium and solute moves from the interstitium into the blood. (This is because the medulla is hypertonic so water travels from the blood capillary to the interstitium to try to dilute the concentration and solute moves from the interstitium into the blood capillary because it’s moving from higher concentration to lower concentration.

In the ascending capillaries, water moves from the interstitium into the capillaries and solute moves from the capillaries out into the interstitium

Question 48

The _____ _____ establishes the osmotic gradient from the cortex to the medulla of the kidney and the _____ _____maintains the osmotic gradient.

Answer 48

countercurrent multiplication, vasa recta

Question 49

Is water reabsorption in the collecting duct passive or active?

Answer 49

Passive. The countercurrent multiplication system establishes a high ion concentration in the medulla. This means that water wants to leave the collecting duct to go into the medulla and this happens as long as there are aquaporin channels open (when vasopressin/ADH is present).

Question 50

Is peritubular capillary oncotic pressure high or low?

Answer 50

It is very high. The glomerulus doesn’t filter most proteins, it just filters waters and solutes. So, the blood in the capillaries is concentrated with proteins and cells which gives it a high oncotic pressure.

Question 51

Solutes and water are reabsorbed from the lumen (urinary space) back into the medulla. How does the water and solutes make it from the medulla back into circulation?

Answer 51

This is driven by starling forces. The high peritubular capillary oncotic pressure attracts water and solutes back into circulation.

Question 52

Explain glomerulo-tubular balance (GFR and volume of ultrafiltrate)

Answer 52

When there is an increase in GFR, there is an increase in the volume of ultrafiltrate. About 65% of the volume, regardless of the increase, is reabsorbed at the proximal convoluted tubule. The remainder of the excess volume is dealt with downstream by an increase in oncotic pressure in the peritubular capillaries. (When GFR increases, blood in the efferent arteriole and peritubular capillaries is more concentrated because more water is being filtered out by the glomerulus. This increase in peritubular capillary oncotic pressure results in more water being taken from the medulla back into the capillaries).

However, these mechanisms aren’t enough to fully compensate. The rest of the balance is done by tubulo-glomerular feedback.

Question 53

What is tubulo-glomerular feedback?

Answer 53

TGF is the mechanism that occurs when excess delivery of NaCl is sensed by the macula densa at the end of the thick ascending limb. The macula densa sends a signal to afferent arterioles to constrict. This lowers GFR which lowers ultrafiltrate volume.

Question 54

How does flow rate influence tubular function?

Answer 54

As solutes and water flow through tubules, they pass different reabsorptive mechanisms. If the flow is faster, there is less time for interaction with the transporters so less solute and water are absorbed. If the flow is slower, there is more time for interaction with transporters so more solute and water are absorbed.