Tubular Transport of NaCl and Water

Question 1

Total excretion is determined by:

Answer 1

Filtration-Reabsorption+Secretion.

Question 2

How many L of fluid may be excreted in a single day?

Answer 2

15L of 180L filtered

Question 3

How much salt excreted in a single day?

Answer 3

Up to 30 grams of 1500g filtered

Question 4

Which of the following is false?

• Renal Tubules are lined by polarized epithelial cells
• The two general means of transport in a tubule are Transcellular and Paracellular
• Paracellular transport is from apical to basolateral membrane.

Answer 4

Paracellular transport is from apical to basolateral membrane.

FALSE:

It flows between cells (mainly through tight junctions)

Question 5

What are the driving forces of simple diffusion?

Answer 5

1. Electrical Gradient (charge difference)
2. Chemical gradient (concentration difference)

Question 6

How does facilitated diffusion differ from simple diffusion?

Answer 6

Facilitated diffusion requires carrier proteins to transfer ions/molecules.

Question 7

The Kidney is 2nd only to heart in O2 consumption (QO2) per gram of tissue. Why?

Answer 7

There is a large amount of active transport going on.

Question 8

What 4 important ions/molecules are reabsorbed in the Proximal Tubule?

Answer 8

1. Na+
2. Cl-
3. Glucose
4. H20

Question 9

What are the main jobs of:

1. Proximal Convoluted Tubule
2. Loop of Henle
3. Distal Convoluted Tubule
4. Collecting Duct

Answer 9

1. Proximal Convoluted Tubule
   
   • Bulk Reabsorption (65% of NaCl and H2O and 100% of Glucose and Amino Acids– and pretty much everything else)
2. Loop of Henle
   
   • Urine Dilution
3. Distal Convoluted Tubule
   
   • Fine-tuning Na Balance
4. Collecting Duct
   
   • Fine tuning of K, Na, H+/HCO3-, H2O

Question 10

True or False:

In the Loop of Henle, dilution of the filtrate occurs because H2O is secreted.

Answer 10

FALSE

Dilution occurs because the thick ascending limb of the LOH is impermiable to water but permiable to ions

Question 11

True or False: NaCl cotransporters in the DCT (distal convoluted tubule) are thiazide sensitive.

Answer 11

True.

Remember Hydrachlorothiazide? This is where its MOA is.

Question 12

In the collecting duct, there are 2 types of cells. Name them and their funcions. And for kicks and giggles, what is the fxn of hte collecting duct again?

Answer 12

Principal cells – fine tune:

• Sodium reabsorption and Potassium secretion (via Aldoserone mediated channels)
• Water reabsorption (via vasopressin or ADH mediated aquaporins)

Intercalated cells (alpha and beta)

• Acid-base balance

Collecting duct does “Fine tuning”

Question 13

What are the molecular effects of Aldosterone and ADH?

Answer 13

Insert more channels:

ADH: Inserts aquaporins (H2O channel) on the luminal side of cells

Aldosterone: Inserts ENaC more channels (which absorb Na)

Question 14

What is the relationship (>,<,=) between H2O and NaCl permiability in the Descending and Ascending Loop of Henle?

Answer 14

Descending: H2O > NaCl

Ascending: NaCl >>>> H2O (impermeable

Note, the deeper you go in the kidney, the more concentrated the peritubular tissue becomes.

Question 15

What is the role of the vasa recta?

Answer 15

Maintain the osmotic gradient in the medula by allowing flow of H2O and Ions.

Look at the picture and ask yourself what is going on. Its a good one to understand.

Question 16

Okay that was a lot… How about a summary? Here ya go…

Answer 16

yup

Question 17

F_ic=K’ (P_int - P_cap+pi_int - pi_cap)

WTF does this mean?

Answer 17

This is an important CONCEPT that basically tells us that the net Hydrostatic(P) and oncotic (pi) pressures are greater in the interstitum of the kidney and therefore will cause reabsorption of H2O and ions into the blood.

The main driver is oncotic pressure because cells and protiens are left in the blood and not filtered into the nephron.

Question 18

If GFR increases by 30 L, why doesnt urine output increase by 30L as well?

Answer 18

Glomerulo-tubular Feedback (2/3 H2O reabsorbed):
Inc GFR results in a inc [blood], and an inc peritubular capillary oncotic pressure. This leads to an inc in REABSORPTION of fluid.

Tubulo-glomerular feedback:

Macula densa cells in the juxtaglomerular apparatus sense excess NaCl delivery in loop of henle. They then tell the afferent arteriole to constrict. This decreases GFR.

Question 19

Which scenerio would allow for the greatest reabsorption of ions (assuming the same number of ion channels)?

Fast Flow through the lumen

Slow Flow

Normal Flow

Answer 19

Slow!

Think of the krispy kreme donut making process. If the donuts go by you fast, a greater % will not get eaten. If they go super SLOW, then you eat until you barf your guts out.