Structure and function of the renal tubule

Question 1

What is the renal tubules main task?

Answer 1

Convert the filtered fluid from the glomerulus into urine. Rate of urine flow = 1ml/min

Question 2

Modification of the filtrate then occurs along the tubule, via reabsorption and secretion of water and various solutes and this changes the composition of the filtrate.. What is reabsorption and secretion?

Answer 2

Reabsorption is movement from the tubular lumen into the peritubular capillary plasma/interstitial fluid.
Secretion is movement from the peritubular capillary plasma into the tubular lumen.

Question 3

Where in the Renal tubule will you be able to find interstitial fluid and what is another name for this?

Answer 3

Between the tubular epithelial cells is interstitial fluid and between the peritubular capillaries and tubule. This is sometimes called the peritubular fluid.

Question 4

What are the two physiological processes involved in the reabsorption and secretion?

Answer 4

Active and passive transfer.

Question 5

What are the 2 types of carrier proteins used in co-transport?

Answer 5

A symport (both substances in same direction) e.g. Na+ -glucose.§
Antiport (moving in opposite directions), e.g. Na+ - H+

Question 6

Where is Na+ freely filtered?

Answer 6

The glomerular capillaries. It moves down its concentration gradient, from the glomerular fluid to the tubular cells.

Question 7

The energy generated by the passive movement of Na+ is used for what substance?

Answer 7

Glucose

Question 8

What is the transporter used to move Glucose against its concentration gradient? What happens when the glucose starts building up in the tubular cells?

Answer 8

Symporter (SGLT2 – like SGLT1, which is in the small intestine). It co transports Na+ and glucose.
As glucose builds up in the cell, it can move passively (by facilitated diffusion) into the tubular capillaries through the GLUT-2 transporter. This is how it is reabsorbed into the blood.

Question 9

Name the condition in which the patient has a mutated gene for the SGLT2 symporter (inactivating it)?

Answer 9

Familial Renal Glycosuria.

Question 10

Name 2 SGLT2 inhibitors and describe how they treat diabetes.

Answer 10

Dapagliflozin and Canagliflozin. This is helpful because it just dumps out a lot of the excess glucose, rather than reabsorbing it.

Question 11

Name the other molecules where the Sodium concentration gradient is being used for their transport.

Answer 11

Amino acids in a symporter (both enter the cell) and hydrogen in an antiporter (Na+ in and H+ out).

Question 12

What are the techniques used to investigate tubular function?

Answer 12

• Micropuncture and isolated perfused tubule
• Electrophysiological Analysis
• Patch clamping

Question 13

How can Electrophysiological Analysis be used to investigate tubule function?

Answer 13

Combine with microperfusion to measure and alter potential difference and measure ion moving with or against electrochemical gradient.

Question 14

What is patch clamping?

Answer 14

Rather than insert a microelectrode through membrane, a blunt-tip pipette is pressed against the cell membrane until a seal forms between electrode tip and membrane surface. Can measure current flow through an individual ion channel so can see types of channels and response to drugs and hormones.

Question 15

What are the 7 segments of the nephron?

Answer 15

1. Proximal convoluted tubule (PCT)
2. Thin descending limb, Loop of Henle
3. Thin ascending limb, Loop of Henle
4. Thick ascending limb, Loop of Henle
5. Distal convoluted tubule (DCT)
6. Collecting/Connecting tubule
7. Medullary collecting duct

Question 16

What are the two types of nephron and describe their structure?

Answer 16

• Cortical nephrons (which make up 85%) and have a short LoH. Entire tubular system is surrounded by and extensive network of capillaries (except LoH)
• Juxta-medullary nephron (15% of nephrons) have a very long LoH that goes deep into the medulla. Very much involved in concentrating urine. Efferent arterioles extend from glomeruli to outer medulla and divided into specialised capillaries (vasa recta) that extend downward into medulla and lie side by side with LoH.

Question 17

Describe how the Proximal Convoluted Tubule (PCT) is designed for reabsorption?

Answer 17

Has a large number of transporters and, large number of mitochondria for active transport (highly metabolic). It also has an extensive brush border on the luminal side, which gives a large surface area for rapid exchange.
**About 65% of the filtrate including all essential nutrients are reabsorbed in the PCT.

Question 18

Glomerular filtrate is protein free but some small proteins do get through. What happens to the proteins that manage to get through?

Answer 18

Proteins are taken up by endocytosis into the tubular cells and are degraded by lysosomal enzymes into amino acids and simple sugars which can then be reabsorbed into plasma.

Question 19

What is Fanconi’s syndrome?

Answer 19

When all PCT reabsorptive mechanisms are defective, so you detect these substances (glucose, AA etc.) in the urine.

Question 20

What are the three distinct functional segments of the Loop of Henle (LoH)? Describe their structure.

Answer 20

Thin descending limb and Thin ascending limb - Made up of thin epithelial cells, with no brush border, few mitochondria and low metabolic activity.
The thick ascending limb - Has thick epithelial cells, extensive intercellular folding, few microvilli and many mitochondria, giving it high metabolic activity.

Question 21

How does the LoH dilute/concentrate urine?

Answer 21

Adjusts the rate of water reabsorption.

Question 22

Which of the limbs is permeable/impermeable to water? What does this mean

Answer 22

The descending limb is very permeable to water, while the ascending limb is virtually impermeable to water. Results in it creating an osmotic gradient

Question 23

What is the significance of the collecting duct transversing the medulla?

Answer 23

The urine gets even more concentrated as water moves out by osmosis.

Question 24

What is a ‘loop diuretic’ and where do they act.

Answer 24

A diuretic is a medicine which increases the amount of water that you pass out from your kidneys. (A diuretic causes an increase in urine (a diuresis). The block Na transport of of the LoH. The thick ascending limb is the site of powerful loop diuretics such as furosemide.