25 13-15

Question 1

Regional specialization of the renal tubules - describe

Answer 1

Nonregulated reabsorption in the PCT:

Apical membrane has many microvilli that increase surface area for absorption.

• Mitochondria for active transport
• Leaky tight junctions (paracellular transport - diffusion of solutes and water)

Regulated reabsorption and secretion in DCT and CD:

Less prominent brush border.
Few mitochondria.
Tight junctions tighter.
Tubular epithlial cells have receptors for homones (ADH/ANP/Aldosterone.)

Water conservation in loop of henle:

Juxta nephrons are specialized to create an osmotic gradient for conserving water and concentrating urine.

Question 2

12. List several substances that are secreted and the direction they move in.

Answer 2

Molecules move from the peritubular capillaries into the renal tubules.

Substances: Excess H+, K+, creatinine, organic anions/cations, prostaglandins, histamine, norepinephrine, chemicals (drugs, food coloring.)

Question 3

Where does secretion take place?

Answer 3

Most active in the PCT, but also in CD and DCT. Dr. J is focusing on DCT/K+ and H+!

Question 4

What is an important substance in secretion?

Answer 4

K+. Kidneys have fine control over K+ concentration (controlled by aldosterone).

K+ plays a key role in membrane electrical activity of excitable tissue so must be held in homeostatic range.

Question 5

What does urine contain?

Answer 5

Both filtered and secreted substances.

Question 6

What is tubular secretion important for? 5

Answer 6

1. last chance elimination - compounds that did not filter out in the renal corpuscle can filter out here.
2. Can get rid of substances that were bound to plasma proteins and therefore couldn’t pass the filtration membrane.
3. Elimination of undesirable substances that have been reabsorbed by the passive process - urea and uric acid.
4. Ridding the body of excess K+ (most K+ is reabsorbed in PCT, aldosterone driven secrection in the DCT gets rid of it)
5. Controlling blood pH.

Question 7

What is secreted/reabsorbed in the tubules and where?

Answer 7

PCT:
Out - 65% of filtrate is reabsorbed, H2o/Na+/HCO3-/glucose/aa/nutrients.
IN - H+/NH4

LOOP OF HENLE:
Out: descending/H2o, ascending/Na+/Cl-/K+. IN: Urea

DCT:
Out (regulated reabsorption) - Na+/Cl-/Ca2
In - (regulated secretion) K+ (aldosterone regulated both in/out).

CD:
Out (regulated reabsorption) H2O/Na+/Cl-/Urea(increased by ADH)
In - Regulated secretion, K+ by aldosterone

Question 8

C+C filtration-reabsorption-secretion

where 
from/to 
what 
active/passive
selective/nonselective

Answer 8

WHERE R. Corpuscle PCT(2/3) DCT/CD

FROM/TO BL–>filt Filt–>BL BL–>filt

WHAT GBU GOOD BU

ACTIVE/PASSIVE Passive Both ??

SELECT/NONselect Non Selective Selective

Question 9

Where is the osmotic gradient greater - outer medulla or inner medulla? Why is this gradient important?

Answer 9

Inner. Varies from 300 to 1200 mOsm. Critical for water absorption from the DCT and CD.

Question 10

What mOsm do the kidneys maintain for the rest of the body?

Answer 10

300, the normal osmotic concentration of blood plasma. Milliosmol

Question 11

14. Describe the mechanisms responsible for the medullary osmotic gradient.

Answer 11

Countercurrent mechanisms determine urine concentration and volume and establish/maintain an osmotic gradient extending from the cortex through the depths of the medulla.

Countercurrent multiplier and exchanger.

Question 12

Countercurrent exchanger

Answer 12

The flow of blood through the vasa recta

Question 13

Countercurrent multiplier

Answer 13

Interaction between the flow of filtrate through the loops of henle in juxta nephrons.

Question 14

Who are the players in the osmotic gradient?

Answer 14

Long loops - create the gradient/multipliers, Vasa recta - preserve the gradient/exchangers, Collecting ducts - use the gradient to adjust osmolality

Question 15

How do the loops create the gradient? 5 steps plus description of positive fb loop.

Answer 15

1. Filtrate entering the loop is isosmotic to both blood plasma and cortical interstitial fluid.
2. Water moves out through aquaporins in the descending limb, down its osmotic gradient.
3. Filtrate reaches its highest concentration (1200) at the bend of the loop.
4. Na+/Cl+ are pumped out of the filtrate in the ascending loop. This increases the interstitial fluid osmolality.
5. Filtrate is at its most dilute as it leaves the nephron loop - 100 mOsm, hypo-osmotic to the interstitial fluid.

Question 16

Describe the positive fb loop in the ascending/descending loop.

Answer 16

Both sides of the loop enhance the activity of the other in a positive fb loop - the more NaCl the ascending limb extrudes, the more the water diffuses out of the descending limb, more water out means a higher concentration of salt in the ascending limb,

Question 17

How does the vasa recta as a exchanger?

Answer 17

It is highly permeable to water and solutes and it follows the shape of the loop in the opposite direction.

Permeability allows the vasa recta to remain nearly isomotic with the surrounding fluid.

Following the shape of the loop allows it to reflect what is happening in the loop preserving the gradient.

Question 18

Formation of Dilute urine

Answer 18

Overhydration/no ADH

↓ Osmolality of extracellular fluids

↓ ADH release from post pit

↓ number of aquaporins in CD

↓ H2O reabsorption in CD

Large volume of dilute urine (100 mOsm)

Question 19

Formation of Concentrated urine

Answer 19

Dehydration/maximal ADH

↑ Osmolality of extracellular fluids

↑ ADH release from post pit

↑ number of aquaporins in late DCT and CD

↑ H2O reabsorption

Small volume of concentrated urine (1200 mOsm)

Question 20

What is the role of urea?

Answer 20

The countercurrent multiplier establishes the gradient, but more solute is needed to maintain it.

Urea from the CD–>IF enters the filtrate by facilitated diffusion in the ascending limb.

This recycling back of urea contributes 40% of the high osmolalit