1. Grow the Gradient! An interactive countercurrent multiplier game
Jessica R. Santangelo
and Peter Daniel

2. Begin by giving an overview or review of content students should already be familiar. Alternatively, students can be asked to identify the parts either by calling out answers, through use of clicker questions, or with a pre-activity quiz.
The kidneys are located on either side of the spine in the lower abdomen. The ureter connects the kidney to the bladder which empties via the urethra.
Note: Image is labeled for noncommercial reuse with modifications (

3. Continue with overview of macroscopic structures, either verbally, via clicker questions, or via a quiz.
(Left image – orients students based on the previous slide) The kidneys are located on either side of the spine in the lower abdomen. The ureter connects the kidney to the bladder which empties via the urethra.
(Right image) Taking a close up look at the kidney, the renal artery brings blood into the kidney, the renal vein brings blood out. The outer part of the kidney is the cortex, the inner part is the medulla.
If we zoom in on the area enclosed by the rectangle, we can see the structure of the functional unit of the kidney, the nephron.
Note: Image is labeled for noncommercial reuse with modifications(

4. Cortex
Medulla
Continuing with the overview, point out the cortex and medulla along with the structures in each area:
Within the cortex we find the glomerulus and Bowman’s capsule, proximal convoluted tubule and distal convoluted tubule.
Within the medulla we find the loop of Henle, the beginning of the distal convoluted tubule, and most of the collecting duct.
The FUNCTION of the loop of Henle is to create a concentration gradient within the medulla. The CORTEX has the same salt concentration as the blood. But, as you travel down into the medulla, the medulla gets saltier and saltier. The loop of Henle is the structure in the nephron that creates this salt gradient.
Note: Image is labeled for noncommercial reuse (

5. Osmolarity of fluid in tissues (mosm/l)
Cortex
proximal conv. tubule
distal conv. tubule
300
glomerulus
collecting duct
300
300
400
600
900
1200
1000
700
500
200
300
300
blood plasma (300 mosm/l)
300
Medulla
NaCl
H2O
400
H2O
500
Osmolarity of fluid in tissues (mosm/l)
600
descending limb
ascending limb
700
800
This is a simplified diagram to demonstrate the movement of water and salt that ultimately results in the salt concentration gradient in the medulla. Note that even though it is “simplified” it’s still pretty complicated. If students are responsible for learning this prior to the class/lab, clicker questions could be used to gauge their level of understanding.
The slide is animated such that the labels come up one at a time, allowing you to walk students through a relatively complicated, dynamic process.
First orient the students to the two major regions (cortex and medulla). You can ask: Which area has the same salt concentration as the blood? (Answer: Cortex) Which area has the salt concentration gradient? (Answer: Medulla). You can draw the students’ attention to the vertical axis that displays the osmolarity of the two regions.
Then orient the students to major structures within the nephron: glomerulus, proximal convoluted tubule, loop of Henle (both the descending and ascending limbs), distal convoluted tubule, and collecting duct.
Then the filtrate concentrations appear. Students should notice that the filtrate starts at the same concentration as the blood (300), increases in concentration as it descends the loop of Henle, then *decreases* in concentration as it moves up through the ascending limb of the loop of Henle. Note that the filtrate concentration in the distal convoluted tubule is LESS THAN that of filtrate in the proximal convoluted tubule. A common misconception is that the loop of Henle concentrates the urine. That can’t be true if the filtrate is LESS concentrated leaving the loop of Henle than it was when entering the loop of Henle.
Then discuss what must be moving out of the descending limb of the loop of Henle (salt or water) to make the filtrate more concentrated. The next click reveals that water moves out. You could ask students by which process water moves out of the descending limb (osmosis).
Then discuss what must be moving out of the ascending limb of the loop of Henle (salt or water) to make the filtrate less concentrated. The next click reveals that salt moves out. You could ask students by which process salt moves out (active transport).
Then discuss what must be moving out of the collecting duct to make the filtrate (soon to be urine) more concentrated. The next click reveals that water moves out. You could ask students by which process water moves out (osmosis).
Note: The copyright for this image is owned by Jessica R. Santangelo.
900
1000
1100
1200
loop of Henle

6. Your game board looks like:
Identify on your game board:
Proximal convoluted tubule
Distal convoluted tubule
Descending limb
Ascending limb
Loop of Henle
Filtrate
Interstitial fluid
Label where Na moves (is this active or passive transport)
Label where H2O moves (is this active or passive transport)
Display the blank game board that students have. They should label it with the terms listed then label where Na and H2O move and whether those are active or passive processes.
Note: The copyright for this image is owned by Jessica R. Santangelo and Peter Daniel.

7. Passive Water movement (osmosis)
Your game board looks like:
Filtrate
Interstitial fluid
From proximal tubule
To distal tubule and collecting duct
Descending limb:
Water permeable only
Ascending limb:
Water IMpermeable
Use this slide to check in with students to make sure their game boards are appropriately labeled.
To avoid students arriving at the misconception that the water and salts simply move across cell membranes, you could ask students: HOW are the water and salts getting across the cell membranes to be able to exist the loop of Henle? Students may be encouraged to draw channels in the loop of Henle to represent the channels through which the water and salt move.
Blue channels (representing protein channels called aquaporins) allow water to move. Note that blue channels only exist in the descending limb making the descending limb permeable to water but NOT to salts.
Red channels (representing active transport protein pumps) allow salt to move. Note that red channels only exist in the ascending limb making the ascending limb permeable to salts but NOT to water.
Note: The copyright for this image is owned by Jessica R. Santangelo and Peter Daniel.
Na Active Transport
Passive Water movement (osmosis)

8. The students must now Grow the Gradient
The students must now Grow the Gradient! They will be playing a game to see if they can create a salt concentration gradient in the medulla.
They should note that we will be starting with NO gradient, which doesn’t actually happen in the mammalian kidney. We are simply starting at NO gradient to give them an opportunity to Grow the Gradient, in the process discovering how water and salt movement out of the loop of Henle creates this gradient.

9. Grow the Gradient Your team’s job:
Use appropriate transport processes to establish a concentration gradient in the kidney.
Follow along as I familiarize you with the rules.
Check in at each stopping point to make sure you are on the right track.
List the expectations for students as you play the game.

10. Rule 1: Move Filtrate 1 space
Filtrate Pump
Rule 1: Move Filtrate 1 space F
300 A G
300 B E
300 B F
300 C D
300 C E
300 D
Use these slides to walk students through the first round of play. Instructors could choose one student to read each step out loud (rotating through students with each step) to encourage students to read the Rule Sheet.
Students should populate their game board with only “300” game pieces. We use a value of 300 to reflect the osmolarity of blood. Tell students that the kidneys in a healthy adult mammal are never in this state, however, since we want them to use the countercurrent multiplier to “Grow the Gradient” in the medulla, the game begins with no gradient.
The first step asks students to simply move the filtrate through the loop of Henle. They will shift game pieces over one spot. Note that game piece “A” should just be set to the side (some students will try to place game piece “A” over where a new piece, “G”, should be put. If they do this, it implies that filtrate from the ascending limb of the loop of Henle somehow moves to the top of the descending limb of the loop of Henle which is not biologically accurate).
You could ask students: what force or mechanism causes the filtrate to always move in this direction (Answer: blood pressure – as blood enters the glomerulus, filtrate is forced out of the leaky glomerular capillaries by blood pressure).

11. Rule 1: Move 1 space; Rule 2: Check Diagram!
Filtrate Pump
Rule 1: Move 1 space; Rule 2: Check Diagram!
300
300
300
300
300
300
The number in the lower right corner indicates the round of play.
Display this slide as you tell students to complete Rule 1 and Rule 2. The numbers on the boards don’t change as all they did was move the filtrate through the loop of Henle.
300
300
300 1

12. Membrane Transport: sodium and water
Rules 3 and 4
400
400
200
400
400
200
The number in the lower right corner indicates the round of play.
Students then transport sodium and water, following the rules on the score card. Sodium moves OUT of the ascending limb INTO the interstitial fluid. That means the filtrate becomes LESS salty while the interstitial fluid becomes MORE salty.
Note that the movement of 100 osmoles of salt is an arbitrary amount selected to make the game more straightforward.
Students should record their values on the Score Card. After checking that their numbers are correct, students are asked if there is a gradient in the interstitial fluid. Answer: not yet.
400
400
200
Is there a gradient in the interstitial fluid? 1

13. Filtrate Pump Rules 1 and 2 300 300 200 400 400 200 400 400 400 2
The number in the lower right corner indicates the round of play.
Students now do a second round, following Rules 1 and 2. When students feel confident that they have the appropriate game pieces on the boards, display the correct values for them to check. They should also record their values on their score card.
400
400
400 2

14. Is there a gradient in the interstitial fluid?
Membrane Transport
Rules 3 and 4
350
350
150
400
400
200
The number in the lower right corner indicates the round of play.
Students now complete Rules 3 and 4. They record their values on the Score Card then the instructor displays the correct values so students can check if they are correct. Students are then asked if there is a gradient in the interstitial fluid. Answer: Yes, there is a gradient of 150 mosmol/liter. They are growing the gradient!
500
500
300
Is there a gradient in the interstitial fluid? 2

15. Filtrate Pump Rules 1 and 2 300 300 200 350 350 300 400 400 500 3
The number in the lower right corner indicates the round of play.
Students complete a third round in the same manner. They follow Rules 1 and 2, record their values, the instructor displays the correct values and students check their results against the correct values.
400
400
500 3

16. Is there a gradient in the interstitial fluid?
Membrane Transport
Rules 3 and 4
350
350
150
425
425
225
The number in the lower right corner indicates the round of play.
Students then complete Rules 3 and 4 of Round 3, recording their values then checking their values against the correct values displayed by the instructor. They are asked if there is a gradient. Answer: Yes there is a gradient of 200 mosm/l.
550
550
350
Is there a gradient in the interstitial fluid? 3

17. What would happen if you completed
more rounds of play?
You may choose to ask the students:
What would happen if they completed more rounds of play and even ask them to do so to find out what happens. Answer: the gradient increases until it reaches an equilibrium in which the gradient persists but does not get larger.

18. What would happen if the loop of Henle was longer?
You may choose to ask the students:
What would happen if the loop of Henle was longer? Answer: a larger gradient develops.
What would happen to the amount of water retained by the body if the loop of Henle was longer? Answer: more water would be retained.
In what type of environment might it be beneficial to retain more water? Answer: a dry environment like a desert

19. What happens to filtrate concentration as it travels down the collecting duct?
You may choose to ask the students:
What happens to the filtrate concentration as the filtrate travels down the collecting duct? You can ask students to draw the collecting duct on a separate sheet of paper that they place to the right of their game bards. As they move filtrate out of the ascending limb of the loop of Henle and down the collecting duct, water moves out of the collecting duct because the collecting duct is surrounded by the medulla which has a salt concentration gradient. The water moves out via special channels called aquaporins (that is, it doesn’t just diffuse across lipid bilayers).