The Kidney

Question 1

What are the 2 major important roles of the kidney?

Answer 1

-excretion
-homeostasis

Question 2

What do the kidneys do?

Answer 2

-Filter nitrogenous waste products out of the blood (UREA)
-Maintain the water potential and pH of the blood and tissue fluid

Question 3

What are nephrons? What are their role in the kidney?

Answer 3

-each kidney is made up of millions of nephrons (1.5 million each)
-they are the functional unit of the kidney, responsible for forming urine, which then passes down the ureters to the bladder for storage
-when full, the sphincter muscles at the base of the bladder opens, allowing urine to flow out of the body via the urethra

Question 4

Draw and label the parts of the kidney structure and the nephron

Question 5

What is the definition of ultrafiltration?

Answer 5

The filtering of substances out of the blood at the molecular level

Question 6

How does the structure of the glomerulus and renal capsule cause substances to be filtered out?

Answer 6

-the afferent arteriole (coming from the arenal artery) enters the glomerulus and has a wider diameter than the efferent arteriole which leaves the glomerulus
-blood enters the glomerulus at a faster rate than it leaves, which creates a high hydrostatic pressure in the glomerulus capillaries, forcing molecules out of the glomerulus and into the renal capsule

Question 7

What are the 3 layers that molecules have to pass through in order to move from the blood into the renal filtrate?

Answer 7

1) the walls of the capillaries (endothelium) in the glomerulus. These have pores called fenestrations which let water and small solutes through
2) the basement membrane of the renal capsule
3) podocytes (special cells)

Question 8

What substances will stay in the blood and why?

Answer 8

-red blood cells
-white blood cells
-platelets
-large plasma proteins

=anything TOO BIG to fit through basement membrane

Question 9

What substances will leave the blood and why?

Answer 9

-water
-amino acids
-glucose
-vitamins
-mineral ions
-urea
-small hormones
-small proteins

=anything SMALL ENOUGH to fit through basement membrane

Question 10

Why is the basement membrane important?

Answer 10

-made of collagen and glycoproteins
-most of the molecules in the blood plasma are small enough to pass through the basement membrane

Question 11

What are podocytes and why are they important?

Answer 11

-they’re an additional filter
-they have extensions called pedicels that wrap around the capillaries forming slits, this ensures that any blood cells or large proteins that did get across the basement membrane doesn’t enter the filtrate

Question 12

What is the GFR?

Answer 12

-Glomerular Filtration Rate
-this is the volume of blood filtered through the kidneys in a given time
-it gives an idea of how well the kidneys are working

Question 13

What is the function of the proximal convoluted tube (PCT)

Answer 13

-selective reabsorption of useful substances back into the blood
-the cells lining this tubule are adapted for the reabsorption of substances from the lumen across them, into the blood capillaries around them

Question 14

How are the cells lining the PCT adapted for their role?

Answer 14

-they have microvilli= increases surface area for reabsorption to occur at a fast rate
-lots of mitochondria= high respiration rate so high production of ATP for active transport during reabsorption

Question 15

Explain the process of selective reabsorption of substances from the PCT back into the blood

Answer 15

1) the cells lining the PCT contain sodium potassium pumps in their membranes. These use ATP to actively pump sodium ions out of the cells and potassium ions in

2) this lowers the concentration gradient of sodium ions inside the PCT cells, so it moves back in (facilitated diffusion) down the concentration gradient from the lumen of the PCT into the cells. It enters through a protein carrier which carries glucose in at the same time (co transporter) by facilitated diffusion. The protein carrier has a complementary shape for both the glucose molecule and the sodium ion at the same time

3) the concentration of glucose then increases inside the cells lining the PCT, causing glucose to move (facilitated diffusion) from the cells into the tissue fluid, then into the nearby blood capillaries, down a concentration gradient

4) as the glucose moves across into the blood it lowers the water potential of the cells lining the PCT and the blood, so water moves in by osmosis, from the PCT lumen into the cells lining the PCT, into the blood capillaries, down a water potential gradient.

5) any very small proteins that were initially filtered out of the blood can be reabsorbed at this point

6) most of the urea stays in the filtrate which becomes urine, but a small amount will diffuse back across to the blood

7) amino acids are selectively reabsorbed back into the blood using the same method as glucose

Question 16

How much of the filtrate will have been absorbed by the time it’s passed along the PCT?

Answer 16

-80-85% of it has been selectively reabsorbed back into the blood
-what’s left in the filtrate is essentially urine

Question 17

Explain what the role of the loop of Henle is in the kidney

Answer 17

-to create a low water potential (high solute concentration) in the tissue fluid around the nephron, in the medulla region
-the loop has a descending limb, a hairpin bend, and an ascending limb

Question 18

What is meant by a counter current multiplier?

Answer 18

-this is where fluid flows in opposite directions, in two vessels close to one another
-it maximises the concentration gradient that can be built up between the two, all the way along the length
-the ascending limb uses ATP to create the concentration gradient needed for water to be removed from the descending limb, moving back into the blood

Question 19

Explain the process that occurs at the loop of henle

Answer 19

1) the renal fluid entering the L of H from the PCT is isotonic to the blood since a lot of water was reabsorbed from the PCT

2) the DESCENDING limb of the L of H is permeable to water. Water can leave the nephron here if there is a water potential gradient. It can then move back into the nearby network of blood capillaries by osmosis, so water is reabsorbed

3) when the renal fluid inside the L of H reaches the hairpin at the bottom, it is now hypertonic to the blood (more concentrated/ lower water) because a lot of water has left. This makes the concentration of Na and Cl ions inside higher

4) the renal fluid now moves into the ASCENDING limb, the walls of which are permeable to salts, but not water. In the LOWER part of the limb, salts diffuse out through facilitated diffusion down a concentration gradient. This creates a lower water potential in the tissue fluid around the L of H

5) HIGHER up in the ascending limb, salts are now actively pumped out into the tissue fluid using ATP, maintaining the low water potential outside the L of H

6) some salts diffuse back into the descending limb to be reused

7) by the time the filtrate inside the nephron reaches the DCT, it is more dilute again due to most of the salts having left (hypotonic)

8) now as the filtrate finally passes down the collecting ducts, it encounters increasingly lower water potential outside, so lots of water moves out along here, by osmosis, and re-enters the surrounding blood capillaries

Question 20

For mammals, what is the result of the role of the loop of henle and why is this advantageous?

Answer 20

-mammals can produce urine which is hypertonic (more concentrated) to the blood plasma because this mechanism allows them to save a lot of water

Question 21

Explain the role of the Distal Convoluted Tubule

Answer 21

-this is where further balancing of the water and salt content occurs
-the concentration of urea is much higher now than it was previously because a lot of water has left the filtrate from the L of H and the PCT back into the blood
-the cells lining the DCT also have microvilli and lots of mitochondria for carrying out active transport
-if the body lacks salt, sodium ions are actively pumped out of the DCT, then chloride ions follow and move back into the blood (electrochemical gradient)
-this tube is also permeable to water, in response to the hormone ADH so more water can be saved

Question 22

Explain the role of the collecting duct

Answer 22

-the walls of the collecting ducts respond to the hormone ADH
-it is here the final volume and concentration of urine is determined

Question 23

Explain the final product of the urine and how this occurred

Answer 23

-final urine produced is hypertonic when compared to blood plasma (high solute concentration, lower water potential)
-this is due to the large volume of water that has been reabsorbed as the filtrate passes along the nephron
-this is very important as it ensures the body does not become severely dehydrated

Question 24

A) what is unusual about the y-axis of the graph and why is it shown this way?
B) Explain the shape of each substances line on the graph

Answer 24

A) it’s a log scale, as goes up in factors of 10 = this allows you to plot data with a huge variation to it so it fits in one graph

B) -glucose and amino acids decrease = selectively reabsorbed back into the blood by the end of the PCT, so none js left in filtrate
-urea increases in concentration as water leaves filtrate and is reabsorbed into the blood in the loop of henle and collecting duct
-sodium enters loop of henle as water goes out, so increases in concentration in descending limb, then decreases in ascending limb as it diffuses out to create the water potential gradient . They then increase in concentration again as water leaves through osmosis back into the blood in the DCT and CD, making a hypertonic filtrate