The Kidney

Question 1

What are the main functions of the kidneys?

Answer 1

• Regulation of water, salts and acid-base
• Removal of metabolic waste (excretion)
• Removal of foreign chemicals (excretion)
• Gluconeogenesis
• Hormone/enzyme production

Question 2

Describe the gross anatomy of the kidney.

Answer 2

Comprised of inner medulla and outer medulla wrapped in a capsule.

Nephrons which span the cortex and medulla feed into the renal pelvis which supply the ureter with urine.

Nephrons are the functional unit.

Question 3

Describe the structure of the nephron along with the role of each portion.

Answer 3

Glomerulus - Capillary that forces water and dissolved particles out of the blood.

Bowman’s Capsule - Catches water forced out of the glomerulus and controls what is filtered out (podocytes)

Proximal convoluted tubule - Responsible for bulk of nutrient reabsorption

Loop of Henle - Countercurrent multiplier that reabsorbs water and Na+

Distal convoluted tubule - Reabsorbs water and ions

Collecting duct - tububles feed together here, reabsorbs water

Question 4

Explain how the glomerulus forces water out of the bloodstream:

Answer 4

• Afferent arteriole is larger than the efferent arteriole
• Raises the pressure causing water and dissolved solutes to be forced out

Question 5

What 3 key process occur in the nephron?

Answer 5

• Glomerular filtration
• Tubular secretion
• Tubular reabsorption

Question 6

What is tubular secretion?

Answer 6

The secretion of substances from the blood into the nephron tubule.

Question 7

What is tubular reabsorption?

Answer 7

The reabsorption of substances from the tubule back into the blood.

Question 8

Explain how the glomerulus and Bowman’s capsule is specialised for filtration:

Answer 8

• Podoctyes and basement cells of Bowman’s ensure no large particles are filtered
• Glomerulus is 1 cell thick, making movement of water and solutes easier

Question 9

What is GFR?

Answer 9

Glomerular filtration rate. The volume of fluid filtered by the nephrons in a given amount of time.

Question 10

How is GFR regulated by the kidneys?

Answer 10

The blood pressure in the afferent and efferent arterioles is adjusted, thereby controlling the amount of water and solutes forced out.

Question 11

Which one is the afferent arteriole?

Which one is the efferent arteriole?

Answer 11

The one flowing into the glomerulus.

The one flowing out of the glomerulus.

Question 12

What effect would constricting the afferent arteriole have on GFR?

Answer 12

Decreases GFR

Question 13

What effect would constricting the efferent arteriole have on GFR?

Answer 13

Increases GFR

Question 14

What effect would dilating the afferent arteriole have on GFR?

Answer 14

Increases GFR

Question 15

What effect would dilating the efferent arteriole have on GFR?

Answer 15

Decreases GFR

Question 16

Describe the movement of Na+ and associated molecules in the proximal tubule:

Answer 16

• Na+ counter transported against H+ into the tubule cells
• Na+ also cotransported with other molecules into tubule cells
• Na+ pumped by Na+/K+ pump into interstitial space
• K+ from pump is recycled from tubule cells to interstitial space by K+ channels
• Other molecules transported independently

Question 17

Describe the movement of Na+ and associated molecules in the loop of Henle:

Answer 17

• Na+, K+ and 2Cl- are cotransported by NKCC into tubule cells
• Na+ from tubule cells is pumped by Na/K+ pump into interstitial space
• 2Cl- is transported by Cl- channel into the interstitial space
• K+ which was transported into the tubule cells is recycled into the tubules and interstitial space

Question 18

Describe the movement of Na+ and associated molecules in the collecting duct:

Answer 18

• Na+ transported by an Na+ channel into the tubule cells from the tubule
• Na+ in the tubule cells is pumped by Na+/K+ pumps into the interstitial space
• K+ has the opposite pathway, being pumped into the tubule cells from interstitial then transported by nK+ channels into the tubule

Question 19

How does aldosterone regulate Na+ reabsorption in the collecting duct?

Answer 19

• Aldosterone absorbed by tubule cells
• Enters nucleus and binds to DNA
• Increases expression of Na+ and K+ channels as well as Na+/K+ pumps necessary for reabsorption

This increases the rate of Na+ reabsorption.

Question 20

Explain how the loop of Henle reabsorbs water:

Answer 20

• Descending loop is water permeable, not ascending loop
• Ascending loop reabsorbs Na+
• Thus decreasing water conc of interstitial space
• By osmosis water diffuses out of descending loop into interstitial space

In doing so, water is reabsorbed from the descending loop of Henle.

Question 21

Explain how the loop of Henle’s circulatory network aid water reabsorption:

Answer 21

• Blood flow is counter to nephron flow
• Thus fresh blood (low in salt) encounters salty ascending loop and absorbs salt
• Now salty blood encounters watery descending loop
• Water diffuses by osmosis into the blood

Works as a countercurrent multiplier.

Question 22

Describe how water is reabsorbed in collecting ducts:

Answer 22

Aquaporin channels line the tubule-tubule cell membrane as well as the tubule cell-interstitial space membrane. This allows for water to flow into the interstitial space.

Question 23

Explain how vasopressin regulates water reabsorption in the collecting ducts:

Answer 23

• Vasopressin binds to receptors on collecting duct tubule cells
• Causes aquaporin-containing vesicles to release aquaporin channels which bind to the membrane
• More aquaporin channels in the membrane increase water reabsorption

Question 24

How does the kidney help regulate body pH?

Answer 24

It regulates HCO3- reabsorption, thereby influencing the body’s pH buffer system.

Question 25

Summarise carbonate reabsorption in the kidneys. What is the net result?

Answer 25

- Carbonic acid is formed by carbonic anhydrase in tubule cells - H+ released by carbonic acid transported into the tubule - Binds with HCO3- forming carbonic acid - Degrades into H20 and CO2 which are transported into tubule cells There's no net change in HCO3-

Question 26

How is H+ secreted by the kidneys? What is the net result?

Answer 26

- Carbonic acid synthesised by carbonic anhydrase releases H+ which is transported into the tubule - H+ binds to phosphate ions and is excreted By excreting H+ released by carbonic acid, the HCO3- ions are free to enter the blood stream. There's a net gain of HCO3-.

Question 27

Describe the process of glutamine metabolism in the kidney. What is the net result of this process?

Answer 27

- Glutamine filtered into the tubule and glutamine from the plasma enters the tubule cells - Glutamine in tubule cells is metabolised into into NH4+ and HCO3- - HCO3- is transported into the body There's a net gain of HCO3-.

Question 28

How do the kidneys generate carbonate ions for the body?

Answer 28

By: - Metabolising glutamine - Secreting H+

Question 29

What is alkalosis?

Answer 29

When the body's pH becomes alkaline due to a loss of H+.

Question 30

What is acidosis?

Answer 30

When the body's pH becomes acidic due to a gain of H+.

Question 31

Why does the body's pH have to be tightly regulated?

Answer 31

Many processes depend on the pH of the body. If it fluctuates, essential processes may not be able to occur.

Question 32

How do the kidneys respond to acidosis?

Answer 32

- All filtered HCO3- is reabsorbed - HCO3- absorbed as H+ excreted bound to phosphate - More glutamine is metabolised, generating HCO3- for the body Overall increase in HCO3- reabsorption thus increasing pH.

Question 33

How do the kidneys respond to alkalosis?

Answer 33

- Less H+ excreted into tubule, increasing HCO3- elimination - Decreased glutamine metabolism Overall decrease in HCO3- reabsorption thus reducing pH.

Question 34

How does urine pH influence drug excretion?

Answer 34

Depending on the condition urine pH can be acidic/basic. This can cause basic/acidic drugs to ionise and thus make them less likely to be reabsorbed.