Homeostasis - kidneys

Question 1

Define osmoregulation (1)

Answer 1

Control of blood water potential via homeostatic mechanisms

Question 2

Describe the structure of a kidney (7)

Answer 2

Fibrous capsule - outer membrane that protects the kidney
Cortex - outer region consists of Bowman’s capsules, convoluted tubules and blood vessels
Medulla - inner region consisting of loop of Henle, collecting ducts and blood vessels
Renal pelvis - funnel-shaped cavity that collects urine into the ureter
Ureter - tube that carries urine into the bladder
Renal artery - supplies kidneys with blood from the heart via the aorta
Renal vein - returns blood to the heart via the vena cava

Question 3

Describe the structure of a nephron (5)

Answer 3

Bowman’s capsule at the start of the nephron - cup shaped, surrounds glomerulus, inner layer of podocytes
Proximal convoluted tubule (PCT) - series of loops surrounded by capillaries, walls made of epithelial cells with microvilli
Loop of Henle - hairpin loops extends from cortex into medulla
Distal convoluted tubule (DCT) - similar to PCT but fewer capillaries
Collecting duct - DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney

Question 4

Describe the blood vessels associated with a nephron (2)

Answer 4

Wide afferent arteriole - from renal artery enters renal capsule and form glomerulus
Efferent arteriole - branches to form capillary network that surrounds tubules

Question 5

Explain how glomerular filtrate is formed (5)

Answer 5

Higher than normal hydrostatic pressure in glomerular causes ultrafiltration in Bowman’s capsule
Small molecules such as urea, water, glucose, mineral ions (Na+/Cl-) pass through basement membrane which acts as a filter
Blood cells and proteins too large so stay behind
Fluid travels through fenestrations in capillary endothelium between podocytes

Question 6

How are cells of the Bowman’s capsule adapted for ultrafiltration (2)

Answer 6

Fenestrations between capillary endothelium
Fluid can pass between and under folded membrane of podocytes

Question 7

Explain what happens during selective reabsorption (3)

Answer 7

Sodium ions actively transported out of cells lining the PCT into blood capillaries to reduce their sodium ion concentration
This allows sodium ions from the filtrate to move into the surrounding cells via facilitated diffusion, but they can only come in via co-transport with another molecule
This allows useful molecules to be reabsorbed back into the blood and ensures they do not leave the body via urine

Question 8

How are cells in the proximal convoluted tubule adapted for selective reabsorption (3)

Answer 8

Microvilli - large surface area for co transporter proteins
Many mitochondria - ATP for active transport
Folded basal membrane - large surface area

Question 9

Describe the process occurring at the loop of Henle (3)

Answer 9

Sodium ions actively transported out of the ascending limb (which then re-enter the descending limb)
This lowers the water potential between the low limbs (interstitial fluid), drawing water out of the descending limb by osmosis into the medulla region
Water from descending limb then enters capillaries returning to the blood

Question 10

Describe the structure of the ascending and descending limb (4)

Answer 10

Ascending limb - Narrow and highly permeable to water
Descending limb - Wider and impermeable to water

Question 11

Describe what happens in the ascending limb of the loop of Henle (2)

Answer 11

Water remains
Na+/Cl- ions removed by active transport

Question 12

Describe what happens in the descending limb of the loop of Henle (2)

Answer 12

Water moves out by osmosis
Na+/Cl- move in via facilitated diffusion

Question 13

Explain the role of the distal convoluted tubule (2)

Answer 13

Reabsorption of:
Water via osmosis
Of ions via active transport

Question 14

How does the distal convoluted tubule make final adjustments to the water and salts that are reabsorbed (1)

Answer 14

Permeability of the walls become altered under the influence of various hormones

Question 15

Explain what happens at the collecting duct (3)

Answer 15

Permeable to water so water moves out by osmosis as the counter current multiplier ensures there is always a water potential gradient along the entire collecting duct
All water that leaves collecting duct moves into the interstitial fluid and then into the blood via surrounding capillaries
The remaining filtrate becomes urine which then leaves the body

Question 16

Explain how urea is concentrated in the filtrate (4)

Answer 16

Reabsorption of water by osmosis
At the PCT/descending limb of loop of Henle
At the DCT/ CD
Active transport of ions

Question 17

Explain why it is important to maintain an Na+ gradient (2)

Answer 17

Counter current multiplier - filtrate in collecting duct is always beside an area of interstitial fluid that has a low water potential
Maintains water potential gradient for maximum reabsorption of water

Question 18

What can cause blood water potential to change? (4)

Answer 18

Level of water intake
Level of ion intake in diet
Level of ions used in metabolic processes or excreted
Sweating

Question 19

Explain osmoregulation (3)

Answer 19

Osmoreceptor cells, found in the hypothalamus, shrink as water moves out by osmosis
ADH secreted into capillaries by the posterior pituitary gland
ADH travels to kidneys in blood and increase the permeability of the cells of the distal convoluted tubule and collecting duct to water

Question 20

How does antidiuretic hormone (ADH) work? (5)

Answer 20

Causes vesicles bound with aquaporin proteins to fuse with cell surface membrane of cells of DCT and CD
Increase permeability of DCT and CD
Also makes cells lining CD more permeable to urea:
Water potential in interstitial fluid decreases
More water reabsorbed = more concentrated urine