control of blood water potential

Question 1

Describe the structure of a nephron

Answer 1

● Nephron = basic structural and functional unit of the kidney (millions in the kidney)
● Associated with each nephron are a network of blood vessels

Question 2

1. Bowman’s / renal capsule

Answer 2

Formation of glomerular filtrate (ultrafiltration)

Question 3

2. Proximal convoluted tubule

Answer 3

Reabsorption of water and glucose (selective reabsorption)

Question 4

Loop of Henle

Answer 4

Maintenance of a gradient of sodium ions in the medulla

Question 5

4. Distal convoluted tubule and collecting duct

Answer 5

Reabsorption of water (permeability controlled by ADH)

Question 6

Describe the formation of glomerular filtrate

Answer 6

1. High hydrostatic pressure in glomerulus
   ○ As diameter of afferent arteriole (in) is wider than efferent arteriole (out)
2. Small substances eg. water, glucose, ions, urea forced into glomerular filtrate, filtered by:
   a. Pores / fenestrations between capillary endothelial cells
   b. Capillary basement membrane
   c. Podocytes
3. Large proteins / blood cells remain in blood

Question 7

Describe the reabsorption of glucose
by the proximal convoluted tubule

Answer 7

1. Na+ actively transported out of epithelial cells to capillary
2. Na+ moves by facilitated diffusion into epithelial cells down a concentration gradient, bringing glucose against its concentration gradient
3. Glucose moves into capillary by facilitated
   diffusion down its concentration gradient

Question 8

Describe the reabsorption of water
by the proximal convoluted tubule

Answer 8

● Glucose etc. in capillaries lower water potential
● Water moves by osmosis down a water potential gradient

Question 9

Describe and explain how features of the cells in the PCT allow the rapid
reabsorption of glucose into the blood

Answer 9

● Microvilli / folded cell-surface membrane → provides a large surface area
● Many channel / carrier proteins → for facilitated diffusion / co-transport
● Many carrier proteins → for active transport
● Many mitochondria → produce ATP for active transport
● Many ribosomes → produce carrier / channel proteins

Question 10

Explain the importance of maintaining a gradient of sodium ions in the
medulla (concentration increases further down)

Answer 10

● So water potential decreases down the medulla (compared to filtrate in collecting duct)
● So a water potential gradient is maintained between the collecting duct and medulla
● To maximise reabsorption of water by osmosis from filtrate

Question 11

Describe the role of the loop of Henle in maintaining a gradient of sodium
ions in the medulla

Answer 11

1. In the ascending limb:
   ○ Na+ actively transported out (so filtrate concentration decreases)
   ○ Water remains as ascending limb is impermeable to water
   ○ This increases concentration of Na+
   in the medulla, lowering water potential
2. In the descending limb:
   ○ Water moves out by osmosis then reabsorbed by capillaries (so filtrate concentration increases)
   ○ Na+ recycled’ → diffuses back in

Question 12

Describe the reabsorption of water by the distal convoluted tubule and
collecting ducts

Answer 12

● Water moves out of distal convoluted tubule & collecting
duct by osmosis down a water potential gradient
● Controlled by ADH which increases their permeability

Question 13

Describe the role of the hypothalamus in osmoregulation

Answer 13

1. Contains osmoreceptors which detect increase OR decrease in blood water potential
2. Produces more ADH when water potential is low OR less ADH when water potential is high

Question 14

Describe the role of the posterior pituitary gland in osmoregulation

Answer 14

Secretes (more / less) ADH into blood due to signals from the hypothalamus

Question 15

Describe the role of antidiuretic
hormone (ADH) in osmoregulation

Answer 15

1. Attaches to receptors on collecting duct (and distal convoluted tubule)
2. Stimulating addition of channel proteins
   (aquaporins) into cell-surface membranes
3. So increases permeability of cells of collecting duct and DCT to water
4. So increases water reabsorption from collecting duct / DCT (back into blood) by osmosis
5. So decreases volume and increases
   concentration of urine produced

Question 16

How is urea removed from the blood

Answer 16

Hydrostatic pressure / description of pressure / description of how pressure generated;

Causes ultrafiltration (Allow description of ultrafiltration) at Bowman’s capsule / glomeruli / renal capsule;

Through basement membrane;

Enabled by small size urea molecu

Question 17

how is urea concnetrated in the filtrate

Answer 17

Reabsorption of water / by osmosis;

At the PCT / descending LoH;

At the DCT / CD;

Active transport of ions / glucose creates gradient (in context);

Ignore references to facilitated diffusion or to selective reabsorption.