the kidneys

Question 1

What is Osmoregulation?

Answer 1

Osmoregulation is the control of the water potential of the blood

Question 2

What substances need to be controlled in order to do this? [3]

Answer 2

• Water
• Salts (sodium ions)
• Urea

Question 3

Symptoms of Dehydration

Answer 3

Mild dehydration can cause dizziness, a dry mouth and
concentrated urine.
Severe dehydration can cause death.

Question 4

What causes dehydration? [6]

Answer 4

• heavy sweating
• low water intake
• eating salty food
• breathing dry air
• caffeine and alcohol
• diarrhoea

Question 5

What does the Urinary System do? [2]

Answer 5

• To filter the blood, remove urea and excess water and
  salts, creating urine
• Any nutrients that are needed by the body are reabsorbed

Question 6

List the parts of the Kidney structure [8]

Answer 6

• Renal Artery
• Renal Vein
• Capsule
• Cortex
• Medulla (organised in pyramids)
• Nephrons
• Pelvis
• Ureter

Question 7

List the parts of the Nephron structure [9]

Answer 7

• Branch of Renal Artery
• Branch of Renal Vein
• Capillary
• Glomerulus
• Renal/Bowman’s Capsule
• Proximal Convoluted Tubule
• Loop of Henlé
• Distil Convoluted Tubule
• Collecting Duct

Question 8

Stages in the Kidney

Answer 8

Ultrafiltration - between the glomerulus and renal capsule

Selective reabsorption - at the proximal convoluted tubule

Reabsorption of water and the counter-current multiplier - at the loop of Henle, distal convoluted tubule and collecting duct

Question 9

Renal (Bowman’s) capsule

Answer 9

Surrounds a network of capillaries (the glomerulus). Inner layer is composed of podocytes

Question 10

Afferent and Efferent Arteriole

Answer 10

Blood vessels entering and leaving the renal capsule. Afferent is wider than efferent, leading to an increase in blood pressure

Question 11

Ultrafiltration in the Glomerulus [8]

Answer 11

• Diameter of efferent arteriole is smaller than afferent arteriole
• build-up of hydrostatic pressure
• water/glucose / ions squeezed out capillary into Bowman’s capsule through pores in capillary endothelium, basement membrane and podocytes
• large proteins too large to pass

Question 12

Selective Re-absorption

Answer 12

Any useful substances are reabsorbed in the proximal convoluted tubule (PCT)

Question 13

Adaptations of the proximal convoluted tubule [3]

Answer 13

Microvilli - increases surface area

Infoldings along the tubule - increases surface area

Protein channels and pumps in the membrane Mitochondria - provide ATP for active transport

Question 14

The proximal convoluted tubule reabsorbs:

Answer 14

• All of the glucose
• All of the amino acids
• Most of the water
• Blood pH is regulated
• Blood salt levels are regulated
  Reabsorbed molecules pass into the surrounding capillaries.
  Urea, some water and some salts are left in the nephron filtrate.

Question 15

Reabsorption - Co-transport [5]

Answer 15

• Sodium ions move into the capillaries from the PCT epithelial cells by active transport
• PCT lining cells have a lower concentration of sodium ions compared to the filtrate
• Sodium ions move down their concentration gradient, using facilitated diffusion from the PCT lumen into the PCT epithelial cells.
• Glucose (or amino acids) move with the sodium ions = co-transport
• The molecules then move into the capillary by facilitated diffusion

Question 16

Explain why glucose is found in the urine of a person with untreated diabetes

Answer 16

In healthy people
- glucose is removed by ultrafiltration
- glucose is ALL reabsorbed by selective reabsorption via co-transport
In people with untreated diabetes…
- More glucose is removed in ultrafiltration due to high glucose concentration
- Channel/carrier proteins required for selective reabsorption
- Channel/carrier proteins can’t work fast enough to reabsorb it all

Question 17

The Counter-Current Multiplier [3]

Answer 17

• Exchange of substances is greater if liquids flow in opposite directions rather than the same (e.g. fish gills)
• The loop of Henle acts to maintain a lower water potential in the interstitial fluid (surrounding the cells of the medulla) and the collecting duct
• This allows reabsorption of water into the blood, and concentration of urine through the collecting duct

Question 18

The Counter-Current Multiplier in the Loop of Henle [3]

Answer 18

• Describes how to maintain a gradient of Na+ in medulla by the loop of Henle.
• Na+ actively transported out ascending limb to medulla to lower water potential
• water moves out descending limb + DCT + collecting duct by osmosis due to this water potential gradient

Question 19

The Counter-Current Multiplier - Summary

Answer 19

The ascending limb - pumps ions into the interstitial fluid and then to the blood, lowering the water potential

The descending limb - the lower water potential of the interstitial fluid and blood cause the water to move from the limb into the blood by osmosis

Question 20

The Loop of Henle

Answer 20

Descending limb = narrow, highly permeable to water

Ascending limb = wider, impermeable to water, lots of protein pumps for ions

Question 21

Difference between osmolarity and water potential

Answer 21

Osmolarity is the tendency of a substance to attract water

Water potential is the tendency of a substance to supply water

Question 22

The Distal Convoluted Tubule [4]

Answer 22

• The purpose of the DCT is to control the pH of the blood by selectively reabsorbing ions
• It also will reabsorb water
• The levels of reabsorption are controlled by hormones
• The cells of the DCT are adapted with microvilli and mitochondria

Question 23

The Collecting Duct [3]

Answer 23

• The collecting duct is permeable so water is reabsorbed into the interstitial fluid and then moves into the capillaries
• As the water is reabsorbed, the water potential of the filtrate lowers
• Due to the counter-current multiplier, the water potential of the interstitial fluid is always lower than that of the filtrate

Question 24

Osmoregulation [4]

Answer 24

• The blood concentration is monitored by osmoreceptors in the hypothalamus
• If the water potential of the blood falls, the osmoreceptor cells shrink (due to osmosis)
• This causes the hypothalamus to produce antidiuretic hormone (ADH)
• ADH is secreted by the posterior pituitary gland

Question 25

Difference bewtween Diuretic and Antidiuretic

Answer 25

Diuretic - makes you wee more

Antidiuretic - reduces the amount of urine produced

Question 26

The Action of ADH [6]

Answer 26

The Action of ADH [6]
- ADH travels in the blood to the kidney
- It binds to receptors on the cell-surface membrane (capillary side)
- This activates an enzyme called phosphorylase
- Phosphorylase causes special vesicles to fuse with the membrane (collecting duct lumen side)
- These vesicles contain proteins called aquaporins, which form channels in the membrane
- This increases the permeability of the collecting duct

Question 27

Osmoregulation and ADH

Answer 27

Stimulus: Reduced water concentration in the blood

Receptor: Osmoreceptors in the hypothalamus

Effector: Hypothalamus produces ADH, secreted by the
posterior pituitary gland

Response: DCT and collecting duct become more permeable

Question 28

The nephron osmoregulation [3]

Answer 28

• The urine released into the pelvis is more or less concentrated depending upon the blood concentration
• Excessive sweating and eating salty food will produce concentrated urine
• Drinking and cold weather will produce dilute urine