Kidneys and Water Regulation

Question 1

Functions of kidney

Answer 1

• Regulation of water, inorganic ion balance and acid-base balance
• Removal of metabolic waste products
• Removal of foreign chemicals
• Gluconeogenesis
• Production of hormones/enzymes

Question 2

Hormone/enzymes produced by the kidney

Answer 2

• EPO
• Renin
• Conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin

Question 3

Structural components of the kidney

Answer 3

• Nephron
• Renal medulla
• Renal cortex
• Ureter
• Renal pelvis

Question 4

• Forms a filtrate from blood free from cells and proteins
• Filtrate leaves corpuscle and enters the tubule
• Substances added/removed in tubule
• Remaining fluid combines in collecting ducts

Answer 4

Renal Corpuscle

Question 5

• Visceral layer of Bowman’s capsule
• Blood in the glomerular is separated from fluid in Bowman’s space by a filtration barrier
  * Single cell lining of the capillary endothelium
  * Non-cellular protein-rich layer of basement membrane
  * Single cell lining of Bowman’s capsule

Answer 5

Podocyte

Question 6

Specialised for filtering

Answer 6

Glomerular capillaries

Question 7

• Supply the nephron/tubules with own blood supply

- Form the veins where blood leaves the kidney

Answer 7

Peritubular capillaries

Question 8

• Renal corpuscle close to cortical-medullary junction
• Henle’s loop plunges deep into medullar
• Generate osmotic gradient for water reabsorption

Answer 8

Juxtamedullary nephrons

Question 9

• Corpuscle located in outer cortex

- Henle’s loop do not plunge deep into medullar

Answer 9

Cortical nephrons

Question 10

• Part of ascending loop of Henle

- Contain sensors that detect changes in blood composition

Answer 10

Macula Densa

Question 11

Functions of juxtaglomerular apparatus

Answer 11

• Secrete renin
• Formation of angiotensin II
• Controls blood pressure
  * Vasoconstriction
  * Sodium/water retention

Question 12

Non-penetrating solutes

Answer 12

• Na+ and Cl- on outside

- K+ on inside

Question 13

Stimuli that cause kidney to secrete renin

Answer 13

• Renal sympathetic nerves
• Intrarenal baroreceptors
• Macula densa

Question 14

Forces that favour filtration

Answer 14

P-GC - Glomerular Capillary BP

Question 15

Forces that oppose filtration

Answer 15

• P-BS - fluid pressure in Bowman’s space

- pi-GC - osmotic force due to protein in plasma

Question 16

Osmotic regulation

Answer 16

1 - Excess H2O ingested
2 - Decreased fluid osmolarity
3 - Decreased firing by hypothalamic osmoreceptors
4 - Posterior pituitary gland - decreased vasopressin secretion
5 - Decreased plasma vasopressin
6 - Collecting ducts - decreased tubular permeability to H2O
7 - H2O reabsorption
8 - Increased H2O excretion

Question 17

Volume regulation

Answer 17

1 - Decreased plasma volume
2 - Decreased venous, atrial & arterial pressure
3 - Posterior pituitary - increased vasopressin secretion
4 - Increased plasma vasopressin
5 - Collecting ducts - increased tubular permeability to H2O
6 - Increased H2O reabsorption
7 - Decreased H2O excretion

Question 18

Water permeability

Answer 18

• Depends on pressure of aquaporins
• Always high in proximal tubule
• In cortical and medullary collecting ducts can vary due to physiological control

Question 19

• Stimulates insertion of aquaporins
• Binding increases production of cAMP
• Activates protein kinase which phosphorylates proteins, increasing rate of fusion of AQP2
• Water into blood vessels through AQP3&4

Answer 19

Antidiuretic Hormone (ADH)

Question 20

Primary Active Na+ Reabsorption

Answer 20

• Out of cells and into interstitial fluid
• Keeps the intracellular concentration of Na+ low compared to tubular lumen
• Na+ moves downhill out of tubular lumen into tubular epithelial cells
• In proximal tubule, Na+ reabsorption drives reabsorption of co-transported substances and secretion of H+

Question 21

Coupling of Water Reabsorption to Na+ Reabsorption

Answer 21

1 - Na+ transported from tubular lumen to interstitial fluid across epithelial cells
2 - Removal of solutes from tubular lumen decreases local osmolarity of tubular fluid adjacent to the cell
3 - Appearance of solute in interstitial fluid outside cell increases the local osmolarity
4 - Difference in water concentration causes net diffusion of water into interstitial fluid
5 - Everything dissolved in interstitial fluid move by bulk flow into peritubular capillaries

Question 22

Countercurrent Multiplier System

Answer 22

• Along ascending limb, Na+ and Cl- are absorbed into interstitial fluid. No water follows, so interstitial fluid becomes hyperosmotic.
• Net diffusion of water out of descending limb into interstitial fluid.
• Osmolarities of descending limb and interstitial fluid are equal and larger than ascending limb.
• The concentrated interstitial fluid draws water out of the collecting ducts if levels of vasopressin are high, which concentrates the urine.
• The osmolarity difference is multiplied as fluid goes deeper.

Question 23

Form hairpin loops that run parallel to loops of Henle and collecting ducts - blood vessels

Answer 23

Vasa recta

Question 24

Medullary Circulation

Answer 24

• As blood flows down the loop, Na+ and Cl- diffuse into the vessel and water out of the vessel
• Process reversed in ascending loop
• Minimises excessive loss of solute from interstitium
• Salt and water reabsorbed are carried away by bulk flow
• Amount leaving is 2 fold higher due to reabsorption