Formation of urine 2

Question 1

Percentage of reabsorption at the proximal tubule

Answer 1

Water- 65%

Sodium- 65%

Glucose- 100%

Proteins and amino acids- 100%

Question 2

The loop of henle

Answer 2

Tubular fluid is further modified here

Aim is to recover fluid and solutes from the glomerular filtrate

Fluid entering is isotonic, by the tip its hypertonic, by the end is hypotonic

Question 3

Two stages of modification in loop of henle

Answer 3

1. Extraction of water in the descending limb

2. Excreation of Na+ and Cl- in the ascending limb

Question 4

Extraction of water in the thin descending limb

Answer 4

Cells are flat, no active transport of salts

Freely permeable to water via aquaporin-1 channels

Also some passive movement of water via tight junctions

Question 5

Extraction of Na+ and CL- in the thick ascending limb

Answer 5

Tubular wall is impermeable to water

Specialised Na+/K+/Cl- co transporters

Transport: Na+, K+, Cl- but no water

Question 6

Countercurrent multiplication

Answer 6

Creates large osmotic gradient within medulla

Facilitated by Na+/K+/2Cl- transport in ascending limb of LOH

Permits passive reabsorption of water from tubular fluid in descending LOH

Question 7

Urea in the countercurrent multiplication

Answer 7

Urea freely filtered at glomerulus

Some reabsorption in proximal tubule but LOH and distal relatively impermeable to urea

Urea diffuses out of collecting duct into medulla down gradient

Adds to osmolality of medullary interstitium

Question 8

Function of distal tubule

Answer 8

Active absorption and secretion of solutes

Na+ and Cl- actively reabsorbed from the tubular fluid

This is in exchange for K+ or H+ which are secreted into the tubular fluid

Question 9

Principal cells

Answer 9

Exchange Na+ and K+ in the late DT and early collecting duct

Principal cells sensitive to aldosterone

Exchange forms part of RAAS

Question 10

Overall effect of aldosterone

Answer 10

More Na+ reabsorbed so more water moves into plasma so BP increases

Question 11

a-intercalated cells

Answer 11

Secretes acid (H+) via H+/Na+ or H+/K+ exchange

Involves ATPase

Reabsorbs HCO3-

Question 12

B-intercalated cells

Answer 12

Secrete HCO3- via pendrin

Reabsorbs acid (H+)

Question 13

The collecting duct

Answer 13

Relatively impermeable to movement of water and solutes

Permeability increased by ADH

Question 14

Antidiuretic hormone

Answer 14

Most important hormone that regulates water balance

Nonapeptide

Also known as vasopressin

Released from posterior pituitary

Plasma half life is 10-15 minutes

Question 15

ADH action

Answer 15

Acts on vaspressin V2 receptors on basal membrane of principal cells in DT and collecting duct cells

Leads to activation of intracellular water channels

Question 16

Maximal circulating ADH

e.g. if severely dehydrated

Answer 16

Collecting duct becomes permeable to water via AQP2

Reabsorbs up to 66% water entering collecting duct

Delivery of fluid to collecting duct is low

Urine volume can be reduced to 300ml/day

Question 17

No circulating ADH

Answer 17

Reabsorption of water at various sites in nephron

Collecting duct walls impermeable to water as no AQP2 so large volume of water is excreted

Lack of ADH: diabetes insipidus

Question 18

Forms of diabetic insipidus

Answer 18

Nephrogenic- due to inability of kidney to respond normally to ADH

Neurogenic- due to lack of ADH production by the brain

Question 19

Nephorgenic diabetes insipidus treatment

Answer 19

Chlortalidone (diuretic)

Indometacin (anti- inflammatory)

Question 20

Neurogenic diabetes insipidus treatment

Answer 20

Desmopressin (ADH analogue)

Vasopressin

Carbamezapine (anti- convulsive)

Question 21

SIADH

Answer 21

Syndrome of inappropriate ADH

Excessive release of ADH (head injury, effects of drugs)

Cause hyponatraemia and possibly fluid overload

Treatment: V2 receptor blockers

Question 22

Agents which increase ADH release

Answer 22

Nicotine

Ether

Morphine

Barbiturates

Question 23

Agents which inhibit ADH release

Answer 23

Alcohol