urinary concentrating and diluting

Question 1

why is control of urine important

Answer 1

• intracellular environments require tightly controlled extracellular environments (concentration of electrolytes and osmolarity)
• ECf is affected by substrate levels and the amount of water that it is dissolved in
• maintains homeostasis

Question 2

what primarily determines the diluteion of urine in the nephron

Answer 2

resorption of ione in the nephron without accompanying water

Question 3

how does the permeability of the ascending limb of the loop of henle differ from the descending limb

Answer 3

the ascending limb is permeable to ions but not water

Question 4

what is the primary effect of sodium resorption in the proximal convoluted tubule

Answer 4

decrease in the volume of proximal tubular fluid without affecting osmolality

Question 5

what condition leads to the kidney excreting concentrated urine

Answer 5

high ADH levels during water deficit

Question 6

what is the role of the counter-current exchange mechanism in the nephron

Answer 6

to amplify the concentration gradient for water resorption

Question 7

what effect does low ADH have on water resorption in the DCT and collecting ducts

Answer 7

decreased water resorption and more dilute urine

Question 8

what type of environment is created in the renal interstitium as a result of the ascending limb’s activity

Answer 8

hypertonic as a result of ion resorption without water
The ascending limb actively transports ions out, creating a hypertonic environment in the renal interstitium.

Question 9

what is the osmotic impact of water resorption in the proximal convoluted tubule

Answer 9

no change in osmolality despite volume reduction

Question 10

what is the primary clinical significance of measuring urine specific gravity

Answer 10

it assesses the kidney’s ability to concentrate urine

Question 11

in which part of the kidney does the dilution of urine primarily occur

Answer 11

loop of henle

Question 12

what condition would lead to the formation of hyposthenuric urine

Answer 12

volume overload

Question 13

why are urine dipsticks deemed inaccurate for measuring urine concentration

Answer 13

they provide a qualitative rahter than quantitative measurement

Question 14

which structure in the distal convoluted tubule monitors and regulates glomerular filtration rate

Answer 14

macula densa

Question 15

how does ADH primarily affect urine concentration

Answer 15

by increasing water permeability

Question 16

what specific receptors does ADH bind to in order toe xert its effects on collecting duct cells

Answer 16

V2 receptors

Question 17

what effect does the distal convoluted tubule have on urine as it reabsorbs ions

Answer 17

urine gets more dilute

Question 18

what role do aquaporins play in the function of ADH

Answer 18

they increase water permeability
Aquaporins are water channels that are inserted into the membrane due to the action of ADH, increasing water permeability.

Question 19

what role does aldosterone play in relation to ADH

Answer 19

enhances the effect of ADH on sodium resorption

Question 20

the PCT is highly permeable to which substance

Answer 20

water

Question 21

discuss the permeability to water in the loop of henle

Answer 21

variable depending on location. counter current exhange mechanism
generally lower than PCT
- descending limb water permeable
- ascending limb impermeable to water as ions are pumped out

Question 22

is the DCT permeable to water

Answer 22

largely no (but some ADH effect)
macula densa and then convoluted stretch with lots of electrolyte active transport. overall filtrate becomes even more dilute

Question 23

are the collecting tubules water permeable

Answer 23

varies depending on ADH
- cells and tight junctions are impermeable to water but the permability is affedcted by ADH which increases water permeability through channels called aquaporins

Question 24

ADH affects

Answer 24

water only

Question 25

where does ADH come from and when

Answer 25

• made in the hypothalamus, released from the posterior pituitary in response to osmoreceptors detecting a rise in plasma osmolality
• ADH binds to V2 receptors on collecting duct cells. water channels (aquaporins) inserted from intracellular vessicles into the cell membrane and water flows out of the tubules due to strong concentration gradient
• urine gets more concentrated
• also causes vasoconstriction and positive feedback to aldosterone resorbing sodium in the DCT

Question 26

what must be functional for dilute urine to be possible

Answer 26

ascending limb of LoH

Question 27

what is essential for the effect of ADH to be so significant

Answer 27

counter current exchange and hypertonic medullary interstitium
- gradient must be high to allow free flow of water through channels

Question 28

explain the role of urea in relation to the hypertonic medulla

Answer 28

• urea is passively resorbed in the PCT
• beyond this and up to the medullary collecting duct is mostly impermeable to urea
• in the presence of ADH lots of water is resorbed and therefore the urea concentration within the tubule increases rapidly
• tubule moves into the inner medullary CD and more water is resorbed so the urea concentration gradient continues to get larger
• urea starts diffusing out of the tubule, aided by urea transporter. ADH activated even more of these transporters
• some urea is then secreted back into the tubule in the thin loop of henle where it starts the journey towards the collecting ducts again
• urea can recirculate through the end portion of the nephron several times before it is excreted
• adds to the hyperosmolar environment in the inner medulla and so favours resorption of water under the effect of ADH and the production of concentrated urine

Question 29

hyposthenuria means the urine is

Answer 29

the formation of hyposthenuric urine means that the kidneys are actively diluting the urine. dilution mainly occurs in the loop of henle where water follows solutes (mostly sodium) out of the tubule
an example of when you should expect hypostheuric urine would be in situations where the body is volume overloadd and therefore needs to actively dilute urine beyond that of ECF in order to maintain homeostasis
1.001-1.008 USG

Question 30

what is isostheuria and when is it expected

Answer 30

urine excreted with a specific gravity not greater than or less than the protein of free plasma (1.008-1.012)
- protein free plasma is basically what the glomerular ultrafiltrate is so in this case not a lot has happened to the urine in terms of changing its overall concentration after the glomerulus
- typically reflect damage to the nephrons or the renal medulla causing a lack of concentration gradient
- the kidney is neither concentrating or diluting

Question 31

what is hypersthenuric urine and when is it expected

Answer 31

• concentrated urine and so it is technically anything above isothenuric range (over 1.012). however the term is usually used to refer to significantly concentrated urine that is classed as normal for the species
• over 1.030 in dogs
• over 1.035 in cats
• over 1.030 - 1.045 in cattle
• over 1.025 -1.060 in horses

making concentrated urine requires
- ADH
- DCT/collecting system to be responsive to ADH
- a hypertonic medullary interstitium (functional PCT and LoH in order to transport sodium and urea to generate the concentration gradient and blood flow to sustain the vasa recta and its counter current exchnage system)
- we expect hypersthenuric urine most of the time in most animals. it should be present when an animal is dehydrated (kidneys SHOULD concentrate urine to minimise water loss if dehydrated)