Water Balance

Question 1

LEARNING OUTCOMES

Answer 1

• Where in the nephron is water managed
• What is the mechanism
• How is it controlled

Question 2

2 routes to water loss

Answer 2

1. Via kidneys
2. Insensible water loss

Question 3

How much urine must we output per day

Answer 3

500 ml, even in situations where we are seriously dehydrated we still must output this to get rid of waste products

Question 4

What is the upper limit of water loss

Answer 4

18L per day - 10% of GFR

Question 5

What is insensible water loss

Answer 5

Loss of water from skin and lungs - can be as high as 3.5L/hr from skin during exercise in hot conditions

Question 6

Water input

Answer 6

Question 7

Water output

Answer 7

Question 8

Water input =

Answer 8

Water output, despite variable water intake

Question 9

How does the body measure our salt requirement

Answer 9

Based on changes in EC fluid vol and pressures

Question 10

How does the body measure our water requirement

Answer 10

Through ECF osmolarity

• If ECF osmolarity is high than we are water STARVED and need to save and source water => thirsty
• If ECF osmolarity is low than we are water RICH and need to remove water => pee more

NOTE: under normal circumstances osmotic pressure inside and outside the cell is =

Question 11

Name and explain the 3 ways we measure water in the body

Answer 11

1. OSMORECEPTORS - sense P_osm, located in the Paraventricular Nucleus (PVN) in the brain
2. [P_ADH] - feedback mechanism regulated by changes in receptor activity
3. DISTAL NEPHRON - ADH responsive cells CNT and P cells

Question 12

Normal osmolarity

Answer 12

280-300 mOsm/L

Question 13

Function of ADH

Where is it produced

Answer 13

Acts against water loss/promotes the saving of water

Produced by neurons in the PVN

Question 14

When do osmoreceptors fire

Answer 14

Above 290 mOsm/L, triggering the release of ADH

Question 15

Impact of an increase in ECF osmolarity on cells

Answer 15

An increase in ECF osmolarity => cells shrink, sending more APs triggering release of ADH

Question 16

Impact of a decrease in ECF osmolarity on cells

Answer 16

A decrease in ECF osmolarity => cells swell, sending less APs and the release of ADH is suppressed

Question 17

Osmolarity and ADH

Answer 17

• High osmolarity triggers ADH release and ADH promotes the saving of water by the kidney
• Low osmolarity stops ADH release and water is lost by the kidney

Question 18

Alternative names for ADH

Answer 18

AVP (Arginine Vasopressin) or VP (vasopressin)

=> ADH promotes vasoconstriction (vasopressive properties)

Question 19

Why might ADH need to save water

Answer 19

1. Osmolarity is high
2. ECF volume is low

In both cases saving water is good and helps the situation but when ECF vol is low, vasoconstriction will help to support BP and thus ADH has 2 roles depending on the reqs to fix water or pressure or both

The body can manage these very quickly as ADH is produced very quickly and broken down very quickly in the liver - thus we get minute to minute control with ADH

Question 20

Despite the presence of ADH, what is the problem

Answer 20

We have no water pumps

So before water can be saved and ADH can have any effect we need to create a water saving environment (which is made regardless of our water requirement)

Kidney works on the basis that we will need to save water at some point so be prepared and let ADH be the rapid response element in the process

Question 21

Osmolarity at the glomerulus

Answer 21

300

Question 22

Osmolarity at the PCT

Answer 22

300

Question 23

Osmolarity at the tip of the LOH

Answer 23

1200

Question 24

Osmolarity entering the DCT

Answer 24

100

Question 25

Osmolarity at the DCT

Answer 25

100

Question 26

Filtrate and interstitial osmolarity

Answer 26

Numbers reflect the osmolarity of the IS fluid bathing the capillaries and nephrons inside the kidney

Question 27

What does the TAL of the LOH do

Answer 27

Actively pumps out salt

Question 28

What does the TDL of the LOH do

Answer 28

Water is pulled out because the osmolarity of the IS fluid bathing the capillaries and nephrons is greater than the filtrate (enhanced by the unique arrangement of the LOH and medullary peritubular capillaries)

Question 29

What supports a higher osmolarity in the IS fluid

Answer 29

Recycling of urea between loop and collecting duct

Question 30

How do we create a hypertonic medulla

Answer 30

THE SINGLE EFFECT - pumping of salt out of the thick ascending limb however this does not account for the large gradient so some other mechanism must be at play

Question 31

What does countercurrent multiplication do

Answer 31

Magnifies the degree of hypertonicity to achieve 1200 mOsm

Question 32

Countercurrent multiplication - how it happens

Answer 32

1. Ascending limb starts to actively reabsorb salt (i.e. move salt out of the nephron and into the IS fluid spaces) 2. As this process continues we see that the osmolarity of the fluid in the nephron in this region drops to 200 mOsm 3. It drops because salt has been reabsorbed and water stays behind 4. Meanwhile as salt is pumped into the IS fluid spaces the osmolarity here increases to 400 mOsm because we are adding salt 5. As the descending limb of the LOH is permeable to water and the fluid in the IS fluid is at a higher osmolarity this pulls water out of the descending limb increasing its osmolarity

Question 33

How does countercurrent multiplication continue

Answer 33

* Process repeats and the gradient builds * However, would the fluid that leaves the descending limb not dilute the IS fluid and so return osmolarity to normal? * Countercurrent multiplication does build a gradient but not a very big one * VASA RECTA comes in to play

Question 34

What is happening in this diagram

Answer 34

* Salt is pumped out of the thick ascending limb * Some salt sits around in the IS space locally and more is picked up by the slow moving vasa recta which drags the salt towards the tip of the LOH * As the vasa recta is slow moving and the flow is towards the tip the gradient builds up substantially with salt concentrating at the tip of the LOH * WATER - as the ascending arm of the vasa recta rises with its salt-rich blood, it pulls water out of the descending LOH BUT because the flow of the vasa recta is up towards the cortex this water is being pulled away from the tip of the LOH ensuring the gradient is not diluted

Question 35

VASA RECTA AND ITS INVOLVEMENT WITH THE GRADIENT 1. net effect of the process on filtrate 2. effect in IS fluid

Answer 35

1. Fluid heading for the distal nephron is now of low osmolarity so rich in water 2. Fluid next to the tip of the LOH is hyper osmotic (1200 mOsm) and the fluid nearer the cortex is isosmotic (300 mOsm)

Question 36

What does the osmotic gradient mean What happens as a result

Answer 36

We have a water rich solution in the distal nephron and on the other side of the nephron wall in the IS space we have a hyperosmotic fluid This osmotic gradient encourages ADH to come in (recall - ADH is released when ECF osmolarity is high)

Question 37

Presence of ADH

Answer 37

Facilitates the movement of water down its conc gradient, saving water for the body and the vol of urine produced is small Urine will be of a high osmolarity and dark in colour

Question 38

Absence of ADH

Answer 38

The movement of water down its conc gradient is blocked Water is lost from the body and the vol of urine produced is large urine will of a low osmolarity and light in colour

Question 39

Osmolarity at each part of the nephron

Answer 39

Question 40

How is ADH release influenced by osmolarity, blood vol and BP

Answer 40

* Osmolarity goes up when we need water - ADH goes to distal nephron - water is saved * 1% increase in ECF osmolarity increases ADH * 2% decrease below 280 mOsm/L decreases ADH to 0 * Blood vol goes down - we need more vol - ADH goes to distal nephron - water/vol saved * BP goes down - we need more vol to support pressure - ADH goes to distal nephron - water/vol saved

Question 41

Concept map 1

Answer 41

Question 42

Concept map 2

Answer 42

Question 43

Concept map 3

Answer 43