OSMOREGULATION

Question 1

Two main factors controlling osmoregulation of body fluids

Answer 1

water intake and water loss
- intake must = amt eliminated in urine or else will lead to disturbed homeostasis
- water excreted in urine is EXACTLY the amount required to maintain fine balance of osmoregulation
–> urine must still ALWAYS be excreted from the body!

Question 2

insensible water loss

Answer 2

water that is continuously lost from the body in sweat and in water vapor from the mouth and nose
- unaware its happening

Question 3

How much of our daily water INTAKE comes from from food and drink?

Answer 3

~ 2.2L/day

Question 4

How much of our daily water INTAKE comes from metabolism?

Answer 4

~0.3L/day
- NOT a significant amount for humans, but for some other species yes

Question 5

How much of our daily water LOSS comes from insensible water loss?

Answer 5

~0.9 L/day
- a decent amount, more than from metabolism (in a healthy person)

Question 7

Describe the classic experiment that implicated a neurohypophyseal factor in the regulation of body fluid osmolality

Answer 7

normal condition: administration of hyperosmotic NaCl soln to dog resulted in temporary decrease in urinary flow rate as homeostatic mechanism to maintain water balance in body

experimental condition: removed neurohypophysis and observed no change in urinary flow rate upon admin of hyperosmotic NaCl soln until posterior pituitary extract was exogenously administered
–> implicated role of posterior pituitary gland and most importantly ADH

Question 8

renal osmotic stratification

Answer 8

different concentrations of solutes are permeable at different parts of the nephron
- this establishes concentration gradient for water/solute reabsorption and removal of waste material
- the 2/3 water reabsorption at the PT establishes concentration gradient

Question 9

standing osmotic gradient hypothesis

Answer 9

describes how the kidneys create a concentration gradient in the medulla crucial for the concentration of urine and conservation of water
- states the significant 2/3 Na+ reabsorption at the PT establishes gradient for subsequent water reabsorption

Question 10

aquaporins

Answer 10

transmembrane proteins responsible for the transcellular reabsorption of water

Question 11

AQP2

Answer 11

in the principle cells of CD stored in endosomes in cells
- regulation requires cAMP-mediated translocation to and from apical membrane to increase water reabsorption

Question 12

principal cells

Answer 12

cells of the late DT and CD responsible for Na+ reabsorption, water reabsorption and K+ secretion

Question 13

alpha-intercalated cells

Answer 13

cells of the late DT and CD responsible acid-base balance in the kidneys via K+ reabsorption and H+ secretion

Question 14

What solutes contribute to the osmotic gradient, and what mechanisms deposit these solutes in the interstitial fluid?

Answer 14

countercurrent multiplication and urea recycling

Question 15

countercurrent multiplication of the LOH

Answer 15

the LOH forms the corticopapillary osmotic gradient by depositing NaCl into the interstitial fluid deep in the medulla

Question 16

urea recycling

Answer 16

urea is freely filtered and 50% reabsorbed at the PT
- in the medulla urea is secreted back into the filtrate, and not reabsorbed in cortical segments, contributing to the volume of urine and conserving water

Question 17

how is urea reabsorbed in the medullary region of the nephron?

Answer 17

via UT1 transporters
- allows the body to put out a low volume, more concentrated urine to conserve water and maintain water homeostasis

Question 18

How does ADH stimulate reabsorption of urea?

Answer 18

when ADH is present, urea permeability at the CD is upregulated which helps produce a concentrated urine
- high ADH –> less urea excreted
- low ADH –> more urea is excreted

Question 19

T or F: at any horizontal level the blood is almost in equilibrium with the interstitial fluid

Answer 19

TRUE!
- solute and water exchange occurs throughout the vasa recta that travel along with the medullary nephron segments

Question 20

vasa recta

Answer 20

section of the peritubular capillaries that serve the medulla, LOH and papilla of the kidney

Question 21

Vasa recta as counter current exchangers

Answer 21

exchange of water and solute occurring throughout these vessels maintains relative equilibrium of medullary interstitium and the blood
- preserves hyperosmotic medullary interstitium

Question 22

Antidiuresis

Answer 22

= concentrating the urine, often a result of water deprivation
- ADH stimulates increased water reabsorption and urea produces a small and concentrated urine
- ADH works at the late DT and CD to increase water reabsorption and produce a more concentrated urine
- obligatory water loss: ~ 0.5L/day to maintain homeostasis

Question 23

Diuresis

Answer 23

= diluting the urine, result of excessive water intake
- want ADH to be low so in the absence of ADH, the corticopapillary osmotic gradient is much smaller
- late DT and CD now impermeable to water
- obligatory water loss ~ up to 18L/day to maintain homeostasis

Question 24

two factors that influence ADH secretion

Answer 24

1- high plasma osmolarity
2- low ECF/blood volume

Question 25

OTHER effects of ADH

Answer 25

-- increases urea permeability in the inner medullary CD enhancing urea recycling - NKCC2 upregulation at TAL enhancing countercurrent multiplication - decreases vasa recta blood flow - increases Na+ reabsorption and K+ secretion by principal cells

Question 26

effect of alcohol on ADH secretion

Answer 26

alcohol inhibits ADH secretion from posterior pituitary--> decreased water reabsorption and increased diuresis

Question 27

diabetes insipidus

Answer 27

condition characterized by excessive thirst and polyuria - water is not reabsorbed in the CD and urine cannot be concentrated --> large volumes of dilute urine --> body fluids and serum osmolarity increase

Question 28

central diabetes insipidus

Answer 28

results in the failure of the posterior pituitary to release ADH - leads to increased diuresis - can be result of head injury,

Question 29

nephrogenic diabetes

Answer 29

results in decreased response of the principal cells to ADH - normal ADH secretion by posterior pituitary, maybe even high - also suggests possible defect in signaling pathway (defect in the receptor, the G protein, or adenylyl cyclase, etc)

Question 30

concept of free water

Answer 30

defined as distilled water free of solutes - in the nephron, free water is generated in the diluting segments where solute is reabsorbed WITHOUT WATER (like in TAL and early DT)

Question 31

free water clearance (CH20)

Answer 31

the amount of free water that the kidneys excreted out of the plasma per day - describes ability of kidneys to dilute/concentrate the urine - the body removes what it doesn't need to avoid overwhelming the kidneys = urine flow rate (V)- Cosm

Question 32

Cosm

Answer 32

= osmolar clearance = (urine osmolarity x urine flow rate)/ plasma osmolarity - under normal conditions: = 2L/day --> (600mOsm/day)/ 300 mOsm/L

Question 33

free water clearance when ADH is low?

Answer 33

solute is reabsorbed at the TAL and early DT but late DT and CD impermeable to water in LOW ADH condition, producing a hypoosmotic urine --> Clearance of water (CH20 IS POSITIVE)

Question 34

free water clearance when ADH is high?

Answer 34

solute is reabsorbed at the TAL and early DT - late DT and CD HIGHLY permeable to water in HIGH ADH condition, producing a hyperosmotic urine --> Clearance of water (CH20 IS NEGATIVE)

Question 35

what would happen to the clearance of water if V= 4L/day?

Answer 35

CH20 would be positive (2L/day) - hyposmostic urine and ADH is low

Question 36

what would happen to the clearance of water if V= 1L/day?

Answer 36

CH20 would be negative - hyperosmostic urine and ADH is high