Lecture 18 Flashcards
The body is around ____ water (_______ for men, ______ for women)
60%
60%
55%
What is the most important function of the kidneys?
water homeostasis
What is the response of the kidneys when there is an increase in body water?
there is an increase in the excretion of water by the kidneys
An increase in body water means what has happened to the osmolarity?
it has decreased
A decrease in the body water means what has happened to the osmolarity?
it has increased
What is the response of the kidneys when there is a decrease in body water?
there is an increase in the reabsorption of water by the kidneys
How do we know that we are thirsty?
Thirst receptors (osmoreceptors and baroreceptors) are detecting changes in the osmolarity of the water
Where are the osmoreceptors located?
in the macula densa cells
What percent of total body water is ICF and what percent is ECF?
66% ICF
33% ECF
What is the ECF made of and what percent do these make up?
interstitial fluid (75%)
plasma (20%)
transcellular (5%)
Where do we get water input from? How many mLs per day does this equate to?
fluid intake
in food
metabolically produced from food
2500mL
Where do we get water output from? How many mLs per day does this equate to?
insensible sweat faeces urine 2500mL
Water intake equals what? This means the net flow is what?
intake = output
net flow = 0
H2O excretion = H2O ______________ - H2O ______________
filtered
reabsorbed
Which segments of the nephron are permeable to water?
Proximal tubule
Thin descending limb loop of Henle
Late distal tubule
Collecting duct
Which parts of the nephron are permeable to H2O but only under the influence of vasopressin/ADH?
late distal tubule
collecting duct
Which segments of the nephron are not permeable to H2O?
both the thick and thin ascending limb of loop of Henle
early distal tubule (there is still debate about this)
Which of the following statements is TRUE?
A. 65%of a male’s body is composed of water.
B. Normally, a person has 4 litres of plasma in their blood.
C. The net water intake and water output of person per day is 0.
D. A person can live without water for 3 months.
C. The net water intake and water output of person per day is 0.
What are the two ways the H2O can move across a membrane?
paracellularly and transcellularly via aquaporin channels
What is water movement dependent on?
the osmotic gradient
Describe how water moves transcellularly across the proximal tubule
There is the transport of a solute, such as Na+ from the apical to the basolateral membrane which increases the osmolarity on the basolateral side which causes H2O to move due to the osmotic gradient. It travels through AQP1 on both the apical and basolateral membrane
Why can the proximal tubule also transport water paracellularly?
because it has leaky epithelium
What are aquaporins?
they are a family of plasma membrane proteins that form channels permitting a high rate of water flow
Where are aquaporins found?
in organisms as diverse as bacteria, plants and aminals
Aquaporins are specific to what and allow what?
specific for H2O and allow single-file movement through the pore
What are aquaporins impermeable to?
charged species
What is the osmotic gradient required for in the medulla?
for concentrating the urine
If 180L of water is entering the nephron, how much of this is reabsorbed in the proximal tubule?
120L
If 180L of water is entering the nephron, how much of this is reabsorbed in the thin descending loop of Henle?
30L
If 180L of water is entering the nephron, how much of this is reabsorbed in the thin ascending loop of Henle?
none
If 180L of water is entering the nephron, how much of this is reabsorbed in the thick ascending loop of Henle?
none
If 180L of water is entering the nephron, how much of this is reabsorbed in the late distal tubule?
15L (only under the influence of ADH)
If 180L of water is entering the nephron, how much of this is reabsorbed in the collecting duct?
14.5L (only under the influence of ADH)
If 180L of water is entering the nephron, how much of this is excreted?
0.5L (to 30L)
How many litres of H2O do the proximal tubules receive per day?
180L
How much of the 180L of H2O that it receives do the proximal tubules reabsorb? This makes up what percentage of the filtered load?
120L
67%
What sort of epithelia does the proximal tubules have? What does this mean for the movement of H2O? What helps this movement of H2O and where are these located?
leaky absorptive epithelium
this allows the paracellular and transcellular movement of H2O
AQP1 which aids the transcellular movement of water (it is present in both the apical and basolateral membranes)
After passing through the proximal tubules, how much water does the thin descending limbs of the loop of Henle receive per day? How much of this does it reabsorb? This makes up what percentage of the filtered load?
60L
about half (30L)
16%
What sort of epithelia does the thin descending limbs of the loop of Henle have? What does this mean for the movement of H2O? What helps this movement of H2O and where are these located?
leaky absorptive epithelium
this allows the paracellular and transcellular movement of H2O
AQP1 which aids the transcellular movement of water (it is present in both the apical and basolateral membranes)
Which segments are not permeable to water?
Thin and Thick Ascending Limb of the Loops of Henle
After passing through the loop of Henle, how much water does the late distal tubules receive per day? How much of this does it reabsorb? This makes up what percentage of the filtered load?
30L
10-20L
5-10% of the filtered load
What is water reabsorption regulated by in the late distal tubules?
vasopressin/ADH
What sort of epithelia do the late distal tubules have? What helps this movement of H2O and where are these located?
tight absorptive epithelia
AQP2 on the apical membrane
AQP3 and 4 are on the basolateral membrane
After passing through the late distal tubule, how much water does the collecting ducts receive per day? How much of this does it reabsorb? This makes up what percentage of the filtered load?
10-20L
15L
8% of the filtered load of H2O
What is water reabsorption regulated by in the collecting ducts?
vasopressin/ADH
What sort of epithelia do the collecting ducts have? What helps this movement of H2O and where are these located?
tight absorptive epithelia
AQP2 on the apical membrane
AQP3 and 4 are on the basolateral membrane
How does the location of AQP3 and AQP4 differ in the late distal tubules and collecting ducts?
neither are on the apical membrane but AQP3 is on the more lateral part of the basolateral membrane and AQP4 is on the basal side
Where can a summary table of AQPs be found?
slide 23 lecture 18
Multiple choice question on water handling by the nephron
Which of the following statements is TRUE?
A. Aquaporins are only found in animals.
B. AQP2 is located on the apical membrane of Thick Ascending limb cells.
C. AQP3 is located in the basolateral membrane of collecting duct cells.
D. An osmotic gradient is NOT required for water movement.
C. AQP3 is located in the basolateral membrane of collecting duct cells.
Where is vasopressin produced?
in the paraventricular and supraoptic nuclei in the hypothalamus
What is the stimulus for release of vasopressin?
increased body osmolarity and decreased plasma volume
Vasopressin is made in the hypothalamus and then stored and released in the what?
posterior pituitary gland
Describe the mode of action of vasopressin when it is released from the posterior pituitary gland
It binds to the V2 receptor on the basolateral membrane of the late distal tubule and collecting duct. This activates adenylate cyclase which increases cAMP levels. cAMP activates PKA which phosphorylates sub-apical AQP2 vesicles and this causes them to move to the membrane via exocytosis. This increases the apical membrane water permeability
How many membrane domains do aquaporins have?
6
When can we have too many AQP channels?
The amount of AQP channels can increase during congestive heart failure and during pregnancy which leads to increased water reabsorption and leads to weight gain
What is diabetes insipidus?
a disease characterised by excessive urination
What are the two types of diabetes insipidus and how do these come about?
- Neurogenic diabetes insipidus which is from failure to produce or secrete vasopressin
- Nephrogenic diabetes insipidus which comes from a mutation of the AQP2 channel or a mutation of the V2 receptor so it can’t bind vasopressin very well
If it doesn’t come up in lecture 23, re watch slide 31 for more detail.
okay bitch
