Lecture 65 - Urine Concentration Flashcards
What is the purpose of the LOH?
To set up a high osmotic force in the medulla
By secreting stuff into the medulla
Why can ACE inhibitor drugs be dangerous
Can lead to renal failure
If a person has an underlying problem with glomerular function, giving an ACE inhibitor can lead to a big decrease in GFR
What determines how much water leaves the collecting duct to be reabsorbed?
Vasopressin
What are the fluid inputs into the body?
Food & Drink
Metabolism
What is average water input daily?
2.5 litres
What are the fluid outputs?
Urine
Lungs
Skin
Faeces
What is average fluid output per day?
2.5 litres
What is a negative water balance in the body most obvious as?
Increased ECF osmolarity
Which cells detect changes in osmolarity?
Where are these cells?
Osmoreceptor cells in the anterior hypothalamus
Describe the function of the osmoreceptor cells
- Increased osmolarity of ECF
- Fluid leaves osmoreceptor cells
- Cells shrivel
- Release of ADH from posterior pituitary
- Increased fluid reabsorption
What is the osmolarity of the body?
300 mmol
What is the concentration of vasopressin at the set point?
Certain level, not zero
This allows us to go higher as well as lower
What is the relationship between vasopressin and osmolarity?
Directly proportional
The higher the body’s osmolarity, the greater the release of vasopressin
What is the difference between diabetes mellitus and diabetes insipidus?
Mellitus: urine is sweet
Insipidus: urine isn’t sweet
What is the relationship of ECF volume and vasopressin release?
Inversely proportional
However, ECF volume has to be very low for vasopressin levels to increase very much
Where are aquaporins found on the loop of Henle?
Where aren’t they?
Thick ascending: never any aquaporins
What is the range of urine osmolarity?
3-1200 mOsm
How much solute must be excreted per day?
600 mOsm
How much water must be excreted per day?
When does this change?
500 mL
Less than 500 ml indicates kidney failure
Describe the relative concentration of water leaving the loop of Henle
Dilute
100 mOsm
Describe the relative concentration of fluid leaving the PCT
Isosmotic
300 mOsm
Describe the relative concentration of urine at the deepest point on the loop of Henle
Concentrated
1200 mOsm
What is being reabsorbed from the descending loop of Henle?
Lots of water
Because the medulla is so concentrated
What is being reabsorbed from the thick ascending loop of Henle?
Where does this go, and what is the result of this?
Solute is being reabsorbed
Goes into the medulla, making it very hyperosmolar
The fluid leaving the ascending loop of Henle is very hyposmolar. How do we make concentrated urine?
Open up the aquaporins in the DCT and the collecting duct
What is the concentration of the medulla due to?
Sodium
Urea
Why is some urea reabsorbed?
Stays in the medulla, creating the high concentration
Describe absorption of solutes in the thick ascending limb
Na/K ATPase on basolateral membrane
- salt pumped into medulla
Na/K/2 Cl cotransporter on the apical membrane
- all these ions pumped into the tubule cell
What is the osmolarity of the medulla interstitial fluid?
300 - 1200 mOsm
Describe the action of vasopressin
- ADH binds to V2 receptor
- G protein activated
- Activation of adenylate cyclase
- cAMP
- AQP2 inserted into the membrane
Describe the osmolarity of the vasa recta
Follows the osmolarity of the region it is in
Near cortex: 300
Deep in medulla: 1200
Where is the collecting duct?
Why is this important?
It drops down into the medulla
This means that if there are aquaporins present, water will move out of the collecting duct
