Creating Urine and Renal Function Flashcards
1
Q
When is dilute urine created?
A
- if there is too much water or too few ions in the body, dilute urine must be made to concentrate the blood
- fatal water intoxication is the result of blood which is too dilute (drinking 6L in less than 3 hours)
- specifically if the water does not have many ions in it this can happen easier
- too much water that ions are too dilute which can lead to heart irregularities and other problems
- to concentrate the blood, the urine made has to be more dilute in ions than the blood (ie. urine must be <300 mOsm/L)
2
Q
What type of urine are short loop nephrons particularly good at creating?
A
- long and short loop nephrons
- one or the other aren’t turned “on” or “off” but the short loop nephrons are good at creating dilute urine
3
Q
A
4
Q
Describe how dilute urine is formed
A
- the glomerulus does not concentrate the urine but simply filters the blood; this means that the concentration in this area is the same as the blood
- the PCT reabsorbs a lot of the ions and glucose but water follows so that you aren’t changing the concentration of the filtrate
- entering the medulla, there are a lot of solutes which causes water to leave and move to area of higher concentration in descending LOH
- at the end of the LOH, the concentration of urine is very high
- ascending LOH and DCT are impermeable to water but ions and other solutes get pumped out but now the water can’t leave
- because water isn’t leaving, we are moving ions out and making the filtrate more dilute (filtrate loses solute and osmolarity goes down)
- osmolarity falls further in DCT
- enters into collecting duct at low osmolarity
- in collecting duct, no ADH around and not many holes in the collecting duct so more solute is pumped out (collecting duct is only permeable to water with ADH)
5
Q
Describe ion/water movement in the ascending and descending LOH
A
- the osmotic gradient is set up primarily by the thick ascending limb of the loop of Henle pumping out ions but not water
- the descending limb loses water but not ions so much of the water is removed by the descending loop of Henle
6
Q
What type of urine are the long loop (juxtamedullary) nephrons best at making?
A
- concentrated urine
- if there is too little water or too many ions in the body, urine must be made which will reestablish ionic balance
- this urine must be more concentrated in ions than the blood (>300mOsm/L)
- we can make urine that is about 1200mOsm/L or about 4 times more concentrated than blood
7
Q
Describe the formation of concentrated urine
A
- filtrate is coming out of glomerulus at 300
- in the PCT solutes are reabsorbed and water follows them out of the tubule
- there is higher solute concentration in the medulla so water leaves the descending LOH so the concentration increases and osmolarity goes up
- the filtrate becomes less concentrated in the ascending LOH
- need to pump solute out in the ascending limb so that there is a high concentration of solute in the medulla to drive the water out in the descending portion
- at the DCT, water can move in so the concentration is reestablished at 300
- the collecting duct is variably permeable
- if we want to make concentrated urine, we want to let water out of the collecting duct
- we are producing ADH if we are dehydrated which creates pores (aquaporin) all along the collecting duct
- lose a lot of water because as you move down the collecting duct, the medulla increases its concentration of solutes
- end up with 1200mOsm/L concentration
8
Q
How do diuretics such as furosemide (LASIX) work?
A
- inhibit Na+-K+-2CL- pumps in the ascending portion
- this results in less ions being pumped into medulla
- this means that there is less water moving out in the descending loop as well as the collecting duct because the ion concentration in the medulla is lower
- ultimately this means there is less water re-uptake and more urine
- mainly used to treat hypertension:
- water going into urine comes from blood so if you have less water in the blood, you have less blood
- this means you have less blood pressure if you have less blood
9
Q
How are collecting ducts selectively permeable to water?
A
-ADH leads to the fusion of aquaporin 2 to the basolateral membrane of the collecting ducts
10
Q
How is ADH secreted?
A
- created in hypothalamus
- neurosecretory cell in hypothalamus are aware of concentration of the blood
- they detect if the blood is too concentrated so to get rid of the solute, ADH is released
- high osmolarity in the plasma and interstitial fluid is detected by the osmoreceptors in the hypothalamus
- this stimulates the neurosecretory cells in the hypothalamus to release ADH to posterior pituitary
- blood supply takes it to the kidney
11
Q
What does ADH do when it is released?
A
- ADH leaves in the blood and leads to the synthesis of water pores (aquaporin 2) in the collecting ducts of nephrons
- this allows water to easily leave the tubular fluid
- the water then dilutes the blood and brings the osmolarity down to normal
- ADH also increases the activity of the Na+ K+ 2Cl- symporter which makes the medulla more solute rich to pull more water out of the filtrate and concentrate the urine
- with maximal ADH release you can make as little as 400-500mL of urine/day
12
Q
What stimulates angiotensin II?
A
- decrease in BP and sympathetic nervous stimulation stimulate the JG apparatus
- JG apparatus causes renin release which turns angiotensinogen into angiotensin I
- angiotenin I is rapidly converted by ACE to angiotensin II
- angiotensin II constricts arterioles to increase blood pressure
13
Q
What effect does angiotensin II have on the afferent arteriole and the kidney?
A
- constricts the afferent arteriole
- decreases the rate of glomerular filtration (less filtrate)
- angiotensin II also increases Na+ reabsorption by Na+/H+ antiporters so that water will follow Na+ into the filtrate
- causes release of aldosterone which increases Na+ and Cl- reabsorption by the collecting ducts (water follows)
- all of these increase blood volume
14
Q
How can kidney function be evaluated?
A
- urinalysis: analyzes the volume, physical, chemical, and microscopic properties of urine
- blood tests: looks at levels of waste products
15
Q
A
- consistently dark urine- might consider hyperbilirubinemia
- blood coloured- might mean there is damage in the kidneys
- milk looking might indicate infection because that could be white blood cells or bacteria
- no colour consistenly with high volume could be indicative of diabetes
- turbidity is if it blocks light or not
- usually urine lets light pass because it does not have many ions
- cloudy could indicate infection
- fruity smell due to ketones can be due to diabetes or ketosis
- foul smell is rare but with serious infections you can get urine that is fermenting
- low pH depending on diet and ketosis
- high pH is rare but really strong alkaline diets can occasionally do this, alkaline urine can be more common with vegetarian diets
19
Q
What does GFR show?
A
- one of the best measures of kidney function and can be used to determine the stage of kidney disease
- measure of flow rate by which the glomeruli filter the blood
- to do this you need a substance that is not reabsorbed nor secreted by the tubules
- inulin is one example
- GFR can be measured by injecting inulin and measuring the rate of urine output and concentrations of inulin in the blood and urine
- different from renal clearance as renal clearance has some reabsorption whereas GFR has no reabsorption
- if you get to 15mL/min you need transplantation or dialysis
20
Q
How is renal clearance measured?
A
UV/P
- first order equation
- the more than you have, the faster the process works
