concentration and dilution of urine

Question 1

what is osmotic pressure

Answer 1

pressure required to maintain an equilibrium with no net movement of solvent

Question 2

what is osmolarity?

Answer 2

the osmotic pressure generated by the disolved solute molecules in 1L of solvent

Question 3

What is osmolality?

Answer 3

number of molecules dissolved in 1Kg of solvent (independent of temp)

Question 4

what is the composition of the proximal tubular filtrate?

Answer 4

it consists of all constituents of plasma except the cells and plasma proteins - therefore, it consists mainly of salt and water

Question 5

when GFR is normal, what percent of the filtrate is reabsorbed in the proximal tubule?

Answer 5

about 70% of the filtrate

Question 6

what mechanism is used in the loop of henle to reabsorbe water?

Answer 6

the countercurrent mechanism

Question 7

what part of the tubule is concerned with salt and water balance?

Answer 7

the distal tubule and collecting duct -

Question 8

sodium reabsorption is regulated by what hormone?

Answer 8

aldosterone

Question 9

what hormone regulates water reabsorption?

Answer 9

ADH

Question 10

What is the minimum urinary output?

Answer 10

700ml/day

Question 11

what is the maximum urinary output?

Answer 11

12ml/min

Question 12

what are the requirments for water reabsorption in the collecting duct?

Answer 12

1. presence of anti-diuretic hormone - makes the epithelium permeable to water
2. a hypertonic interstitium surrounding the CD - provides the osmotic gradient which is necessary to make water move from the tubular lumen into the intersititum

Question 13

what does hypertonic interstitum mean?

Answer 13

it means that the osmolarity of the interstitium is greater than that of the interstitium in other tissues (which is 300 mosmol/l)

Question 14

what is the function of the loop of henle?

Answer 14

to generate and maintain the hypertonic interstitium so that water may be reabsorbed , by osmosis from the CD - the loop of henle operates as a countercurrent multiplier system- concentrating the kidney intersitium

Question 15

do nephrons with longer or shorter loops have a greater ability to concentrate the interstitium?

Answer 15

long loops have the greatest ability to concentrate the interstitium

Question 16

what 3 characteristics of the loop of henle allow it to operate as a countercurrent multiplier?

Answer 16

1. countercurrent flow- filtrate flowing down in descending loop and up in ascendingloop
2. descending limb permeable to water
3. thick ascending limb of the loop is impermeable to water, but lined with salt pumps which reabsorb salt from the filtrate and deposit the salt in the interstitium - this maintains the the salt concentration between the filtrate and the surrounding interstitium

Question 17

what type of pump is the salt pump in the loop of henle?

Answer 17

Na+, K+ 2Cl- pump -= most important

Question 18

what drugs inhibit the Na+, K+, 2Cl- pump in the loop of henle?

Answer 18

loop diuretics like (furosemide/bumetanide) - inhibition results in reducing the hypertonic interstitium and consequently reducing the volume of water which may be reabsorbed by osmosis from the CD

Question 19

why is the hypertonic interstitium not “washed away “ by the blood?

Answer 19

only about 10% of the blood flow to the kidney perfuses to the medulla. The rest perfuses to the cortex- also the vasa recta capillaries have a hair pin configuration that act as a countercurrent exchange which prevents dissipation of the gradient

Question 20

Describe the pressure in the vasa recta

Answer 20

there is a net inward pressure of 15mmHg in the vasa recta - salt and water in the interstitium is therefore drawn into the vesels

Question 21

what forces govern the reabsorption of sodium and water into the peritubular capillaries?

Answer 21

Starling forces operating between the capillaries and the interstitial fluid - b/c blood in the capillaries has passed through the glomerulus and a proportion of the fluid has been filtered into the nerphon , peritubular capillary blood displays a high oncotic pressure and a low hydrostatic pressure.

Question 22

describe how vasa recta act as counter-current exchangers

Answer 22

1. Vasa recta vessels resemble other capillaries in their high permeability to sodium, chloride, urea, and water
2. Blood in the vasa recta capillaries entering the medulla loses water and gain NaCl and urea progressively as it descends
3. When blood flow in the vasa recta capillaries reverses direction and ascends from the medulla to the cortex, it progressively gains water and loses NaCl and urea
4. The blood leaving the vasa recta is approximately iso-osmotic with normal plasma, and it carries away from the medulla the reabsorbed NaCl and water. Thus, blood leaves the medulla at about the same osmolality that it entered by the mechanism of countercurrent exchange
5. This anatomical arrangement, combined with the relatively low blood flow to the medulla, permits sufficient perfusion for nutrition without destroying the high osmolality of the medulla

Question 23

what is the importance of the loop configuration of the vasa recta?

Answer 23

The loop configuration allows the vasa recta to

continuously remove salt and water from the

interstitium without dissipating the gradient in

the interstitium.