Counter C urrent Mechanism - PowerPoint Presentation

Slide1

Counter Current MechanismMechanism of dilution and concentration of urine.

dr. Rida Shabbir

DPT IPMR KMU

Slide2

IndexDefinition Components Mechanism

Factors

Functions

Slide3

Definition A counter current system refers to a system in which the inflow runs counter to, and in close proximity to the out flow for some distance.

The counter current flow system is formed by U shaped tubules.

Slide4

Components

Counter current

multiplier

- loop of henle, responsible for production of hyperosmolality & a gradient in renal medulla.

Counter

current

exchanger

– vasa recta, responsible for maintenance of the medullary gradient & hyperosmolality.

Slide5

MEDULLARY HYPEROSMOLALITY AND MEDULLARY GRADIENT

The

interstitial fluid of the medulla is critically important in concentrating the urine ,because the osmotic pressure of this fluid provides the driving force for reabsorbing water from both the descending thin segment and collecting duct

.

Normal osmolality of plasma and other body fluids is about 300 mosm/kg H

2

O.

The interstitial fluid of the renal cortex has the same osmolality as that of plasma .i.e., 300mosm/kg H

2

O with virtually all osmoles attributable to NaCl.

Slide6

The osmolality of the renal medulla is higher than the plasma and that it goes on increasing progressively from about 300mosm/kg H

2

O at cortico medullary junction to about 1200mosm/kg H

2

O at papilla were a maximally concentrated urine is excreted.

Slide7

Major factors contributing to the solute concentration into the renal medulla are

1. Active transport of Na

+

and co-transport of K

+

, Cl

-

, and other ions out of the thick portion of the ascending limb of the loop of Henle into the medullary interstitium.

2. Active transport of ions from the collecting ducts into the medullary interstitium.

3. Facilitated diffusion of large amounts of urea from the inner medullary collecting ducts into the medullary interstitium.

4. Diffusion of only small amounts of water from the medullary tubules into the medullary interstitium, far less than the reabsorption of solutes into the medullary interstitium.

Slide8

Special Characteristics of Loop of Henle That Cause Solutes to Be Trapped in the Renal Medulla

Slide9

The most important cause of the high medullary osmolarity. Active transport of Na+ and co-transport of K+

, Cl

-

, and other ions out of the thick portion of the ascending limb of the loop of Henle into the medullary interstitium.

This pump is capable of establishing about a

200- milliosmole

concentration gradient between

the tubular

lumen and the interstitial fluid. Because

the thick

ascending limb is virtually impermeable to

water, the

solutes pumped out are not followed by

osmotic flow

of water into the interstitium.

Thus

, the

active transport

of sodium and other ions out of the

thick ascending

loop adds solutes in excess of water to

the renal

medullary interstitium

.

Slide10

There is some passive reabsorption of sodium chloride from the thin ascending limb of Henle’s loop, which is also impermeable to water, adding further to the high solute concentration of the renal medullary interstitium.The

descending limb of Henle’s loop, in contrast

to the

ascending limb, is very permeable to water, and

then tubular

fluid osmolarity quickly becomes equal to

the renal

medullary osmolarity.

Therefore

, water

diffuses out

of the descending limb of Henle’s loop into

the interstitium

, and the tubular fluid

osmolarity gradually rises

as it flows toward the tip of the loop of Henle

.

Slide11

Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium.

Slide12

First, assume that the loop of

Henle is filled with fluid with a concentration of

300 mOsm/L, the same as that leaving the proximal

tubule (Figure step 1).

Slide13

Next, the active pump of

the

thick ascending limb on the loop of Henle is turned

on, reducing the concentration inside the tubule and

raising

the interstitial concentration; this pump establishes

a 200-mOsm/L concentration gradient between

the tubular fluid and the interstitial fluid (step 2).

Slide14

Step

3

The

tubular fluid in the

descending

limb of

the loop of Henle and the interstitial fluid

quickly

reach osmotic equilibrium

because of osmosis of

water out

of the descending limb.

The

interstitial

osmolarity is

maintained at 400 mOsm/L because of

continued transport

of ions out of the thick ascending loop

of Henle

.

Slide15

Step 4

Is

additional flow of fluid into the loop

of Henle

from the proximal tubule, which causes

the hyperosmotic

fluid previously formed in the

descending limb

to flow into the ascending limb.

Slide16

Once this fluid is in the ascending limb, additional ions are

pumped into

the interstitium, with water remaining

behind, until

a 200-mOsm/L osmotic gradient is

established, with

the interstitial fluid osmolarity rising

to 500

mOsm/L (step

5).

Slide17

step 6 - Then, once again, the fluid in the descending limb reaches equilibrium with the hyperosmotic medullary

interstitial fluid

and

as

the

hyperosmotic tubular fluid from the descending

limb of

the loop of Henle flows into the ascending limb, still

more solute is continuously pumped out of the

tubules and

deposited into the medullary interstitium.

Slide18

step 7. These steps are repeated over and over, with the

net effect

of adding more and more solute to the

medulla in

excess of water; with sufficient time,

this

process gradually

traps solutes in the medulla and

multiplies the

concentration gradient established by the

active pumping

of ions out of the thick ascending loop

of Henle

, eventually raising the interstitial fluid

osmolarity to

1200 to 1400

mOsm/L.

Slide19

The repetitive reabsorption of NacI in the thick ascending loop of Henle and continued inflow

of new

NacI

from the

proximal tubule

into the loop of Henle is called the

Counter Current Multiplier

.

The NacI

reabsorbed

from

the

ascending loop of Henle keeps adding to the

newly arrived NacI,

thus “multiplying” its

concentration in

the medullary interstitium

Slide20

Role of Distal Tubule andCollecting Ducts in Excreting aConcentrated Urine

The

tubular

fluid when

leaves the loop of Henle

and flows

into the distal convoluted tubule in the

renal cortex

, the fluid is dilute, with

an osmolarity

of

only about

100 mOsm/L (

Figure

) .

Slide21

Role of urea

A person usually excretes about 20 to 50 per cent of the filtered load of urea. Urea contributes about 40 to 50 per cent of the osmolarity (500-600 mOsm/L) of the renal medullary interstitium when the kidney is forming a maximally concentrated urine.

When there is water deficit and blood concentrations of ADH are high, large amounts of urea are passively reabsorbed from the inner medullary collecting ducts into the interstitium.

Then, as the tubular fluid flows into the inner medullary collecting ducts, still more water reabsorption takes place, causing an even higher concentration of urea in the fluid.

This high concentration of urea in the tubular fluid of the inner medullary collecting duct causes urea to diffuse out of the tubule into the renal interstitium.

This diffusion is greatly facilitated by specific

urea transporters. One of these

urea transporters, UT-AI, is activated by ADH, increasing transport of urea out of the inner medullary collecting duct even more when ADH levels are elevated.

Slide22

Recirculation of Urea Contributes to Hyperosmotic Renal Medulla

Slide23

Countercurrent Exchange in the VasaRecta Preserves Hyperosmolarityof the Renal Medulla

Two

special features of the

renal medullary

blood flow that contribute to the

preservation of

the high solute concentrations:

1.

Low

medullary blood flow

,

accounting

for

<5

per cent of the total renal blood

flow. This is

sufficient to

supply the

metabolic needs of the tissues but helps

to minimize

solute loss from the

medullary interstitium

.

2. The vasa recta serve as counter current exchangers, minimizing

washout of solutes from the

medullary interstitium

.

Slide24

Slide25

The vasa recta do not create the medullary hyperosmolarity, but they do prevent it from being dissipated.Minimizes loss of solute from the interstitium

Does not prevent the bulk flow of fluid and solutes into the blood through colloid osmotic and hydrostatic pressures.

Absorbs small amount of solute and water from the medullary tubules.

The high concentration of solutes established by the countercurrent mechanism is maintained.

Slide26

Factors influencing CCM

Number of GM nephrons &length of loop of henle is directly proportional osmotic gradient.

Rate of flow in the collecting duct – rapid flow decreases urea absorption & there by gradient.

Rate of renal blood flow – inversely proportional.

Urea availability – concentrating ability of the kidney increases with the urea.

Slide27

Mechanism of concentration and dilution of urine.Kidneys possess unique property of regulating the volume and osmolality of the urine

by

the mechanism of concentrating and diluting

urine.

Main purpose

is to

maintain the osmolality and volume of body fluids within the normal

range

The kidney can produce urine with osmolality as low as 30mOsm/Kg H

2

0 to as high as 1400mOsm/Kg H

2

0 by changing the water excretion as high as 23.3L/day to as low as 0.5L/day

Slide28

At least 87% of filtered water is reabsorbed even when the urine volume is 23.3L/day.The reabsorption of the remainder of the filtered water be varied without affecting total solute excretion i.e. when the urine is concentrated, water is retained in excess of solutes, and when dilute, water is lost from the body in excess of solutes

.

Principal factors

:

Antidiuretic hormones &

Hyperosmolality and osmolality gradient in medullary interstitium of kidneys.

Slide29

Mechanism of urine dilution and concentration

CONDITIONS

IN WHICH DILUTE URINE IS FORMED

Dilute urine is called hypotonic urine, in which urine osmolality is less than blood osmolality

. It

is produced under the following conditions:

1. Low levels of ADH

2. ADH

is ineffective

The principal factors governing formation of dilute and concentrated urine are hyperosmolality medullary gradient and the presence or absence of ADH.

Slide30

Slide31

Production of concentrated urine:Concentrated urine is also called hyperosmotic urine, osmolality is greater than that of blood.

It is produced when circulating ADH levels are high

e.g

Water deprivation,Haemorrahage, Syndrome

of inappropriate antidiuretic hormone (SIADH)

Slide32

Principal factors governing formation of concentrated urineThe high level of ADH is the main factor for governing the formation of concentrated urine; because

it increases

the size of hyperosmolarity medullary gradient

Augments the urea cycling from the inner medullary collecting ducts into the medullary interstitial fluid.

Slide33

Segmental changes in the tubular fluid during formation of concentrated urine.

From proximal tubule to ascending thick limb and even early distal tubule, the changes occurring in the tubular fluid is same as during formation of dilute urine.

Only change is that the ADH increases the size of medullary gradient by counter current multiplier, which is important for the formation of conc urine.

Tubular fluid entering the late distal tubule is hypo-osmolar (osmolality about 150mOsm/L)

In the presence of ADH, H

2

0 permeability of the principal cells is increased and consequently H

2

0 is reabsorbed until the osmolality of distal tubular fluid equals that of the surrounding cortical interstitium (300m0sm/L)

Hence osmotic equilibrium occurs in the presence of ADH.

Slide34

The initial portion of the collecting duct is impermeable to urea, hence it remains the tubular fluid, its conc in the tubular fluid inceases.

In the presence of ADH, urea permeability of the last portion of the medullary collecting duct is increased. Hence urea diffuses out into the medullary interstitium.

The final osmolalityof urine is about 1200m0sm/L and high conc of urea and other non reabsorbed solutes

.

Slide35

Thank

You