Counter Current System -1

Regulation of Extracellular Fluid Osmolarity and Sodium Concentration

• For the cells of the body to function properly

they must be bathed in extracellular fluid with

a relatively constant concentration of

electrolytes and other solutes

Total Concentration Of Solutes

• Osmolarity is determined by the amount of

solute divided by the volume of the extracellular

fluid.

• The extracellular fluid sodium concentration and

osmolarity are regulated by the amount of

extracellular water

Function of Kidneys

• The normal kidney has tremendous capability

to vary the relative proportions of solutes and

water in the urine in various states

• Kidney can excrete a large volume of dilute

urine or small volume of concentrated urine

without major changes in rates of excretion of

solutes such as sodium and potassium

• This ability to regulate water excretion

independently of solute excretion is necessary

for survival especially when fluid intake is

limited

Formation of Dilute and Concentration Urine

• Osmolarity of glomerular filtrate is same as

that of plasma i.e 300 mOsm/L

• Urine can be concentrated till its osmolarity is

four times more than that of plasma i.e. 1200

mOsm/L

The formation of a dilute or concentrated

urine depends upon two factors:

• Medullary gradient.

• Antidiuretic hormone.

Anti-Diuretic Hormone

Rate of ADH secretion to a large extent

determines, whether dilute or conc. urine

is going to be excreted

Osmolarity Of The Body Fluids:

• Conc. body fluids due to increase solutes

• Secretion of ADH by Posterior pituitary

• Increase permeability of water from distal tubules and collecting ducts

• Absorption of large amounts of water

• Decreased urine volume ( without altering rate

of renal excretion of the solutes)

• Osmolarity Of The Body Fluids:

• Excess water in the body fluids

• Decrease ADH secretion

• Reduce permeability of water from distal tubule and collecting ducts

• Excretion of large amount of dilute urine

Medullary Hyperosmolarity

• Osmolarity of the interstitial fluid in the renal

medulla near the cortex is 300 mOsm/L

• Towards the inner part of medulla, it increases

gradually and reaches the maximum at the

inner most part of medulla near renal sinus i.e

1200 mOsm/L

Medullary Gradient

• This type of gradual increase in the osmolarity

of the medullary interstitial fluid is called the

medullary gradient

• The vertical osmotic gradient remains constant

regardless of the fluid balance of the body

Development and Maintenance of Medullary Gradient

• Kidney has some unique anatomical

arrangements, which are responsible for the

development of medullary gradient and for the

hyperosmolarity of interstitial fluid in the inner

medulla. These arrangements are together

called Counter current system

Counter Current System

• In kidney the structures which form the

counter current system are the Loop of Henle

and the Vasa recta

• It is a system of “U” shaped tubules in which

the flow of fluid is in opposite direction in

different limbs of the “U” shaped tubules

• In both the direction of flow of fluid in the

descending limb is just opposite to that in the

ascending limb

• Loop of Henle forms the Counter Current

Multiplier

• Vasa recta form the Counter Current

Exchanger

MOA Of The Counter Current

System

• Long loops of Henle establish the vertical

osmotic gradient

• Vasa recta prevent the dissolution of this

gradient while providing blood to the renal

medulla

• Collectively this entire functional organization

is known as the Medullary Counter Current

System

• Collecting tubules in conjunction with the

vasopressin use the gradient to produce urine

of varying concentrations

At The Level Of The Proximal Tubule

• Immediately after the filtrate is formed,

uncontrolled osmotic reabsorption of filtered

water occurs in the proximal tubule secondary

to active Na+ reabsorption.

• By the end of the proximal tubule, about 65%

of the filtrate has been reabsorbed

• 35% remaining in the tubular lumen still has

the same osmolarity as the body fluids

• Therefore, the fluid entering the loop of Henle

is still isotonic

At The Level Of loop of Henle

• An additional 15% of the filtered H2O is

obligatorily reabsorbed from the loop of Henle

during the establishment and maintenance of

the vertical osmotic gradient, with the

osmolarity of the tubular fluid being altered in

the process

Descending Limb Of Loop Of Henle

It carries fluid from the proximal tubule down

into the depths of the medulla.

• Is highly permeable to water

• Does not actively extrude Sodium ions

Ascending Limb Of Loop Of Henle

Carries fluid up and out of the medulla into the

distal tubule

•Actively transports NaCl out of the tubular

lumen into the surrounding interstitial fluid

•Impermeable to water so salt leaves the tubular

fluid without water osmotically following along

Ability of kidney to form a urine that

is more concentrated than plasma is

essential for survival

• Water is continuously lost from the body

through various routes:

• Lungs

• Gastrointestinal tract

• Skin

• kidneys

• Fluid intake is required to match this loss,

ability of the kidney to form a small volume of

concentrated urine minimizes the intake of

fluid required to maintain homeostasis, this

function is especially important when water is

in short supply

Obligatory Urine Volume

• A normal 70-kg human must excrete about

600 milliosmoles of solute each day

• Maximal urine concentrating ability is 1200

mOsm/L

• Minimal volume of urine excreted is called

Obligatory urine volume

Obligatory Urine Volume

• It is calculated by:

• End Of Todays Lecture!!!