Counter Current Mechanism by dr anita teli

Counter Current Mechanism: 

Counter C urrent M echanism Speaker : Dr Anita Teli pg Chairperson : Dr Sumangala Patil Asso Prof Date & Time : 5 th Jan 2011, 10.30 am BLDEUs Shri B.M.Patil Medical College

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Definition Components Mechanism Factors Functions Other systems of counter current mechanism Applied aspects

Definition : 

Definition A counter current system refers to a system in which the inflow runs parallel to, counter to, and in close proximity to the out flow for some distance. The counter current flow system is formed by U shaped tubules. The counter current system can be best understood by studying the effect of a heater on a straight water pipe and pipe bent in U shape.

COUNTER CURRENT SYSTEM: 

COUNTER CURRENT SYSTEM

Components : 

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 .

Medullary hyperosmolality and medullary gradient: 

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 .

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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.

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

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 .

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

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

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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 .

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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 .

Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium.: 

Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium .

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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).

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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).

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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 .

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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.

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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).

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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 .

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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.

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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

Role of Distal Tubule and Collecting Ducts in Excreting a Concentrated Urine: 

Role of Distal Tubule and Collecting Ducts in Excreting a Concentrated 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 ) .

Role of urea : 

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.

Recirculation of Urea Contributes to Hyperosmotic Renal Medulla: 

Recirculation of Urea Contributes to Hyperosmotic Renal Medulla

Countercurrent Exchange in the Vasa Recta Preserves Hyperosmolarity of the Renal Medulla: 

Countercurrent Exchange in the Vasa Recta Preserves Hyperosmolarity of 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 .

Factors influencing CCM: 

Factors influencing CCM Number of GM nephrons &length of loop of henle is directly proportional osmotic gradient. Proximation of the tubules- directly proportional 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.

Mechanism of concentration and dilution of urine.: 

Mechanism of concentration and dilution of urine. Kidneys possess unique property of regulating the volume and osmolality of the urine formed by the mechanism of concentrating and diluting urine as per the body. Main purpose 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

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Effects of concentration and dilution mechanism on volume and osmolality of urine (in each case, the osmotic load excreted is same 700m0sm/day ) Character of urine formed GFR (ml/min) Percentage filtered H 2 0 reabsorbed Urine vol (L/day) Urine conc (m0sm/Kg H 2 0) Gain or loss of H 2 0 in excess of solute (L/day Urine isotonic to plasma 125 98.7 2.4 290 - Concentrated urine (vasopressin: max antidiuresis) 125 99.7 0.5 1400 1.9 gain Diluted urine no vassopressin complete diuresis DI 125 87.1 23.3 30 20.9 loss

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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.

Mechanism of urine dilution and concentration: 

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 ,central diabetes insipidus 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.

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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)

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Principal factors governing formation of concentrated urine The 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.

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

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 stimulates NaCl reabsorption in the thick ascending limb and 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.

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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 .

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Collecting ducts: As the tubular fluid flows through the collecting ducts, it passes through the corticopapillary gradient( regions of increasingly higher osmolality , from 300m0sm/L to 1200m0sm/L) that is previously established by counter current multiplier and urea cycling . Consequently H 2 0 is reabsorbed from the collecting ducts until the osmolality of tubular fluid equals that of the surrounding interstitial fluid . Osmolality may reach upto 1400m0smol/L, almost five times that of plasma, with a total of 95.7% or more of the filtered water being reabsorbed. Urine under this condition can be as low as 0.5L/day.

Other systems of counter current mechanism : 

Other systems of counter current mechanism Heat exchange taking place between arteries & veins of limbs. Human intestinal villi – diaadvantageous Brain – for regulation of brain temperature. Testes – for maintaning high levels of testoterone .

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Assessment of renal diluting and concentrating ability: Measurement of urine osmolality, Measurement of urine specific gravity, The urine concentration test, The urine dilution test and Estimation of free water clearance test.

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Applied aspect: Clinical Serum ADH Ser Osm/ Ser Na+ Urine osmolality Urine flow rate Free water clearence Primary polydypsia decreased decreased hyposmotic high positive Central diabetes insipidus decreased increased hyposmotic high positive Nephrogenic DI increased increased hyposmotic high positive Water deprivation increased High/nomal hyperosmo low negative SIADH Markedly increased decreased hyperosmo low negative

References : 

References Text book of medical physiology- Guyton Review of medical physiology- Ganong Indu Khurana . Text book of medical physiology. Text book of physiology – A.K.Jain Fundamentals of medical physiology – L Prakasham Reddy.

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