PENGATURANOSMOLALITAS URIN

Rahmatina B. HermanBagian Fisiologi

Fakultas Kedokteran Universitas Andalas

Regulation of ECF Osmolarity

ECF osmolarity depends on the relative amount of H2O compare to soluteThe body fluids are isotonic at an osmolarity of 300 mosm/literVertical osmotic gradient is uniquely maintained in the interstitial fluid of medulla of each kidneyConcentration of the interstitial fluid progressively increases from the cortical boundary down through the depth of the medulla until it reaches a maximum of 1200 mosm/liter at the junction of renal pelvisKnown as the medullary countercurrent system

Vertical osmotic gradient in the renal medulla

Medullary Vertical Osmotic Gradient

Is established by countercurrent multiplicationUnique anatomic arrangements and complex functional interactions between the various nephron components in the renal medulla establish and use the vertical osmotic gradientLoop of Henle in juxtamedullary nephrons the loops plunge through the entire depth of medulla so that the tip of the loop lies near the renal pelvisVasa recta of juxtamedullary nephrons follow the same deep hairpin loop as the long loop of Henle

Countercurrent Mechanism

Countercurrent mechanism produces hyperosmotic renal medullary interstitium concentrated urineBasic requirements for forming a concentrated urine:- Distal tubule and collecting ducts impermeable to

urea and water- High level of ADH increases permeability of distal

tubules and collecting ducts to water avidly reabsorb water

- High osmolarity of renal medullary interstitial fluid osmotic gradient necessary for water reabsorption to occur in the presence of high levels of ADH

…..Countercurrent Mechanism

Major factors that contribute to build up of solute concentration into renal medulla:

1. Active transport of Na+ and co-transport of K +, Cl - and other ions out of thick limb into medullary interstitium

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

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

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

Tubule Characteristics – Urine Concentration

Segment of TubulesActiveNaCl

Transport

Permeability

H2O NaCl Urea

Thin descending limb 0 +++++ + +

Thin ascending limb 0 0 + +

Thick ascending limb +++++ 0 0 0

Distal tubule + + ADH 0 0

Cortical collecting tubule + + ADH 0 0

Inner medullary collecting tubule + + ADH 0 +++

…..Countercurrent Mechanism

…..Countercurrent Mechanism

…..Countercurrent Mechanism

…..Countercurrent Mechanism

Benefits of Countercurrent Multiplication

1. It establishes a vertical osmotic gradient in the medullary interstitial fluid. This gradient in turn is used by the collecting ducts to concentrate the tubular fluid so that a urine more concentrated than normal body fluids

2. The fact that the fluid is hypotonic as it enters the distal parts of the tubule enables the kidneys to excrete a urine more dilute than normal body fluids

Role of Vasopressin (ADH)

Countercurrent Exchange Within the Vasa Recta

Countercurrent exchange in vasa recta does not establish the concentration gradientBecause blood enters and leaves the medulla at the same osmolarity as a result of countercurrent exchange, the medullary tissue is nourished with blood, yet the incremental gradient of hypertonicity in the medulla is preservedSo that, countercurrent exchange within vasa recta conserves the medullary vertical osmotic gradient

…..Countercurrent Exchange Within the Vasa Recta

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