- Kidney performs a unique function of concentration & dilution of urine according to the needs of the body
- Purpose?
- To maintain osmolality and volume of body fluids: by regulating excretion of water and NaCl
There is production of
- Concentrated urine during dehydration with Osmolality: Hyperosmotic (1400 mOsm/L) and Volume: < 0.5L
- Dilute urine during overhydration with Osmolality: Hypoosmatic (30 mOsm/L) and Volume: > 23.5 L
Achieved by
- Counter current mechanism
- Anti-Diuretic Hormone
What is Osmolality?
Osmolality of body fluids (ICF & ECF) is 300 mOsm/L.
Tonicity - 3 types
- Isotonic
- Hypertonic
- Hypotonic
What is Counter Current System?
A system in which flow runs
- parallel to
- counter to
- close contact for some distance
- Variable permeability of the nephrons
- Medullary osmotic gradient
- Direction and proximity of flow of fluid in Loop of Henle and Vasa recta
Variable permeability of the nephrons
Osmotic gradient in Kidneys
Types of nephrons
- Cortical nephrons having Peritubular capillaries
- Juxta-medullary nephrons having Vasa recta
Cortical vs Juxtamedullary nephrons
Components of Counter current system
- Descending limb of loop of Henle
- Thin and thick ascending limb of Loop of Henle
- Medullary interstitium
- Distal convoluted tubule
- Collecting Duct
- Vasa Recta
Counter current system consists of
- Counter current multiplier
- Counter current exchanger
Counter current multiplier
It has
- Single effect
- Multiple effect Single effect
Single effect
Proximal convoluted tubule
- 80% of filtered water and solutes reabsorbed
Descending limb of loop of Henle (Concentrating segment)
- Highly permeable to water
- Water reabsorbed freely into the interstitium
- Relatively impermeable to solutes
- Solute conc. increases (NaCl)
- Osmolality in the tubule increases (1200 mOsm.)
Ascending limb of loop of Henle (Diluting
segment)
- Thin ascending limb
- Impermeable to water
- Permeable to NaCl and urea
- Thick ascending limb
- Impermeable to water and other solutes
- Actively transport the NaCl out of the lumen to interstitium by
- Na-2Cl-K Symport
- Na-K ATPase Pump
Interstitial fluid becomes hyper-osmotic
Tubular fluid becomes more & more
hypo-osmotic
Distal convoluted tubule & Collecting duct
Cortical and outer medullary portion of
Collecting duct
- Permeable to water
- Impermeable to Urea and NaCl
Urea free water reabsorption occurs in Collecting duct
Increase in Urea conc. in tubular fluid
Inner medullary portion of collecting duct
Urea along with water moves out passively along the concentration gradient into medullary interstitium
↓
Gets trapped by counter current exchange in vasa recta (Urea trapping)
↓
Maintains high osmolarity of medullary interstitium
Multiple effect
Osmotic concentration gradient is created
Counter current exchanger
Is due to Vasa recta
Because of Counter current multiplier effect
▼
Osmotic gradient created by NaCl and Urea
▼
Not last long if removed by circulation
- Osmotic gradient is maintained by
- Very slow blood flow in the vasa recta
- Arrangement of descending and ascending limbs of vasa recta in close proximity to each other
Vasa recta
In Descending limb
Due to concentration gradient water moves
out into the interstitium
Contains nonfenestrated endothelim
↓
Facilitated transport of NaCl and Urea to the lumen of the vessel
In Ascending limb
Due to movement of water outside,
concentration of plasma proteins ↑, more water enters the lumen
Contains fenestraed endothelim
↓
NaCl and Urea returned to the interstitium (Urea trapping)
Counter current multiplier
- Due to Loop of Henle
- Establish osmotic concentration gradient in the tubule
Counter current exchanger
- Due to Vasa recta
- Maintains osmotic concentration gradient in the interstitium
- The greater the length of the Loop of Henle, the greater the osmolality that can be reached at the tip of the pyramid
- Concentrate and dilute the urine according to the requirements of the body
- Medullary osmolality is inversely proportional to medullary blood flow
Other examples of counter-current system
- Heat exchanges between the arteries and veins in the limbs
- Villous blood vessels in the small intestine
Role of ADH in Collecting duct
In the presence of ADH
Absorption of more water
↓
Excretion of low volume, hypertonic urine
In the absence of ADH
Less reabsorption of water
↓
Excretion of high volume, hypotonic urine
Applied Physiology
Diuresis
- Excretion of large quantity of water through urine
Osmotic diuresis
- Diuresis induced by the osmotic effects of solutes like glucose, in diabetes mellitus
Polyuria
- Increased urinary output with frequent voiding, diabetes insipidus: Tubules fail to reabsorb water because of ADH deficiency
Hemodialysis
Q. Describe the mechanism of urine formation involving processes of filtration, tubular reabsorption and secretion; concentration and diluting mechanism.