Regulation of Extracellular Fluid Osmolarity and Sodium Concentration

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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.

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Total Concentration Of Solutes

• Osmolarity is determined by the amount of solute
  divided by the volume of the extracellular fluid.
• Thus, to a large extent, extracellular fluid
  sodium concentration and osmolarity are regulated
  by the amount of extracellular water.

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Level Of Body Water

• It is controlled by
• Fluid intake, which is regulated by factors that
  determine thirst.
• Renal excretion of water, which is controlled by
  multiple factors that influence glomerular
  filtration and tubular reabsorption.

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Functions of Kidney

• The normal kidney has tremendous capability to
  vary the relative proportions of solutes and
  water in the urine in various states.

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• 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.

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• This ability to regulate water excretion
  independently of solute excretion is necessary
  for survival, especially when fluid intake is
  limited.

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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 to a maximum of
  1200mOsm/L.

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• The formation of a dilute or concentrated
  urine depends upon two factors
• Medullary gradient.
• Antidiuretic hormone.

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Anti-Diuretic Hormone

• Rate of ADH secretion to a large extent
  determine, whether dilute or conc. urine is
  going to be excreted.

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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

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• Decreased urine volume ( without altering rate of
  renal excretion of the solutes).

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• 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.

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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

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Medullary Gradient

• The gradual increase in the osmolarity of the
  medullary interstitial fluid is called the
  medullary gradient.
• The vertical osmotic gradient remains constant.

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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.

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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.

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• 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.

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MOA Of The Counter Current System.
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• Long loops of Henle establish the vertical
  osmotic gradient.
• Vasa recta , prevent the dissolution of this
  gradient while providing blood to the renal
  medulla.

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• 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.

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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.

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• 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.

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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.

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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.

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Ascending Limb Of Loop Of Henle
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• 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.

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• Ability of kidney to form a urine that is more
  concentrated than plasma is essential for
  survival.

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• Water is continuously lost from the body through
  various routes
• Lungs
• Gastrointestinal tract
• Skin
• kidneys

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• 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.

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Obligatory Urine Volume
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• 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.

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Obligatory Urine Volume

• It is calculated by

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• End Of Todays Lecture!!!