6. Determine how the osmolarity of the fluid in each structure compares to blood

Question

6. Determine how the osmolarity of the fluid in each structure compares to blood plasma. Is the fluid in the structure isgosmotic, hyperosmotic or bypoosmotic to blood plasma? a. Bowman's Space: The fluid in Bowman's space ("filtrate") isto plasma. b. Why? c. (answered for youEnd of PCT/beginning of descending limb of the LH: The filtrate is isosmotic to plasma. d. if certain processes happened to the filtrate in the PCT, why is it isosmotic? e. Bottom of the LH: The filtrate here isto plasma. f. Why? to plasma. Beginning of the DCT: The filtrate here is g. h. Why? i. Cortical Collecting Duct: The filtrate here is to plasma . This filtrate can just be excreted as urine, or something can happen to it in the medullary collecting duct. What.can happen to it in the medullary collecting duct?

Explanation / Answer

Ans) Osmolarity is the concentration of osmotic solution which is measured in osmoles. Plasma osmolarity is the measure of hydration state and is of normal value as 280 - 300 mOs/kg.

a) Fluid in Bowman's space or filterate is isosmotic to plasma. Isosmotic means having same concentration or value.

b) Reason of being isosmotic is because the value of fluid or filterate entering Bowman's capsule is same as plasma 300 mOs/kg. Bowman's capsule filterate is an ultrafiltrate of plasma and thus only difference of not having Proteins in filterate. In plasma proteins are also present but the albumin does not play significant role in maintaining osmolarity of fluid.

d) Osmolarity of fluid in proximal convolutes tubule is also isosmotic to plasma because the proximal convoluted tubule is first of all taking the solution from Bowmans capsule filterate which itself is isosmotic. This is the tubular reabsorption in Proximal tubule which contains large numbers of Mitochondria and is thus highly energetic region thus absorption of fluid with sodium actively in first step but in second step there is backflow of these substances with water back to the bloodstream making the solution again isosmotic to plasma. So the Tubular reabsorption and backflow of ions into the bloodstream is the reason of isosmotic concentration fluid in convoluted tubule.

e) Filtrate in bottom of loop of Henle is Hyperosmotic  to plasma.

f) Loop of henle cells are less energetic than proximal tubule and is thus having high permeability in the descending limb to water and less permeable to ions and salts like sodium and chloride . Thus water flows out of the limb to the interstitial fluid making the fluid highly concentrated.

g) Filtrate in beginning of distal convoluted tubule is hyposmotic to plasma

h) Water , urea and salts in ascending limb of loop of henle pass to the distal convoluted tubule. This area is more permeable to water which is also regulated by Anti diuretic hormone which allows more reabsorption of water and so more backflow of water into the blood making urine more concentrated.

i) Filtrate in cortical collecting duct is hyperosmotic to plasma.

j) In collecting duct secretion process occurs as distal convoluted tubule. Hyposmotic solution from distal tubule enters the collecting duct , where potassium ions are secreted into teh filtrate and sodium ions are rabsorbed into blood. This makes tubular fluid more concentrated and thus hyperosmotic to plasma. In medullary collecting duct the body has last chance to take or squeeze out all water which could be done by presence of Antidiuretic hormone which increases Aquaporins in the apical membrane hence more permeablity for water to reabsorbed and high cencentrated urine is then ready for excretion.

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