Chemistry Basics: Osmolarity, Osmolality and Tonicity 

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@@Penguinprof cONGRATS AND THANK YOU BECAUSE I have seen a lot of confsusion between tonicity and osmolarity. Your explanation is clear!!!

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YES they are very different. In the US and a few other countries, plasma or serum urea concentration is expressed as the amount of urea nitrogen, while in most other countries it is expressed simply as urea in mmol/L. Since BUN represents only the nitrogen in the urea molecule (MW 28) while urea itself has a MW of 60, total urea is approx. twice BUN. To convert: BUN mg/dL multiplied by 0.357 = urea (mmol/L) Urea (mmol/L) divided by 0.357 = BUN (mg/dL) Approximate reference (normal) range: Serum/plasma urea 2.5-7.8 mmol/L Serum/plasma BUN 7.0-22 mg/dL It's confusing at first, but I hope that helps!

@@Penguinprof Thanks ma'am. Got it.So it mainly depends on which is used in which country.Please post this type of videos as much as possible.They are really informative.

Glucose is non-penetrating (it requires a carrier protein to enter cells). As for your second question, definition #2 below is best. But can you BELIEVE what a mess this topic is? Varying definitions of tonicity in a sample of physiology textbooks: Michael D. Johnson, Human Biology: Concept and Current Issues (8th Edition) (9). “Tonicity refers to the relative concentrations of solutes in two fluids. Extracellular fluid that is isotonic has the same solute concentration as the intracellular fluid. When cells are placed in a hypertonic solution, one with a concentration of solutes higher than the intracellular fluid, water diffuses out of the cells and the cells shrink.” Cindy L. Stanfield, William J. Germann, Principles of Human Physiology (3rd Edition) (17). “Whereas a solution’s osmolarity is based solely on its total solute concentration, its tonicity is determined by how it affects cell volume, which depends not only on the solute concentration but also on the solute permeability of cell membranes. A solution is said to be isotonic when it does not alter cell volume. This refers to the cell’s final volume. Under certain conditions a cell may shrink or swell initially, even if the solution is isotonic.” Richard W. Hill, Gordon A. Wyse, Margaret Anderson, Animal Physiology (7). “If a cell membrane or epithelium is impermeable to a solute and the solute is more concentrated on one side than the other, the solute creates a persistent difference of osmotic pressure across the cell membrane or epithelium.” Arthur C. Guyton, John E. Hall, Medical Physiology (10th Edition) (6). “If a cell is placed into a hypotonic solution that has a lower concentration of impermeant solutes, water will diffuse into the cell, causing it to swell; water will continue to diffuse into the cell, diluting the intracellular fluid while also concentrating the extracellular fluid until both solutions have about the same osmolarity.” Walter F. Boron, Emile Boulpaep, Medical Physiology: A Cellular and Molecular Approach (2nd Edition) (2). “The difference between effects of mannitol and urea on the final cell volume illustrates the need to distinguish between total osmolality and effective osmolality (also known as tonicity).” Kenneth S. Saladin, Anatomy & Physiology: The Unity of Form and Function (4th Edition) (14). “Tonicity is the ability of a solution to affect the fluid volume and pressure in a cell. If a solute cannot pass through a plasma membrane, but remains more concentrated on one side of the membrane than on the other, it causes osmosis.”

@@Penguinprof Thank you very much!! I appreciate your help this makes sense! And yea haha I get the overall idea of this topic but it’s always the little things that confuse my mind

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Haha! I try...

Glucose doesn't dissociate at all.

Glucose is made of C, H, O which are non metals. Non metals usually forms covalent bond. Covalent bonds need high energy to break apart. Nacl is ionic compound. Na = alkali metal Cl= non metal They both form ionic bond, a weak bond, which can be broken by just random motion of water molecules( BROwnian motion) in container or any vessel. As they can easily break, they increase the osmolarity of solution

As if science wasn't hard enough...

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