Osmolality, Osmolarity, and Tonicity - Physiology, Biology, and Chemistry 

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I just love listening to you talk. The material sticks so well in my head! ;)

While it does not really have practical implications in the field of medicine, it is not correct to simplify the difference between the concepts of osmolarity and osmolality to just /kg and /L. The way these ideas are presented to students is often confusing and I can certainly remember my own confusion about the two. To get rid of the feeling of vagueness surrounding these concepts it is useful to look at what they are exactly. The term osmolaRity refers to the number of particles of solute per liter of _solution_ , whereas the term osmolaLity refers to the number of particles of solute per kilogram of _solvent_ . The first one, osmolaRity, thus gives the number of particles per liter of solution which these same particles are a part of. So, for something that is extremely soluble in water (say, ammonium nitrate in boiling water), you could make a liter of solution with barely any water in it, resulting in a very high osmolarity. And yet you are limited to only one liter, so there is a limit to how high you can go. There is only so many particles you can put in a liter of solution. OsmoLality, on the other hand, gives you the number of particles per kilogram of solvent. The particles themselves are not a part of that kilogram. So we are not limited by how much of the soluble compound we can add. If we continue with the previous example and add a liter of ammonium nitrate (about 1,72 kg) to the kilogram of our solvent we end up with the numerical value of OsmolaLity being the same as the numerical value of osmolaRity in the previous example. The number of solute particles is the same after all (the osmolaRity is of course different in the latter case as we have more water per particle). But this time we are not limited to just this number. We can add more. 10 kilograms, 100 kilograms, a metric ton. There are solids that are almost infinitely soluble in water at certain temperatures, so you can add more almost indefinitely, resulting in your osmolaLity shooting through the roof, something you would not be able to do with osmolaRity.

very helpful, and engaging - you are an amazing teacher!

Great lessons! And funny! That's what makes it more engaging. Thanks a bunch!

been searching answers to my questions about these minute details.. got so many basics cleared... a big thank u. ❤️

Are there other molecules that don't affect tonicity other than urea?

Thank you so much but I just have a question the Na+ passes through the channels of the plasma membrane as it is a small ion passing with water and also durnig the active transport through the Na, K pump so how it is an effective osmole?

What a great explanation, love the vibe!

Thank u for making this easy and enjoyable

Hi, Thank you so much for your explanation! I am preparing for the CSMLS exam. Do you know any source of questions that can help me!

What great video! Thanks misteRRRR

Love listening to you teach 🥰🙏🏾

Very nice video :) I have just one question i not really get. In the nephron we have urea transporters (for example in the end of the collecting duct). For transport of urea back in the system right? And how we get an urea concentration in the nephron when its always moves through cells?

If you heat 100 mLs of water the volume (osmolarity) will expand but the weight (osmolality) will not. The lightbulb just went on! 💡 (PS: After university, I lived in Edinburgh -- love Scotland, my ancestral homeland.)

You mentioned ethanol talking about effective osmoles. Is it an effective or ineffective osmole?

Love the way to teach you explain you speak Everything 💞

Thank you once again.

you are the best

Can you explain what is osmoregularity ?

you never disappoint me thank you best teacher

This will be perfect when its time for sem 1 revision

Thank you for making medicine fun....a big big fan

👌👌👌very good

In the slide at minute 1:26, it says both that Osmolality is per volume (l) and per mass (kg). Wrong? Because the first is the definition of osmolarity, right?

Hello! Our teacher , always fascinated by the way you explain your lessons , thank you I have a question please ,i m preparing my exam about renal physiology ,i m searching for questions MCQ about that item ,can you give me a website for Free to evaluate myself . keep going your nice work.

Thank you so much!. As a first year medical student i find his extremely helpful.

Haha I won't ever forget the difference between osmolality and osmolaRity . Damn accents and clear confusion too !

Its Ok that practically Osmolarity and Osmolality is not that much different but hey what is "Osmolatity" at 4:23 :D just a mistake?

What's that cc used in 100cc

Thanks a lot ❤

SIRRRR, THANKYOU SO MUCHHHHH I CANT BELIEVEEE I NEVER KNEW WHAT ALL OF THIS BS MEANT UNTIL TODAYYY! ALSO LOVE YOUUU! ALSO, HOW WOULD YOU DEFINE A SUBSTANCE THAT IS OSMOTICALLY ACTIVE? WOULD MEAN A LOT IF YOU REPLY HEHE

Nice video and good channel and teacher 😊👍

I love this so much 😘😘

WE DON'T LET PHYSIO PROFESSOR NEAR PATIENTS 😂😂😂😂 LMAO

0.9 Ns osmolarity is 308.. Which is higher than plasma osmolarity... how it is isotonic fluid.. Plz explain

You said for osmolality when you heat the weight will not change but on heating it is vapourized then how it will change the weight..it will reduce in weight

legit best for quick review 6:28 exercise Z says 500 cc it means 1/2 a liter if i'm correct?

haha love the way you explain this! love the way you talk.

Isn’t osmolarity number of osmoles of solute in every liter of solution not solvent

Is it osmolality or tonicity that affects the secretion of ADH? It is usually said that ADH is secreted by high osmolality, but it is confusing because Urea does not affect the secretion of ADH.

You are an extraordinary teacher, believe me you are better than my text book "Harrison" and "tietz"

underrated.

THANK YOU SIR 1:35 my fav part

In this minute 4:17 you said that osm. Of IS =osm. of plasma= osm. Of ICF But IS +plasma =ECF And ECF osm.should equal ICF osm. How is this correct ?

i am so glad that i found your Channel , good job me🙂

3:10 now that make sense 😂😂

Came to this confused, left educated with a few laughs.

I wish my lectures were more like this💪🏿Urea is BS😂😂

Sir is the answer for problem Z at 6:56 is 30 Osmole/L

Dear sir your speed should be little bit slow, thank you

Why can’t molarity be used instead of osmolarity? I understand that osmolarity calculates the number of particles instead of molecules but I’m still not quite sure why can’t we use molarity?

Early 💜

No osmometer is harmed during this video😁😁😁😁🤣

Why in 1:33 you say osm/volume is osmolality Though osmolality is osm/kg

i couldnt stop laughing at the hard Rs hahaha

I believe I send you a message through Facebook? I’m not sure because I just created the account to ask you a to help me understand my test results Thank you for trying to help me and answering to my comment in the other video

💜❤️

Poor physiology professor 🤭

only the bored people care hahah