osmolarity of the filtrate the same as plasma really?

[MudPhud20XX]

Kaplan says:

- fluid entering Bowman's space is an ultrafiltrate of plasma: that is, the filtrate has the same concentration of dissolved substances as plasma, except proteins.

- osmolarity of the filtrate is 300 mOsm/L.

I thought protein such as albumin plays an important role in osmolarity and here's what Wiki says below. So my question is how come the filtrate has a similar osmolarity with plasma when filtrate contains no albumin which does contribute to the oncotic pressure in our body? What am I missing here? Many thanks in advance.

The large majority of oncotic pressure in capillaries is generated by the presence of high quantities of albumin which constitute approximately 80% of the total oncotic pressure exerted by blood plasma on interstitial fluid. The total oncotic pressure of an average capillary is about 28 mmHg with albumin contributing approximately 22 mmHg of this oncotic pressure. Because blood proteins cannot escape through capillary endothelium, oncotic pressure of capillary beds tends to draw water into the vessels.

---

Members don't see this ad.

 

[MudPhud20XX]

what the heck? something's wrong with the website, I didn't type like that lol!

 

[Successful BM]

Kaplan says:
So my question is how come the filtrate has a similar osmolarity with plasma when filtrate contains no albumin which does contribute to the oncotic pressure in our body? What am I missing here? Many thanks in advance.
.

Your line of reasoning is that the osmolarity of the filtrate should be higher than that of blood plasma because because blood plasma has a much higher concentration (>>>0) of albumin?

Base on the wiki, serum albumin contributes to a majority of the oncotic pressure, but not all of it. The rest comes from other solutes, like ions and waste products, correct? I'm not sure if this is where you're having a problem, but it seems that, assuming the osmolarity of the blood plasma and filtrate are the same, that the osmolarity of the filtrate is due to those other solutes being much more concentrated in the filtrate that in the blood plasma. This equalizes the osmolarities.

So why is it necessary for the osmolarities to be equal? It's not, but it's very helpful. Hydrostatic pressure is what forces fluid into the bowman's capsule from the blood plasma. You wouldn't want the oncotic pressure to be higher in the filtrate, because that would compete with the hydrostatic pressure in the plasma. Filtration would be much less efficient.

This is just my line of reasoning.

 

[Anastomoses]

Kaplan says:

- fluid entering Bowman's space is an ultrafiltrate of plasma: that is, the filtrate has the same concentration of dissolved substances as plasma, except proteins.

- osmolarity of the filtrate is 300 mOsm/L.

I thought protein such as albumin plays an important role in osmolarity and here's what Wiki says below. So my question is how come the filtrate has a similar osmolarity with plasma when filtrate contains no albumin which does contribute to the oncotic pressure in our body? What am I missing here? Many thanks in advance.

The large majority of oncotic pressure in capillaries is generated by the presence of high quantities of albumin which constitute approximately 80% of the total oncotic pressure exerted by blood plasma on interstitial fluid. The total oncotic pressure of an average capillary is about 28 mmHg with albumin contributing approximately 22 mmHg of this oncotic pressure. Because blood proteins cannot escape through capillary endothelium, oncotic pressure of capillary beds tends to draw water into the vessels.

I'm having a hard time figuring out exactly what you're asking. Your wiki article is all accurate.
1. albumin is the majority of capillary oncotic pressure.
2. Blood proteins are mainly responsible for the drawing of fluid into the capillaries.
3. When oncotic pressure is more than hydrostatic pressure, the fluid is drawn into the capillaries; when hydrostatic pressure is more...fluid moves out of the capillary, simultaneously lowering the hydrostatic pressure.

Is it the hydrostatic pressure that you're forgetting about? Or is it that you are having trouble with the number of molecules being about even? Revisit the Gibbs-Donnan effect.

 

[MudPhud20XX]

I'm having a hard time figuring out exactly what you're asking. Your wiki article is all accurate.
1. albumin is the majority of capillary oncotic pressure.
2. Blood proteins are mainly responsible for the drawing of fluid into the capillaries.
3. When oncotic pressure is more than hydrostatic pressure, the fluid is drawn into the capillaries; when hydrostatic pressure is more...fluid moves out of the capillary, simultaneously lowering the hydrostatic pressure.

Is it the hydrostatic pressure that you're forgetting about? Or is it that you are having trouble with the number of molecules being about even? Revisit the Gibbs-Donnan effect.

My apology if my question was not clear enough. I am basically arguing that the osmolarity between plasma and filtrate entering Bowman's space should NOT be similar since the filtrate contains no albumin, which does contribute to the oncotic pressure in our body. That's my question. Kaplan was explaining that the osmolarity of the filtrate is about 300mOsm/L, which is almost identical to the plasma osmolarity. (and the guy in the video lecture was saying that the osmolarity between filtrate and plasma is identical)

 

[Anastomoses]

My apology if my question was not clear enough. I am basically arguing that the osmolarity between plasma and filtrate entering Bowman's space should NOT be similar since the filtrate contains no albumin, which does contribute to the oncotic pressure in our body. That's my question. Kaplan was explaining that the osmolarity of the filtrate is about 300mOsm/L, which is almost identical to the plasma osmolarity. (and the guy in the video lecture was saying that the osmolarity between filtrate and plasma is identical)

The fenestrations are huge in the glomerulus; the pressure is higher than other capillaries of the body, forcing the plasma filtrate out and leaving only Hb and albumin sized proteins. It's the impressively high glomerular capillary pressure overcoming the albumin oncotic pressure, I believe.

 

[DeeJay2728]

They're only referring to NaCl, I believe. Just like 0.9% Normal Saline is isotonic (about 300 mOsm/L.

 

[Anastomoses]

They're only referring to NaCl, I believe. Just like 0.9% Normal Saline is isotonic (about 300 mOsm/L.

They're referring to all the solutes: ions, amino acids, glucose, small proteins etc.

 

[DeeJay2728]

It would make sense that they would be referring to all the solutes, but they only two solutes responsible for the osmolarity gradient are Na and Cl. Urea plays a part deeper in the medulla.

 

[Nephronlearner]

Here is my view of it (I'm not 100% sure though)

Oncotic pressure = c * R * T * correction number (related to how easily something passes through the membrane -> if it doesn't pass it at all then the number is 1, and the easier it passes through the lower the number is)
While in the capillaries, the correction number for sodium is close to 0 and for albumine close to 1, albumin indeed contributes a lot to the oncotic pressure.

But, the same applies to the Bowman space -> whatever can't leave it will indeed contribute to oncotic pressure of urine. And since nothing leaves the space until it reaches the proximal tubule -> every solute contributes to pressure in the Bowman space (including sodium). Thereby sodium could be responsible for the pressure in Bowman space.

 

[dyspareunia]

Kaplan says:

- fluid entering Bowman's space is an ultrafiltrate of plasma: that is, the filtrate has the same concentration of dissolved substances as plasma, except proteins.

- osmolarity of the filtrate is 300 mOsm/L.

I thought protein such as albumin plays an important role in osmolarity and here's what Wiki says below. So my question is how come the filtrate has a similar osmolarity with plasma when filtrate contains no albumin which does contribute to the oncotic pressure in our body? What am I missing here? Many thanks in advance.

The large majority of oncotic pressure in capillaries is generated by the presence of high quantities of albumin which constitute approximately 80% of the total oncotic pressure exerted by blood plasma on interstitial fluid. The total oncotic pressure of an average capillary is about 28 mmHg with albumin contributing approximately 22 mmHg of this oncotic pressure. Because blood proteins cannot escape through capillary endothelium, oncotic pressure of capillary beds tends to draw water into the vessels.

At any given moment there is a lower volume of solvent (i.e. water) in the filtrate than in the GC plasma. Lower volume = needs less solute to have the same osmolarity as a higher volume = the reason why albumin doesn't "win" and make the GC plasma hypertonic relative to the filtrate.

 

[Brosmosis]

I think the reason you are confused is because albumin is not dissolved in plasma and is too large to be filtered into the nephron. The dissolved ions are going to be the same concentration in the plasma and filtrate. For the purpose of this question albumin contributes to pressure gradients, but not the osmolarity of plasma or filtrate.

 

[Anastomoses]

I think the reason you are confused is because albumin is not dissolved in plasma and is too large to be filtered into the nephron. The dissolved ions are going to be the same concentration in the plasma and filtrate. For the purpose of this question albumin contributes to pressure gradients, but not the osmolarity of plasma or filtrate.

Although albumin and globulins exert a negativity that causes that other phenomenon, I forget what it's called...that causes cations to be attracted back into the capillary. But I know Bowman's capsule has a negative basement membrane so I'm not sure how that effects this. Ugh, I need to look this up now.

 

[dyspareunia]

Although albumin and globulins exert a negativity that causes that other phenomenon, I forget what it's called...that causes cations to be attracted back into the capillary. But I know Bowman's capsule has a negative basement membrane so I'm not sure how that effects this. Ugh, I need to look this up now.

The fused basal lamina (it isn't a basement membrane) is negatively charged, but there's speculation now that it doesn't play a significant role. In the past it (negative charged BL) was believed to be the main mechanism keeping albumin from filtering.

 

[Anastomoses]

The fused basal lamina (it isn't a basement membrane) is negatively charged, but there's speculation now that it doesn't play a significant role. In the past it (negative charged BL) was believed to be the main mechanism keeping albumin from filtering.

http://www.nature.com/ki/journal/v73/n3/full/5002659a.html

I'm pretty sure GBM stands for glomerular basement membrane.

You have more up-to-date studies?

 

[dyspareunia]

http://www.nature.com/ki/journal/v73/n3/full/5002659a.html

I'm pretty sure GBM stands for glomerular basement membrane.

You have more up-to-date studies?

No, I just know basic histology better than you.

Basement membrane = primarily type III collagen.
Basal lamina = primarily type IV collagen.

GBM is mainly COL IV, hence the issues associated with Alports. It's inaccurately named a BM. In reality it is two fused BL.

 

[Anastomoses]

No, I just know basic histology better than you.

Basement membrane = primarily type III collagen.
Basal lamina = primarily type IV collagen.

GBM is mainly COL IV, hence the issues associated with Alports. It's inaccurately named a BM. In reality it is two fused BL.

Now you're just being silly. What it is "inaccurately" named is not of any concern. It is named GBM and it includes the layers of the basal lamina. So what?

 

[dyspareunia]

Now you're just being silly. What it is "inaccurately" named is not of any concern. It is named GBM and it includes the layers of the basal lamina. So what?

It doesn't "include" layers of basal lamina, it is only layers of two basal lamina.

 

[Oblique]

It doesn't "include" layers of basal lamina, it is only layers of two basal lamina.

Three

 

[deleted500612]

Kaplan says:

- fluid entering Bowman's space is an ultrafiltrate of plasma: that is, the filtrate has the same concentration of dissolved substances as plasma, except proteins.

- osmolarity of the filtrate is 300 mOsm/L.

I thought protein such as albumin plays an important role in osmolarity and here's what Wiki says below. So my question is how come the filtrate has a similar osmolarity with plasma when filtrate contains no albumin which does contribute to the oncotic pressure in our body? What am I missing here? Many thanks in advance.

The large majority of oncotic pressure in capillaries is generated by the presence of high quantities of albumin which constitute approximately 80% of the total oncotic pressure exerted by blood plasma on interstitial fluid. The total oncotic pressure of an average capillary is about 28 mmHg with albumin contributing approximately 22 mmHg of this oncotic pressure. Because blood proteins cannot escape through capillary endothelium, oncotic pressure of capillary beds tends to draw water into the vessels.

I read this the other day too and had the exact same confusion as you, I don't think most people in the thread are understanding what you are confused about. Honestly, I think it just comes down to semantics... the filtrate is going to be isotonic in terms of solutes. I think when they are making that statement they are ignoring oncotic pressure, which as you mention, is technically a form of osmotic pressure. This had me confused as well because the filtrate is not technically isotonic then. I always thought isotonic referred to osmolarity (which it does) but according to some dictionaries it can also be used to refer solely to sodium concentration, which I assume is the context they used it in for this example.

Just remember that the filtrate is isotonic to plasma in terms of everything except proteins

 

[deleted500612]

No, I just know basic histology better than you.

Basement membrane = primarily type III collagen.
Basal lamina = primarily type IV collagen.

GBM is mainly COL IV, hence the issues associated with Alports. It's inaccurately named a BM. In reality it is two fused BL.

Sure you can argue that logically it "should" be called something else, but it IS called the glomerular basement membrane, so I don't think its really fair to criticize others for calling it that.

 

[MudPhud20XX]

I read this the other day too and had the exact same confusion as you, I don't think most people in the thread are understanding what you are confused about. Honestly, I think it just comes down to semantics... the filtrate is going to be isotonic in terms of solutes. I think when they are making that statement they are ignoring oncotic pressure, which as you mention, is technically a form of osmotic pressure. This had me confused as well because the filtrate is not technically isotonic then. I always thought isotonic referred to osmolarity (which it does) but according to some dictionaries it can also be used to refer solely to sodium concentration, which I assume is the context they used it in for this example.

Just remember that the filtrate is isotonic to plasma in terms of everything except proteins

Additional to proteins also, lipid-soluble substances transported in the plasma attached to proteins such as lipid soluble bilirubin, T4 (thyroxine), other lipid-soluble hormones are not freely filtered.

You read my mind. Thanks man and thanks to everyone who helped me out with the concept!

 

[dyspareunia]

Three

Three basal lamina? The GBM is the basal lamina of the podocyte fused with the basal lamina of the endothelial cell. One plus one equals two.

Sure you can argue that logically it "should" be called something else, but it IS called the glomerular basement membrane, so I don't think its really fair to criticize others for calling it that.

Originally I was only pointing out that the GBM is a basal lamina (COL IV) whereas a true BM is primarily COL III. There's a difference and it's significant when you consider Alport's and other COL IV disorders.

 

[Anastomoses]

Three basal lamina? The GBM is the basal lamina of the podocyte fused with the basal lamina of the endothelial cell. One plus one equals two.



Originally I was only pointing out that the GBM is a basal lamina (COL IV) whereas a true BM is primarily COL III. There's a difference and it's significant when you consider Alport's and other COL IV disorders.

He's referring to the three layers of the GBM. lamina rara interna, lamina densa, lamina rara externa. Only lamina densa is collagen IV. the rest is heparan sulfate.

 

[Oblique]

Originally I was only pointing out that the GBM is a basal lamina (COL IV) whereas a true BM is primarily COL III. There's a difference and it's significant when you consider Alport's and other COL IV disorders.

Can you please explain what you mean by true basement membrane? I did some googling but couldn't find anything that goes along what you are saying.
http://books.google.com/books?id=RM-FVY47NEgC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q=collagen III&f=false
"Type IV collagen is the only collagen specific for basement membrane"