I have been workshopping this one for awhile in my mind and today I carved out a few hours to create it.
It starts here
Part of the inspiration for this came from an epic message on the Channel Your Enthusiasm back channel by Roger Rodby
Here is the draft of the script.
Diffusive vs Convective clearance
I was teaching the third year medical students about acute kidney injury and the lecture begins with a brief history of extracorporeal dialysis for AKI. And I asked a student what extracorporeal dialysis was, and he correctly identified it as “dialysis outside the body.”
Since the ironclad law of the Socratic Method is that every correct answer is rewarded with another, harder, question, I replied, “Can you think of any example of intracorporeal dialysis?” The right answer is peritoneal dialysis, but he said , “The kidney?”
And off on a tangent we went…
Does the kidney even do dialysis? No. The kidney does not use diffusion to clean the blood. Clearance is provided by convection at the glomerulus. Plasma is squeezed through the slit diaphragms of the podocytes in the glomerulus but besides the lack of protein, the solute composition on both sides of that membrane is essentially identical.The kidney does not clear the blood by diffusion, the defining characteristic of dialysis, but rather by convection. How does that work? Glad you asked. Take Creatinine. The creatinine on both sides of the podocyte is the same, 4.4 mg/dL in this example.
4.4 mg per dL x a GFR of 25 mL per minute x 1440 minutes in a day divided my 100 mL in a dL comes to 1584 mg of creatinine filtered.
That is just about the amount of creatinine produced by a typical person a day.
So convective clearance can clear all of the creatinine produced everyday, the additional creatinine secreted in the proximal tubule is just gravy.
What about sodium?
138 mEq/L x a GFR of 25 mL per minute x 1440 minutes in a day divided by 1000 mL in a L comes to 4968 mE of creatinine filtered.
This is a problem since we only consume around 100-200 mEq of sodium a day. So this where the tubules earn their stripes by reabsorbing all the excess filtered sodium to keep us from peeing ourselves to death.
So these two examples demonstrate an important principle of convective clearance, it is better for clearing things at a high concentration than at a low concentration. In fact, a GFR of 1 is enough to clear a typical sodium daily load.
138 x 1 ml/min x 1440 min/day divided by 1000 ml/L = 198 mEq/day
This why even a tiny residual renal function makes a huge difference in dialysis patients.
But that same GFR of 1 would only clear
4.4 x 1 ml/min x 1440 divided by 100 ml/dL = 63 mg of creatinine only about 4% of the daily creatinine load.*
*This calculation is highly dependant on the serum Cr concentration, which would be a lot higher than 4.4 if the GFR was 1, but since a GFR of 1 in incompatible with life, the patient would also be getting renal replacement therapy, so it is hard to know where the serum Cr would actually be.So after explaining that the kidney didn’t actually do dialysis, or anything remotely close to dialysis. I asked if there was an organ that did do dialysis? Or, more specifrically, used diffusion for clearance.
Answers from the crowd:Liver > nope
Spleen > nope
Skin > nope
And finally, Lung? Yup.
The lung clears carbon dioxide from the body and absorbs oxygen by setting up a setting where the gasses move down their respective concentration gradients across a semipermeable membrane. You know, like dialysis.
A ventilator is not really like an artificial lung, in the way a dialysis machine replaces the core function of a kidney. It provides flow, but no clearance. We still are dependent on the alveolar membrane for oxygen absorption and carbon dioxide clearance.
But ECMO is an artificial lung and fully replaces the alveoli and uses the principles of dialysis to clear carbon dioxide and move oxygen into the blood. So at some level, ECMO is closer to the lung than dialysis is to the kidney.
One final note on this thread is in regards to dialysis and convection. The kidneys work by convective clearance but our primary means of replacing them is by diffusive clearance. However this summer we saw a randomized controlled trial of modifying dialysis to use convection rather than diffusion…and the result? Significant reduction in total mortality.
We don’t get a lot of wins in dialysis, so when we get one, we pay attention.
The script isn’t exact because I have to do some edits to meet the character limits of tweets.
Here are the Keynote slides that I used to create the gifs.