This soup is a concentrated separated product of multiple unremunerated donors, which is heated at 60 degrees for 10 hours.
The Australian supplier is CSL, and they have this handy data sheet here. As you can see, not only albumin comes in the bottle. You get some sodium octanoate as well. The 20g of albumin has its own osmolality, and is dissolved in the electrolyte solution which supplies most of the remaining osmoles, but still in comparison to human plasma this solution is hypo-osmotic. The manufacturer data reports a nominal osmolality of 130mOsm/Kg. Given that osmolality relates to the number of particles, one makes sense of this by recalling that 20 grams equates to only 0.3 mmol of albumin (its molecular mass is 66.438 kDa), so most of that 130mOsm comes from electrolytes.
The oncotic effects of concentrated albumin infusion
The oncotic pressure exerted by the albumin is due to the fact that it is a plasma colloid, trapped forever in the intravascular compartment. Normal albumin values exert about 75% of the total 20-30mmHg of oncotic pressure; it stands to reason that a 5-times concentrated infusion of albumin should exert 75-113mmHg of oncotic pressure.
This pressure sucks water out of the interstitial space, which contributes to the plasma-expanding effects of albumin. An American company’s prescribing information reveals that every 100ml of 20% human albumin draws an additional 250ml from the interstitial fluid over 15 minutes.
Whats with all the octanoate? Who put octanoate in my albumin?
Back in the days of World War 2, people started adding sodium caprylate (now known as sodium octanoate) to stabilise the albumin for transport and storage. Additionally, octanoate is weakly antimicrobial and antifungal; it also keeps the albumin from denaturing during the ten hours of 60-degree heat treatment.
It has the delicious distinction of being a common constituent of mammalian milk, and it is named after goats (caprylate, derived from capris). This poster from Germany warns us that octanoate is not very well metabolised by people who have dysfunctional livers.