Gelofusine is 4% solution of succinylated bovine gelatine, delivered in normal saline.

Bovine gelatin, you say.

Yes, the bones and skin of cattle are used to create this nutritious broth.
Sounds like alchemy? Hogan had first used a gelatin as a resuscitation fluid in 1915; of course his gelatin was “straight from the cow”, with a high molecular weight (about 100 kDa), and tended to solidify hilariously at room temperature, forming a bag full  of  flavourless Aeroplane Jelly.

This disadvantage was overcome by decreasing the molecular weight; the smaller molecules failed to jellify, and these days companies like Braun have settled on an average molecular weight of around 30,000 Daltons. This product is then chemically modified by succinylation. 

Succinylation is the addition of a succinyl group (a four-carbon molecule) to the lysine residue of a protein. In this case, the result is a change in the charge of the lysine residue from a (+1) to a (-1), thereby increasing the anionic charge of the protein. This has several desirable effects, including improved emulsion activity, emulsion stability and increased water adsorption capacity, all of which should theoretically improve the fitness of this product for its purpose.

The effect of this can be seen on the contents of the gelofusine bag. The additional negative charge of the gelatin decreases the amount of chloride required to maintain electroneutrality; thus the “normal saline” carrier fluid is in fact not normal at all.

Contents of a bag of gelofusine

This is a positive feature; the increased strong ion difference results in less hyperchloraemic acidosis among patients into whom massive amounts of gelofusine are infused.

References

HOGAN, JAMES J. "The intravenous use of colloidal (gelatin) solutions in shock." Journal of the American Medical Association 64.9 (1915): 721-726.

Braun have a product information pamphplet, detailing the contents and properties of their brand of Gelofusine.

Alias, Nur Aliyana Binti, et al. "Improving the Physicochemical Properties of Commercial Bovine Gelatin using Succinylation." Asia Pacific Journal of Sustainable Agriculture, Food and Energy 5.1 (2017): 2-6.