All of the following statements about colloids are true EXCEPT:

Colloids are solutions that contain large molecules, such as proteins or starches, which are typically unable to cross cell membranes due to their size and molecular weight. This characteristic allows colloids to exert oncotic pressure, helping to retain fluid within the vascular space and preventing it from leaking into surrounding tissues.

The statement that colloids are unable to hold fluid in the vascular space is incorrect because one of the primary functions of colloids is indeed to help maintain fluid balance in this area. By holding onto fluid, colloids contribute to maintaining blood volume and pressure, which is crucial for proper circulation and organ function.

Thus, the incorrect statement is that they are unable to hold fluid in the vascular space. All other provided statements accurately describe the properties and effects of colloids, contributing to a better understanding of their role in veterinary pharmacology and fluid therapy.