A 19-year-old woman complains of swelling of her eyelids, abdomen, and ankles. At bedtime, there are depressions in her legs at the location of the elastic in her socks. A chest X-ray shows bilateral pleural effusions. Urine protein electrophoresis demonstrates 4+ proteinuria. A percutaneous needle biopsy of the kidney establishes the diagnosis of minimal change nephrotic syndrome. Soft tissue edema in this patient is most likely caused by which of the following mechanisms of disease?