The liquid which collects in the cavity of Bowman's capsule is
blood plasma minus blood proteins
Urine formation begins with the filtration of blood through the epithelial walls of the glomerulus and Bowman's capsule. The fluid portion of the blood, which consists of water, urea, ions, nutrients and small proteins, is able to move across the capillary wall. Blood cells and larger proteins, however, cannot cross and are retained in the blood.