The urinary bladder is able to expand as urine accumulates within it due to the presence of a specialized mucosa called

The urinary bladder is able to expand as urine accumulates within it due to the presence of a specialized mucosa called urothelium (a form of transitional epithelium).    
The urinary bladder is a hollow muscular organ which collects and stores urine. The outside of the bladder is protected by a serous membrane, there is a smooth muscle layer below it and the inner side of the bladder is lined with a mucosal membrane, the basement membrane, and the lamina propria. The mucosal have a protective role and offers a urothelial barrier against the passing of infections.