The kidney is a paired organ, mostly paired, located in the abdomen behind the peritoneum, between the 11th thoracic vertebra and the 3rd lumbar vertebra (for the left kidney) or between the 12th thoracic vertebra and the space between the 3rd and 4th lumbar vertebrae (for the right kidney). By filtration and excretion of urine, the kidneys provide water and salts balance (homoeostasis) of the blood and of the body in general.
The bladder is a hollow pelvic organ with walls composed of an outer smooth muscle (the detrusor, which adjusts to filling) and a pseudo-stratified epithelium on the inside surface, called  the urothelium. This wall is distensible to allow the storage of urine between mictions. Its usual maximum filling provides a physiological capacity of 400-500 ml in adults.