Though the filter membrane contains many wastes, it also houses some useful substances.  These substances, such as proteins, may be reabsorbed into the blood stream.  The process of useful substances moving from the filtrate membrane, to the blood stream is known as tubular reabsorption.  Tubular reabsorption begins when the filtrate enters the convuluted tube.  Tubule cells act like transporters and move useful substances from the filtrate to the extracellular space.

     Tubular secretion is the converse of tubular reabsorption.  Waste products move from the blood, to the tubule cells, to the filtrate.  This process provides a way for waste substances to move from the blood stream to the filtrate.

     The ureters are thin, slender tubes that are about 25 cm. to 30 cm.  There are two ureters that run from the hilius of each kidney to the posterior are of the bladder.  The ureters job is to transport the freshly collected urine from the kidneys to the bladder.  Layers of muscle tissue line the walls of the ureters.  These muscles help "push" urine from the kidneys to the bladder.  When urine has entered the bladder, ther are a number of valve-like mucosa that prevent urine from traveling up the ureters.

     The urinary bladder is a large hollow, muscular organ that temporarily holds urine.  The urinary bladder is located retroperitoneally in the pelvis, and is posterior to the pubic symphysis.  There are three openings in the bladder, two of them are for the ureters, and one is for the urethra.  The triangular base of the bladder is known as the trigone.  Most infections of the bladder occur at the trigone.  The male prostate gland is located in the bladder near the urethra's opening.  The walls of the bladder are made up of three layers of muscle tissue.  Collectively, these muscles are known as the detrosur muscle.  When the bladder is empty it is about 7.5 cm. to 5 cm. thick.  As urine collects in the bladder the walls of the bladder expand and rise.  A bladder that is full can hold some 500 ml. of urine, and be expanded to 12.5 cm. thick.

     The urethra is a narrow, hollow passageway that flows from the bladder to the to an opening outside the body.  At the junction of the bladder and urethra, smooth muscle forms the internal urethral sphincter.  The internal urethral sphincter is an involuntary muscle that prevents urine from flowing up the urethra to the bladder.  At the end of the urethra is the external urethral sphincter.  This sphincter is voluntary and regulates urine flowing from the bladder to an opening outside the body.