The urethra is a tube that allows urine to leave the body. It is part of the urinary system, which includes the kidneys, bladder, ureters, and urethra. The urethra is a passageway located in the pelvic region of the body. The walls of the tube are thin and made up of epithelial tissue, smooth muscle cells, and connective tissue. The urethra is different in males and females. In males, the urethra is about 18 to 20 cm long and transports both urine and semen. The first part of the male urethra passes through the prostate gland and is called the prostatic urethra. The second part, a short region that penetrates the pelvic floor and enters the penis, is called the membranous urethra. The third part, the spongy urethra, is the longest region and extends the entire length of the penis. In females, the urethra is much shorter, only 3 to 4 cm long, and opens in front of the vagina. The urethra is surrounded by two sphincters, the internal urethral sphincter, which is smooth (involuntary) muscle, and the external urethral sphincter, which is skeletal (voluntary) muscle. The internal urethral sphincter surrounds the beginning of the urethra, where it leaves the urinary bladder, while the external urethral sphincter encircles the urethra where it goes through the pelvic floor.