Male urogenital system, vertical side section, artwork. The base of the spine is at upper right. At centre is the urinary bladder. The urethra (red) is the tube down which urine and semen are expelled from the body. Semen contains sperm, which is produced in the testes (grey, bottom), mixed with fluids from the seminal vesicle (very small, pink, at right of bladder) and the prostate gland (below bladder). The penis (left) is composed of erectile tissue (blue), which fills with blood when aroused, causing it to become hard and erect. The light blue area above the penis is the pubic bone. At right are the rectum and anus, through which faeces are expelled.