The prostate is a small walnut-sized gland situated at the base of the bladder, surrounding the urethra. Its main function is to provide the prostatic fluid which, when combined with the sperm in the seminal vesicles, provides the semen for ejaculation. 

 

It can be, literally, a pain in the arse. As men age, the prostate naturally enlarges (benign prostatic hyperplasia, BPH) and this can constrict the urethra, causing difficulty in passing urine and, at the worst, urinary retention. Fortunately, medication is available to help, although surgery in some form is still occasionally required.