The male reproductive system

The male system sits in the pelvis and has a pair of testes, a set of accessory ducts, accessory glands and the external genitalia.

The testes lie outside the abdominal cavity inside a pouch, the scrotum, which keeps them about 2–2.5 °C cooler than the internal body temperature; this lower temperature is needed for sperm production.

An adult testis is oval, roughly 4–5 cm long and 2–3 cm wide, and is divided internally into about 250 compartments called testicular lobules.

Each lobule holds one to three tightly coiled seminiferous tubules, where sperms are made. Their lining carries two cell types: male germ cells (spermatogonia), which divide by meiosis to form sperms, and Sertoli cells, which nourish the developing germ cells.

The spaces between the tubules (interstitial spaces) contain small blood vessels, Leydig (interstitial) cells that make androgens such as testosterone, and some immunologically competent cells.

Sperms leave the testis through a duct sequence: rete testis → vasa efferentia → epididymis (on the back surface of the testis) → vas deferens. The vas deferens loops over the urinary bladder, receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct.

The urethra starts at the urinary bladder, runs through the penis and opens outside at the urethral meatus; it therefore carries both urine and semen.

The penis is made of special tissue that helps it become erect for insemination. Its enlarged end, the glans penis, has a loose fold of skin over it called the foreskin.

There are three kinds of accessory glands: paired seminal vesicles, a single prostate and paired bulbourethral glands. Together their secretions make up the seminal plasma, rich in calcium, fructose and certain enzymes; the bulbourethral secretions also help lubricate the penis.