The male reproductive system

The primary sex organs make the gametes and the sex hormones: the testes in males, the ovaries in females. The rest are accessory parts: ducts, glands and the external genitalia.

The two testes are oval, about 4-5 cm long and 2-3 cm wide. They hang outside the abdomen in a pouch of skin, the scrotum. This keeps them about 2-2.5 °C cooler than the inside of the body, and that lower temperature is needed to make sperms.

Picture a testis as a hotel:

• It is divided into about 250 rooms, the testicular lobules.
• Each room has one to three guests: long, coiled seminiferous tubules, where sperms are made.
• The tubules open into a network, the rete testis, like the hotel lobby.
• Fine tubes, the vasa efferentia, lead from the lobby to the epididymis, a long coiled tube along the back of the testis, like a car park where the sperms wait and mature.

Inside a seminiferous tubule there are two kinds of cells:

• Spermatogonia (male germ cells), which divide to form sperms.
• Sertoli cells, larger cells that nourish the developing sperms.

Between the tubules lie the interstitial or Leydig cells. They make the male hormones, the androgens (mainly testosterone). Some immune cells are found there too.

From the epididymis, the vas deferens runs up into the abdomen and loops over the urinary bladder. It joins a duct from the seminal vesicle to form the ejaculatory duct, which opens into the urethra. The urethra carries both urine and semen, and opens at the tip of the penis. So the sperm's route is: seminiferous tubules → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra.

Three accessory glands add the fluid part of semen, the seminal plasma: the paired seminal vesicles, the single prostate and the paired bulbourethral glands. Seminal plasma is rich in fructose (the sperms' energy source), calcium and certain enzymes. The bulbourethral glands also help lubricate the penis.