Testicles (AKA testes): the male reproductive organ where sperm and testosterone are produced. There are usually 2 that sit inside the scrotum. The testicles descend into the scrotum in early male fetal development (before birth).
Scrotum: skin sac on the outside of the body that houses the testicles
Epididymis: a thin tube that connects the testicles to the vas deferens. The sperm finish maturing while traveling through here.
Vas deferens: a thin tube that connects the epididymis to the ejaculatory duct. The sperm travel through here during an ejaculation.
Ejaculatory Duct: a thin tube that connects the vas deferens to the urethra. The sperm travel through here during an ejaculation and are coated with an alkaline fluid known as semen.
Seminal vesicles, prostate gland, and bulbourethral glands: glands that secrete the contents of semen into the ejaculatory duct.
Bladder: muscular organ that fills with urine, which exits the body through the urethra and out the tip of the penis. Note: semen should not flow into the bladder due to a small sphincter (valve) that closes when an ejaculation occurs to block sperm from entering the bladder.
Urethra: tube-like structure inside the penis that both urine and semen travel through (but typically not at the same time).
Penis: made up of spongy tissue that fills with blood during an erection. The urethra runs through the penis and acts as an exit route for urine and semen.
The testes have 2 main functions: produce testosterone and sperm. Males begin secreting testosterone during fetal development (yes, before they were even born) and continue to secrete low amounts of it until puberty begins.
Once puberty begins, cells inside the testes (called Leydig cells) begin to produce large amounts of testosterone, which aids in the development of muscles and facial hair, among other things. But most importantly, the high amount of testosterone allows for sperm production (spermatogenesis) to occur inside the testicles in these little tubes called seminiferous tubules. More specifically, sperm production occurs inside Sertoli cells, which are located inside the seminiferous tubules.
Once the sperm are created in the seminiferous tubules, they travel to the epididymis. This is where the sperm will finish maturing (now they can move on their own) and will remain stored until an ejaculation occurs.
An ejaculation begins with an erection, during which time the penis hardens as it fills with blood. This triggers a few things:
The sperm leave the epididymis and travel through the vas deferens and into the ejaculatory duct.
The prostate gland, seminal vesicles, and bulbourethral glands release the contents that make up semen into the ejaculatory duct as the sperm move through it.
The sperm, now in the semen, travel through the ejaculatory duct and urethra, and then exit the body at the tip of the penis (AKA the urethral opening).
Good question! They're actually controlled by a pretty cool balancing act that involves both the testes and the brain.
One part of the brain (the hypothalamus) releases this hormone called GnRH, which then causes another part of the brain (the anterior pituitary gland) to release these hormones called LH and FSH. LH stimulates testosterone production, while FSH stimulates spermatogenesis.
So, the testes are getting this LH and FSH from the brain and they're making testosterone and sperm. Cool...at least for a little while.
But eventually there will be too much testosterone being produced. The brain will realize that there is a lot of testosterone floating around, and it will stop releasing the GnRH that it was releasing before. No GnRH means no more FSH and LH, which means no more testosterone and sperm production. At least for a little bit.
Eventually, the brain will realize that the testosterone levels are low in the body again, and the process will repeat.
Sperm production begins at puberty and continues throughout a male's lifetime. It takes roughly 50-75 days for new sperm to be produced from start to finish.
Physiology, Male Reproductive System - StatPearls - NCBI Bookshelf (nih.gov)
Anatomy and Physiology of the Male Reproductive System | Anatomy and Physiology II (lumenlearning.com)