Anatomy of a Penis
The penis is the male sexual organ.
The shaft is the longest part of it. The head or glans is at the end of the shaft. The opening at the tip of the head, where urine and semen come out, is called the meatus.
Inside, two cylinder-shaped chambers called the corpora cavernosa run the length of the penis. They have a maze of blood vessels, tissue, and open pockets.
The urethra, the tube that urine and semen flow through, runs along the underside of them, in the spongy tissue of the corpus spongiosum.
Two main arteries (one in each of the corpora cavernosa) and several veins move blood in and out. Nerves relay messages to and from other parts of your body.
What Is an Erection?
An erection starts in your brain. Something you saw, felt, smelled, heard, or thought makes your nerves send chemical messages to the blood vessels in your penis. The arteries relax and open up to let more blood flow in; at the same time, the veins close up. Once blood is in the penis, pressure traps it within the corpora cavernosa. Your penis expands and holds the erection.
When the inflow of blood stops and the veins open, your penis becomes soft.
What Is Ejaculation?
When you're aroused, tubes called the vas deferens squeeze sperm from the testes toward the back of the urethra. The seminal vesicles also release fluid there.
The urethra senses the sperm and fluid mixture. Then, at the height of sexual excitement, it sends signals to your spinal cord, which in turn sends signals to the muscles at the base of your penis. These contract powerfully and quickly, every 0.8 seconds. This forces the semen out of the penis as you climax.