Describe the internal male reproductive structures that produce the fluids that semen is composed of.

The male reproductive system is responsible for producing the fluids that make up semen. Here are the main structures involved:

1. Testes: These are the primary male reproductive organs. They produce sperm and testosterone, a hormone that plays a key role in the development of male reproductive tissues. The sperm produced in the testes makes up only about 2-5% of the semen volume.

2. Seminal Vesicles: These are a pair of glands located behind the bladder. They produce a thick, yellowish fluid that is rich in fructose. This fluid provides energy for the sperm and makes up about 70% of the semen volume.

3. Prostate Gland: This gland is located below the bladder and surrounds the urethra. It produces a thin, milky fluid that is alkaline. This fluid neutralizes the acidity of the vaginal tract, prolonging the lifespan of sperm. The fluid from the prostate makes up about 25% of the semen volume.

4. Bulbourethral Glands (Cowper's Glands): These are two small glands located below the prostate. They produce a clear, slippery fluid that lubricates the urethra and neutralizes any acidity due to residual urine. This fluid is released before ejaculation and does not typically contribute a significant volume to the semen.

5. Vas Deferens: This is a long, muscular tube that transports mature sperm to the urethra in preparation for ejaculation.

6. Urethra: This is the tube that carries urine from the bladder to outside of the body. During ejaculation, it also carries semen.

These structures work together to produce and transport semen, which is composed of sperm and various fluids.