Erection and Ejaculation

Certain mammals, including many primates, have a penile bone called baculum within the penis. This bone enables erection, which is the stiffness of the penis necessary during copulation or sexual intercourse. However, humans are an exception since they do not have a penile bone.

Erection occurs in distinct phases:

1. Following sexual arousal triggered by erotic or sexually pleasurable stimuli (images, sounds, smells, physical contact), male erection occurs.

2. It is maintained by significant blood accumulation, especially in the corpus cavernosum, but also in the corpus spongiosum. This is possible due to a dilation of the arterioles leading to an increased blood flow to the tissues of the penis. At the same time, this dilation leads to a contraction of the venules of the penis, which reduces the outflow of blood from the organ.

3. Through this whole process, the penis gets bigger, longer and stiffer.

The purpose of erection is to maintain the necessary stiffness of the penis to enter the vagina.

During arousal, the Cowper’s gland, also called the bulbourethral gland, produces pre-ejaculatory fluid that lubricates the urethra and neutralizes acidity from residual urine. The same nerve impulses that trigger the erection continue to increase in intensity until a certain critical threshold is reached that leads to ejaculation and orgasm.

Ejaculation occurs in three phases:

1. First, there is the emission, during which the genital tract and accessory glands contract and the liquids they contain are released into the urethra.

2. Then, the urethral sphincter contracts to prevent the expulsion of urine and the reflux of semen into the bladder.

3. Finally, rhythmic contractions of the urethra and penile muscles occur, allowing semen to be expelled.

The average volume of an ejaculate is between 2 mL and 5 mL and contains about 350 million spermatozoa.

Semen is composed of spermatozoa and fluid from the various accessory glands. Seminal fluid makes up about 50% of semen. The fluid from the prostate gland makes up about 35% of semen. Spermatozoa and fluid from the bulbourethral gland make up the rest of the semen. It is whitish, thick and slightly sticky.