Why do men get hard?

Sex education has gradually entered the classroom, and many minors have also begun to be exposed to healthy sex knowledge. In our cognition, the manifestation of male sexual desire is that the penis will become hard, which is a sign that the other party can clearly see. There are many situations in which men will become hard, such as seeing a beautiful girl, or watching a movie plot related to sex, etc. Why do men's penises become hard?

Erection refers to the state and process of the penis, clitoris or nipples of animals swelling and hardening. Generally speaking, after being stimulated, the male penis relaxes in a short time, quickly becomes engorged with blood, and blood is perfused into the venous blood vessels in the corpus cavernosum until the pressure rises to a certain limit. The corpus cavernosum filled with blood will prop up the penis, making it harder and longer. At this time, the erect penis can be inserted into the female vagina more easily to perform piston movement - sexual intercourse.

The corpus cavernosum of the penis is composed of smooth muscle cells and connective tissue to form the corpus cavernosum trabeculae. The corpus cavernosum nerve of the penis contains two components, sympathetic and parasympathetic. The former comes from the spinal cord T11 to L2, and the latter comes from the spinal cord S2-4. Stimulating the sacral parasympathetic nerves can cause the penis to swell, while stimulating the thoracolumbar sympathetic nerves will cause the penis to become flaccid. The dorsal penis nerve transmits sensations to the corpus cavernosum and glans scalp, as well as the urethra and corpus cavernosum.

Penile erection is a complex psycho-physiological process, which is essentially a series of neurovascular activities.

When the parasympathetic nerves are excited, the smooth muscle cells of the small arteries and vascular sinuses in the corpus cavernosum of the penis relax, the corpus cavernosum vascular sinuses dilate, the blood flow in the arteries increases, and the corpus cavernosum of the penis becomes congested and swollen. The swollen corpus cavernosum of the penis compresses the small veins under the tunica albuginea, closing the venous outflow tract. The contraction of the pelvic floor muscles can also compress the corpus cavernosum, causing it to further swell and harden, resulting in an erection. When the sympathetic nerves are excited, the smooth muscle cells of the small arteries and vascular sinuses contract, the pressure of the corpus cavernosum decreases, the veins open, and the penis begins to become soft. Therefore, smooth muscle relaxation, arterial blood flow and blood velocity, and venous blood outflow resistance are the three elements of penile erection.

When the smooth muscle cells of the arteries and vascular sinuses in the corpus cavernosum of the penis relax, the brain or local part of the penis receives sexual stimulation, and impulses are sent from the hypothalamus or the lower central nervous system of the sacral cord. The nerve impulses are transmitted to the corpus cavernosum of the penis, and the synthesis and release of nitric oxide (NO) by the parasympathetic nerve endings and vascular endothelial cells under the catalysis of nitric oxide synthase (NOS) increases. NO enters the smooth muscle cells and activates guanylate cyclase (GC), which increases the cGMP in the smooth muscle cells. The latter activates proteinase K and acts on the calcium ion channel, reducing the calcium ion concentration in the cells and causing the smooth muscle cells to relax. The cGMP in the smooth muscle of the corpus cavernosum of the penis is degraded into GMP by phosphodiesterase 5 (PDE5) and loses its activity.

In addition to NO, substances related to smooth muscle relaxation and penile erection include acetylcholine, vasoactive intestinal peptide, calcitonin gene-related peptide, PGE2, cAMP, etc.; substances related to smooth muscle contraction and penile flaccidity include norepinephrine, endothelin, PGF2α, etc.

Due to individual differences, the erection angle of each person is also different. The size of the erection angle is related to the pressure in the cavernous body of the penis. Generally speaking, the erection angle of men is the largest when they are around 20 years old.

The erection angles of men vary. Most men have an erection angle greater than 90 degrees (horizontally upward), ranging from 120 to 130 degrees. About 10% of men have their penises close to their bellies when erect, and about 5% of men have an erection angle less than 90 degrees (downward). Generally speaking, it is normal to be able to complete sexual intercourse and obtain sexual satisfaction. Most people have their penises erected vertically and forward, but some people have their penises slightly bent to the left or right, which is also normal.

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