Penis Anatomy and Testicles: Erections, Semen and Health Questions

The shaft contains erectile tissues surrounding the urethra. The glans is the sensitive head of the penis. In an uncircumcised penis, foreskin covers some or all of the glans when flaccid and normally retracts to varying degrees.

Most of the penis contains two columns of erectile tissue called the corpora cavernosa and a third erectile structure, the corpus spongiosum, which surrounds the urethra and expands into the glans. The foreskin, when present, covers part or all of the glans when the penis is not erect, and the frenulum is a sensitive fold of tissue on the underside.

The urethra carries urine from the bladder and also provides the pathway for semen during ejaculation, although these functions normally occur at different times. Blood vessels, nerves and connective tissues throughout the penis are essential to sensation and erectile function.

Nerve signals relax blood vessels and increase blood flow to the erectile tissue. Venous outflow is restricted so firmness is maintained. An erection can occur spontaneously or during sleep and does not automatically reflect conscious sexual interest.

An erection occurs when nerve signals cause blood vessels in erectile tissue to relax and allow increased blood flow into the penis. As the erectile spaces fill, the veins that normally drain blood are partly compressed, helping the penis remain firm until blood flow returns toward its resting pattern.

Erections can happen during sexual arousal, sleep or without an obvious sexual trigger. They are partly controlled by the autonomic nervous system, which means an erection does not necessarily prove sexual interest or consent, and the absence of an erection does not by itself mean someone lacks desire.

The testicles produce sperm and testosterone. Sperm mature in the epididymis and then travel through the vas deferens. A change in fertility cannot be diagnosed from penis or testicle size alone.

The testicles sit in the scrotum outside the main body cavity, where a somewhat cooler environment supports sperm production. Sperm develop within tiny structures in the testicles and then move into the epididymis, a coiled structure behind each testicle where they continue to mature before traveling through the vas deferens.

The testicles also produce testosterone, which contributes to sexual development, bone and muscle health, red blood cell production and sexual function. Sperm production and testosterone production are related but distinct processes, so a change in one does not always predict an identical change in the other.

Semen contains sperm plus fluids from glands including the seminal vesicles and prostate. Pre-ejaculate is a different fluid released before ejaculation and may still be relevant to pregnancy and STI risk.

Semen is not made entirely of sperm. Most of the fluid comes from reproductive glands, including the seminal vesicles and prostate, and this fluid mixes with sperm before ejaculation. Semen appearance and consistency can change temporarily with hydration, frequency of ejaculation and other ordinary factors.

Pre-ejaculate is a separate fluid released before ejaculation. Because sperm can sometimes be present in or mix with fluid in the urethra, relying on withdrawal or the appearance of pre-ejaculate cannot guarantee prevention of pregnancy. Sexually transmitted infections can also be transmitted through genital fluids or skin-to-skin contact even when ejaculation does not occur.

Penis size and erection angle vary widely. One testicle often hangs lower. A mild lifelong curve can be normal, while a new painful curve or plaque-like area may need assessment.

Penis size, shape, skin color, vein visibility, erection angle and degree of curvature vary widely. Mild differences between the left and right sides of the scrotum are also common, and one testicle often hangs lower than the other. These variations are usually anatomical differences rather than signs of disease.

Changes that are new are more important clinically than simply being different from someone else's anatomy. A newly developing hard testicular lump, rapidly increasing curvature, persistent swelling or a new change that causes pain or interferes with sexual or urinary function deserves medical assessment.

Sudden severe testicular pain can indicate testicular torsion, which requires rapid treatment. A painful erection lasting around four hours can be priapism and also requires emergency care.

Sudden severe pain in one testicle, especially with swelling, nausea or a testicle appearing higher than usual, can be a sign of testicular torsion. Torsion can interrupt blood flow and requires emergency evaluation rather than waiting to see whether the pain resolves on its own.

An erection that remains prolonged and painful for several hours without resolving also needs urgent medical care because prolonged loss of normal blood drainage can damage erectile tissue. Major penile trauma, a sudden injury with severe pain and swelling, uncontrolled bleeding or an inability to urinate are additional reasons to seek urgent evaluation.