The penis is the external reproductive and urinary organ of the male body. It serves two primary functions: carrying urine out of the body and delivering semen during sexual intercourse. Its structure is built around spongy tissues that fill with blood to produce an erection, a dual-channel system that handles both waste removal and reproduction through the same opening.
External Anatomy
The visible parts of the penis include the shaft, the glans (or head), and, in uncircumcised individuals, the foreskin. The shaft makes up the main body of the organ. At its tip sits the glans, which contains a high concentration of nerve endings and is the most sensitive part. The urethral opening at the tip of the glans is where both urine and semen exit the body.
The foreskin is a sleeve of highly vascular skin that covers most of the glans. Its outer surface is continuous with the skin of the shaft, while its inner surface is lined with a softer, mucosal tissue. A small band of tissue called the frenulum connects the underside of the foreskin to the glans. In circumcised individuals, most or all of the foreskin has been surgically removed, leaving the glans permanently exposed.
Internal Structure and Erections
Inside the penis are three columns of spongy tissue. Two of these, called the corpora cavernosa, run side by side along the top of the shaft. A third column, the corpus spongiosum, runs along the underside and surrounds the urethra (the tube that carries urine and semen). These tissues are made of connective fibers, smooth muscle, and hollow spaces that can expand with blood.
An erection begins in the brain. Visual, physical, or mental stimulation sends nerve signals that trigger the release of a chemical messenger called nitric oxide in the penile blood vessels. This causes the arteries feeding the penis to relax and widen, allowing a rush of blood into the hollow spaces within the corpora cavernosa. As those chambers fill and expand, they compress the surrounding veins, trapping the blood inside and making the penis rigid. When stimulation stops, the arteries constrict, blood drains out, and the penis returns to its soft, flaccid state.
One notable feature of the human penis compared to most other mammals: it contains no bone. Many primates and other mammals have a structure called a baculum, a penile bone that aids in mating. Humans lost this bone over the course of evolution and rely entirely on blood pressure to achieve rigidity.
How It Functions in Reproduction
Sperm are produced in the testes and stored in a coiled tube called the epididymis. During ejaculation, smooth muscle contractions push sperm through a long duct called the vas deferens, up and over the pelvic bone, and into the area near the prostate gland. Along the way, fluid from the seminal vesicles and a milky secretion from the prostate are added. The resulting mixture is semen.
Semen is propelled through the urethra and out of the penis. During an erection, the engorged erectile tissues press against the part of the urethra that normally carries urine, effectively blocking urine flow. This is why urine and semen don’t mix during ejaculation.
Development During Puberty
The penis is present from birth in a pre-pubertal form but doesn’t reach adult size until puberty is complete. Male puberty typically begins between ages 9 and 14, with the first visible sign being growth of the testicles rather than the penis itself. Penile growth accelerates in the middle stages of puberty, around the same time as the peak growth spurt in height. Sperm production usually begins in the later stages, and full adult development is typically complete by the late teens. If no signs of puberty have appeared by age 14, it may be worth discussing with a healthcare provider.
Average Size
A large meta-analysis pooling data from tens of thousands of men found the following averages. In a flaccid (soft) state, the average length is about 9.2 cm (3.6 inches) with a circumference of roughly 9.1 cm (3.6 inches). When erect, the average length is approximately 13.8 cm (5.4 inches) with a circumference of about 11.9 cm (4.7 inches). There is significant natural variation, and these are midpoint figures across global populations.
Hygiene Basics
For uncircumcised individuals, routine care involves gently retracting the foreskin and rinsing the area underneath with water. Soap and other hygiene products can irritate the sensitive mucosal skin of the inner foreskin and the glans, so plain water is sufficient. After cleaning, always slide the foreskin back over the glans to its natural position. Leaving it retracted can cause a painful condition called paraphimosis, where the tight foreskin band traps behind the glans and restricts blood flow.
Cleaning the area at least two to three times per week with gentle retraction has been shown to reduce the buildup of smegma, a whitish substance made of dead skin cells and natural oils that can accumulate under the foreskin. Regular retraction and rinsing also lowers the risk of inflammation, adhesions, and tightening of the foreskin over time. For circumcised individuals, hygiene is simpler since the glans is already exposed, but regular washing with water remains important.
Common Conditions
Several conditions can affect the penis at various stages of life:
- Phimosis occurs when the foreskin is too tight to retract over the glans. It is normal in young children, as the foreskin is naturally fused to the glans in infancy and gradually becomes retractable. In adults, persistent phimosis can cause discomfort and hygiene difficulties.
- Balanitis is inflammation of the glans, often caused by poor hygiene, irritation from soaps, or infection. It typically presents as redness, swelling, and soreness at the tip of the penis.
- Peyronie’s disease involves the formation of hard, fibrous plaques beneath the skin of the penis, causing it to curve noticeably during erection. Symptoms can include pain during erections or intercourse, changes in penile shape such as narrowing or shortening, and difficulty maintaining an erection. The pain often improves over time, but the curvature may persist. A urologist can usually feel the plaques during a physical exam without the need for imaging.
- Erectile dysfunction is the inability to achieve or maintain an erection firm enough for intercourse. Because erections depend on healthy blood flow and nerve signaling, conditions like diabetes, cardiovascular disease, and nerve damage are common contributing factors.