The penis is the male external organ that serves functions in both the urinary and reproductive systems. It is a complex structure composed of several distinct tissues that work in concert to facilitate urination and sexual intercourse. The organ’s form and capabilities are the result of its specific anatomical components, both visible externally and situated internally. Its dual role in waste elimination and reproduction underscores its integration with multiple physiological systems within the body.
External Penile Structures
The main length of the organ is the shaft, or body, which connects the pubic region to the tip. The shaft’s skin is loose and elastic, allowing for changes in size during an erection. At the end of the shaft is the glans, a cone-shaped structure often referred to as the head of the penis. The glans is highly concentrated with nerve endings, making it particularly sensitive.
Separating the glans from the shaft is a rounded, projecting border known as the corona. At the very tip of the glans is the urethral meatus, which is the opening through which both urine and semen exit the body. In uncircumcised individuals, a retractable layer of skin called the foreskin, or prepuce, covers the glans. This double layer of skin attaches to the ventral surface of the glans and can be surgically removed in a procedure known as circumcision.
Internal Penile Structures
Internally, the penis is composed of three cylindrical columns of erectile tissue. The two larger columns, positioned side-by-side along the dorsal, or upper, side of the shaft, are the corpora cavernosa. These structures are made of a sponge-like tissue containing countless blood-filled spaces. The corpora cavernosa are the primary erectile bodies; during arousal, they fill with blood to produce the rigidity associated with an erection, holding approximately 90% of the blood involved.
Running along the bottom of the shaft is the third column, the corpus spongiosum. This single cylinder of spongy tissue surrounds the urethra, the tube that carries urine and semen. The main purpose of the corpus spongiosum during an erection is to prevent the urethra from being compressed, ensuring it remains an open channel for ejaculation. It terminates at the tip of the penis, where it expands to form the glans.
The Erection Process
An erection is a neurovascular event that begins with sensory or mental stimulation, such as touch or fantasy. These triggers prompt the brain to send nerve signals through the autonomic nervous system to the penis. The parasympathetic nerves, specifically, become active, leading to the release of neurotransmitters like nitric oxide within the erectile tissues.
The release of these neurotransmitters causes the smooth muscles in the walls of the penile arteries to relax. This relaxation allows the arteries to widen, dramatically increasing blood flow into the corpora cavernosa. As these spongy columns engorge with blood, they expand and press against the tunica albuginea, a dense fibrous sheath surrounding them. This pressure compresses the veins that normally drain blood from the penis, effectively trapping the blood inside and causing the penis to become firm and elongated.
The process of losing an erection, known as detumescence, occurs when the initial stimulation ceases or following ejaculation. The sympathetic nervous system takes over, releasing neurotransmitters like norepinephrine that cause the arteries to constrict again. This reduces blood flow into the penis, while the trapped blood is released as the veins decompress. As the blood flows out, the penis returns to its flaccid, or non-erect, state.
Anatomical Variations
Size is one of the most discussed variations, with notable diversity in both length and girth in flaccid and erect states. Studies based on measurements by health professionals show the average erect length is approximately 13.12 cm (5.17 inches). The size of a flaccid penis is not a reliable predictor of its erect size, as some smaller flaccid penises elongate more substantially than larger ones.
Beyond size, the shape of the penis can also vary. It is common for an erect penis to have a slight curvature, which can be directed up, down, or to either side. A curve of up to 30 degrees is generally considered within the normal range and typically does not interfere with function. More pronounced curvature may be associated with conditions like Peyronie’s disease, where scar tissue develops internally.
The general appearance of the penis, including the glans and scrotum, also differs from person to person. Skin color can vary, and the prominence of blood vessels on the shaft can change, often becoming more noticeable during an erection due to increased blood volume. Some individuals may also have small, harmless bumps on the corona of the glans called pearly penile papules.