The human penis contains no bone, no cartilage, and very little fat. It is built almost entirely from spongy erectile tissue, connective tissue, blood vessels, nerves, and smooth muscle, all wrapped in some of the thinnest skin on the body. Understanding what these tissues are and how they work together explains both how erections happen and why the organ is vulnerable to certain injuries and conditions.
Three Cylinders of Spongy Tissue
The core structure of the penis consists of three long, tube-shaped chambers that run its full length. Two of these sit side by side along the top and are called the corpora cavernosa. The third runs along the underside, surrounding the urethra, and is called the corpus spongiosum. All three are made of spongy tissue riddled with tiny blood-filled spaces called sinusoids, similar in concept to a dense, fluid-filled sponge.
The corpora cavernosa do the heavy lifting during an erection. They are made mostly of connective tissue (collagen and elastin fibers), smooth muscle cells, and a network of small blood vessels. Smooth muscle is a relatively small fraction of the overall tissue volume, but it plays an outsized role: when those muscle cells relax, tiny corkscrew-shaped arteries called helicine arteries open wide and flood the sinusoids with blood. The tissue expands, stiffens, and an erection results. When the smooth muscle contracts again, blood drains out through veins at the surface, and the penis returns to its soft state.
The corpus spongiosum stays softer during an erection than the other two chambers. This is by design. It surrounds the urethra and keeps it from being pinched shut, which would block both urination and ejaculation. Small mucus-producing glands called the glands of Littre line the inside of the urethra within this chamber, secreting a slippery fluid that lubricates and protects the urethral lining.
The Tough Outer Casing
Each of the three spongy chambers is wrapped in a dense, fibrous sheath called the tunica albuginea. This layer is what gives the erect penis its rigidity and shape. It is made primarily of collagen fibers (types I and III), which are extremely strong but not very stretchy, woven together with elastic fibers that can stretch to 150 to 200 percent of their resting length. That combination allows the tunica to expand during an erection but resist bursting under high internal pressure, then snap back to its original size afterward.
Think of it like a tough balloon: the elastic component lets it inflate, while the collagen framework prevents it from over-inflating. Damage to this layer, whether from injury or age-related loss of elasticity, is what causes conditions like Peyronie’s disease, where scar tissue creates an abnormal curve.
How Blood Creates Rigidity
An erection is a hydraulic event. When the brain sends arousal signals through the nervous system, chemical messengers cause the smooth muscle inside the corpora cavernosa to relax. The helicine arteries dilate, and blood rushes into the sinusoids. As these spaces fill, the expanding tissue compresses the small veins that normally drain blood away, trapping it inside. Internal pressure climbs, and the penis becomes rigid.
The entire process depends on a healthy balance between blood flowing in and blood being prevented from flowing out. This is why cardiovascular health is so closely linked to erectile function. Anything that damages blood vessels or reduces blood flow, such as high blood pressure, diabetes, or smoking, can interfere with this mechanism.
Nerve Endings and Sensation
The penis is one of the most nerve-rich structures in the body. The glans (head) contains scattered sensory receptors, but the most sensitive area is actually the frenular delta, the small ridge of tissue on the underside where the glans meets the shaft. A 2024 study providing the most detailed mapping of penile nerves to date found that specialized touch receptors called sensory corpuscles cluster in groups of up to 17 in the frenular delta, while they appear only in isolated, spread-out patterns across the rest of the glans.
Among these receptors are Krause corpuscles, which detect the tiny vibrations generated when skin rubs against skin. These are thought to be the primary mediators of sexual pleasure. The shaft, by contrast, has far fewer specialized receptors and relies more on general pressure and stretch-sensing nerve fibers.
Skin, Fascia, and the Dartos Layer
Penile skin is unusually thin and loose compared to skin elsewhere on the body. It contains almost no subcutaneous fat, which is why the penis doesn’t accumulate fat the way the abdomen or thighs do. Just beneath the skin sits the dartos layer, a thin sheet of smooth muscle that also extends into the scrotum. The dartos muscle can contract in response to cold, pulling the skin tighter against the shaft.
Below the dartos lies a layer of loose connective tissue called Buck’s fascia, which runs from the base of the penis to the rim of the glans. This fascia acts as a sleeve around the deeper structures, helping to hold the three erectile chambers and their blood vessels in position. In uncircumcised men, the foreskin (prepuce) is a double-layered fold of this same thin skin, with the dartos muscle continuing into it.
No Bone, Unlike Most Mammals
Many mammals, including dogs, bears, walruses, and most other primates, have a penis bone called a baculum that provides structural support for mating. Humans lost this bone at some point during evolution. The leading hypothesis, published in Mammal Review, suggests that as early human ancestors developed greater cognitive ability and began using targeted aggression against rivals, a boneless penis became an advantage. A bone inside the penis is a liability during a fight: a fracture there could permanently remove a male from the gene pool. Males born without the bone, or with a reduced one, may have had a survival edge in social groups where physical conflict was common.
Instead of a bone, the human penis relies entirely on blood pressure within the corpora cavernosa for rigidity. This hydraulic system works well but is more sensitive to disruption from vascular disease, nerve damage, or hormonal changes than a bone-supported structure would be.
How These Tissues Change Over Time
The composition of penile tissue shifts with age. Collagen gradually replaces some of the smooth muscle and elastic fibers in the corpora cavernosa, making the tissue stiffer and less able to expand. The tunica albuginea loses elastic fibers, reducing its ability to trap blood efficiently. These changes are a normal part of aging and contribute to the gradual decline in erectile firmness that many men experience in their 50s and beyond.
Conditions like diabetes accelerate these changes by damaging the small blood vessels and nerves that control the erection process. Smoking has a similar effect, stiffening arterial walls and reducing blood flow. Because the penis depends so heavily on vascular health, it often serves as an early warning system: erectile difficulties can appear years before more serious cardiovascular problems like heart attack or stroke.