The penis is made of three cylinders of spongy erectile tissue, wrapped in layers of connective tissue, and covered with skin. It contains no bone, no cartilage, and very little fat. Instead, it relies on a hydraulic system of blood vessels and smooth muscle to shift between its soft and erect states. Understanding what’s inside helps explain how erections work, why they sometimes don’t, and what makes the penis structurally unique among organs.
The Three Cylinders of Erectile Tissue
If you were to slice a cross-section through the shaft, you’d see three distinct columns running its length. Two of them sit side by side on top: the corpora cavernosa. These are the main structural workhorses responsible for rigidity during an erection. Below them, a single column called the corpus spongiosum surrounds the urethra (the tube that carries urine and semen) and extends to form the glans, or head, at the tip.
All three cylinders are made of spongy tissue filled with tiny blood-filled spaces called sinusoids, similar in concept to a dense sponge. The walls of these spaces are lined with a thin layer of cells that control blood flow, and they’re surrounded by smooth muscle cells arranged in circular and longitudinal layers, typically two to three cells thick along the sinusoid walls. In a healthy penis, smooth muscle makes up roughly 37% or more of the tissue inside the corpora cavernosa. That percentage matters: when smooth muscle content drops, erectile function declines.
The Tunica Albuginea: A Built-In Pressure Jacket
Each of the three cylinders is encased in a tough, fibrous sheath called the tunica albuginea. This is what gives the penis its ability to become rigid without having a bone. The tunica is built from two layers of collagen fibers stacked at right angles to each other, with elastic fibers woven between them. Think of it like a high-pressure hose: flexible when relaxed, but capable of containing significant internal pressure without bursting or ballooning out of shape.
The tunica around the corpora cavernosa is substantially thicker than the one around the corpus spongiosum. This difference is intentional. The corpus spongiosum needs to stay softer during an erection so it doesn’t compress the urethra shut. The thicker walls of the corpora cavernosa, meanwhile, trap blood under high pressure to create rigidity.
How Blood Flow Creates an Erection
Erections are fundamentally a blood pressure event. Each corpus cavernosum has an artery running through its center called the cavernosal artery. These arteries branch into smaller helicine arteries that open directly into the spongy sinusoids. During arousal, nerve signals trigger the release of nitric oxide, a signaling molecule produced by both nerve endings and the cells lining blood vessels. Nitric oxide sets off a chemical chain reaction that relaxes the smooth muscle cells in the artery walls and sinusoid linings, allowing blood to rush in.
As the sinusoids fill and expand, they press against the tunica albuginea from the inside. This compression squeezes the small veins that normally drain blood out of the penis, trapping it inside. The result is a self-reinforcing loop: blood flows in, pressure builds, drainage gets cut off, and the penis becomes rigid. The entire process depends on healthy smooth muscle cells being able to relax on command. This is also why medications for erectile dysfunction work the way they do: they amplify the nitric oxide signaling pathway to help smooth muscle relax more effectively.
The penis has three separate venous drainage systems (superficial, intermediate, and deep) that handle blood flow when the organ is in its soft state. About 28 to 35% of people also have an accessory artery contributing extra blood supply to the penis, originating from nearby pelvic vessels.
Nerves and Sensation
The penis has a dense and complex nerve supply that serves two purposes: triggering erections and providing sensation. Sensory fibers run primarily through the dorsal nerves (on top) and ventral nerves (on the underside), each carrying a mix of nerve types that both promote and regulate erections.
Two types of sensory receptors handle touch and pressure. Free nerve endings, which detect pain, temperature, and light touch, are densely concentrated around the urethral opening and the frenulum (the sensitive strip of tissue on the underside of the glans). Encapsulated receptors handle different jobs: Pacinian corpuscles, which sense deep pressure and vibration, cluster around the erectile bodies and in the foreskin. Ruffini corpuscles, which respond to sustained pressure and stretching, are rarer and sit just beneath the surface of the glans and foreskin.
This distribution explains why the frenulum and glans are far more sensitive than the shaft, and why sensation varies from one area to another.
Skin and Surface Tissue
The skin covering the shaft is unusually thin, loose, and elastic compared to skin elsewhere on the body. It needs to accommodate significant changes in size. It lacks the subcutaneous fat layer found in most other skin, which is why veins on the shaft are often visible.
The glans is covered by a different type of tissue: a dense layer of stratified squamous epithelium over a thick bed of connective tissue that functions like a dermis. This gives the glans a smoother, slightly mucosal quality compared to the shaft skin. The structural differences between glans and shaft tissue contribute to their distinct sensitivity profiles and appearance.
Ligaments That Hold It in Place
The penis doesn’t just hang freely. It’s anchored to the pubic bone by a system of ligaments at its base. The suspensory ligament proper is a triangular band that connects the shaft to the point where the left and right pelvic bones meet. It splits around the large dorsal vein on top of the penis and rejoins deeper in the erectile tissue. Overlaying it is the fundiform ligament, a stretchier band made of fat and connective tissue that wraps around the base. Behind both sits the arcuate sub-pubic ligament, adding a third layer of support.
Together, these ligaments determine the angle of the erect penis and keep the organ stable during use. They’re the reason the penis points the direction it does rather than flopping freely in all directions.
No Bone, Unlike Most Mammals
One of the more unusual things about the human penis is what it doesn’t contain: a bone. Most primates and many other mammals have a baculum, a stiffening bone inside the penis that assists with penetration independent of blood flow. Humans lost the baculum over the course of evolution, making erections entirely dependent on the hydraulic blood-trapping mechanism described above. In very rare cases, bone tissue has formed in the soft tissue of a human penis as an abnormality, but this is not a true baculum.
This means the human erection is, structurally, a pressurized fluid event held in place by collagen and smooth muscle. It’s an elegant system, but one with more potential failure points than a bone-assisted design, which partly explains why erectile difficulties are so common with aging, vascular disease, or nerve damage.