Physiologically, an erection is required for a male to effect penetration or sexual intercourse and is triggered by the parasympathetic division of the autonomic nervous system, causing the levels of nitric oxide (a vasodilator) to rise in the trabecular arteries and smooth muscle of the penis.

The arteries dilate causing the corpora cavernosa of the penis (and to a lesser extent the corpus spongiosum) to fill with blood; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood.

Absence of nocturnal erection is commonly used to distinguish between physical and psychological causes of erectile dysfunction and impotence.

[1][2] An erection occurs when two tubular structures, called the corpora cavernosa, that run the length of the penis, become engorged with venous blood.

Parasympathetic branches extend from the sacral plexus into the arteries supplying the erectile tissue; upon stimulation, these nerve branches release acetylcholine, which in turn causes the release of nitric oxide from endothelial cells in the trabecular arteries.

The ischiocavernosus and bulbospongiosus muscles also compress the veins of the corpora cavernosa, limiting the venous drainage of blood.

[9] After ejaculation or cessation of stimulation, erection usually subsides, but the time taken may vary depending on the length and thickness of the penis.

[10] The cerebral cortex can initiate erection in the absence of direct mechanical stimulation (in response to visual, auditory, olfactory, imagined, or tactile stimuli) acting through erectile centers in the lumbar and sacral regions of the spinal cord.

[11] The cortex may suppress erection, even in the presence of mechanical stimulation, as may other psychological, emotional, and environmental factors.

[13][14][15][16] Though an erection can have many causes, it is most commonly seen as an indicator of sexual arousal and is therefore considered taboo or inappropriate for a public setting in many societies.

This may cause physical and psychological effects for the affected individual, which could include erectile dysfunction or pain during an erection.

[23][26] Erectile dysfunction, tied closely as it is to cultural notions of potency, success and masculinity, can have devastating psychological consequences including feelings of shame, loss or inadequacy.

[28] Priapism is a painful condition in which the penis does not return to its flaccid state, despite the absence of both physical and psychological stimulation.

Protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure.

[34] Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis.

[38] A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall and being erected by lymph, not blood.