An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. The primary physiological mechanism that brings about erection is the autonomic dilation of arteries supplying blood to the penis, which allows more blood to fill the three spongy erectile tissue chambers in the penis, causing it to lengthen and stiffen. The now-engorged erectile tissue presses against and constricts the veins that carry blood away from the penis. More blood enters than leaves the penis until an equilibrium is reached where an equal volume of blood flows into the dilated arteries and out of the constricted veins; a constant erectile size is achieved at this equilibrium. When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract. Although many erect penises point upwards, it is common and normal for the erect penis to point nearly vertically upwards or nearly vertically downwards or even horizontally straight forward, all depending on the tension of the suspensory ligament that holds it in position (Pic. 1). Depending on the flaccid dimensions of a penis, it can increase in size slightly or greatly during erection, with the average length of an erect penis measuring approximately 15 cm.

Mechanism of erection

Penile erection is managed by two different mechanisms: 

  • The reflex erection, which is achieved by directly touching the penile shaft. 
  • The psychogenic erection, which is achieved by erotic or emotional stimuli.

The former uses the peripheral nerves and the lower parts of the spinal cord, whereas the latter uses the limbic system of the brain. In both circumstances, an intact neural system is required for a successful and complete erection. Stimulation of penile shaft by the nervous system leads to the secretion of nitric oxide (NO). Nitric oxide is released from nerve endings near blood vessels within the corpora cavernosa and spongiosum (Pic. 2). Release of NO activates a signaling pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the penis and induces the endothelial cells in the penile arterial walls to also secrete NO and perpetuate the vasodilation. Additionally, adequate levels of testosterone (produced by the testes) and an intact pituitary gland are required for the development of a healthy erectile system.

Voluntary and involuntary control

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

Spontaneous or random erections

Spontaneous erection, also known as involuntary, random or unwanted erection, is commonplace and a normal part of male physiology. Socially, such erections can be embarrassing if they happen in public or when undesired. Erections can occur spontaneously at any time of day, and if clothed may cause a bulge which (if required) can be disguised or hidden by wearing close-fitting underwear, a long shirt, or baggier clothes.

Priapism 

It is a potentially painful medical condition in which the erect penis does not return to its flaccid state, despite the absence of both physical and psychological stimulation, within four hours. There are two types of priapism: low-flow (ischemic) and high-flow (nonischemic); 80% to 90% of clinically presented priapisms are low-flow disorders. Low-flow involves the blood not adequately returning to the body from the penis. High-flow involves a short-circuit of the vascular system partway along the penis. Treatment is different for each type. Early treatment can be beneficial for a functional recovery.

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Sources

Erection ―sourced from Wikipedia licensed under CC BY-SA 3.0
Erectile dysfunction ―sourced from Wikipedia licensed under CC BY-SA 3.0
Anatomy of the Female Reproductive System ―sourced from Lumen licensed under CC BY 4.0
Penis ―sourced from Lumen licensed under CC BY 4.0
Erectile Dysfunction and the Blue Pill ―sourced from Boundless licensed under CC BY-SA 4.0
Male sexual response ―sourced from Lumen licensed under CC BY 4.0
Priapism ―sourced from Wikipedia licensed under CC BY-SA 3.0
Erection development ―by OrlandoDL licensed under CC BY-SA 3.0
The corpus spongiosum ―by Carter licensed under CC0 1.0
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