Penile erection or penile tumescence is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. 



It  is the result of a interaction of psychological, neural, vascular, and endocrine factors.



It is often associated with sexual arousal or sexual attraction.



Around one in ten men experience recurring impotence problems at some point in their lives.



Erections can also be spontaneous. 



Erection shape, angle, and direction vary considerably in humans.



Physiologically, it is triggered by the parasympathetic division of the autonomic nervous system.



The parasympathetic nervous system causes the levels of vasodilator nitric oxide to rise in the trabecular arteries and smooth muscle of the penis. 



Parasympathetic nervous system branches extend from the sacral plexus into the arteries supplying the erectile tissue.



When stimulated , these nerve branches release acetylcholine, which in turn causes release of nitric oxide from endothelial cells in the trabecular arteries.



Nitric oxide dilates the arteries causing the corpora cavernosa of the penis, and to a lesser extent the corpus spongiosum to fill with blood.



In addition, and simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the exiting and circulation of this blood.



When parasympathetic activity reduces to baseline.the erection subsides.



It is an autonomic nervous system response, and  may result from sexual stimulation and sexual arousal, and is therefore not entirely under conscious control. 



Erections during sleep or upon waking up are known as 



Nocturnal penile tumescence  relates to erections during sleep or upon waking 



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



When the corpora cavernosa, that run the length of the penis, become engorged with venous blood an erection occurs.



Such engorgement occurs from  physiological stimuli, also known as sexual stimulation and sexual arousal. 



During erection the corpus spongiosum, a single tubular structure located just below the corpora cavernosa, which contains the urethra, through which urine and semen pass during urination and ejaculation respectively, may also become slightly engorged with blood, but less so than the corpora cavernosa.



The scrotum may usually tightens during erection. 



During erection the foreskin automatically and gradually retracts, exposing the glans.



With mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system with minimal input from the central nervous system. 



When parasympathetic stimulation is  eases  the erection subsides.



Stimulation from the sympathetic division of the autonomic nervous system causes constriction of the penile arteries and cavernosal sinosoids, forcing blood out of the erectile tissue via erection-related veins.



The cerebral cortex can initiate erection in response to visual, auditory, olfactory, imagined, or tactile stimuli in the absence of direct mechanical stimulation.



The cerebral cortex acts through erectile centers in the lumbar and sacral regions of the spinal cord.



The cortex may suppress erection, despite mechanical stimulation, as may other psychological, emotional, and environmental factors.



Nocturnal penile tumescence may occur during sleep or be erect on waking up. 



Erection is an indicator of sexual arousal and is required for vaginal penetration and sexual intercourse.



Erections occur commonly in children and infants, and even occur before birth.



Following puberty, erections occur much more frequently.



Spontaneous erection, is commonplace and a normal part of male physiology. 



The length of a flaccid penis does not necessarily correspond to the length of the penis when it becomes erect.



Smaller flaccid penises may grow much longer, while some larger flaccid penises grow comparatively less.



The size of an erect penis is fixed throughout post-pubescent life. 



Penis size may be increased by surgery, although penile enlargement is controversial



The average length of an erect human penis is 13.12 cm (5.17 inches) long.



The average circumference of an erect human penis is 11.66 cm (4.59 inches).



Depending on the tension of the penile suspensory ligament the erect penis may point upwards, nearly vertically upwards or nearly vertical downwards, or even horizontally straightforward.



An erect penis can also take on a number of  shapes, ranging from a straight tube to a tube with a curvature up or down or to the left or right. 



An increase in penile curvature can be caused by is associated with Peyronie’s disease is associated 



Increased penile curvature could be accompanied by physical and psychological effects, erectile dysfunction or pain during an erection. 



Erectile dysfunction is a sexual dysfunction characterized by the inability to develop and/or maintain an erection.



The  cause for erectile dysfunction is that not enough nitric oxide (NO) is released by the vascular endothelium of the branches of the perineal artery.



Erectile dysfunction may be due to physiological or psychological reasons: 


diabetes, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease -including arterial insufficiency and venogenic erectile dysfunction, neurologic disease which collectively account for about 70% of ED cases.



Some drugs used to treat other conditions, may cause erectile dysfunction




Erectile dysfunction, can have psychological consequences including feelings of shame, loss or inadequacy.



Priapism is a painful condition in which the penis does not return to its flaccid state.



Priapism lasting over four hours is a medical emergency.




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