In males, the majority of testosterone is secreted from the testes, hence the term “testosterone”. The hormone is also produced in small amounts by the adrenal gland. The production of this hormone is controlled by the hypothalamus and pituitary gland in the brain. The pituitary gland receives instructions from the hypothalamus on how much testosterone needs producing and passes this information onto the testicles via chemicals and hormones circulating in the bloodstream.
A number of research groups have tried to further define the relationship of testosterone and body composition by artificial alteration of testosterone levels in eugonadal populations. Induction of a hypogonadal state in healthy men (Mauras et al 1998) or men with prostate cancer (Smith et al 2001) using a gonadotrophin-releasing-hormone (GnRH) analogue was shown to produce increases in fat mass and decreased fat free mass. Another experimental approach in healthy men featured suppression of endogenous testosterone production with a GnRH analogue, followed by treatment with different doses of weekly intramuscular testosterone esters for 20 weeks. Initially the experiments involved men aged 18–35 years (Bhasin et al 2001) but subsequently the study was repeated with a similar protocol in men aged 60–75 years (Bhasin et al 2005). The different doses given were shown to produce a range of serum concentrations from subphysiological to supraphysiological (Bhasin et al 2001). A given testosterone dose produced higher serum concentrations of testosterone in the older age group (Bhasin et al 2005). Subphysiological dosing of testosterone produced a gain in fat mass and loss of fat free mass during the study. There were sequential decreases in fat mass and increases in fat free mass with each increase of testosterone dose. These changes in body composition were seen in physiological and supraphysiological treatment doses. The trend was similar in younger versus older men but the gain of fat mass at the lowest testosterone dose was less prominent in older patients (Bhasin et al 2001; Bhasin et al 2005). With regard to muscle function, the investigators showed dose dependent increases in leg strength and power with testosterone treatment in young and older men but there was no improvement in fatigability (Storer et al 2003; Bhasin et al 2005).
It appears that testosterone has NOS-independent pathways as well. In one study, castrated rats were implanted with testosterone pellets and then divided into a group that received an NOS inhibitor (L-nitro-L-arginine methyl ester [L-NAME]) and a control group that received no enzyme.  The castrated rats that were given testosterone pellets and L-NAME still had partial erections, a result suggesting the presence of a pathway independent of NOS activity.
For obvious reasons, ED can be a sensitive subject, one that until relatively recently men were more likely to try to hide than to deal with. Fortunately, a deeper understanding of the variety of causes of erectile dysfunction has led to medications, therapies, and other treatments that can be more individualized and more likely to be effective—and more open discussion about addressing the concern.
Overall, few patients have a compelling contraindication to testosterone treatment. The majority of men with late onset hypogonadism can be safely treated with testosterone but all will require monitoring of prostate parameters HDL cholesterol, hematocrit and psychological state. It is also wise to monitor symptoms of sleep apnea. Other specific concerns may be raised by the mode of delivery such as local side effects from transdermal testosterone.
Alprostadil (also known as prostaglandin E1 [PGE1]) is the prominent known smooth-muscle dilator of the corpus cavernosum. Its mechanism of action is believed to be the promotion of intracellular accumulation of cyclic adenosine monophosphate, thereby causing decreased intracellular accumulation of calcium and resulting smooth muscle relaxation. Alprostadil can be delivered to the erectile tissue either via an intraurethral suppository that is massaged and then absorbed across the corpus spongiosum of the urethra to the corpora cavernosa, or directly injected into the corpora cavernosa. When administered urethrally, doses are substantially higher than when directly injected (typical dosing is 500 mcg to 1 mg intraurethral compared with 2.5 mcg to 20 mcg intracavernosal).
Testosterone is used as a medication for the treatment of males with too little or no natural testosterone production, certain forms of breast cancer, and gender dysphoria in transgender men. This is known as hormone replacement therapy (HRT) or testosterone replacement therapy (TRT), which maintains serum testosterone levels in the normal range. Decline of testosterone production with age has led to interest in androgen replacement therapy. It is unclear if the use of testosterone for low levels due to aging is beneficial or harmful.
When stimulated by the nerves, the spongy tissue arranges itself in such a way that more blood can be stored in the penis. The veins running through the outer sheath of the penis then compress which stops the blood from leaving the penis. As the blood is stopped from flowing out, the penis fills with blood and stretches within the outer casing, giving an erection.
Erectile dysfunction is known to be associated with general health status, thus, lifestyle modification improves erectile function and decreases the rate of decline of function with aging. One year after discontinuation of smoking, patients were found to have a 25% improvement in erectile quality.16 In addition, multivariate analysis found obesity is associated with erectile dysfunction with an approximately 50% increase in ED in obese men as compared with normal weight men.17
Her remark was entirely destructive of poetry, since it was to the effect that poetry had nothing whatever to do with her; all her friends spent their lives in making up phrases, she said; all his feeling was an illusion, and next moment, as if to taunt him with his impotence, she had sunk into one of those dreamy states which took no account whatever of his existence.
So what is this Big T, anyway? Derived from cholesterol, testosterone is a steroid hormone—called an androgen—that causes the development and maintenance of masculine characteristics. It's mainly secreted by the testicles in males, although the adrenal cortex and ovaries in females also secrete testosterone—though only about one-tenth the amount as in healthy males.
An analysis of 14 studies involving more than 90,000 patients with ED confirmed the relation between ED and an increased risk of cardiovascular events and mortality.  Compared with patients without ED, those with ED had a 44% increased risk of cardiovascular events, a 25% increased risk of all-cause mortality, a 62% increased risk of MI, and a 39% increased risk of cerebrovascular events. Treatment of ED, either through lifestyle interventions or by pharmacologic means, may improve prognosis and reduce risk.
A common and important cause of ED is vasculogenic. Many men with ED have comorbid conditions such as hyperlipidemia, hypercholesterolemia, tobacco abuse, diabetes mellitus, or coronary artery disease (CAD).  The Princeton III Consensus recommends screening men who present with ED for cardiovascular risk factors; ED may be the earliest presentation of atherosclerosis and vascular disease. 
Performance anxiety can be another cause of impotence. If a person wasn’t able to achieve an erection in the past, he may fear he won’t be able to achieve an erection in the future. A person may also find he can’t achieve an erection with a certain partner. Someone with ED related to performance anxiety may be able to have full erections when masturbating or when sleeping, yet he isn’t able to maintain an erection during intercourse.
Conditions that may be associated with ED include diabetes, [25, 26, 27] hypertension,  , and CAD, as well as neurologic disorders, endocrinopathies, benign prostatic hyperplasia,  , sleep apnea  , COPD,  and depression (see Table 1 below). [32, 33, 34, 35] In fact, almost any disease may affect erectile function by altering the nervous, vascular, or hormonal systems. Various diseases may produce changes in the smooth muscle tissue of the corpora cavernosa or influence the patient’s psychological mood and behavior.
Exercise is the original testosterone booster, and it’s one of the most powerful full-body hacks around. Men see a sharp increase in both testosterone and human growth hormone (HGH) after lifting weights, and the boost is greater with shorter rest time between sets (1 minute rest outperforms 3 minutes rest) . With the shorter rest time, women also get a large boost in HGH.
Although not proven, it is likely that erectile dysfunction can be prevented by good general health, paying particular attention to body weight, exercise, and cigarette smoking. For example, heart disease and diabetes are problems that can cause erectile dysfunction, and both are preventable through lifestyle changes such as sensible eating and regular exercise. Furthermore, early diagnosis and treatment of associated conditions like diabetes, hypertension and high cholesterol may prevent or delay erectile dysfunction, or stop the erectile dysfunction from getting more serious.
Conflicting results have been obtained concerning the importance of testosterone in maintaining cardiovascular health. Nevertheless, maintaining normal testosterone levels in elderly men has been shown to improve many parameters that are thought to reduce cardiovascular disease risk, such as increased lean body mass, decreased visceral fat mass, decreased total cholesterol, and glycemic control.
Testosterone is the principle sex hormone responsible for the development of reproductive function in male vertebrates. Testosterone is one of the hormones referred to as androgens, which are also known of as anabolic steroids. As a steroid hormone, testosterone is derived from cholesterol and the structure of this hormone is similar across all mammals, reptiles, birds and fish.
Capogrosso, P., Colicchia, M., Ventimiglia, E., Castagna, G., Clementi, M. C., Suardi, N., ... Salonia, A. (2013, July). One patient out of four with newly diagnosed erectile dysfunction is a young man — worrisome picture from the everyday clinical practice. The journal of sexual medicine. 10(7), 1833–1841. Retrieved from https://onlinelibrary.wiley.com/doi/full/10.1111/jsm.12179