15.5 — The Physiology of Sex

Sexual function is one of the areas of medicine where people are least likely to ask questions and most likely to get their information from unreliable sources. It is also an area where accurate physiology dissolves a large amount of anxiety, because most of what people worry about is either normal variation or a recognised and treatable condition.

This chapter covers the mechanisms, and it covers what is established and what is asserted.

The response cycle

William Masters and Virginia Johnson described a four-stage model in 1966 — excitement, plateau, orgasm, resolution — based on direct laboratory observation, which had essentially never been done before.

Helen Singer Kaplan added desire as a preceding stage in the 1970s.

And the linear model has since been substantially revised, particularly for women.

Rosemary Basson proposed a circular model in 2000 in which desire frequently follows arousal rather than preceding it. A person may begin from a position of emotional closeness or willingness rather than spontaneous desire, become aroused, and experience desire as a result.

This matters clinically. The linear model implies that absent spontaneous desire is a disorder. The circular model recognises responsive desire as normal, and it fits the reported experience of a large proportion of women — and of many men in long-term relationships.

Which reframes a common complaint. "I never feel like it until we start" is not a dysfunction.

What physically happens

The changes are broadly parallel in both sexes, which is unsurprising given that the structures develop from the same embryonic tissue (Chapter 4.5).

Vasocongestion — blood flowing into erectile tissue faster than it drains.

In men: erection (Chapter 15.1).

In women: engorgement of the clitoris — including the internal crura and bulbs (Chapter 15.3) — and of the labia and vaginal walls. Vaginal lubrication is transudate: plasma forced through the vaginal wall by the engorgement, since the vagina has no glands.

Which is why lubrication is a vascular event rather than a glandular one, and why anything affecting blood flow affects it.

Myotonia — increasing muscle tension throughout the body, not only in the pelvis.

Systemic changes — heart rate rising to 100 to 180, blood pressure rising, breathing rate increasing, and the sex flush, a measles-like reddening seen in around 50 to 75 percent of women and 25 percent of men.

Orgasm — rhythmic contractions of the pelvic floor muscles at roughly 0.8-second intervals, with contraction of the uterus or of the vas, prostate and seminal vesicles.

And the same muscles are involved in both sexes, with the same rhythm — a good indication of the shared embryological origin.

Ejaculation and orgasm are separable events. They usually coincide and do not have to: men can experience orgasm without ejaculation (before puberty, after some prostate surgery, and in retrograde ejaculation), and ejaculation without the subjective experience of orgasm occurs in some neurological conditions.

Resolution — return to baseline.

And here there is a genuine sex difference. Men have a refractory period during which further orgasm is not possible, ranging from minutes in young men to hours or longer with age. Women generally do not, which is why multiple orgasm is physiologically straightforward for some women and not for men.

The mechanism of the male refractory period is not fully established. Prolactin release after orgasm is the leading candidate, and the evidence is suggestive rather than conclusive.

The nervous control

Chapter 11.9 gave the crude summary: parasympathetic for arousal, sympathetic for orgasm.

Which has a direct consequence: anxiety impairs arousal, because sympathetic activation opposes the parasympathetic vasodilation.

And it produces the vicious circle that underlies a large proportion of sexual difficulty. A single episode of difficulty causes anxiety about the next; the anxiety produces sympathetic activation; the activation causes the difficulty; and the pattern establishes itself — performance anxiety, and it is a genuine physiological loop rather than merely a psychological one.

Which is exactly why treatment frequently involves removing the demand for performance rather than adding a drug.

Spinal reflexes exist independently of the brain. People with complete spinal cord injuries above the relevant level can have reflex erections and lubrication from direct stimulation, without sensation, because the sacral reflex arc is intact and disconnected. And psychogenic arousal, from thought alone, uses a different pathway from the thoracolumbar cord, so which is preserved depends on the level of injury.

Sexual function after spinal injury is a major concern for patients and is frequently not discussed by clinicians, which is a documented gap in care.

The G-spot and female ejaculation

Two topics where confident assertions considerably exceed the evidence, and where the honest position is worth stating.

The "G-spot" was described in 1950 and popularised in 1982. Anatomical studies have not identified a discrete structure, and multiple imaging and dissection studies have failed to find one.

What does exist is the internal clitoral structure — the crura and bulbs — lying against the anterior vaginal wall, together with the urethra and the periurethral glands. Stimulation through the anterior vaginal wall reaches those structures.

So the sensitivity is real and the "spot" as a distinct anatomical entity is not established. The current framing is of a clitoro-urethro-vaginal complex rather than a discrete organ.

Female ejaculation — the expulsion of fluid at orgasm — is reported by a proportion of women, and two distinct phenomena appear to be conflated.

A small volume of thick fluid from the periurethral (Skene's) glands, which are homologous to the prostate and which contain PSA.

And a larger volume of dilute fluid which analysis indicates is substantially urine, expelled from the bladder.

Both are normal and neither indicates a problem. The main practical value of stating this is that women who experience it are frequently distressed and believe they have a continence problem.

Sexual dysfunction

Common, treatable, and under-reported. Around 40 percent of women and 30 percent of men report a sexual problem at some point, and a minority raise it with a clinician.

And it is worth noting that a problem is only a disorder if it causes distress. Low desire that does not trouble the person or their relationship is not a condition requiring treatment, and pathologising it has been a genuine problem in this field.

In men

Erectile dysfunction — Chapter 15.1, and the cardiovascular warning bears repeating: it precedes cardiac symptoms by three to five years on average, and a man presenting with it should have his cardiovascular risk assessed.

Premature ejaculation — the commonest male sexual problem, affecting around 20 to 30 percent.

Defined by three elements together: ejaculation occurring sooner than desired, an inability to delay it, and resulting distress. A time threshold of about one minute is used for the lifelong form, and it is the distress rather than the stopwatch that defines the condition.

Treatments that work: behavioural techniques — the stop-start and squeeze methods — topical anaesthetics, applied and then removed to avoid transferring numbness; SSRIs, taken daily or on demand, which delay ejaculation as a side effect that is here the intended effect; and treating any coexisting erectile dysfunction, because men who are anxious about losing an erection often rush.

Delayed or absent ejaculation — commonly caused by SSRIs, and by alcohol, opioids, diabetes and prostate surgery.

Low libido — testosterone deficiency, depression, relationship factors, medication, and chronic illness. Testosterone helps when levels are genuinely low and not otherwise (Chapter 12.6).

Peyronie's disease — fibrous plaques in the penis causing curvature and pain. Around 3 to 9 percent of men. Treatments include injections of a collagen-dissolving enzyme and surgery.

In women

Low desire — the commonest complaint, and the one with the most complicated causes: relationship factors, fatigue, stress, depression, medication (particularly SSRIs and hormonal contraception in some women), menopause, and pain.

And the treatment order matters. Pain, if present, is addressed first, because desire predictably falls in anticipation of pain and no amount of desire-directed treatment will overcome that.

Arousal difficulty — reduced lubrication and engorgement. Common after menopause from oestrogen deficiency, and vaginal oestrogen is highly effective, very low dose, minimally absorbed, and substantially underused (Chapter 12.6).

Orgasmic difficulty — affecting around 10 to 15 percent of women consistently.

And accurate information resolves a proportion of it. Around 70 to 80 percent of women do not reach orgasm from vaginal penetration alone, and require direct clitoral stimulation.

That is a description of normal anatomy, not a dysfunction. The expectation that penetration alone should produce orgasm has caused a great deal of unnecessary distress in both partners, and correcting it is a legitimate and often sufficient intervention.

Dyspareunia — painful sex.

Superficial pain at entry — vaginal dryness, infection, skin conditions such as lichen sclerosus, scarring after childbirth, or vaginismus.

Deep pain — endometriosis (Chapter 15.3), pelvic inflammatory disease, fibroids, or ovarian pathology.

And painful sex is never something to be endured. It has a cause, and most causes are treatable. The frequency with which women are told it is normal or psychological is a recognised failing.

Vaginismus — involuntary contraction of the pelvic floor muscles preventing penetration.

It is a reflex, not a choice, and the person cannot simply relax. It is often triggered initially by pain, fear or trauma, and then persists as a conditioned response — which is why it continues after the original cause is gone.

Treatment works well: pelvic floor physiotherapy, graded dilator use under guidance, and psychological therapy where relevant. Success rates are high with proper treatment, which is worth knowing given how distressing and isolating it is.

Vulvodynia — chronic vulval pain without an identifiable cause, often burning, and often provoked by touch. A neuropathic pain condition (Chapter 11.13), treated accordingly with amitriptyline, gabapentin, topical agents and physiotherapy rather than with antifungals, which are frequently prescribed repeatedly without benefit.

What affects sexual function generally

Drugs — a substantial and under-discussed list. SSRIs cause sexual side effects in 30 to 70 percent of users, and this is one of the commonest reasons people stop antidepressants without telling anyone. Antihypertensives, particularly beta-blockers and thiazides; antipsychotics, through prolactin; finasteride; opioids; and alcohol.

Discussing this proactively when prescribing improves adherence, because a side effect that was warned about is tolerated far better than one that appears unexplained.

Chronic disease — diabetes, cardiovascular disease, kidney failure, arthritis, and neurological conditions.

Mental health — depression reduces desire; anxiety impairs arousal; and the treatments for both frequently do the same, which is a genuine therapeutic dilemma.

Ageing — real changes occur and they are not the end of sexual function. Slower arousal, less firm erections and longer refractory periods in men; reduced lubrication and thinner tissue in women. Both are largely manageable, and sexual activity in later life is far commoner than younger people assume.

Pregnancy and postpartum — sex is safe in uncomplicated pregnancy. After delivery, resumption is guided by comfort and healing rather than by a fixed interval, and difficulty is very common and usually temporary. Breastfeeding suppresses oestrogen and causes vaginal dryness, which is easily addressed and rarely mentioned.

A note on what belongs in a medical account

Sexual orientation is not a medical condition. It was removed from the American psychiatric classification in 1973 and from the World Health Organization's in 1992. Attempts to change it — "conversion therapy" — are ineffective and are associated with substantial harm, and are prohibited or condemned by every major medical body.

Gender identity is likewise not a disorder. The relevant diagnostic term concerns the distress that can arise from incongruence, not the identity itself. Clinical care in this area is genuinely complex, evolving, and contested in parts, and what is not contested among medical bodies is that the people involved deserve competent, respectful care.

And sexual difficulty following sexual violence is a recognised and treatable consequence, not a character flaw. Specialist services exist and are effective, and knowing they exist is sometimes the only barrier.

What the next page fixes

Chapter 15.6 covers what happens when conception succeeds — the placenta, the trimesters, what changes in the mother's body, what can go wrong, and what antenatal care is actually looking for.