Physiological variables covering many different aspects of relationships have now been explored by scientific studies.

Love As Embodied Medicine

Warm partner contact is related to lower cardiovascular reactivity

Influence of a “Warm Touch” Support Enhancement Intervention Among Married Couples on Ambulatory Blood Pressure, Oxytocin, Alpha Amylase, and Cortisol

Prognostic significance of spouse we talk in couples coping with heart failure

Sex and death: are they related? Findings from the Caerphilly cohort study

More frequent partner hugs and higher oxytocin levels are linked to lower blood pressure and heart rate in premenopausal women

Kissing reduces allergic skin wheal responses and plasma neurotrophin levels

Between- and within-sex variation in hormonal responses to psychological stress in a large sample of college students

The impact of physical proximity and attachment working models on cardiovascular reactivity: Comparing mental activation and romantic partner presence

Trends in Sexual Activity and Associations With All-Cause and Cause-Specific Mortality Among US Adults

Declines in Sexual Activity and Function Predict Incident Health Problems in Older Adults: Prospective Findings from the English Longitudinal Study of Ageing

Touch is a key physiological variable

Touch between romantic partners: Being stroked is more pleasant than stroking and decelerates heart rate

Blood pressure reactivity to stress is better for people who recently had penile-vaginal intercourse than for people who had other or no sexual activity

Effects of partner support on resting oxytocin, cortisol, norepinephrine, and blood pressure before and after warm partner contact

Effects of different kinds of couple interaction on cortisol and heart rate responses to stress in women

Stable partnership and progression to AIDS or death in HIV infected patients receiving highly active antiretroviral therapy: Swiss HIV cohort study

The influence of marital adjustment on 3-year left ventricular mass and ambulatory blood pressure in mild hypertension

Blood pressure reactivity to stress is better for people who recently had penile-vaginal intercourse than for people who had other or no sexual activity

Prognostic importance of marital quality for survival of congestive heart failure

Here’s a smattering of oxytocin research, which helps elucidate its effects:

Anxiety, cortisol, and attachment predict plasma oxytocin

Effects of route of administration on oxytocin-induced changes in regional cerebral blood flow in humans

Oxytocin for the treatment of drug and alcohol use disorders

The Role of Oxytocin in Alcohol and Drug Abuse