cardiology, including the study of all circulatory organs

circulation / circulation of blood etc.

circulate / for blood to circulate

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend

container, vessel / a tool, a utensil / one's degree of ability: talent, calibre / female genitalia: a vagina

scholarship; learning; knowledge

a vicious cycle, a vicious circle

nitrogen cycle