Object clauses refer to the predicate (verb or adjective) of the principal clause or to the non-finite forms of the verb (the infinitive, the gerund and the participle).
Object clauses are introduced by the conjunctionsthat, if, whether, lest, by the conjunctive pronouns and adverbs who, whose, what, which, where, when, how, why and asyndetically – the conjunction thatis often omitted especially in colloquial speech.
e.g. I know that the posterior pituitary releases antidiuretic hormone and oxytocin.
John asks if glucagon raises the blood sugar level.