Which of the following is true regarding endocrine glands?

A. Gated channels: Some signaling molecules influence ion channels, but hormones do not act only on gated channels - many act via receptors that trigger intracellular cascades.
B. Receptors: Hormones (like neurotransmitters) act only on cells that have specific receptors to which the hormone binds (membrane receptors for peptide hormones or intracellular receptors for lipophilic hormones). This is the correct answer.
C. Enzymes: Hormones can modulate enzymes inside cells, but they require receptors to target cells first; enzymes are not the general targeting mechanism.
D. Cofactors: Cofactors assist enzymatic reactions but are not the specific binding target that determines hormone action on particular cells.
E. Metabolic pathways: Hormones influence metabolic pathways, but they act via receptors on/inside target cells - metabolic pathways are downstream, not the specific binding site.

A. They have an unusually low density of blood capillaries: Endocrine glands have a high density of capillaries so hormones can quickly enter the bloodstream.
B. They secrete their products by way of ducts: Glands that use ducts are exocrine glands; endocrine glands are ductless.
C. Their secretions may be released onto the body surface: That describes exocrine secretions (e.g., sweat, saliva); endocrine secretions are released internally.
D. They release their secretions into the blood: Endocrine glands secrete hormones directly into the bloodstream so the hormones can travel to target cells.
E. They secrete substances that do not alter the metabolism of their target cells, but extracellular effects: Hormones typically alter the metabolism or activity of their target cells (intracellular signaling); endocrine secretions commonly produce metabolic/physiologic changes.