What are biochemical substances that circulate in the bloodstream and only affect target organs?

Hormones are biochemical substances that are produced by glands in the endocrine system and then released into the bloodstream. Their primary function is to regulate various physiological processes in target organs located far from their site of origin. This targeted nature of hormones ensures that they can influence specific functions such as growth, metabolism, reproduction, and mood, among others.

For example, insulin, a hormone produced by the pancreas, helps regulate blood sugar levels by acting on the liver, muscles, and fat tissues. Similarly, adrenaline, released by the adrenal glands, prepares the body for a 'fight or flight' response. The ability of hormones to travel through the bloodstream and exert their effects on specific target organs is what distinguishes them from other biochemical substances such as enzymes, which catalyze reactions within cells, and neurotransmitters, which typically act over a very short distance at synapses.