Which type of chemical messengers is secreted into the blood stream and acts on cells far away?

Hormones are chemical messengers that are secreted directly into the bloodstream. This allows them to travel over varying distances throughout the body to reach target cells or organs, where they exert their effects. Hormones are produced by various glands in the endocrine system, such as the pituitary, thyroid, adrenal glands, and pancreas.

The function of hormones is crucial for regulating many physiological processes, including metabolism, growth and development, tissue function, and mood. They can influence cells far away from their site of release, making them distinct from other types of chemical messengers that typically act over shorter distances.

Neurotransmitters, for example, are chemicals released by neurons at synapses to communicate with other neurons; their actions are localized to the immediate vicinity. Enzymes primarily catalyze biochemical reactions and do not function as messengers. Cytokines are signaling molecules used by the immune system for communication between cells, but they generally have a more localized effect rather than acting over long distances like hormones. Thus, the role of hormones as systemic regulators emphasizes their importance in the body's overall functioning.