If a ligand released from a cell travels through the circulatory system to reach the responding cell, what is this called?

When a ligand travels through the circulatory system to reach a responding cell, this process is identified as endocrine communication. In this type of signaling, hormones (which are often the ligands involved) are released into the bloodstream by endocrine glands and then transported over varying distances to target cells or tissues that possess specific receptors for these hormones. This method allows for widespread effects throughout the body, as a single hormone can influence multiple cells and tissues simultaneously.

In contrast, paracrine communication involves signals that affect nearby cells rather than traveling over long distances through the bloodstream. Exocrine communication pertains to secretions released into ducts that lead to external environments, such as glands secreting substances through a duct. Lastly, synaptic communication occurs when neurotransmitters are released at synapses between nerve cells to transmit signals directly to other nerve cells or muscle cells. Thus, the distinguishing feature of endocrine communication is the utilization of the circulatory system to reach distant targets, making it the correct choice in this context.