24.2: Hormones travel through the circulatory system to influence cells elsewhere in the body.

You wouldn’t use the postal service to notify the fire department that your house is burning down. Similarly, you probably wouldn’t use the phone to convey the complex details of a contractual agreement or architectural plans—the mail would be preferable, to ensure that the details are conveyed precisely. For almost all types of communication, some modes of delivery are more effective than others.

Animals need ways to respond to the outside world: ways of sending signals about certain conditions and giving instructions to tissues to respond to those conditions. There are two systems in animals for carrying out this internal communication and regulation (FIGURE 24-2). As we saw in Chapter 23, the nervous system generally is responsible for controlling rapid movement and sensations in response to environmental changes. In multicellular animals, including humans, there is another system as well, with a special type of messenger. Hormones (from the Greek word meaning “that which sets in motion or urges forward”) are a type of chemical messenger, secreted by cells into the circulatory system (the bloodstream or, in insects, the hemolymph), that influences the actions of cells elsewhere in the body, as part of an internal communication and regulation system. Oxytocin, described in Section 24-1, is an example of an animal hormone.

Cells that secrete hormones are called endocrine cells, and the cells that receive their signals are called target cells. The hormone-secreting cells may be individual cells such as the endocrine cells in the lining of the stomach and small intestine, which aid in digestion. Larger collections of hormone-secreting cells—including the pituitary gland, the pancreas, and the ovaries and testes—are called endocrine glands. Together, all of the hormone-secreting cells in an animal make up its endocrine system (FIGURE 24-3).

While the nervous system controls rapid movements and sensations, the endocrine system controls chemical signals that influence cells close to and far from the secreting cells. In contrast to the nervous system, the endocrine system is generally responsible for longer-term, slower regulation, such as growth and development, as well as a variety of secretions (see Figure 24-2). It’s important to note, however, that there are some nervous system cells that secrete hormones, so the systems are not completely distinct but rather can overlap somewhat in function. Interestingly, although they don’t have nervous systems, most plants do have hormones (see Chapter 19), suggesting that hormone systems predate nervous systems evolutionarily.

When a hormone gets to a target cell, it elicits a response. Commonly, the hormone’s effect on a target cell is to alter the animal’s physiology to help the organism maintain homeostasis. In doing so, the hormone may alter the target cell’s metabolism or growth, spur cell division, or initiate some developmental pathway.

Another type of chemical messenger, called a pheromone, is considered an “ecto-hormone,” because it is transported to the outside of an animal and can cause a behavioral or physiological change in another individual. Pheromones serve many different purposes. They include territorial pheromones, such as those found in dogs’ urine, which mark the boundaries of a territory (FIGURE 24-4); alarm pheromones, such as those that signal the presence of a predator and can trigger aggression or flight in a group of individuals; and trail-marking pheromones, such as those produced by ants that serve as a guide to the colony. The first pheromones discovered were those signaling sexual receptivity. In some butterfly species, for example, males can detect a female’s sex pheromones from more than six miles away and fly in the direction of increasing pheromone concentration to locate the female.