In the last chapter, we learned about the retail end of cell - cell communication - the precise signaling of one neuron to one or a few other nerve or muscle cells. In this section, we will learn about wholesale communication using hormones to carry signals to a large number of cells that may be distant from the signaling cell. Like neurotransmitter molecules, hormones are released from one cell and detected by another. Unlike neurotransmitters, hormones are released into the blood where they can in principle be received by any cell in the body. Pheromones are another kind of signaling molecules. These molecules are detected by receptors on the cell surfaces of a different animal.

The major sites of hormone release in the human body are shown in figure 33.2. We will be concerned mainly with hormones secreted by the pituitary gland, pancreas, and gonads. Before we look in detail at the hormone messages sent by these glands, though, consider figures 33.3 and 33.4. These figures show the mode of action for two different kinds of hormones. The first type, steroid hormones, are lipid soluble, and therefore able to cross the phospholipid bilayer. Steroid hormones, like estrogen and testosterone, once inside the cell, bind to receptor proteins that, in concert with their hormone partners, are able to regulate gene expression. Other hormones, like insulin and the other peptide hormones listed in table 33.1 are protein molecules. These hydrophilic molecules are unable to cross the membrane, and must bind to receptors on the outside of the cell and signal their presence to the inside, where regulation of gene expression can occur. The receptors are transmembrane proteins that, when occupied by hormone, generate a second messenger inside the cell. In the case of the transmembrane glucagon receptor in figure 33.4, the second messenger is cAMP, the same molecule used to signal the availability of glucose in the E. coli lac operon regulation. In eukaryotic cells, cAMP is the first in a series of steps in the cell that culminate in a response by the cell. In this case enzyme activity is regulated. In many other cases, it is gene expression that is regulated.

The pituitary gland is a part of the brain where nerve cells secrete hormones into the blood. In this respect, the cells of the pituitary gland are akin to neurons releasing neurotransmitters at synapses. The pituitary gland releases a large array of hormones, many of which act to regulate the release of other hormones by other endocrine glands. One hormone released by the posterior pituitary, ADH, is important in regulating the amount of water released by the kidneys. Another, somatotropin (STH), or growth hormone, is released by cells in the posterior pituitary, and is one of the most important signals regulating the division of cells. Excessive STH results in gigantism, while too little results in dwarfism (figure 33.7).

The pancreas is the major regulator of glucose homeostasis (see figure 33.12). If blood sugar (glucose) is low, the beta cells of the pancreas release the hormone glucagon, which stimulates the liver to release glucose into the blood. If blood sugar levels rise, the alpha cells of the pancreas release insulin, causing the liver to take up and store glucose, removing it from the blood. Together, these activities keep the glucose in the blood at a constant level. Note that in the absence of insulin (type I) or an inability to respond to insulin (type II) diabetes (abnormally high blood sugar) results. This disease damages the kidneys, brain and circulatory system.

Sections 34.9 and 34.10 show the hormonal control of the female reproductive cycle. The cycle depends on the timed release of FSH and LH from the anterior pituitary, along with the release of estrogen from the ovary and the corpus luteum to maintain the uterine lining in a receptive state for pregnancy. In the absence of pregnancy, the lining breaks down and a new cycle begins. If pregnancy does occur, progesterone secreted by the developing embryo is responsible for maintaining the uterine lining.