Hormones Are Chemicals Produced by SpecificTissuesThat AreTransported by the Vascular System to Affect Other Tissues at Low Concentrations
The endocrine system has evolved to allow physiological processes to be coordinated and regulated. The system uses chemical messengers called hormones. Hormones have traditionally been defined as 'chemicals that are produced by specific endocrine organs, are transported by the vascular system, and are able to affect distant target organs in low concentration.” Although this definition is useful from a veterinary medical point of view, it should be recognized that some substances, such as prostaglandins and somatomedins, are produced by many other tissues and are still considered hormones.
Other types of control systems use chemical substances that are not transported in the vascular system to influence distant cell activity. These systems serve as means of local integration among or between cells, as follows:
1. Paracrine effectors, in which the messenger diffuses through the interstitial fluids, usually to influence adjacent cells; if the messenger acts on the cell of its origin, the substance is called an autocrine effector (Figure 33-1).
FIGURE 33-1 Types of cell communication via chemical mediators. (From Hedge GA, Colby HD, Goodman RL: Clinical endocrine physiology, Philadelphia, 1987, Saunders.)
FIGURE 33-2 Comparison of functional arrangements of an ordinary neuron releasing its neurotransmitter (NT) into a synapse and a neurosecretory neuron releasing its neurohormone (NH) into a blood vessel. (From Hedge GA, Colby HDz Goodman RL: Clinical endocrine physiology, Philadelphia, 1987, Saunders.)
2. Neurotransmitters, which affect communication between neurons, or between neurons and target cells; the substances are limited in the distance traveled and the area of the cell influenced (Figure 33-2).
3. Exocrine effectors, such as hormones produced by the pancreas, are released into the gastrointestinal tract.