This chapter will cover general physiologic considerations concerning the endocrine system as they pertain to the practice of anesthesia.
GENERAL PHYSIOLOGIC CONSIDERATIONS
Studying the physiology of the endocrine system starts with specifying the definitions of endocrine and hormone. The nonspecific term “endocrine” refers to a ductless gland that secretes directly into the surrounding extracellular (interstitial) space.1,2 Exocrine glands release their secretions onto the skin or into the GI tract or other hollow organs via ducts. While sounding similar, eccrine glands refer specifically to sweat glands which are but one type of exocrine gland. The term hormone (from the Greek word for “impetus”) broadly refers to a ductless gland cell’s chemical secretion, which affects the activity of a target organ. If the secretion acts upon the secreting cell, it is described as autocrine or autocoid. Secretions acting upon adjacent cells are called paracrine. These substances are also called local factors or local hormones.
The oldest scientific knowledge of hormones and the endocrine system is only a little over 100 years.3,4 The intimate interaction of the various bodily systems to the endocrine system is most apparent in its relationship to the nervous system (the neuroendocrine system). Neuroendocrine systems involve the hypothalamus, the pituitary, and a specific target organ or gland. These systems control many of the most basic physiological processes such as metabolism, reproduction, growth, and adaptations to stress. Several other hormones and endocrine glands (parathyroid, pancreas) are not so directly linked to the nervous system and function much more autonomously. The fact that the endocrine glands are scattered throughout the body and secrete multiple hormones with vastly different actions can make it difficult to determine if some cellular secretions should be considered hormones or something else. Early scientists included the thymus as an endocrine gland due to its secretion of various remote-acting substances such as immunoglobins and other cytokines. Now, however, it is considered a separate part of the immune system (although the hormone cortisol does affect immune function).
Neurotransmitters are differentiated from hormones, although some chemicals such as epinephrine or serotonin can perform both roles.5 Although neurotransmitters can affect the releasing neuron or adjacent neurons, they function very rapidly (in milliseconds). Activity is usually terminated rapidly, as well.
Hormone effects can be excitatory or inhibitory but usually act upon an adjacent cell or cells in an all-or-none manner. They take seconds to days to exert their effects, the magnitude of which varies depending upon the quantity of hormone released. The great majority of the cells of the brain are neurotransmitter-releasing neurons, but some are modified to release their secretions into the circulation (at sites such as the hypothalamus where there is little to no blood-brain barrier [BBB]). When epinephrine is released from a neuron and acts upon the releasing neuron or an adjacent neuron, it is called a neurotransmitter. But when ...