Comparison of the Endocrine and Nervous Systems

The endocrine system, along with the nervous system, functions in the regulation of body activities. The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion. The effect is of short duration, measured in seconds, and localized. The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities. The action of the endocrine system is measured in minutes, hours, or weeks and is more generalized than the action of the nervous system.

Comparison of Exocrine and Endocrine Glands

The two major categories of glands in the body are exocrine and endocrine. Exocrine glands have ducts that carry their secretory product to a surface. These have a variety of functions and include the sweat, sebaceous, and mammary glands and the glands that secrete digestive enzymes. The endocrine glands do not have ducts to carry their product to a surface. They are called ductless glands. The word “endocrine” is derived from the Greek terms endo, meaning “within,” and krine, meaning “to separate or secrete.” The secretory products of endocrine glands are called hormones and are secreted directly into the blood and then carried throughout the body where they influence only those cells that have receptor sites for that hormone. Other cells are not affected. Endocrine glands have an extensive network of blood vessels, and organs with the richest blood supply include some of the endocrine glands, such as the thyroid and adrenal glands.

Characteristics of Hormones

Each hormone produced in the body is unique. Each one is different in its chemical composition, structure, and action. In spite of the differences, there are similarities in these molecules.

Mechanism of Hormone Action

Hormones are potent substances. This means that small amounts of a hormone may have profound effects on metabolic processes. Hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells that respond to a given hormone have receptor sites for that hormone. This is sort of a lock and key mechanism. If the key fits the lock, then the door will open. If a hormone fits the receptor site, then there will be an effect (Figure 11-1). If a hormone and a receptor site do not match, then there is no reaction. All the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases the target tissue is localized in a single gland or organ. In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected. Hormones bring about their characteristic effects on target cells by modifying cellular activity.

Endocrine Glands and their Hormones

The organs of the endocrine system are the glands that secrete hormones. Figure 11-2 illustrates that the eight major endocrine glands are scattered throughout the body; however, they are still considered to be one system because they have similar functions, similar mechanisms of influence, and many important interrelationships.

Some glands also have nonendocrine regions that have functions other than hormone secretion. The pancreas is one of these glands. It has a major exocrine portion that secretes digestive enzymes and an endocrine portion that secretes hormones. The ovaries and testes secrete hormones and also produce the ova and sperm. Some organs, such as the stomach, intestines, and heart, produce hormones, but their primary function is not hormone secretion. These organs are discussed in more detail in the chapters dealing with their predominant function. Table 11-1 summarizes the major endocrine glands and their hormones.

Table 11-1

Principal Endocrine Glands and Their Hormones

Gland	Hormone	Target Tissue	Principal Actions
Anterior lobe of pituitary	Growth hormone (GH)	Most tissues in body	Stimulates growth by promoting protein synthesis
	Thyroid-stimulating hormone (TSH)	Thyroid gland	Increases secretion of thyroid hormone; increases size of thyroid gland
	Adrenocorticotropic hormone (ACTH)	Adrenal cortex	Increases secretion of adrenocortical hormones, especially glucocorticoids, such as cortisol
	Follicle-stimulating hormone (FSH)	Ovarian follicles in females; seminiferous tubules of testis in males	Follicle maturation and estrogen secretion in females; spermatogenesis in males
	Luteinizing hormone (LH); also called interstitial cell–stimulating hormone (ICSH) in males	Ovary in females, testis in males	Ovulation; progesterone production in females; testosterone production in males
	Prolactin	Mammary gland	Stimulates milk production
Posterior lobe of pituitary	Antidiuretic hormone (ADH)	Kidney	Increases water reabsorption (decreases water lost in urine)
	Oxytocin	Uterus; mammary gland	Increases uterine contractions; stimulates ejection of milk from mammary gland
Thyroid gland	Thyroxine and triiodothyronine	Most body cells	Increases metabolic rate; essential for normal growth and development
	Calcitonin	Primarily bone	Decreases blood calcium by inhibiting bone breakdown and release of calcium; antagonistic to parathyroid hormone
Parathyroid gland	Parathyroid hormone (PTH) or parathormone	Bone, kidney, digestive tract	Increases blood calcium by stimulating bone breakdown and release of calcium; increases calcium absorption in digestive tract; decreases calcium lost in urine
Adrenal cortex	Mineralocorticoids (aldosterone)	Kidney	Increases sodium reabsorption and potassium excretion in kidney tubules; secondarily increases water retention
	Glucocorticoids (cortisol)	Most body tissues	Increases blood glucose levels; inhibits inflammation and immune response
	Androgens and estrogens	Most body tissues	Secreted in small amounts so that effect is generally masked by hormones from ovaries and testes
Adrenal medulla	Epinephrine, norepinephrine	Heart, blood vessels, liver, adipose	Helps cope with stress; increases heart rate and blood pressure; increases blood flow to skeletal muscle; increases blood glucose level
Pancreas (islets of Langerhans)	Glucagon	Liver	Increases breakdown of glycogen to increase blood glucose levels
	Insulin	General, but especially liver, skeletal muscle, adipose	Decreases blood glucose levels by facilitating uptake and use of glucose by cells; stimulates glucose storage as glycogen and production of adipose
Testes	Testosterone	Most body cells	Maturation and maintenance of male reproductive organs and secondary sex characteristics
Ovaries	Estrogens	Most body cells	Maturation and maintenance of female reproductive organs and secondary sex characteristics; menstrual cycle
	Progesterone	Uterus and breast	Prepares uterus for pregnancy; stimulates development of mammary gland; menstrual cycle
Pineal gland	Melatonin	Hypothalamus	Inhibits gonadotropin-releasing hormone, which consequently inhibits reproductive functions; regulates daily rhythms, such as sleep and wakefulness

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