7 Endocrine glands

Learning objectives

After reading this chapter you should understand:

• the function of hormones

• the control of hormone secretion

• the function of specific glands

The endocrine glands are organs producing secretions called hormones, which are poured directly into the blood stream from the glandular cells. It is for this reason that they are also known as ductless glands. This chapter is mainly concerned with the overall control of the endocrine system, which is composed of all the endocrine glands, by the pituitary in the brain. The function of the thyroid and parathyroid glands will be considered in some detail along with the adrenal glands, although the gonads, which are also endocrine glands, are considered in Chapter 25.

Hormones are organic compounds manufactured by the glands from substances carried in the blood. They are mainly protein derivatives, although some are steroids. They are then carried by the blood stream to other parts of the body where they have a specific effect.

The endocrine glands have been investigated and are described as though they are separate entities; however, their functions are closely interrelated. Initially the functions of the hormones were deduced by observing the effects of disease, destruction or overgrowth of glands. Hormones have been isolated, obtained in pure form, analysed and, in some cases, successfully synthesized. The glands secrete their hormones continuously but the quantity of secretion can be increased or decreased according to body needs. Control of hormone secretion occurs in several different ways:

• Nerve cells produce chemical substances that are carried to the gland and cause secretion.

• The gland responds to impulses from the autonomic nervous system.

• One gland produces a hormone, which affects a second gland. The second gland produces its hormone, which influences the secretion of the first gland. This is called a ‘feedback’ mechanism.

• The gland responds to blood levels of substances other than hormones.

The pituitary

The pituitary gland lies in the hypophyseal fossa of the sphenoid bone in the base of the skull. It is attached by a neural stalk to the optic chiasma at the base of the brain (Fig. 7.1).

Fig. 7.1 The pituitary gland, shown in section, lying in the sella turcica or hypophyseal fossa. Note the stalk joining it to the base of the brain.

The gland consists of an anterior lobe, or adenohypophysis, and a posterior, or neural, lobe. The anterior lobe is an endocrine gland in the true sense, while the posterior lobe is derived from the brain and consists of nervous tissue; it is connected directly to the hypothalamus. The two lobes are essentially two different endocrine glands and are commonly called the anterior and posterior pituitary glands.

The anterior lobe of the pituitary is sometimes referred to as the ‘master gland’ of the endocrine system because it has an important influence in regulating the function of other glands. However, the glands really act in concert, one becoming active if the others are not producing sufficiently and output of hormones being decreased when other glands are active.

The anterior lobe produces a number of hormones:

• thyroid-stimulating hormone (TSH or thyrotrophic hormone)

• adrenocorticotrophic hormone (ACTH)

• somatotropin

• follicle-stimulating hormone (FSH)

• luteinizing hormone (LH)

• prolactin.

1. TSH influences all aspects of thyroid gland function, including stimulation of the accumulation of iodine in the gland for conversion into thyroid hormones, manufacture of the hormones and their release into the circulatory system. Through its action on the thyroid gland, TSH is involved in the regulation of metabolic rate, breakdown of fat and the increase of the water content of certain tissues.

2. ACTH regulates the development, maintenance and secretion of the cortex of the suprarenal glands. General metabolic effects of this hormone include mobilization of fats, leading to hypoglycaemia and increase in muscle glycogen. It is also involved in body resistance to stress.

3. Somatotrophic (growth) hormone exerts its influence mainly on the hard tissues of the body, though there is some effect on the soft tissues. The hormone increases the rate of growth and maintains size once maturity has been reached. It controls the rate of growth in epiphyses and other ossification centres of the skeletal system. Oversecretion of this hormone causes overgrowth of the long bones in children (gigantism) and acromegaly in adults. In acromegaly the bones cannot increase in length because the epiphyseal plates have closed so they become thicker and coarser; the lower jaw, hands and feet are particularly affected. Undersecretion of the growth hormone results in dwarfism. People who are very short or very tall, due to under- or oversecretion of this hormone, are usually of normal intelligence, unlike those who suffer from undersecretion of the thyroid gland. Metabolic effects of this hormone include an increase in the conversion of carbohydrates to amino acids and in the uptake of amino acids by the cells, mobilization of fat from the storage areas with increased fat metabolism and an increased blood sugar level.

4. FSH controls the maturation of ovarian follicles in the female and the production of sperm in the male.

5. LH causes changes in the female which lead to the formation of the corpus luteum; it also helps to prepare the breasts for the secretion of milk. In the male, a comparable hormone is called interstitial cell-stimulating hormone (ICSH), which acts on the testes and controls the secretion of the male sex hormone testosterone.

6. Lactogenic hormone (prolactin) is one of several hormones involved in the production of milk by the breasts and appears to function only in females.

The posterior lobe releases two hormones: (1) oxytocin; and (2) antidiuretic hormone (ADH or vasopressin).

It is important to note that these hormones are produced in the hypothalamus and only stored in, and released from, the posterior lobe.

1. Oxytocin exerts its effects mainly on the smooth muscle of the pregnant uterus and the cells around the ducts of the breasts, although it does also promote a generalized contraction of unstriped muscle throughout the body.

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