LESSON 11 The Human Endocrine System.

LESSON ASSIGNMENT Paragraphs 11-1 through 11-18.

LESSON OBJECTIVES After completing this lesson, you should be able to:

11-1. Given a hormone, identify the endocrine organ that produces it.

11-2. Match the names or types of hormones with the body functions affected.

SUGGESTION After completing the assignment, complete the exercises at the end of this lesson. These exercises will help you to achieve the lesson objectives.

ENDO = within

CRINE = secrete

The endocrine system (Figure 11-1) is a loose collection of organs called endocrine glands.

Figure 11-1. The endocrine glands of the human body and their locations.

a. The endocrine glands are organs of internal secretion. Since they lack a duct system, they are often referred to as ductless glands.

b. Since their secretions pass into the blood, they are usually well supplied with blood vessels.

The secretion of an endocrine organ is called a hormone. The hormone is a chemical required in very small amounts for the proper development and/or functioning of the body. (Note the similarity of this definition to that of a vitamin. However, the hormone is produced within the body, and the vitamin is acquired from without.)

When the hormone is secreted by the endocrine organ, it is carried by the blood to the appropriate organ, the target organ. In addition, the level of activity of the target organ often affects the activity of the endocrine organ. Thus, there is a feedback mechanism that causes the endocrine organ to secrete just the right amount of hormone.

a. The pituitary body is located immediately under the brain. It is in a special hollow of the floor of the cranial cavity. The pituitary body is actually two glands: the posterior pituitary gland and the anterior pituitary gland.

b. As a whole, the pituitary body produces a large number of hormones. These affect many tissues of the body. Many of these hormones are referred to as tropins (or trophins) because they cause development or activity of the tissues.

In the embryo, the posterior pituitary gland develops as an outcropping (hypophysis) of the inferior part of the brain. Later in life, the posterior pituitary gland remains connected to the forebrain-stem. There is a series of nuclei in the fore-brainstem which are together referred to as the hypothalamus. The hormones of the posterior pituitary gland are actually produced in the hypothalamus. The hormones pass from the hypothalamus to the posterior pituitary gland by way of neurosecretory fibers. From the posterior pituitary gland, the hormones are secreted into the blood. The main hormones of the posterior pituitary gland are:

a. Antidiuretic hormone (ADH). ADH is involved with the resorption or salvaging of water within the kidneys. Antidiuretic hormone is produced under thirst conditions.

b. Oxytocin.  Oxytocin has several specific effects, particularly upon smooth muscle. It is involved with contractions of smooth muscle in the uterus and with milk secretion.

In the embryo, the anterior pituitary gland develops from the roof of the pharynx. Eventually, it lies in front of and attached to the posterior pituitary gland. Certain cells of the hypothalamus produce specific secretions called releasing factors. A special venous portal system carries these releasing factors to the anterior pituitary gland. There, they stimulate the cells of the anterior pituitary gland to secrete their specific hormones.

a. Somatotropin (Somatotrophic Hormone; Growth Hormone). Somatotropin stimulates the growth of the body in general. When this hormone is deficient, dwarfism results. When it is present in excess amounts, giantism results.

b. Thyrotropin. Thyrotropin stimulates the thyroid gland to produce its hormones.

c. Adrenocorticotropic Hormone (ACTH).  Adrenocorticotropic hormone stimulates the adrenal (suprarenal) cortex to produce its hormones.

d. Luteinizing Hormone (LH).  Luteinizing hormone stimulates ovulation and luteinization of ovarian follicles in females and promotes testosterone production in males.

e. Follicle-Stimulating Hormone (FSH).  Follicle-stimulating hormone stimulates ovarian follicle growth in females and stimulates spermatogenesis in males.

f. Prolactin.  Prolactin stimulates milk production and maternal behavior in females.

The pineal gland is located just above the brainstem. It is between the cerebral hemispheres.

The details of the secretions and functions of the pineal gland are still not fully understood. Apparently, they are associated with sexual drive and reproduction. At least in lower animals, the pineal gland is influenced by the cumulative number of hours of light passing into the eyes each day.

a. Location and Structure.  The thyroid gland is located around the trachea, just below the larynx. It consists of two lobes, left and right. They are connected across the front of the trachea by an isthmus.

b. Hormones.

(1) Thyroxin. The most important hormone produced by the thyroid gland is thyroxin. Thyroxin affects the basal metabolic rate (BMR), the level of activity of the body. Since iodine is an important element in the structure of thyroxin, the dietary intake of iodine is very important. When the gland is not functioning properly, it may become enlarged (goiter). Insufficient or excess thyroxin has serious effects on the body.

(2) Calcitonin. A second hormone of the thyroid gland is calcitonin. It is involved with calcium metabolism in the body.

On the posterior side of each thyroid lobe is a pair (2 + 2 = 4) of tiny bodies called the parathyroid glands. The hormone of the parathyroid glands is parathoromone. It is important in maintaining the calcium levels of the body. When excess thyroid tissue is removed in surgery, the surgeon takes care not to remove the parathyroid glands.

There are small groups of cells, known as islets, distributed through the substance of the pancreas. These cells function independently of the pancreas and produce their own hormones.

Insulin and glucagon are two important hormones of the islets. These hormones are concerned with the glucose levels in the body.

As seen in a previous lesson, the kidneys are attached to the upper posterior abdominal wall by a combination of fat and fascia. The adrenal (suprarenal) gland is embedded in the fat immediately above each kidney. Each is triangular or crescent shaped. Each adrenal gland has a central medulla and an outer cortex.

The central portion of the adrenal gland produces two hormones: epinephrine (Adrenalin) and norepinephrine (noradrenalin). These hormones mobilize the energy-producing organs of the body and immobilize the others. This is important during the stress reaction ("fight or flight").

The outer portion (the cortex) of the adrenal gland produces a variety of hormones which can be grouped into three categories:

a. Mineralocorticoids (for example, aldosterone), which are concerned with the electrolyte and water balance of the body.

b. Glucocorticoids (for example, cortisol), which are concerned with many metabolic functions. They are especially known for their anti-inflammatory effects.

c. Sex Hormones (adrenal androgens and estrogens).

We have already seen that the primary sex organs (gonads) produce sex hormones in addition to sex cells (gametes). These hormones help to determine an individual's actual sex (male or female) and promote the sexual development of the individual.

In the male, certain cells of the testes produce the male sex hormones, known as androgens (for example, testosterone). Androgens are concerned with male sexuality.

The sex hormones of the female are known as the estrogens and progesterone. In the female, these hormones are secreted in a cyclic sequence, the menstrual cycle. During this cycle, the hormones affect a number of tissues of the female body. These tissues include the endometrium of the uterus, the milk-producing portions of the mammary glands, and so forth. During pregnancy, the placenta continues the production of progesterone.