The endocrine system is the second major controlling system of the body. Acting with the nervous system, it helps coordinate and integrate the activity of the body’s cells. However, the nervous system employs electrochemical impulses to bring about rapid control, while the more slowly acting endocrine system employs chemical “messengers,” or hormones, which are released into the blood to be transported throughout the body.

The term hormone comes from a Greek word meaning “to arouse.” The body’s hormones, which are steroids or amino acid-based molecules, arouse the body’s tissues and cells by stimulating changes in their metabolic activity. These changes lead to growth and development and to the physiologic homeostasis of many body systems. Although all hormones are blood borne, a given hormone affects only the biochemical activity of a specific organ or organs. Organs that respond to a particular hormone are referred to as the target organs of that hormone. The ability of the target tissue to respond seems to depend on the ability of the hormone to bind with specific receptors (proteins) occurring on the cells’ plasma membrane or within the cells.

Although the function of some hormone-producing glands (the anterior pituitary, thyroid, adrenals, parathyroids) is purely endocrine, the function of others (the pancreas and gonads) is mixed---both endocrine and exocrine. Both types of glands are derived from epithelium, but the endocrine or ductless glands release their product (always hormonal) directly into the blood or lymph. The exocrine glands release their products at the body’s surface or outside an epithelial membrane via ducts. In addition, there are varied numbers of hormone-producing cells within the intestine, stomach, kidney, and placenta; organs whose functions are primarily non-endocrine. Only the major endocrine organs are considered here.

Figure 9.1

Sketch and Label the General Endocrine System

Pituitary gland, Pineal body, Pancreas, Thyroid gland, Parathyroid glands, Adrenal glands, Ovaries, Testes

The pituitary gland, or hypophysis, is located in the sphenoid bone. It consists largely of two functional areas, the adenohypophysis, or anterior pituitary, and the neurohypophysis, consisting mainly of the posterior pituitary. The pituitary gland is attached to the hypothalamus by a stalk called the infundibulum.

Anterior Pituitary Hormones

The anterior pituitary, or adenohypophysis, secretes a number of hormones. Four of these are tropic hormones.
In each case, a tropic hormone released by the anterior pituitary stimulates its target organ, which is also an endocrine gland, to secrete its hormones. Target organ hormones then exert their effects on other body organs and tissues.

Figure 9.2

The tropic hormones include:

Follicle-stimulating hormone (FSH)

Luteinizing hormone (LH)
regulate gamete production and hormonal activity of the gonads (ovaries and testes)

Adrenocorticotropic hormone (ACTH)
regulates the endocrine activity of the cortex portion of the adrenal gland

Thyrotropic hormone (TSH)
influences the growth and activity of the thyroid gland.

The three other hormones produced by the anterior pituitary are not directly involved in the regulation of other endocrine glands of the body.

Growth hormone (GH)
is a general metabolic hormone that plays an important role in determining body size. It affects many tissues of the body; however, its major effects are exerted on the growth of muscle and the long bones of the body

Prolactin (PRL)
stimulates breast development, promotes and maintains lactation by the mammary glands after childbirth. Its function in males is unknown

Melanocyte-stimulating hormone (MSH)
which stimulates melanocytes to increase their synthesis of melanin pigment, does not appear to be of major significance in humans

The anterior pituitary controls the activity of so many other endocrine glands that it has often been called the master endocrine gland. However, the anterior pituitary is not autonomous in its control because release of the anterior pituitary hormones is controlled by neurosecretions, releasing or inhibiting hormones, produced by the hypothalamus.

The posterior pituitary is not an endocrine gland in a strict sense because it does not synthesize the hormones it releases. Instead, it acts as a storage area for two hormones transported to it from the hypothalamus.

Oxytocin
stimulates powerful uterine contractions during birth and coitus and also causes milk ejection in the lactating mother

Anti-diuretic hormone (ADH)
causes the distal and collecting tubules of the kidneys to reabsorb more water from the urinary filtrate, thereby reducing urine output and conserving body water. It also plays a minor role in increasing blood pressure because of its vasoconstrictor effect on the arterioles.

Sketch and Label the Pituitary Gland

Anterior Pituitary, Posterior Pituitary, Infundibulum, Hypothalamus

Observe the general structure of the pituitary gland and differentiate between the glandular anterior pituitary and the neural posterior pituitary

Sketch and Label the Anterior Pituitary (Microscopic)

Basophils, Acidophils

Figure 9.3	Figure 9.4	Figure 9.5	Figure 9.6

The thyroid gland is composed of two lobes. It is located in the throat, just inferior to the larynx. It produces two major hormones, thyroid hormone and calcitonin.

 Figure 9.7

Thyroid hormone (TH)
Actually two physiologically active hormones known as T₄ (thyroxine) and T₃ (triiodothyronine). Because its primary function is to control the rate of body metabolism and cellular oxidation, TH affects virtually every cell in the body.

Calcitonin
decreases blood calcium levels by stimulating calcium deposit in the bones. It acts antagonistically to parathyroid hormone, the hormonal product of the parathyroid glands.

Observe Figures 9.8 and 9.9 of the thyroid gland , noting the follicles, spherical sacs containing a pink stained material. The parafollicular, or C, cells you see between the follicles are responsible for calcitonin production.

Sketch and Label the Thyroid (Microscopic)

Follicles, Follicle cells

Figure 9.8	Figure 9.9

The parathyroid glands are found embedded in the posterior surface of the thyroid gland. Typically, there are two small oval glands on each lobe, but there may be more and some may be located in other regions of the neck.

Figure 9.10

Parathyroid hormone (PTH)
The most important regulator of calcium-phosphate ion homeostasis of the blood. When blood calcium levels decrease below a certain critical level, the parathyroids release PTH, which causes release of calcium from bone matrix and prods the kidney to reabsorb more calcium and less phosphate from the filtrate.

Observe the parathyroid tissue below in Figure 9.11 and view its two major cell types, the chief cells and the oxyphil cells. The chief cells, which synthesize parathyroid hormone (PTH), are small and abundant, and arranged in thick branching cords. The function of the scattered, much larger oxyphil cells is unknown

Sketch and Label the Parathyroid (Microscopic)

Chief cells, Oxyphil cells

Figure 9.11

The two bean-shaped adrenal glands are located atop or close to the kidneys.

Adrenal medulla

The adrenal medulla is directly controlled by sympathetic nervous system neurons. The medullary cells respond to this stimulation by releasing:

Epinephrine (80%)

Norepinephrine (20%)
act in conjunction with the sympathetic nervous system to elicit the “flight or fight” response to stressors

Adrenal cortex

The adrenal cortex produces three major groups of steroid hormones, collectively called the corticosteroids.

Mineralocorticoids
Chiefly aldosterone, regulate water and electrolyte balance in the extra-cellular fluids, mainly by regulating sodium ion re-absorption by kidney tubules.

Glucocorticoids (cortisone, hydrocortisone, and corticosterone)
enable the body to resist long-term stressors, primarily by increasing blood glucose levels.

Gonadocorticoids (or sex hormones)
Produced by the adrenal cortex are chiefly androgens (male sex hormones), but some estrogens (female sex hormones) are formed. The gonadocorticoids are produced throughout life in relatively insignificant amounts; however, hypersecretion of these hormones produces abnormal hairiness (hirsutism), and masculinization occurs.

Sketch and Label the Adrenal Gland

Adrenal cortex, Adrenal medulla