Adrenalorrhaphy

Adrenalotomy

Pituitary Gland

The pituitary gland, also known as the hypophysis, is a tiny gland located behind the optic nerve in the cranial cavity in a depression in the sphenoid bone called the sella turcica. The infundibulum, named for its funnel-like appearance, is the structure that attaches the pituitary to the hypothalamus directly superior to it in the brain. Sometimes called the master gland because of its role in controlling the functions of other endocrine glands, the hypophysis is composed of anterior and posterior lobes, each with their own function.

The anterior lobe, or adenohypophysis, is composed of glandular tissue and secretes hormones in response to stimulation by the hypothalamus. The hypothalamus sends hormones through blood vessels, which cause the adenohypophysis either to release or to inhibit the release of specific hormones. The adenohypophysis has a wide range of effects on the body, as Figure 15-2 and the table below illustrate.

The posterior lobe (neurohypophysis) of the pituitary gland is composed of nervous tissue. The hormones that it secretes are produced in the hypothalamus, transported to the neurohypophysis directly through the tissue connecting the organs, and released from storage in the posterior lobe by neural stimulation from the hypothalamus. The two hormones released by this lobe are antidiuretic hormone (ADH) and oxytocin (OT). See the following table and Figure 15-2 for the hormones secreted by the neurohypophysis and their effects.

Thyroid Gland

The thyroid gland is a single organ, but is divided into right and left lobes that are joined by a thin structure termed the isthmus (Fig. 15-3). It is located in the anterior part of the neck and is bounded by the trachea behind it and the thyroid cartilage above it. It regulates the metabolism of the body and normal growth and development, and controls the amount of calcium (Ca) deposited into bone. The thyroid gland is composed of small sacs, called follicles, that absorb iodine. The sacs are surrounded by follicular cells that produce triiodothyronine (T₃) and thyroxine (T₄). Parafollicular cells in the thyroid produce and secrete calcitonin, which controls the amount of calcium in the blood. Thyroid-stimulating hormone (TSH), released by the anterior pituitary gland, causes the thyroid to release T₃ and T₄.

Parathyroid Glands

The parathyroids are four small glands (right and left, superior and inferior) located on the posterior surface of the thyroid gland in the neck. They secrete parathyroid hormone (PTH) in response to a low level of calcium in the blood. When low calcium is detected, the PTH increases calcium by causing it to be released from the bone, which results in calcium reabsorption by the kidneys and the digestive system. PTH is inhibited by high levels of calcium.

Adrenal Glands (Suprarenals)

The adrenal glands, also called the suprarenals, are paired, one on top of each kidney. Different hormones are secreted by the two different parts of these glands: the external portion called the adrenal cortex and an internal portion called the adrenal medulla.

The adrenal cortex secretes three hormones that are called steroids.

The adrenal medulla is the inner portion of the adrenal gland. It produces sympathomimetic hormones that stimulate the fight-or-flight response to stress, similar to the action of the sympathetic nervous system.

Pancreas

The pancreas, located inferior and posterior to the stomach, is a gland with both exocrine and endocrine functions (Fig. 15-4). The exocrine function is to release digestive enzymes through a duct into the small intestines. The endocrine function, accomplished through a variety of types of cells called islets of Langerhans, is to regulate the level of glucose in the blood by stimulating the liver. The two main types of islets of Langerhans are alpha and beta cells. Alpha cells produce the hormone glucagon, which increases the level of glucose in the blood when levels are low. Beta cells secrete insulin, which decreases the level of glucose in the blood when levels are high. Insulin is needed to transport glucose out of the bloodstream and into the cells. In the absence of glucose in the cells, proteins and fats are broken down, causing excessive fatty acids and ketones in the blood. Normally, these hormones regulate glucose levels through the metabolism of fats, carbohydrates, and proteins. See Figure 15-5 for a diagram explaining the effects of insulin and glucagon.

Thymus Gland

The thymus gland is located in the mediastinum above the heart. It releases a hormone called thymosin, which is responsible for stimulating key cells in the immune response. For more details, see Chapter 9 on the circulatory and lymphatic systems.

Ovaries and Testes

The ovaries and testes, the female and male gonads, also act as endocrine glands, which influence reproductive functions.

Pineal Body

The pineal body (gland) is located in the center of the brain, functioning to secrete the hormone melatonin, thought to be responsible for inducing sleep.