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Endocrine Glands

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THE PITUITARY GLAND

The Pituitary Gland is small and round with a diameter of approximately half an inch, it is pea shaped gland that is located in the inferior aspect of the brain attached via the Infundibulum. The Pituitary gland has a very rich blood supply that is furnished by the Circle of Willis. It is an endocrine gland that secretes several hormones for the regulation of multiple body processes such as digestion, reproduction and various other metabolic activities such as water balance maintenance, sugar and fat metabolism and core body temperature regulation. The secretions of the Pituitary Gland are controlled by the Hypothalamus and another process called negative feedback inhibition.

The Pituitary gland is divided into an anterior lobe anatomically termed the Adenohypophysis and posterior lobe the Neurohypophysis, both of which receive hormones from the hypothalamus and both of which have their own communicating circulatory pathways with specific regions of the hypothalamus. Their different in the fact that the anterior lobe is capable of receiving and secreting its own hormones, which it does in response to specific messages received from the hypothalamus. The posterior lobe though, is unable to secrete its own hormones; it can only receive hormones from again specific regions of the hypothalamus.

Of the nine hormones secreted by the Pituitary gland, seven are secreted by the anterior lobe. These include:

1. Thytrophin, Thyroid Stimulating Hormone or TSH which acts on the thyroid gland which affects the thyroid hormones.

2. Adrenocorticotropin, Corticotrophin or ACTH which acts upon the adrenal glands triggering the adrenal cortical hormones.

3. Luteotrophin, Leutinizing Hormone or LH in the female and Interstitial Cell Stimulating Hormone or ICSH in the male have the same functions as Folliculotrophin below.

4. Folliculotrophin, Follicle Stimulating Hormone or FSH which allow the ovaries to release oestrogen and progesterone and affects the male testes to release testosterone.

5. Prolactin or Luteotropic Hormone again affects the ovaries to release oestrogen and progesterone as well as affecting the breast for milk ejection and suckling.

6. Somatotrophin, Growth Hormone or GH affects all body organs including bone, muscle, fat etc.

7. Melanocyte Stimulating Hormone or MSH is involved with the maintenance and functions of the skin.

The two hormones of the posterior lobe of the Pituitary Gland include:

1. Antidiuretic hormone or ADH which instructs the kidneys to conserve water.

2. Oxytocin or the hormone of labour instructs the breasts for ejection of milk.

THYROID GLAND

The thyroid gland is a very vascular organ that is located in the neck. It consists of two lobes, one on each side of the trachea, just below the larynx or voice box. The two lobes are connected by a narrow band of tissue called the isthmus. Internally, the gland consists of follicles, which produce thyroxine and triiodothyronine hormones both of which contain iodine.

About 95 percent of the hormone secreted by the thyroid gland is thyroxine with triiodothyronine making up the remaining 5-10%. Neither of these hormones have the capacity to function and synthesize without the presence of iodine. And if there is a deficiency in iodine levels the anterior pituitary gland receives a signal to secrete its thyroid-stimulating hormone. This in turn will result in a growth of the thyroid gland in an attempt for it to secrete more hormones. This is impossible for it to do though as the vital ingredient (iodine) is unavailable for its utilisation. When the thyroid gland becomes enlarged in this manner it is known as simple goiter or iodine deficiency goiter.

The parafollicular cells of the thyroid gland also secrete a hormone known as calcitonin, this hormone’s role is to reverse the function of the parathyroid gland. The parathyroid gland secretes parathyroid hormone or parathormone which attempts to counteract low blood calcium levels where as calcitonin attempts to lower high blood calcium levels.

THE THYMUS GLAND

The Thymus Gland is dual lobed with outer cortex and inner medulla, it is a lymphatic gland that is located in the anterior Mediastinum, posterior to the sternum, anterior to the aorta and central to the lungs. The size may fluctuate with each individual but generally the gland will not exceed 40 grams. It is large in the infant, hits maximum size when puberty is reached then begins to atrophy and be replaced by fat and connective tissue as maturation process begins. It is fully atrophied once maturation is complete. A number of hormones secreted by the thymus gland may help regulate the immune system.

THE ADRENAL GLAND

Located adjacent to each of the kidney are the paired adrenal glands. Each of these glands consists of two individual sections, the outer cortex and the inner medulla. The adrenal cortex’ presence is critical to life, but removal of the inner medulla will result in no undesired effects to the individual’s health. Both of these portions of the adrenal gland are greatly influenced by the functions of the hypothalamus of the brain.

The adrenal cortex is divided into three sections; each of which secreting its own form of hormone. These hormones are titled as corticoids and are categorised as steroid.

The most exterior layer of the adrenal cortex is responsible for secreting mineral corticoids, the main mineral corticoid being aldosterone, its primary function being to store sodium ions and water in the body. The middle region of the adrenal cortex releases a form of hormone known as Glucocorticoids, the principal Glucocorticoid is cortisol that acts to increase blood glucose levels. The third and final hormone group secreted by the adrenal cortex would be the gonadocorticoids, or the hormones associated with the individual’s sex. They are secreted by the innermost region of the adrenal cortex.

Two hormones are secreted from the adrenal medulla, adrenaline and noradrenaline. Secreted in response to messages sent by the sympathetic nervous system to allow the body to react and function with the force and energy required to carry out a fight or flee response.


THE ENDOCRINE GLANDS OF THE PANCREAS

Not only does the pancreas have an exocrine gland that secretes digestive enzymes into the pancreatic duct that in turn releases these enzymes directly into the duodenum. The endocrine portion consists of the pancreatic islets, which secrete glucagon and insulin.

Glucagon is secreted by the alpha cells of the pancreatic Islets in response to low blood glucose levels in an attempt to normalise them. Glucagon triggers the release of stored glycogen from the liver into the blood stream.

Beta cells of the pancreatic islets secrete insulin in an effort to lower the high concentration of glucose in the blood by allowing the glucose to be absorbed into the cells of the body to be used as energy or stored in the liver as glycogen.