General Science - Human Glandular System

Human Glandular System

There are two basic types of glands in the human body:

Exocrine glands [exo-, outside + crin, secrete] secrete substances through a duct or tube, usually to an outside surface. Examples are tear glands, salivary glands, and sweat glands.

Endocrine glands [endo-, within + crin, secrete] are "ductless glands" which secrete substances directly into the blood stream, which carries it to target tissues or organs where it stimulates some type of action. The substances secreted by endocrine glands are called hormones, from the Greek word meaning "to urge on" or "to set in motion." Examples of endocrine glands are the pineal, pituitary, thyroid, parathyroid, thymus, pancreas, adrenals and the sex glands (the ovaries and testes).

The pancreas is both an exocrine and an endocrine gland. Its exocrine function is to secrete pancreatic juice through the pancreatic duct into the small intestines where it plays an important role in the digestion of food. (This is covered in the lesson on the digestive system.) The endocrine function of the pancreas includes the secretion of insulin and glucagon into the bloodstream to help regulate blood sugar levels and carbohydrate metabolism.

In the remainder of this lesson we will be limiting our discussion to the endocrine glands.

1.Tear Glands

 It is defined as an acinous gland that is about the size and shape of an almond, secretes tears, and is situated laterally and superiorly to the bulb of the eye in a shallow depression on the inner surface of the frontal bone called also tear glands

2.Salivary Glands

A gland that secretes saliva, especially any of three pairs of glands, the parotid, submandibulary, and sublingual, that dischargesecretions into the oral cavity of humans and most other mammals.

Parts of Salivary glands

1.Parotid Glands

The parotid gland is a major salivary glands in many animals. In humans, the two parotid glands occur near the mouth and the left ear and the mouth and right ear. They are the largest of the salivary glands. Each parotid is wrapped around the mandibular ramus, and secretes saliva through the Stensen duct (or parotid duct ) into the mouth, to facilitate mastication and swallowing and to begin the digestion of starches.

2.Submandibular Glands

The paired submandibular glands are major salivary glands located beneath the floor of the mouth  They weigh about 15 grams and contribute to around 60–67% of unstimulated saliva secretion, but on stimulation, the parotid secretion rises to 50%

3.Sublingual Glands

a small salivary glandon each side of the mouth lying beneath the mucous membrane in a fossa in the mandible near the symphysis called also sublingualsalivary gland.

3.Sweat Glands

The sweat (sudoriferous) glands are small tubular structures situated within and under the skin (in the subcutaneous tissue). They discharge sweat by tiny openings in the surface of the skin.

The sweat is a transparent colorless acidic fluid with a peculiar odor. It contains some fatty acids and mineral matter. It is also called perspiration.

4.The Pineal Glands

 A small gland that is located near the center of the brain. This gland secretes melatonin, and it may therefore be part of the body's sleep regulation apparatus. Also known as pineal body.

5.Pituitary Glands

The main endocrine gland. It is a small structure in the head. It is called the master gland because it produces hormones that control other glands and many body functions including growth. The pituitary consists of the anterior and posterior pituitary.

The anterior pituitary is the front portion of the pituitary. Hormones secreted by it influence growth, sexual development, skin pigmentation, thyroid function, and adrenocortical function. These influences are exerted through the effects of pituitary hormones on other endocrine glands except for growth hormone which acts directly on cells.

The effects of underfunction of the anterior pituitary include growth retardation (dwarfism) in childhood and a decrease in all other endocrine gland functions normally under the control of the anterior pituitary (except the parathyroid glands). The results of overfunction of the anterior pituitary include overgrowth (gigantism) in children and a condition called acromegaly in adults.

The posterior pituitary is the back portion of the pituitary. It secretes the hormone oxytocin which increases uterine contractions and antidiuretic hormone (ADH) which increases reabsorption of water by the tubules of the kidney. Underproduction of ADH results in a disorder called diabetes insipidus characterized by inability to concentrate the urine and, consequently, excess urination leading potentially to dehydration. The urine is "insipid" (overly dilute).

6.Thyroid Glands

A gland that makes and stores hormones that help regulate the heart rate, blood pressure, body temperature, and the rate at which food is converted into energy. Thyroid hormones are essential for the function of every cell in the body. They help regulate growth and the rate of chemical reactions (metabolism) in the body. Thyroid hormones also help children grow and develop.

The thyroid gland is located in the lower part of the neck, below the Adam's apple, wrapped around the trachea (windpipe). It has the shape of a butterfly: two wings (lobes) attached to one another by a middle part called the isthmus.

The thyroid uses iodine, a mineral found in some foods and in iodized salt, to make its hormones. The two most important thyroid hormones are thyroxine (T4) and triiodothyronine (T3). Thyroid stimulating hormone (TSH), which is produced by the pituitary gland, acts to stimulate hormone production by the thyroid gland. The thyroid gland also makes the hormone calcitonin, which is involved in calcium metabolism and stimulating bone cells to add calcium to bone.

7.Parathyroid Glands

The parathyroid glands are four tiny glands, located in the neck, that control the body's calcium levels. Each gland is about the size of a grain of rice (weighs approximately 30 milligrams and is 3-4 millimeters in diameter). The parathyroids produce a hormone called parathyroid hormone (PTH). PTH raises the blood calcium level by:

1. breaking down the bone (where most of the body's calcium is stored) and causing calcium release
2. increasing the body's ability to absorb calcium from food
3. increasing the kidney's ability to hold on to calcium that would otherwise be lost in the urine.

Normal parathyroid glands work like the thermostat in your home to keep blood calcium levels in a very tightly controlled range. When the blood calcium level is too low, PTH is released to bring the calcium level back up to normal. When the calcium level is normal or gets a little too high, normal parathyroids will stop releasing PTH. Proper calcium balance is crucial to the normal functioning of the heart, nervous system, kidneys, and bones.

8.Thymus Glands

The thymus gland is the main organ of the lymphatic system. Located in the upper chest region, the primary function of this gland is to promote the development of specific cells of the immune system called T lymphocytes. T lymphocytes or T-cells are white blood cells that protect against foreign organisms (bacteria and viruses) that have managed to infect body cells, They also protect the body from itself by controlling cancerous cells.

From infancy to adolescence, the thymus is relatively large in size. After puberty, the thymus begins to decrease in size and continues to shrink with age.

Thymus Anatomy

The thymus is a two lobed structure that is positioned in the upper chest cavity. It partially extends into the neck region. The thymus is situated above the pericardium of the heart, in front of the aorta between the lungs below the thyroid, and behind the breastbone. The thymus has a thin outer covering called a capsule and consists of three types of cells. Thymic cell types include epithelial cells, lymphocytes, and Kulchitsky cells, or neuroendocrine cells

• Epithelial cells - tightly packed cells that give shape and structure to the thymus.
• Lymphocytes - immune cells that protect against infection and stimulate an immune response.

Kulchitsky cells - Hormone-releasing cells.

Thymus Function

The thymus functions chiefly to develop T lymphocytes. Once mature, these cells leave the thymus and are transported via blood vessels to the lymph nodes  and spleen. T lymphocytes are responsible for cell mediated immunity, which is an immune response that involves the activation of certain immune cells to fight infection. T-cells contain proteins called T-cell receptors that populate the T-cell membrane and are capable of recognizing various types of antigens (substances that provoke an immune response). T lymphocytes differentiate into three major classes in the thymus. These classes are:

• Cytotoxic T cells - directly terminate antigens.
• Helper T cells - precipitate the production of antibodies by B-cells and also produce substances that activate other T-cells.
• Regulatory T cells - also called suppressor T cells, these cells suppress the response of B-cells and other T-cells to antigens.

The thymus produces hormone-like proteins that help T lymphocytes mature and differentiate. Some thymic hormones include thympoeitin, thymulin, thymosin, and thymic humoral factor (THF). Thympoeitin and thymulin induce differentiation in T-lymphocytes and enhance T-cell function. Thymosin increases immune responses. It also stimulates certain pituitary gland hormones (growth hormone, luteinizing hormone, prolactin, gonadotropin releasing hormone, and adrenocorticotropic hormone (ACTH)). Thymic humoral factor increases immune responses to viruses in particular.

Summary

The thymus gland acts to regulate the immune system through the development of immune cells responsible for cell mediated immunity. In addition to immune function, the thymus also produces hormones that promote growth and maturation. Thymic hormones influence structures of the endocrine system, including the pituitary gland and adrenal glands, to assist in growth and sexual development. The thymus and its hormones also influence other organs and organ systems including the kidneys, spleen, reproductive system, and central nervous system.

9.Pancreas

A spongy, tube-shaped organ that is about 6 inches long and is located in the back of the abdomen, behind the stomach. The head of the pancreas is on the right side of the abdomen. It is connected to the upper end of the small intestine. The narrow end of the pancreas, called the tail, extends to the left side of the body. The pancreas makes pancreatic juices and hormones, including insulin and secretin. Pancreatic juices contain enzymes that help digest food in the small intestine. Both pancreatic enzymes and hormones are needed to keep the body working correctly. As pancreatic juices are made, they flow into the main pancreatic duct, which joins to the common bile duct, which connects the pancreas to the liver and the gallbladder and carries bile to the small intestine near the stomach. The pancreas is thus a compound gland in the sense that it is composed of both exocrine and endocrine tissues. The exocrine function of the pancreas involves the synthesis and secretion of pancreatic juices. The endocrine function resides in the million or so cellular islands (the islets of Langerhans) that are embedded between the exocrine units of the pancreas. Beta cells of the islets of Langerhans secrete insulin, which helps control carbohydrate metabolism. Alpha cells of the islets of Langerhans secrete glucagon, which counters the action of insulin.

10.Adrenals

The adrenal glands are small glands located on top of each kidney. They produce hormones that you can't live without, including sex hormones and cortisol. Cortisol helps you respond to stress and has many other important functions.

With adrenal gland disorders, your glands make too much or not enough hormones. In crushings syndrome there's too much cortisol, while with addison’s disease there is too little. Some people are born unable to make enough cortisol.

Causes of adrenal gland disorders include

• Genetic mutations
• Tumors including pheochromocytes
• Infections
• A problem in another gland, such as the pituitary, which helps to regulate the adrenal gland
• Certain medicines

Treatment depends on which problem you have. Surgery or medicines can treat many adrenal gland disorders.