The Endocrine System

 

The endocrine system consists of glands widely separated from each other with no direct anatomical links. Endocrine glands consist of group of secretory cells surrounded by an extensive network of capillaries which facilitates diffusion of hormones from the secretory cells into the blood stream. They are commonly referred to as the ductless glands because the hormones are secreted and diffuses directly into the blood stream. A hormone is formed in one organ or gland and carried in the blood to another organ. Target organ or tissue is probably quite distance, where it influences cellular activity, especially growth and metabolism. Hormones of the internal environment is maintained partly by the autonomic nervous system and partly by the endocrine system. The autonomic nervous system is concerned with rapid changes, while the hormones of the endocrine system are mainly involved in slower and more precise adjustments.

 

Endocrine glands

-        One pituitary gland

-        One thyroid gland

-        Four parathyroid glands

-        Two adrenal glands

-        Pancreatic islets

-        One pineal gland

-        Thymus gland

-        Two ovaries in female

-        Two testes in male

 

The pituitary gland and the hypothalamus

            The pituitary gland and the hypothalamus act as a unit regulating the activity of most of the other endocrine glands. The pituitary gland lies in the hypophyseal fossa of the sphenoid bone below the hypothalamus to which it is attached by a stalk.

Pituitary gland consists of 3 distinct parts that originate from different types of cells. The anterior pituitary is an upgrowth of glandular epithelium from the pharynx. Posterior pituitary is down growth of nervous tissue from the brain. There is a network of nerve fibers between the hypothalamus and the posterior pituitary. Between these lobes there is a thin stipe of tissue called as intermediate lobe. (function in human is not known).

Some of the hormones secreted by the anterior lobe of the pituitary gland stimulate or inhibit secretion by other endocrine glands – tropic hormones

Some hormones have direct effect on target tissues – growth hormone

The release of anterior pituitary hormone is due to stimulation of the gland by a specific releasing hormone produced by the hypothalamus and conveyed to the gland through the pituitary portal system of blood vessels.

         

                

Thyroid gland

The thyroid gland is situated in the neck in front of the larynx and trachea. Resembles butterfly in shape, consisting of two lobes one on either side of the thyroid cartilage and upper cartilaginous rings of the trachea. The lobes are joined by a narrow isthmus, lying in front of the trachea. The gland is composed of cuboidal epithelium and forms spherical follicles. These secrete and store colloid, a thick sticky protein material, thyroid hormones in the form of thyroglobulin are stored here. Between the follicles there are other cells found singly or in small groups: parafollicular cells also called C cells, which secrete the hormone Calcitonin.

 

Parathyroid glands

Parathyroid glands secrete parathyroid hormone. Function of parathyroid hormone is to increase blood calcium level when it is low.