Hormones are chemicals produced by the ductless endocrine glands. Small amounts of these chemicals are carried around the body in the blood. The organ on which a particular hormone acts is called target organ. Hormones speed up or slow down or alter the activities of these organs. Hormones are chemical messengers which regulate and control a number of activities of the body. The hormonal system is slower acting and regulates processes that occur over days or even month. Why it is beneficial for an animal to have two different systems for coordinating activities? Having two systems allow an animal to respond to two different types of stimuli - those requiring immediate action and those that depend on a long- term response.
The gland that secretes a substance out through a duct is called exocrine gland. The secretion is either inside the body or on a surface of the body.
Following major endocrine glands are present is man: 1. Pituitary gland 4. Pancreas
2. Thyroid gland
5. Adrenal glands
3. Parathyroid glands
6. Gonads
Diagram image pending upload by Admin.
Diagram image pending upload by Admin.
5.9.1 Pituitary Glands
Pituitary gland is attached with the hypothalamus. It is about the size of a pea, and consists of two distinct lobes, the anterior and the posterior lobe. It is called master gland because it controls the activity of other endocrine glands.
Anterior Lobe
It mainly affects, regulates and maintains the development and functions of the endocrine glands. It is the larger one and secretes many hormones. Thyroid stimulating hormones control development and maintenance of the thyroid glands including production of thyroid hormone. Growth hormone or somatotropic hormone affects height of the individual. If less growth hormone is produced during childhood, the individual becomes pituitary dwarf. If too much growth hormone is produced the individual, is a pituitary giant. When there is overproduction of growth hormone in the adult a condition called acromegaly results. Since only the feet, hands and face can respond, these portions of the body become overly large. The other hormones of the anterior lobe influence gonads and adrenal glands.
Posterior lobe
It secretes two hormones, oxytocin and antidiuretic hormone.
Oxytocin: It causes powerful contractions of uterus during birth. It causes the ejection of milk. Antidiuretic hormone. It causes reabsorption of water from the nephrons. It's under secretion results in large quantity of dilute urine. This disease is called diabetes insipidus.
5.9.2 Thyroid Gland
The thyroid gland is located in the neck and fits closely around and sides of trachea just below larynx. The thyroid gland has two lobes. The thyroid hormones are thyroxin and calcitonin.