Let us learn about endocrine glands, hormones secreted by endocrine glands and their functions in detail. In animals, the message, communicated in the form of nerve impulses, from receptors (sensory neurons) to the central nervous system and from the latter to the effectors (muscles and glands) is very quick. But cells cannot continuously generate and transmit nerve impulses. They take some time to reset their mechanisms before a new impulse is generated and transmitted. So most multicellular organisms use another means of communication between cells, commonly termed “ chemical communication”.
The stimulated cells release a chemical directly into the blood. Other body tissue cells detect this chemical using special molecules (receptors) present either on their surfaces or inside their cytoplasm. The message is then transmitted and these chemicals produce their effects.
Example: Squirrel has to prepare for either fighting on running away when they are in a scary situation. This requires wide-ranging changes. A chemical signal reaches all cells of the body and provides wide-ranging changes. These chemicals are called hormones which are the secretion of endocrine glands.
What are the endocrine glands?
The glands that lack ducts and pass their secretions into the surrounding blood for transport to the site of action are called ductless or endocrine glands. Their secretions are known as hormones.
Hormones are chemical messengers that regulate the biological processes in the living organisms.
The human body possesses a large number of endocrine glands. These are:
- Thyroid gland
- Parathyroid gland
- Adrenal glands
- Pineal gland
- Testes (in males) and ovaries (in females).
The cells of endocrine glands release hormones into the bloodstream. The blood carries them from the site of production to the site of action. These hormones act on specific organs called target organs.
The blood contains all the hormones but the cells of a target organ pick up the required hormone only and ignore all others. The target cell has on its surface or in its cytoplasm, a specific protein molecule called a receptor. The receptor can recognize and pick out the specific hormone capable of acting in that cell. Non –target cells, on the other hand, lack these receptors and so do not respond to the circulating hormones.
Let’s now discuss the hormones secreted from these endocrine glands & their action.
1. Hypothalamus gland: The Hypothalamus gland is situated at the base of the brain and is composed of nervous tissue. It secretes several neurohormones such as:
(1) Releasing hormones (RH) & Inhibiting hormones (IH): These neurohormones are carried to the pituitary gland to stimulate or inhibit the secretion of anterior pituitary hormones.
(2) Oxytocin: it induces contraction of smooth muscles of the uterus during the birth of the child.
(3) Vasopressin: It decreases the loss of water in the urine.
2. Pituitary gland: The pituitary gland is present just below the brain & attached to hypothalamus of the brain by a stalk or infundibulum.
It produces many hormones, such as:
(1) Follicle-stimulating hormone (FSH):– that stimulates sperm formation in the male & growth of ovarian follicles in females.
(2) Luteinising hormones (LH):– that stimulates the formation of sex hormones testosterone in males and estrogen & progesterone in females. This brings the changes associated with puberty.
(3) Thyroid-stimulating hormones(TSH):– that stimulates the growth of the thyroid gland & production of thyroid hormones.
(4) Adrenocorticotrophic hormones (ACTH):- that stimulates the adrenal cortex to grow and secrete its hormone.
(5) Growth hormone (GH):– that stimulates growth and development of all tissues by accelerating protein synthesis and cell division, and by retaining calcium in the body. Improper secretion of GH produces three important disorders – Dwarfism, Gigantism, or Acromegaly.
(6) Prolactin hormone(PH):– that stimulates the growth of milk glands during pregnancy and the secretion of milk after delivery of the child.
(7) Melanocyte stimulating hormone (MSH):- that stimulates the synthesis of the black pigment melanin in the skin.
3. Thyroid gland:- It is the largest endocrine gland and is situated in the neck region.
It secretes the following two hormones:-These two hormones maintain the basal metabolic rate (BMR) of the body by regulating the rate of oxidation and production of energy. These promote balanced growth of body tissues & the development of mental faculties.
1. Thyroxin: Thyroxine controls the working of the kidney. Iodine is essential for the synthesis of thyroxin. The deficiency of iodine in the diet causes goiter. A swollen neck is one of the symptoms of this disorder
2. Triiodothyronine: It is a thyroid hormone that plays crucial role in the maintaining body’s metabolic rate, heart and digestive functions, muscle control, brain development and function, and bone maintenance.
3. Calcitonin:– It regulates the concentration of calcium & phosphorus in the blood.
4. Parathyroid gland: These are four small, flat, oval glands situated on the posterior surface of the thyroid gland, two in each lobe of the thyroid. They secrete parathormone (PTH). When the blood calcium level is below normal, then parathormone is released. Parathormone and calcitonin regulate the calcium-phosphorus balance in the blood.
5. Adrenal glands:- These are a pair of glands situated on the upper side of each kidney. Its secretion regulates the metabolism of minerals, carbohydrates, proteins, fats and stimulates the development of secondary sexual character both in males and females. The hormone adrenaline secreted from the Adrenal glands is termed an emergency hormone.
In normal situation these hormones are secreted in small amounts but when a person faces stress or danger, adrenaline is secreted in large amounts to prepare the body to face emergency situation.
These increase the rate of heart beat to supply more oxygen to muscles.
Also, the blood supply to the digestive system and skin is reduced due to the contraction of muscles around small arteries in these organs to direct more blood to our muscles. The breathing rate also increases due to the contraction of the diaphragm and rib muscles. All these responses together enable the animal body to be ready to deal with a situation.
6. Pancreas:- The pancreas lies below the stomach. It is an elongated, yellowish gland consisting largely of lobules.
It secretes two hormones.
Insulin – It helps to lower the blood glucose level.
Glucagon – It increases the blood glucose level
Deficiency of insulin hormone in the body causes a disease known as diabetes mellitus in which the sugar level in the blood rises. In this disease the patient excretes sugar (glucose) in urine, feels excessive thirst and also does excessive urination.
The timing and amount of hormone released are regulated by feedback mechanisms. If the sugar levels in blood rise, they are detected by the cells of pancreas which respond by producing more insulin. As the blood sugar level falls, insulin secretion is reduced.
7. Pineal Gland: It is very small and is situated between the two cerebral hemispheres of the brain. It secretes the melatonin hormone and regulates the working of gonads.
8. Thymus gland: The thymus gland is situated in the upper chest near the front side of the heart. It is a prominent gland in the young child. However, it gradually atrophies in the adult. It secretes the thymosin hormone which stimulates the development and differentiation of lymphocytes and thereby increasing resistance to infection.
9. Testes: Testes are part of the male reproductive system. It secretes the sex hormone testosterone.
Testosterone stimulates the growth and development of male accessory glands like prostate, penis, etc. The development of secondary sexual characters in males. eg mustache, beard, hair on the body, deepening of the voice, broadening of shoulders, enlarged and stronger bones and muscles.
Ovaries: A pair of ovaries lie in the abdomen in females. They secrete three female hormones.
(1) Estrogen- that stimulates the formation of ova in the ovary and development of accessory sex characters such as enlargement of breasts and broadening of the pelvis.
(2) Progesterone– that stimulates the thickening of uterine epithelium during the menstrual cycle and maintains the pregnancy by suspending ovulation.
(3) Relaxin helps in the easy birth of the young one.
So we saw that endocrine glands and their secretion are very important.