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Hormones in Animals

Chemical substances are released directly into the nearby blood lymph vessels, from where they are carried to various organs where the exert their effect. These glands are termed as endocrine glands or ductless glands. The secretions of the ductless hormones are called as the hormones.

Characteristics of Animal Hormones:

Endocrine glands also known as ductless glands are secrete hormones.

•Hormones are directly secreted into the blood stream. 

•Glands of the endocrine system secrete hormones in very minute quantities. 

•Often the effect of the hormones is at a different site from the site of their secretion.

•The action of the hormone is on specific tissues or organs known as the target tissues or organs. 

•They are known as chemical messengers; they modify and regulate the activity of the target tissues or organs. 

•Most of the animal hormones are water-soluble and are derived from amino acids, peptides or proteins. These molecules are hydrophilic in nature and large and they cannot pass through the cell membrane. These hormones interact with receptor molecules seen on the surface of the membranes. 

•Hormones like the steroid hormones, thyroid hormones are hydrophobic in nature and they readily diffuse through the cell membranes and bind to the receptor inside the cell. 

Endocrine glands:

Different types of endocrine glands present in our body are the pituitary gland, the pineal gland, the hypothalamus, the thyroid, the parathyroid, the thymus, the adrenal gland, the pancreas, the testes and the ovary.

The pituitary gland:

 It is a pea-shaped gland located at the base of the brain.
• It is considered to be master gland as it secretes many hormones to regulate the organs as well as the other glands.
• Different hormones secreted by this gland include Growth hormone, TSH, FSH, LH, ACTH, MSH, Vasopressin and Oxytocin.

The hypothalamus:

• It is a neuro-endocrine part of the brain.
• It links the nervous system and the endocrine system through the pituitary gland.
• Different hormones secreted by this gland include TRH, GnRH, GHRH, CRH, Stomatostatin, and Dopamine.

The thyroid gland:

• It is located in the neck, ventral to the larynx.
• It is the one of the largest endocrine glands.
• The principal hormones produced by this gland are triiodothyronine and thyroxin.
• Thyroxin is a hormone that regulates the metabolism of carbohydrates, proteins and fats in the body.  Hypo secretion of thyroxin leads to cretinin in children, and myxoedema in adults.  Hyper secretion of thyroxin leads to exophthalmic goiter in adults.  Goiter is caused due to deficiency of iodine in food.  Iodine is essential for the synthesis of thyroxin.

Parathyroid glands:

• These are two pairs of small, oval-shaped glands embedded on the dorsal surface of the thyroid gland present in the neck.
• They secrete parathormone.  Parathormone helps in regulation of calcium and phosphate ions in the bones and blood.
• Hypo secretion leads to parathyroid tetany and hyper secretion causes osteoporosis.

The adrenal glands:

• These are located above the kidneys and hence are called as suprarenal glands.
• Two regions of the adrenal gland are adrenal cortex and adrenal medulla.
• Adrenal cortex secretes the hormones like cortisol, aldosterone and androgens.
•Adrenal medulla secretes the hormones like adrenaline and noradrenaline.
Adrenaline is also called the “hormone of fight ” or the emergency hormone. It prepares the body to face an emergency condition of physical stress, like danger, anger and excitement.

The pancreas:

• It is located just below the stomach within the curve of the duodenum.  It is both exocrine and endocrine in function.
• It secretes hormones such as insulin, glucagon, somatostatin and pancreatic polypeptide.
• Insulin regulates the sugar level in our blood.  Insulin  secreted in small amounts increases the sugar level in our blood which in turn causes a disease called diabetes mellitus.


Two types of gonads present in human beings are female gonads and male gonads.

Female gonads

• A pair of ovaries forms the gonads in female.  
• Ovaries are the female sex organs that lie one on either side of the abdominal cavity. Ovaries produce two hormones, namely, oestrogen and progesterone.
• Oestrogen controls the changes that occur during puberty, like feminine voice, soft skin and development in mammary glands.
• Progesterone controls the uterine changes in the menstrual cycle, and helps in the maintenance of pregnancy.

Male gonads

• A pair of testes forms the gonads in males.
• A pair of testes is the male sex organ located in the scrotum, which is outside the abdomen.
• Testes produce the hormone testosterone.
• Testosterone controls the changes, which occur during puberty, like deeper voice, development of penis, facial and body hair.

The pineal gland

• It is located near the centre of the brain, dorsal to the diencephalon.
• It produces the hormone melatonin.
• Melatonin affects reproductive development, modulation of wake and sleep patterns, and seasonal functions.

The thymus gland

• It is located in front of the heart, in the upper part of the sternum.
• It produces the hormone thymosine.
• It helps in the maturation of T-lymphocytes.