Human Endocrine System- Definition, Major Endocrine Glands & their Functions

Why do we breathe heavily when we are afraid? Why do certain changes take place in our body when we reach puberty? Have you ever thought about what mechanism or which system is actually regulating all these? The answer is the Human Endocrine System. The nervous system plays an important role in the control and coordination of the body. But the effect of this system does not reach each and every cell of the body. Its effect is of short duration, and it cannot pass continuously.

These shortcomings are overcome in the Endocrine System. Human Endocrine System secretes chemicals that diffuse throughout the body and can influence all the cells of the target. Multiple effects can also be produced. Let’s dig into the Human Endocrine System to know more.

What is Human Endocrine System?

The human endocrine system is the system that comprises different ductless glands. These glands secrete chemicals called hormones which can control different distant target organs. Branch of science that deals with the study of endocrine glands, hormones is known as endocrinology.

Endocrine Glands & Exocrine Glands

On the basis of the presence or absence of ducts, the gland is of two types:-

1. Endocrine glands – It refers to the glands which secrete their secretions directly into the blood without any duct. These are called ductless glands. Its target organ can be far away from the source of secretion. It can be of two types-

a. Holocrine glands- It refers to the glands which have only endocrine cells that secrete only hormones. Examples are the Pituitary gland, Thyroid gland, Adrenal gland, etc.

b. Heterocrine glands- It refers to the glands which have both endocrine and exocrine cells that can secrete hormones as well as enzymes. Examples are the pancreas, testes and ovaries.

2. Exocrine glands– Glands that secrete their secretions and transfer them to the target organ through ducts are called exocrine glands. These are called duct glands. Examples are the liver, salivary glands, gastric glands, etc.

Fig: Endocrine gland and Exocrine gland

Characteristics of Hormones

1. Hormone, a chemical, was first discovered by Baylis and Starling.
2. ‘Secretin’ is the first hormone discovered.
3. These chemicals are transferred from the source to their target organ through blood.
4. Here, the target organ is generally away from the source.
5. These chemicals are produced in very small quantities but show long-lasting effects.

Major Endocrine Glands and their Functions

Fig: Components of Endocrine system

The main human endocrine glands are:

1. Hypothalamus
2. Pituitary gland
3. Thyroid gland
4. Parathyroid gland
5. Thymus
6. Pancreas
7. Adrenal gland
8. Gonads

Hypothalamus

Location: It is located on the floor of diencephalon. It is a part of the nervous system.

Function: It produces hormones or neurohormones that are transferred to the pituitary glands. Hypothalamus controls the master gland, i.e. pituitary gland. So, hypothalamus is considered as the Control center of the endocrine system.

It secretes releasing and inhibitory hormones, which are to be passed on to the pituitary gland. But oxytocin and vasopressin hormones are produced by the hypothalamus, which are transferred to the posterior pituitary for the direct secretion in blood.

Pituitary Gland

Location: It is located below the hypothalamus. It is about the size of a pea. It is the smallest endocrine gland. This gland is also called the master gland, as it controls all other glands.

Function: This gland is divided into two lobes called anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis).

Following are hormones secreted by the anterior pituitary gland-

1. Growth hormone– This hormone is for the normal growth of a person. If this hormone is secreted too much, then the person becomes too tall, and the disease is called gigantism. On the other hand, if it is secreted in less amount, then the person becomes dwarf, and the disease is called dwarfism.
2. Prolactin– This hormone is responsible for the growth of breasts or mammary glands in mothers. Milk secretion is also controlled by this hormone. In a pregnant and lactating woman, this hormone level is high.
3. Gonadotropic hormones (GnRH)– It regulates the secretion of hormones from gonads. Like oestrogen in females, testosterone in males.
4. Thyroid Stimulating Hormones (TSH)- It stimulates the thyroid gland to secrete its hormone.
5. Adrenocorticotropic Hormones (ACTH)- It stimulates adrenal glands to secrete its hormone.

Fig: Pituitary gland.

Following are hormones secreted by the posterior pituitary gland-

1. Oxytocin– This hormone is responsible for uterine contraction during the birth of a child. So, it is also called a birth hormone.
2. Vasopressin– It helps in the reabsorption of water from tubules of the nephron and thus concentrating the urine.

High secretion of this hormone results in constriction of the blood vessels.
Less secretion of this hormone causes diabetes insipidus that results in frequent dilute urination, which may cause dehydration and thirst.

Thyroid Gland

Location: It is a bilobed structure that looks like a butterfly and is situated just below the larynx attached to the trachea, in front of the neck. It is the largest endocrine gland.

Function: The hormone secreted by this gland is called thyroxine hormone and calcitonin hormone. Thyroxine formation requires iodine from outside while it is not required for the formation of calcitonin.

If this iodine deficiency is not met, then it causes a deficiency disease of thyroxine hormone called goitre.

Cretinism is the term given to the deficiency disease in children, caused due to the deficiency of thyroxine hormone, while in adults, it is called myxedema.

If this thyroxine hormone is oversecreted, then that disease is called exophthalmia, in which eyes protrude out.

Fig: Thyroid and parathyroid gland

Common salt provides the required amount of iodine in our daily diet.

Parathyroid Gland

Location: These glands are oval in shape, and two pairs of it are present, one pair on each lobe of the thyroid gland at its dorsal side.
Function: These secrete parathormone, which balances calcium and phosphate level in blood.
Excessive secretion of this hormone causes Osteoporosis.
Insufficient secretion of this hormone causes hypocalcemic tetany, which results in spasms and cramps.

Thymus

Location: It is a pinkish gland situated near the heart. Its size is bigger during puberty, while its size constricts in old age.
Function: It secretes the thymosin hormone. Thymosin helps in the formation of lymphocytes or WBCs that helps to fight infections.
In the thymus, maturation of T-lymphocytes occurs.

Fig: Thymus

Pancreas

Location: It is a heterocrine gland and is situated below the stomach under the ‘C’ shaped duodenum. It has both endocrine and exocrine parts. Islets of Langerhans are endocrine cells of the pancreas.
Function: Endocrine cells of the pancreas secretes blood regulating hormones called insulin and glucagon. Insulin is secreted by β-cells of Islets of Langerhans, and glucagon is secreted by the α-cells of the Islets of Langerhans.
Insulin reduces the level of glucose, while glucagon increases the level of glucose in the blood.
Deficiency of insulin in the blood causes diabetes mellitus. While deficiency of ADH causes diabetes insipidus.

Adrenal Gland

Location: These are pyramid-shaped glands that lie above the kidneys. So named adrenal glands. “Ad” means “above” and “renal” means kidneys. Each gland has an outer cortex and an inner medulla.
Function: Adrenal cortex secretes three hormones collectively called corticoids, while the adrenal medulla secretes two types of hormones adrenaline (epinephrine) and nor-adrenaline (nor-epinephrine) hormones.

Corticoids– i. Glucocorticoids- For carbohydrate metabolism.

                   ii. Mineralocorticoids- For mineral balance.

                  iii. Sex-corticoids- For maintenance of external sex characters.

Adrenaline hormone is called emergency hormone or 3F (Fight, Fright and Flight) hormone. This hormone prepares our body for stress situations by increasing heart rate, respiration, blood pressure, etc. Like joy, fear, anger, emotional stress, etc.

Gonads

Under gonads comes male and female sex organs.
Testes
Location: Testes are a paired structure, or one pair of the testis is present in males in the scrotum outside the body.
Function: It produces a male sex hormone called testosterone. It helps in the development of secondary sexual character in males like the development of moustache, beard, hoarse voice, pubic hairs, etc. It also helps in the development of male sex organs.
Ovary
Location: Ovary is a part of the female reproductive system.
Location: Ovary produces progesterone and oestrogen. Oestrogen helps in the development of secondary sexual characters like the development of breast, pubic hairs, underarm hairs, etc. It also helps in the development of female sex organ.
Progesterone helps in the development of the placenta during pregnancy. Its level is high throughout the gestation period and helps in maintaining pregnancy.

Human Endocrine System Diagram

Fig: Human Endocrine System

Human Endocrine System- Associated Diseases

Summary

The human endocrine system comprises different ductless glands which secrete their chemical called hormones. For proper control and coordination of the body, with the nervous system, the endocrine system is also needed. Different glands of the endocrine system are the pituitary gland, thyroid and parathyroid, thymus, pancreas, liver, adrenal, etc. The pituitary gland is called the master gland, which directs the functioning of other endocrine glands.

Frequently Asked Question (FAQs) on Human Endocrine System

Q1. What are the 7 endocrine glands?
Ans: Following are the endocrine glands:-
a. Pituitary gland
b. Thyroid gland
c. Parathyroid gland
d. Thymus
e. Pancreas
f. Adrenal gland
g. Gonads

Q2. Explain the Human Endocrine System with an example?
Ans: Hormone is secreted through endocrine glands, which is transferred to the target organs through blood. Like transfer of TSH or thyroid-stimulating hormone from the pituitary gland to the thyroid gland occurs through blood.

Q3. What is the major function of the Human Endocrine System?
Ans: Human endocrine system releases hormones that are transferred to the far situated target organ through blood and show their long time effect.

Q4. What are the 7 hormones?
Ans: Thyroxine, Oxytocin, vasopressin, prolactin, progesterone, estrogen, insulin, etc.

Q5. What are the 5 main functions of the endocrine system?
Ans: Functions of the endocrine system are:-
a. Growth and development
b. Reproduction
c. Metabolism
d. Controls Blood pressure
e. Controls Heart rate

We hope this detailed article on Human Endocrine System helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.