Human Endocrine Glands
Endocrine glands in animals possess a versatile communication system to coordinate biological functions. Exocrine glands and endocrine glands are two kinds of glands found in animals. Endocrine glands are found in different regions of the body of animals as well as human beings. These glands are called ductless glands. Their secretions are called hormones which are produced in minute quantities. heT secretions diffuse into the blood stream and are carried to the distant parts of the body. They act on specific organs which are referred as target organs.
Exocrine glands have specific ducts to carry their secretions e.g. salivary glands, mammary glands, sweat glands.
Endocrine glands present in human and other vertebrates are
a) Pituitary gland
b) Thyroid gland
c) Parathyroid gland
d) Pancreas (Islets of Langerhans)
e) Adrenal gland (Adrenal cortex and Adrenal medulla)
f) Gonads(Testes and Ovary)
g) Thymus gland
The pituitary gland or hypophysis is a pea shaped compact mass of cells located at the base of the midbrain attached to the hypothalamus by a pituitary stalk. The pituitary gland is anatomically composed of two lobes and perform different functions.
They are the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The intermediate lobe is non-existent in humans.
The pituitary gland forms the major endocrine gland in most vertebrates. It regulates and controls other endocrine glands and so is called as the “Master gland”.
The anterior pituitary is composed of different types of cells and secrete hormones which stimulates the production of hormones by other endocrine glands. The hormones secreted by anterior pituitary are
a) Growth Hormone
b) Thyroid stimulating Hormone
c) Adrenocorticotropic Hormone
d) Gonadotropic Hormone which comprises the Follicle Stimulating Hormone and Luteinizing Hormone
GH promotes the development and enlargement of all tissues of the body. It stimulates the growth of muscles, cartilage and long bones. It controls the cell metabolism.
The improper secretion of this hormone leads to the following conditions.
Dwarfism: It is caused by decreased secretion of growth hormone in children. The characteristic features are stunted growth, delayed skeletal formation and mental disability.
Gigantism: Oversecretion of growth hormone leads to gigantism in children. It is characterised by overgrowth of all body tissues and organs. Individuals attain abnormal increase in height.
Acromegaly: Excess secretion of growth hormone in adults may lead to abnormal enlargement of head, face, hands and feet.
TSH controls the growth of thyroid gland, coordinates its activities and hormone secretion.
ACTH stimulates adrenal cortex of the adrenal gland for the production of its hormones. It also influences protein synthesis in the adrenal cortex.
The gonadotropic hormones are follicle stimulating hormone and luteinizing hormone which are essential for the normal development of gonads.
Follicle stimulating hormone (FSH)
In male, it stimulates the germinal epithelium of testes for formation of sperms. In female it initiates the growth of ovarian follicles and its development in ovary.
Luteinizing hormone (LH)
In male, it promotes the Leydig cells of the testes to secrete male sex hormone testosterone. In female, it causes ovulation (rupture of mature graafian follicle), responsible for the development of corpus luteum and production of female sex hormones estrogen and progesterone.
PRL is also called lactogenic hormone. This hormone initiates development of mammary glands during pregnancy and stimulates the production of milk after child birth.
Hormones secreted by the posterior lobe (Neurohypophysis) of pituitary
The hormones secreted by the posterior pituitary are
1. Vasopressin or Antidiuretic hormone
In kidney tubules it increases reabsorption of water. It reduces loss of water through urine and hence the name antidiuretic hormone.
Deficiency of ADH reduces reabsorption of water and causes an increase in urine output (polyuria). This deficiency disorder is called Diabetes insipidus.
It helps in the contraction of the smooth muscles of uterus at the time of child birth and milk ejection from the mammary gland after child birth.
The thyroid gland is composed of two distinct lobes lying one on either side of the trachea. The two lobes are connected by means of a narrow band of tissue known as the isthmus. This gland is composed of glandular follicles and
lined by cuboidal epithelium.The follicles are fsilled with colloid material called thyroglobulin.
An amino acid tyrosine and iodine are involved in the formation of thyroid hormone. The hormones secreted by the thyroid gland are
a. Triiodothyronine (T3)
b. Tetraiodothyronine or Thyroxine (T4)
The functions of thyroid hormones are
· Production of energy by maintaining the Basal Metabolic Rate (BMR) of the body.
· Helps to maintain normal body temperature.
· Influences the activity of central nervous system.
· Controls growth of the body and bone formation.
· Essential for normal physical, mental and personality development .
· It is also known as personality hormone.
· Regulates cell metabolism.
When the thyroid gland fails to secrete the normal level of hormones, the condition is called thyroid dysfunction. It leads to the following conditions
It is caused due to the decreased secretion of the thyroid hormones. The abnormal conditions are simple goitre, cretinism and myxoedema.
It is caused due to the inadequate supply of iodine in our diet. This is commonly prevalent in Himalayan regions due to low level of iodine content in the soil. It leads to the enlargement of thyroid gland which protrudes as a marked swelling in the neck and is called as goitre.
It is caused due to decreased secretion of the thyroid hormones in children. The conditions are stunted growth, mental defect, lack of skeletal development and deformed bones. They are called as cretins.
It is caused by deficiency of thyroid hormones in adults. They are mentally sluggish, increase in body weight, puffiness of the face and hand, oedematous appearance.
It is caused due to the excess secretion of the thyroid hormones which leads to Grave’s disease. The symptoms are protrusion of the eyeballs (Exopthalmia), increased metabolic rate, high body temperature, profuse sweating, loss of body weight and nervousness.
The parathyroid glands are four small oval bodies that are situated on the posterior surface of the thyroid lobes. The chief cells of the gland are mainly concerned with secretion of parathormone.
The parathormone regulates calcium and phosphorus metabolism in the body.They act on bone, kidney and intestine to maintain blood calcium levels.
The secretion of parathyroid hormone can be altered due to the following conditions.
Removal of parathyroid glands during thyroidectomy (removal of thyroid) causes decreased secretion of parathormone. The conditions are
· Muscle spasm known as Tetany (sustained contraction of muscles in face, larynx, hands and feet).
· Painful cramps of the limb muscles.
Pancreas is an elongated, yellowish gland situated in the loop of stomach and duodenum. It is exocrine and endocrine in nature. The exocrine pancreas secretes pancreatic juice which plays a role in digestion while, the endocrine portion is made up of Islets of Langerhans.
The Islets of Langerhans consists of two types of cells namely alpha cells and beta cells. The alpha cells secrete glucagon and beta cells secrete insulin.
A balance between insulin and glucagon production is necessary to maintain blood glucose concentration.
· Insulin helps in the conversion of glucose into glycogen which is stored in liver and skeletal muscles.
· It promotes the transport of glucose into the cells.
· It decreases the concentration of glucose in blood.
· Glucagon helps in the breakdown of glycogen to glucose in the liver.
· It increases blood glucose levels.
The deficiency of insulin causes Diabetes mellitus. It is characterised by
· Increase in blood sugar level (Hyperglycemia).
· Excretion of excess glucose in the urine (Glycosuria).
· Frequent urination (Polyuria).
· Increased thirst (Polydipsia).
· Increase in appetite (Polyphagia).
The adrenal glands are located above each kidney. They are also called supra renal glands.
The outer part is the adrenal cortex and the inner part is the adrenal medulla. The two distinct parts are structurally and functionally different.
The adrenal cortex consists of three layers of cells. They are zona glomerulosa, zona fasciculata and zona reticularis
The hormones secreted by the adrenal cortex are corticosteroids. They are classified into
The glucocorticoids secreted by the zona fasciculata are cortisol and corticosterone
· They regulate cell metabolism.
· It stimulates the formation of glucose from glycogen in the liver.
· It is an anti-inflammatory and anti-allergic agent.
The mineralocorticoids secreted by zona glomerulosa is aldosterone
· It helps to reabsorb sodium ions from the renal tubules.
· It causes increased excretion of potassium ions.
· It regulates electrolyte balance, body fluid volume, osmotic pressure and blood pressure.
The adrenal medulla is composed of chromaffin cells .They are richly supplied with sympathetic and parasympathetic nerves.
It secretes two hormones namely
a. Epinephrine (Adrenaline)
b. Norepinephrine (Noradrenaline)
They are together called as “Emergency hormones”. It is produced during conditions of stress and emotion. Hence it is also referred as “flight, fright and fight hormone”.
· It promotes the conversion of glycogen to glucose in liver and muscles.
· It increases heart beat and blood pressure.
· It increases the rate of respiration by dilation of bronchi and trachea.
· It causes dilation of the pupil in eye.
· It decreases blood flow through the skin.
Most of its actions are similar to those of epinephrine.
The sex glands are of two types the testes and the ovaries. The testes are present in male, while the ovaries are present in female.
Testes are the reproductive glands of the males. They are composed of seminiferous tubules, Leydig cells and Sertoli cells. Leydig cells form the endocrine part of the testes. They secrete the male sex hormone called testosterone.
· It influences the process of spermatogenesis.
· It stimulates protein synthesis and controls muscular growth.
· It is responsible for the development of secondary sexual characters (distribution of hair on body and face, deep voice pattern, etc).
The ovaries are the female gonads located in the pelvic cavity of the abdomen. They secrete the female sex hormones
Estrogen is produced by the Graafian follicles of the ovary and progesterone from the corpus luteum that is formed in the ovary from the ruptured follicle during ovulation.
· It brings about the changes that occur during puberty.
· It initiates the process of oogenesis.
· It stimulates the maturation of ovarian follicles in the ovary.
· It promotes the development of secondary sexual characters (breast development, high pitched voice etc).
· It is responsible for the premenstrual changes of the uterus.
· It prepares the uterus for the implantation of the embryo.
· It maintains pregnancy.
· It is essential for the formation of placenta.
Thymus is partly an endocrine gland and partly a lymphoid gland. It is located in the upper part of the chest covering the lower end of trachea. Thymosin is the hormone secreted by thymus.
· It has a stimulatory effect on the immune function.
· It stimulates the production and differentiation of lymphocytes.