The endocrine system in humans, constitute the endocrine glands & hormones producing diffuse tissue/ cells located in different parts of our body.
The endocrine glands are ductless glands ie. lack ducts. The secretion of these glands is called hormones.
They pour their secretion into the surrounding blood for transport to the site of action or distantly located target organ.
Types of Endocrine Glands
- Pure endocrine glands (Pituitary etc.)
- Partial endocrine glands:- (Kidney, liver etc.)
Structure & functions of major endocrine glands:-
- Located in basal part of diencephalon (forebrain).
- Contains several groups of neuro secretory cells which secrete hormones that regulate the synthesis and secretion of pituitary hormones.
It stimulates the secretion of pituitary hormones. For ex- Gonadotropin releasing hormone (GnRH). It stimulates the synthesis & release of gonadotropins (FSH & LH) from the anterior pituitary.
Inhibit the release of growth hormone from the pituitary i.e., regulate the function of anterior pituitary. For ex- Somatostatin.
- Located in the bony cavity called "sella- tursica" & attached to the hypothalamus by a stalk.
Types of pituitary:-
Adeno- Hypophysis (Anterior pituitary):
It consist of two portions called pars distalis and pars- intermedia.
Hormones from Pars- Distalis:-
Hormone from pars intermedia:
- Growth Hormones (GH)
- Prolactin (PRL)
- Thyroid stimulating hormone (TSH)
- Adreno Cortico trophic hormone (ACTH)
- Luteinizing hormone (LH)
- Follicle stimulating hormone (FSH)
Melanocyte stimulating hormone. (MSH)
Release two hormones i.e.,. Oxytocin and Vasopressin which are actually synthesized by hypothalamus & transported to neurohypophysis.
Excess secretion of growth hormone in adults mainly in middle age can result in severe disfigurement called acromegaly, which may lead to serious complications and premature death if unchecked.
An impairment affecting synthesis or release of ADH results in diminished ability of the kidney to conserve water, leading to water loss and dehydration. This condition is known as Diabetes Insipidus.
Pancreas (Composite Gland)
Both exocrine & endocrine
The endocrine part of the pancreasis called islets of langerhans, which contain two types of cells-
$\alpha$ - cells
$\alpha$cells secrete glucagon while $\beta $cells secretes insulin.
- It is hyperglycemic hormone i.e., Increases the level of glucose in the blood.
- Act on hepatocytes (liver cells) & stimulates glycogenolysis.
- Stimulates gluconeogenesis i.e., Synthesis of glucose from amino acids.
- Reduces glucose uptake & utilization by cells.
- It is hypoglycemic ie. reduces the level of glucose in the blood.
- Stimulates uptake & utilization of glucose by tissue cells.
- Acts on hepatocytes & stimulates glycogenesis i.e., the conversion of glucose into glycogen.
- Prevents glycogenolysis.
Deficiency of insulin causes hyperglycemia which leads to diabetes mellitus. Its symptoms are:-
- Loss of glucose through urine.
- Formation of Ketone bodies.
- Excess thirst etc.
It has two types of tissues:-
Hormones from Adrenal Medulla
- Inner centrally located Adrenal medulla.
- Outer Adrenal cortex.
- Adrenaline or epi- nephrine
- Nor- Adrenaline or Nor- epihephrine
- These are commonly called as catecholamines.
- They are secreted in response to stress during emergency situations & are called emergency hormones or hormones of fight or flight.
Hormones from Adrenal cortex:-
- These hormones increases alertness, dilation of pupil, sweating, piloerection ( raising of hairs) etc.
- Also increase heart- beat rate, strength of heart contraction & the rate of respiration.
- Stimulate breakdown of fats, proteins & glycogen into glucose.
Adrenal cortex can be divided into three layers:-
- Inner zona reticularis
- Middle zona fasciculata
- Outer zona glomerulosa
Hormones by the adrenal cortex are collectively called corticoids.
Located on the dorsal side of forebrain.
It plays a very important role in the regulation of 24 hour rhythm of our body i.e., it helps in maintaining sleep-wake cycles (Diurnal), body temperature, pigmentation & menstruation cycle.
In addition, it also influences metabolism as well as a defence capability.
It is located on either side of the trachea and composed of follicular cells and stromal tissues. It is connected with a thin flap of connective tissue called Isthmus.
Thyroid Gland is composed of follides & stromal tissues.
The follicle cells synthesis two hormones:-
Functions of Thyroid Hormones:-
- Thyroxine or Tetraiodothyronine (T4)
- Triiodothyronine (T3)
Hormones from Stromal tissues:
- Regulate basal metabolic rate.
- Control the metabolism of carbohydrates, fats & proteins.
- Influence the maintenance of water & electrolyte balance.
- Support the process of RBC formation.
Stromal cells (Para follicle cells) secretes thyrocalcitonin (TCT). When the level of calcium ions (Ca2+) in plasma increases, it regulates the blood calcium level & is called hypocalcemic hormone.
Role of iodine in thyroid hormone:-
Iodine is the main component of thyroid hormone. It’s deficiency results in hypothyroidism which leads to goitre i.e., the enlargement of the thyroid gland.
hypothyroidism during pregnancy or early childhood result in disorder called Cretinism that is stunted growth, mental retardation, abnormal and low IQ (intelligence quotient).
Abnormally, high secretion of thyroid hormone leads to hyperthyroidism due to cancer or formation of nodules in thyroid gland.
Graves' disease: It is an example of hyperthyroidism or exophthalmic goitre that is enlargement of the thyroid gland, protrusion of eyeball, increase BMR and weight loss.
They are four small sized parathyroid gland present on the back side of the of the thyroid gland, one pair each in the two lobes of the thyroid gland.
Parathyroid hormone, a peptide hormone that increases the level of calcium ions (Ca2+) in the blood and are called as hypercalcemic hormone. Along with thyrocalcitonin, it plays an important role in the calcium balance in the body.
It is located on the dorsal side of the heart and aorta.