Endocrine Gland And Hormones
Ernest Starling was the first person who coined the term “Hormone”. The endocrine glands lack ducts and are hence are called ductless glands. The secretions of the ductless gland are called hormones.
Hormones can be scientifically* (new definition from NCERT) defined as Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
Hormones are secreted from their source directly into the blood. Blood carries the hormone to the specific target cells which respond to it. Hormones are produced in very small quantities and are biologically very active. For example, adrenaline is active even at a concentration of 1 in 300 million parts.
Human Endocrine System
In the human body, the Endocrine system consists-
- pancreas- composite gland which acts as both exocrine and endocrine gland.
- thymus and
- gonads (testis in males and ovary in females)
***Some other organs also secrete hormones e.g. gastrointestinal tract, liver, kidney, the heart also produces hormones.
The Hypothalamus is the basal part of the diencephalon, the forebrain which regulates many body functions having groups of neurosecretory cells called nuclei that produce hormones. The hormones produced by the hypothalamus regulate the synthesis and secretion of pituitary hormones. Hypothalamus produces two types of hormones:
- the releasing hormones (which stimulate secretion of pituitary hormones) Ex- Gonadotrophin releasing hormone (GnRH) stimulates the pituitary synthesis and release of gonadotrophins
- the inhibiting hormones (which inhibit secretions of pituitary hormones). Ex- somatostatin from the hypothalamus inhibits the release of growth hormone from the pituitary.
Pituitary — the master gland
The pituitary gland (also called hypophysis) is a small projection (about the size of a pea) located in a bony cavity called Sella tursica and is attached to the hypothalamus by a stalk. The pituitary divided in two parts– adenohypophysis and neurohypophysis.
Adenohypophysis: It consists of two parts pars distalis- It is commonly called anterior pituitary and pars intermedia – Pars intermedia secretes only one hormone called melanocyte-stimulating hormone (MSH). Acts on the melanocytes (melanin-containing cells) and regulates pigmentation of the skin
Neurohypophysis- Neurohypophysis (pars nervosa) also known as posterior pituitary, stores, and releases two hormones called oxytocin and vasopressin.
Pituitary hormones, their action, and abnormalities:
The action, function and abnormalities are given in table below:
The Pineal Gland
This gland is located on the dorsal side of the forebrain. The Pineal gland secretes a hormone called melatonin. Melatonin helps in maintaining the normal rhythms of the sleep-wake cycle, body temperature, and 24-hour (diurnal) rhythm of our body.
It is composed of two lobes located on either side of the trachea, interconnected with a thin flap of connective tissue called the isthmus. It secretes two hormones by follicular cells theses are- Tetra iodothyronine or thyroxine (T4) and Triiodothyronine (T3).
The thyroid gland also secretes a protein hormone called Thyrocalcitonin (TCT) which regulates the blood calcium levels
- Deficiency of iodine in our diet results in hypothyroidism in which the thyroid gland enlarged called Goitre.
- In pregnant women, Hypothyroidism causes stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism in babies.
- Increased to abnormally high levels leading to a condition called hyperthyroidism.
- A form of hyperthyroidism Exophthalmic goiter is characterized by enlargement of the thyroid gland, protrusion of the eyeballs, increased basal metabolic rate, and weight loss, it is also called Graves’ disease.
Two pair parathyroid glands are present on the backside of the thyroid gland, one pair in every two lobes of the thyroid gland. It secretes a peptide hormone called Parathyroid hormone (PTH). parathyroid hormone increases the Ca2+ levels in the blood so-called PTH is a hypercalcemic hormone.
It is a lobular structure located between lungs behind the sternum on the ventral side of the aorta. The thymus gland secretes the peptide hormones called Thymosins it plays a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity. It provides humoral immunity by promoting the production of antibodies. As thymus degenerates in an old person so the production of antibodies reduced due to this immunity becomes weak.
There is one pair of adrenal glands in our body. Located one at the anterior part of each kidney. Adrenal glands are composed of two types of tissues, centrally located tissue is called the adrenal medulla, and outside of this lies the Adrenal cortex.
Adrenal medulla– Adrenaline or epinephrine and noradrenaline or norepinephrine secreted by the adrenal medulla and these are commonly called as catecholamine. During emergency and in the situation of stress Adrenaline and noradrenaline secret rapidly so these are called emergency hormones or hormones of Fight or Flight.
Adrenal Cortex-. It can be divided into three layers –
- zona reticularis (inner layer)
- zona fasciculata (middle layer) and
- zona glomerulosa (outer layer
The adrenal cortex secretes many hormones, commonly termed corticoids. These are also of two types –
- The corticoids involved in carbohydrate metabolism called glucocorticoids.
- In our body the main glucocorticoid is cortisol. Glucocorticoids stimulate gluconeogenesis, lipolysis, and proteolysis, and inhibit cellular uptake and utilization of amino acids.
- maintaining the cardiovascular system as well as the kidney functions managed by cortisol.
- Cortisol produces anti-inflammatory reactions and suppresses the immune response.
- Cortisol stimulates the RBC production
- These are corticoids that regulate the balance of water and electrolytes in our body.
- In our body, Aldosterone is the main mineralocorticoid.
- Aldosterone acts on renal tubules and stimulates the reabsorption of Na+ and water and excretion of K+ and phosphate ions.
- In the maintenance of electrolytes, body fluid volume, osmotic pressure, and blood pressure aldosterone play an important role.
*** Adrenal cortex also secretes small amounts of androgenic steroids which play role in the growth of axial hair, pubic hair, and facial hair during puberty.
In the human body, Pancreas is a composite gland, which acts as both exocrine as well as and endocrine glands. As endocrine pancreas consists of ‘Islets of Langerhans’. There are two main types of cells in the Islet of Langerhans are called a-cells (Alpha cells) and b-cells (beta cells).
The glucose homeostasis in the blood is thus maintained jointly by the two –glucagon and Insulin.
- Glucagon hormone is secreted by alpha cells.
- It is a peptide hormone that plays an important role in maintaining the normal blood glucose levels acting on liver cells.
- Glucagon reduces cellular glucose uptake and utilization. Thus, glucagon is known as a hyperglycaemic hormone.
- Insulin is secreted by beta cells.
- Insulin is a peptide hormone and helps in glucose homeostasis.
- It acts mainly on hepatocytes and cells of adipose tissue (adipocytes) enhances uptake and utilization of cellular glucose.
- In the target cells, insulin stimulates the conversion of glucose to glycogen (glycogenesis).
*** Prolonged hyperglycemia can cause a complex disorder diabetes mellitus, it is associated with loss of glucose through urine and formation of harmful compounds known as ketone bodies. Diabetic patients are successfully treated with insulin therapy.
In males, a pair of the testis is present in the scrotal sac (outside abdomen). It performs both functions as a primary sex organ and an endocrine gland. The testis is made up of seminiferous tubules and stromal or interstitial tissue. It is present in the intertubular spaces produces a group of hormones called androgens mainly testosterone.
- Androgens steroidal hormone plays a major role in the process of spermatogenesis (formation of spermatozoa). Androgens act on the central neural system and influence male sexual behaviour.
- The testosterone stimulates the development of the male characters during puberty which starts developing facial hair, and their voice cracks and deepens.
Females have a pair of ovaries found in the abdomen. It is the primary female sex organ that produces one ovum during each menstrual cycle.
Ovaries in females produce two kinds of hormones—estrogen and progesterone these are of the steroidal group.
- Itis secreted by the follicles of the ovary.
- It stimulates the development of secondary sexual organs eg. breasts, the high pitch of voice, and fat deposition on the hip in a mature woman.
- Estrogen prepares the wall of the uterus for receiving the fertilized egg.
- Itis secreted by the corpus luteum (follicle which is left after the release of the ovum).
- progesterone supports a pregnancy.
- It brings about the final changes in the uterus for the retention and growth of the fetus during pregnancy.
- Progesterone also acts on the mammary glands and stimulates the formation of alveoli and secretion of milk.
***Some hormones of other organs
Few hormones are also secreted by some tissues which are not endocrine glands. Some are listed here-
- A very important peptide hormone called atrial natriuretic factor (ANF) is secreted by the atrial wall of our heart. It reduces blood pressure.
- A peptide hormone called erythropoietin produced by juxtaglomerular cells of the kidney. It stimulates erythropoiesis (formation of RBC).
- The different parts of our gastro-intestinal tract secrete four major peptide hormones, namely gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP).
- Gastrin acts on gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen.
- Secretin stimulates the secretion of water and bicarbonate ions acting on the exocrine pancreas.
- cholecystokinin (CCK) stimulates the secretion of pancreatic enzymes and bile juice, respectively acting on both the pancreas and gall bladder
- gastric secretion and motility are inhibited by gastric inhibitory peptides (GIP).
for more about the hormones click the link given – Chemical classification of hormones
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