Endocrine Gland And Hormones

Ernest Starling was the first person who coined the term “Hormone”.

Definition of Endocrine Gland And Hormones:

The endocrine glands lack ducts and are hence 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.
Endocrine system in male and female

The endocrine system in male and female

Human Endocrine System

In the human body, the Endocrine system consists of the following glands:

  • Pituitary
  • pineal
  • thyroid
  • adrenal
  • pancreas- composite gland which acts as both exocrine and endocrine glands.
  • parathyroid
  • thymus and
  • gonads (testis in males and ovary in females)

***Some other organs also secrete hormones e.g. gastrointestinal tract, liver, kidney, and heart also produce hormones.

The Hypothalamus:

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 is divided into two partsadenohypophysis and neurohypophysis.

Pituitary gland- The master gland: Endocrine Gland And Hormones

Pituitary gland- The master gland

Adenohypophysis: It consists of two parts pars distalis-  It is commonly called the 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.

The action of Pituitary hormones and abnormalities caused by them:

The action, function, and abnormalities are given in the table below:

Pituitary hormones their actions and deficiencies: Endocrine Gland And Hormones

Pituitary hormones actions and deficiencies

The Pineal Gland

The pineal 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.

Thyroid Gland

The thyroid gland 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

Thyroid gland

Thyroid gland

Deficiency caused by Thyroid:

  • Deficiency of iodine in our diet results in hypothyroidism in which the thyroid gland is enlarged called Goitre.
  • In pregnant women, Hypothyroidism causes stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, and 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.
Parathyroid Gland

Two pairs of 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.

Thymus

The thymus is a lobular structure located between the 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 the thymus degenerates in an old person so the production of antibodies is reduced due to this immunity becomes weak.

Adrenal Gland

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 are secreted by the adrenal medulla and these are commonly called catecholamine.

During emergencies and in the situation of stress Adrenaline and noradrenaline are secret rapidly so these are called emergency hormones or hormones of Fight or Flight.

Adrenal gland

Adrenal gland

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. There are also two types:

  • glucocorticoids
  • mineralocorticoids

Glucocorticoids:

  • The corticoids involved in carbohydrate metabolism are 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

Mineralocorticoids:

  • These are corticoids that regulate the balance of water and electrolytes in our bodies.
  • In our body, Aldosterone is the main mineralocorticoid.
  • Aldosterone acts on renal tubules and stimulates the reabsorption of Na+ and water and the 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.

Pancreas

In the human body, Pancreas is a composite gland, which acts as both exocrine as well as 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.

Endocrine Gland And Hormones

pancreas – the composite gland, both endocrine and exocrine glad

Glucagon:

  • Glucagon hormone is secreted by alpha cells.
  • It is a peptide hormone that plays an important role in maintaining normal blood glucose levels acting on liver cells.
  • Glucagon reduces cellular glucose uptake and utilization. Thus, glucagon is known as a hyperglycaemic hormone.

Insulin:

  • Insulin is secreted by beta cells.
  • Insulin is a peptide hormone that helps in glucose homeostasis.
  • It acts mainly on hepatocytes and cells of adipose tissue (adipocytes) and enhances the 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 the formation of harmful compounds known as ketone bodies. Diabetic patients are successfully treated with insulin therapy.

Testis

In males, a pair of 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 and 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.
  • Testosterone stimulates the development of the male characters during puberty which starts developing facial hair, and their voice cracks and deepens.

Ovary:

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.

  • Estrogen:
    • It is 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.
  • Progesterone:
    • It is secreted by the corpus luteum (follicle which is left after the release of the ovum).
    • progesterone supports 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 the secretion of milk.

***Some hormones of other organs

A 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 is produced by the juxtaglomerular cells of the kidney. It stimulates erythropoiesis (formation of RBC).
  • The different parts of our gastrointestinal 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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