Chemical Coordination and Integration
The neural system and the endocrine system together coordinate and regulate thephysiological functions in the body.
Endocrine system is composed of endocrine glands, which are ductless glands.
They secrete chemicals called hormones. Endocrine system is composed of hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, and gonads (testis in males and ovary in females).
Hypothalamus contains several groups of neurosecretory cells called nuclei, which secrete hormones.These hormones are of two types:Releasing hormonesand inhibiting hormones.These hormones reach the pituitary gland through hypophyseal portal vein and through certain axons to the posterior pituitary.
The pituitary gland is located in a sellatursica and is attached to the hypothalamus by an infundibulum.It is divided into an adenohypophysis and a neurohypophysis.
The two portions of adenohypophysis are pars distalis and pars intermedia.
• Pars distalis produces growth hormone (GH), prolactin (PRL), thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinizing hormone (LH), and follicle stimulating hormone (FSH)
• Over-secretion of GH leads to gigantism while, low secretion of GH results in dwarfism.
• ACTH stimulates the synthesis and secretion of glucocorticoids from the adrenal cortex.
• LH and FSH stimulate gonadal activity and hence are called gonadotrophins.
• Pars intermedia secrete only one hormone called melanocyte stimulating hormone (MSH), which acts on the melanocytes.
Neurohypophysisstores and releases two hormones: oxytocin and vasopressin.
Pineal glandsecretes a hormone called melatonin.
Thyroid gland is composed of follicles and stromal tissues or parafollicular tissues.
The follicular cells synthesise two hormones:Thyroxine or tetraiodothyronine(T4) andTriiodothyronine(T3).
The rate of the synthesis and secretion of the thyroid hormones may become abnormal that leads to following conditions:
Hypothyroidism and hyperthyroidism.
The parathyroid glands produce parathyroid hormone.
Thymus secretes peptide hormones called thymosins.
Adrenal medulla secretes hormones called adrenaline or epinephrine and Noradrenaline or norepinephrine.
They areemergency hormones or hormones of fight.
The adrenal cortex can be divided into three layers: zonareticularis, zonafasciculata and zonaglomerulosa.
It secretes many hormones commonly called as corticoids (like glucocorticoids).
Mineralocorticoid hormones regulate salt and water balance in the body.
Pancreas is an exocrine gland that secretes pancreatic juice and insulin.
Endocrine pancreas consists of ‘Islets of Langerhans’, which have two main types of cells: α-cells and β-cells.
The α-cells secrete glucagons while, β-cells secrete insulin.
Testis produces sperm and male hormones.
The interstitial cells of Leydigproduce androgens, mainly testosterone.
Ovaries produce ova and in vertebrates, estrogen and progesterone.
Hormones of Heart:
The atrial wall of our heart secretes atrial natriuretic factor (ANF).
Hormones of Kidney:
The juxtaglomerular cells of kidney produce an erythropoietin hormone.
Hormones of GI tract:
The endocrine cells present in different parts of the gastro-intestinal tract secrete four major peptide hormones.
They are: Gastrin, secretin, Cholecystokinin (CCK) and Gastric inhibitory peptide (GIP)
On the basis of their chemical nature, groups of hormones are:
• Peptide, polypeptide, protein hormones
• Amino-acid derivatives