Chapter Outline
11.1 Endocrine glands and hormones
11.2 Human endocrine system
11.3 Hypo and hyper activity of endocrine glands and related disorders
11.4 Mechanism of hormone action.
Learning Objectives:
• Understands the positions of the various endocrine glands and their secretions.
• Learns the mechanism of hormone action.
• Understands the disorders related to hypo and hyper activity of the endocrine glands.
• Learns the role of gastro intestinal hormones.
While hearing your test marks, some may have anxiety and some may hesitate to hear and some may be worried. Do you know the reasons for such immediate changes? While seeing any unexpected happenings, we get goose bumps. Do you know the reason, why? These are all due to the biochemical changes happening in our body, Which are created by the endocrine system. The Klotho an anti-aging hormone makes people smart enhances cognitive abilities and longevity. above mentioned biochemical changes are due to the hormone adrenalin (flight, fright and fight hormone).
11.1 Endocrine glands and hormones
Physiological functions of our human body is regulated and coordinated by both neural and endocrine systems. The endocrine system influences the metabolic activities by means of hormones (hormone means to excite) which are chemical messengers released into the blood and circulated as chemical signals and acts specifically on certain organs or tissues called target organs or target tissues. Hormones may speed up or slow down or alter the activity of the target organs. The hormones secreted do not remain permanently in the blood but are converted by the liver into inactive compounds and excreted by the kidneys.
maintain homeostasis. Hormones are composed of water soluble proteins or peptides or amines or fat soluble steroids.
11.2 Human endocrine system
There are two glandular systems such as the exocrine glands and the endocrine glands. The exocrine glands secrete enzymes, saliva and sweat and have ducts that carry their substances to the membrane surfaces. Example: salivary
gland and gastric gland. The endocrine glands, called ductless glands produce hormones and lack ducts; they release their hormone to the surrounding tissue fluid. The hormones circulate around the body and eventually reach the target organs. Endocrine glands (Figure: 11. 1) include the pituitary,thyroid, parathyroid, pineal, adrenal,thymus and are also known as exclusive endocrine glands. The hypothalamus along with its neural function also produces hormones and is considered as a neuro endocrine gland. In addition several organs such as pancreas, gastro intestinal tract epithelium, kidney,heart, gonads and placenta are also have endocrine tissues and are known as partial endocrine glands.
11.2.1 Hypothalamus
Hypothalamus is a small cone shaped structure that projects downward from the brain ending into the pituitary stalk.It interlinks both the nervous system and endocrine system. Though pituitary gland is known as master endocrine glands that controls the other endocrine glands, but it is, in turn controlled by the hypothalamus. Hypothalamus contains groups of neurosecretory cells.It produces neurotransmitters which regulate the secretions of the pituitary (Figure11. 2). The hormones produced by the hypothalamus act either as a releasing hormone or as an inhibitory hormone.
In the basal region of the brain, the hypothalamic hypophyseal portal blood vessel connects hypothalamus and anterior pituitary. It allows hypothalamic hormones to control anterior pituitary secretion. The posterior pituitary is connected with hypothalamus by a nerve bundle called hypothalamic hypophyseal axis. It produces nerve signal that control the posterior pituitary secretion.Hypothalamus maintains homeostasis,blood pressure, body temperature, cardio and fluid electrolyte balance of the body. As the part of limbic system it influences various emotional responses.
11.2.2 Pituitary gland or Hypophysis
The pituitary gland (means to grow under) is ovoid in shape and is located in the sella turcica, a bony cavity of the sphenoid bone at the base of brain and connected to the hypothalamic region of the brain by a stalk called infundibulum. It is about one centimetre in diameter and 0.5 gm in weight. The pituitary consists of two lobes, anterior glandular adenohypophysis and posterior neural neurohypophysis.The anterior lobe originates from the embryonic invagination of pharyngeal epithelium called Rathke’s pouch and the posterior lobe is originates from the base of the brain as an outgrowth of hypothalamus. Anatomically the adenohypophysis has three lobes or zones namely pars intermedia, pars distalis and pars tuberalis. The neurohypophysis is otherwise known as pars nervosa.The anterior lobe of pituitary secretes six tropic hormones such as growth hormone (GH), thyroid stimulating hormone (TSH), adreno corticotropic hormone (ACTH), follicle stimulating hormone (FSH), luteinizing hormone (LH), luteotropic hormone (LTH) and melanocyte stimulating hormone (MSH) (in lower animals only). The posterior lobe of pituitary secretes the hormones namely vasopressin and oxytocin.
