Zoology Assignment Fourth Semester 2018 Submitted To: Dr. Amrit Kaur Bansal Submitted By: Himanshu Goyal 

 

Thyroid Gland 

Location 

The thyroid gland surrounds the front of the larynx and upper part of the trachea in the 

neck. 

 

 

Structure 

● Thyroid is the largest endocrine gland and weighs about 28 gram. It is bilobed, 

highly vascular organ. 

● The two lobes are connected by a narrow band, the isthmus. 

● The gland is composed of a irregularly rounded, about 0.1 mm wide follicles held 

together by connective tissue very rich in blood capillaries. 

● A follicle has a wall of cuboidal epithelium resting on a basement membrane and 

is filled with a clear, gelatinous colloid. 

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● The follicular cells become columnar and bear microvilli on the free surface when 

the gland is activated by thyroid stimulating hormone. 

 

Physiology 

The thyroid gland produces the hormones- thyroxine (T4) and Triiodothyronine (T3), 

which regulate metabolic body processes, cellular respiration, Total energy expenditure, 

growth, and maturation of tissues, and turnover of hormones substrates, and vitamins. 

Thyroperoxidase (TPO) is one of the primary enzyme produced in the thyroid. It is 

synthesized within the endoplasmic reticulum of thyrocyte and oxidizes iodine, thereby 

facility the formation of T3 and T4. 

Release of hormones into the bloodstream involve the negative feedback system of the 

hypothalamic-pituitary-thyroid axis. A low metabolic rate or a decrease in serum T3 

and/or T4 level signals the hypothalamus to secrete thyrotropin releasing hormone 

(TRH), which travels to the anterior pituitary gland and stimulate secretion of thyroid 

stimulating hormone (TSH). TSH, in turn, stimulates the thyroid gland to manufacture 

and release T3 and T4 until the metabolic rate is normalised. An elevated T3 serum level 

inhibits the release of TRH and TSH. 

 

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Parathyroid 

Location 

Parathyroid glands are situated in the back of the thyroid. 

 

Structure 

● The parathyroids are four small, rounded glands, two on each side. 

● The cells of parathyroids are arranged in compact mass, not in follicles, like those 

of thyroid. 

● Healthy parathyroid glands generally weigh about 30 mg in men and 35 mg in 

women. 

 

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Physiology 

The parathyroid secrete a single hormone called parathormone (PTH), also called the 

Collip’s hormone after its discoverer. It is a long peptide of 84 amino acid units. 

Parathormone and thyroid hormone calcitonin work antagonistically to regulate the 

calcium-phosphorus balance in the blood. The release of parathormone increases blood 

calcium to normal by drying calcium from the bones into the plasma, by increasing 

calcium absorption in the digestive tract, and by reducing loss of calcium in the urine. It 

lowers blood phosphate by increasing elimination of phosphate in the urine. Proper 

calcium-phosphorus balance is necessary for the growth of bones and teeth. Calcium is 

vital for blood clotting, for muscle tone and for normal nervous activity. It is also needed 

for the activities of many enzymes. 

Parathyroids are under the feedback control of blood calcium level. A fall in blood 

calcium level stimulates them to secrete parathormone which restores blood calcium 

level. A rise in blood calcium level inhibits secretion of parathormone. 

Adrenal Glands 

Location 

The adrenal glands, also called suprarenal glands, are paired glands placed in front of 

the upper parts of the kidney. 

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Structure 

● The adrenals are conical, yellowish bodies, each weighing about 5 gram.  

● Each gland has two distinct regions, having different embryonic origin, structure 

and function. 

● The two regions are called adrenal cortex and adrenal medulla, both produce 

different hormones. 

● The adrenal cortex is an external firm, pale-yellowish-pink structure covered by a 

fibrous capsule. It forms 80% of the gland. It is formed from mesoderm. 

● The adrenal medulla is an internal soft, dark-reddish-brown tissue drive from the 

ectoderm. It consists of strands of cells surrounded by blood capillaries. 

 

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Physiology 

Adrenal Cortex: 

● Mineralocorticoids: These hormones are secreted by the outer region of adrenal 

cortex. They regulate mineral metabolism. Their secretion is stimulated by a fall 

in the sodium concentration in the Plasma or in the volume of blood. The major 

mineralocorticoid, aldosterone tends to increase the reabsorption of sodium 

from urine, saliva, bile and sweat to reduce its loss from the body. 

● Glucocorticoids: These hormones are secreted by the middle region of adrenal 

cortex. They regulate the metabolism of Carbohydrates, proteins and fats. They 

increase the blood glucose level by converting proteins and fats into 

carbohydrates which are in turn converted to glucose. 

● Sexcorticoids: These hormones are secreted by both the middle and inner 

regions of the adrenal cortex. They include small amounts of both male and 

female sex hormones. Testosterone stimulates the development of male 

secondary sexual characters and oestrogen stimulates the appearance of female 

secondary sexual characters. 

Adrenal Medulla: Cells of adrenal medulla secrete two similar hormones: adrenaline 

(epinephrine) and noradrenaline (norepinephrine). The adrenal medulla enable the 

animal to face physical and emotional stress which require more energy which is 

provided by increasing Heartbeat, blood pressure, respiratory rate, sugar level of blood 

and blood supply of heart. The hormones also causes contraction of spleen to squeeze 

out the reserve supply of blood, decrease in clotting time of blood and dilation of Pupil. 

The above role of the two hormones is often called “Fight or Flight" reaction. Because of 

the above role of their hormones, the adrenals are known as the “glands of emergency”. 

 

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Hypothalamus 

Location 

Hypothalamus is the base of the diencephalon, a part of the forebrain. 

 

Structure 

● The hypothalamus is composed of nervous tissue. 

● It comprises many masses of grey matter, called hypothalamic nuclei scattered in 

white matter. 

● It is connected with the anterior lobe of the pituitary gland by hypophyseal portal 

blood vessels and with the posterior lobe of the pituitary gland by axons of its 

neurons. 

● The portal vessels and axons pass through a stalk to the pituitary lobes. 

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Physiology 

Neurosecretory cells of the hypothalamus secrete hormones called neurohormones. 

These are short peptides, having 3 to 14 amino acids. The neurohormones are carried 

by the portal blood to the anterior lobe of the pituitary gland and stimulate the 

hypothalamus to release its hormones. On this account, such hypothalamic hormones 

are also called releaser hormones (RH). Certain hypothalamic hormones inhibit the 

secretion of some pituitary hormones. These are termed inhibitory hormones the 

well-known releasing and inhibitory hormones (IH). The well known releasing and 

inhibitory hormones of hypothalamus are given below: 

● Thyrotropin-Releasing hormone (TRH) 

● Adrenocorticotropic-Releasing Hormone 

● Follicle-Stimulating hormone-Releasing hormone (FSH-RH) 

● Luteinizing hormone-Releasing hormone (LH-RH) 

● Growth hormone-Releasing hormone (GHRH) 

● Growth hormone Release-Inhibiting Hormone (GH-RIH) 

● Prolactin-Releasing hormone (PRH) 

● Prolactin release-Inhibiting Hormone (PR-IH) 

● Melanocyte-Stimulating hormone-Releasing Hormone (MSH-RH) 

● Melanocyte-Stimulating hormone Release-Inhibiting Hormone (MSHR-IH) 

 

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Hypothalamus is able to control endocrine secretion because it can monitor metabolites 

and hormone levels in the blood. The information so gathered and also the information 

reaching from other parts of the brain is passed to the pituitary via releasing hormones. 

The hypothalamic-pituitary system is of paramount importance for homeostasis as it 

regulate the most major physiological activities in the body. 

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