Topic 2 Endocrine System

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X INTRODUCTIONIn this topic, we will proceed with the discussion on the endocrine system. Theendocrine system executes various physiological processes through chemicalmessengers called hormones. The endocrine system is a collection of glands thatsecrete hormones which are necessary for normal bodily functions. Theendocrine system, as shown in Figure 2.1, comprises the pituitary, parathyroid,thyroid, adrenals, pancreas and ovaries or testes. The hypothalamus in the brainreceives information from nerve cells about both the internal condition of the

body and the external environment. It then responds to those conditions over the

endocrine system by using the pituitary gland to relay directions to other glands.

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EndocrineSystem

By the end of this topic, you should be able to:

1. Explain the role of hormones in humans;

2. Describe animal hormones and their functions;

3. Identify plant hormones and their functions; and

4. Differentiate between animal and plant hormones.

LEARNING OUTCOMES


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TOPIC 2 ENDOCRINE SYSTEM W 33

Figure 2.1: The endocrine systemSource: http://www.drstandley.com

ROLE OF HORMONES IN HUMANS

An endocrine gland or ductless gland is a group of specialised cells that producesand secretes hormones into the bloodstream. Only small concentrations ofhormones are needed for their effectiveness. Table 2.1 lists several endocrineglands and some of their secreted hormones which help to maintain homeostasiswithin the human body.

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X TOPIC 2 ENDOCRINE SYSTEM34

Table 2.1: Major Human Endocrine Glands and Their Secreted HormonesSite of secretion Hormone

Pituitary gland ADH, Oxytocin, GH, TSH, FSH, Prolactin

Thyroid gland Thyroid hormone

Parathyroid gland Parathyroid hormone

Adrenal Adrenalin

Pancreas Insulin, Glucagon

Ovaries Estrogens, Progesterone

Testes Testosterone

Endocrine glands are located in strategic areas around the body. Their hormonesregulate a vast range of activities including blood glucose concentration, heartrate, metabolism, growth rate, reproduction and water balance.

To further our understanding of endocrine glands and hormones, let us take alook at the following scenario.

Fight or FlightImagine that you suddenly see a tiger in a jungle. What will you do? Do youwant to fight with the tiger? Maybe you should run as fast as you can, awayfrom the tiger? Do you want to fight or take flight? Whatever your decision, ithas something to do with interaction between the endocrine system andnervous system. Impulses are sent through the nervous system so the adrenalglands get the message to secrete adrenaline hormone. Consequently, the

breathing and heart rate increase which causes the transportation of oxygenand glucose into the body cells to become rapid. Eventually, more blood ispumped to the muscles and the brain. As a result, a quick decision can bemade by the brain, and there is an increase in energy production and bloodpressure.


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ANIMAL HORMONES

Although hormones will reach the entire parts of the body through blood

circulation, each hormone will have its own target organ. Due to this, eachhormone has its own specific function. Lets take a look at Table 2.2 on animalsmain hormones and their functions.

Table 2.2: Main Animal Hormones and Their FunctionsHormone FunctionsAntidiuretichormone (ADH)

x Promotes reabsorption of water by kidneys

Oxytocin x Stimulates contraction of uterus

x Stimulates milk flow from the mammary glands

Growth hormone(GH)

x Stimulates growth

x Stimulates metabolic functions

Thyroidstimulatinghormone (TSH)

x Regulates growth of thyroid glands and its hormone secretion

Folliclestimulatinghormone (FSH)

x Stimulates production of ova and sperm

x Stimulates estrogens secretion in ovaries

x Stimulates testosterone secretion in testes

Prolactin x Stimulates mammary glands to secrete milk

Thyroid hormone x Stimulates and maintain metabolic processes

Parathyroidhormone

x Raises blood calcium level

x Regulates bone formation

Adrenaline x Prepares body for action such as increasing blood glucose, heartrate and metabolic rate

Insulin x Lowers blood glucose concentration

Glucagon x Raises blood glucose concentration

Estrogens x Promote development and maintenance of female secondary sexcharacteristics

x Stimulate uterine lining growth

x Control menstrual cycle and pregnancy

Progesterone x Controls menstrual cycle and pregnancy

Testosterone x Promotes development of testes and male secondary sexcharacteristics

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Study, think and matchthese keywords!Get more information by borrowing books in the library or surfing theInternet to group these keywords. These keywords represent endocrineglands, hormones and functions of those hormones. Then, summarisethem in the table below.

Endocrine gland Hormone Function

ACTIVITY 2.1


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PLANT HORMONES

Plant hormones are also known as plant growth substances. Unlike animalhormones, plant hormones are not manufactured in special organs. Instead, theyare made by cells in many different parts of the plant. They regulate manyaspects of plant growth and development, from seed formation and germinationto the maturity and death of the plant. There are five major types of planthormones: auxins, gibberellins, cytokinins, abscisic acid and ethene. Table 2.3

below provides a brief explanation of their respective functions.

Table 2.3: Plant Hormones and Their FunctionsHormone Production Site and Functions

Auxins Synthesised in the apical meristems to promote primary growthby increasing the rate of cell elongation.

Gibberellins Formed in young leaves to stimulate the growth of shoots andleaves.

Cytokinins Synthesised in the roots and transported to other parts of theplant. When combined with auxins, it stimulates cell division.

Abscisic acid Synthesised in chloroplast to promote dormancy in some seedsand stimulates the closing of stomata.

Ethene/Ethylene Released from ripening fruit, nodes of stems, ageing leaves and

flowers. Involved in seed dormancy, fruit ripening and leafabscission.

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ACTIVITY 2.2

Test your knowledge on the major types of plant hormones bymatching the hormones below to each function.

Auxin Ethylene Cytokinins

Function HormonesRipen fruit

Shoot growth

Increase stem length


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Endocrine system is a collection of glands that secretes hormones, which arenecessary for normal human bodily functions such as digestion, metabolism,growth, reproduction, heart rate and water balance.

Plant hormones act either to stimulate or to inhibit certain parts of plants.

Animal hormones

Endocrine gland

Endocrine system

Plant hormones

Kent, M. (2000). Advance biology. New York: Oxford University Press.

Mader, S. S. (2010). Biology. New York: McGraw-Hill.

SELF-CHECK 2.1

Describe the role of hormones in the cycle of a girls life until shebecomes a woman, gets married and gives birth.

Topic 2 Endocrine System

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