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The Human Endocrine System

Hormonal coordination involves the endocrine system which is made up of endocrine glands and hormones. Hormonal coordination may work independently or together with nervous system.

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Hormones

Hormones are chemical substances secreted by endocrine glands. They are secreted in small quantities directly into the bloodstream. Usually, hormones are secreted in one place but used in places far from their origin. Although hormones are carried by the blood to all parts of the body, only targeted tissues or organs will respond to them.

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The roles played by hormones are: a) increasing or decreasing the rate of body process and an example is increasing the rate of heartbeat b) causing physical and physiological changes in the body c) influencing body growth. Excessive or insufficient amounts of specific hormones have undesirable effects on the health of an individual. Examples of hormones are thyroxin adrenaline, oestrogen, testosterone and insulin.

The Endocrine System

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Endocrine Glands

 Endocrine glands are also known as ductless glands. They secrete hormones directly into the bloodstream. The endocrine glands I in both males and females are the same except for the testis (male) and the ovary (female). The majority of endocrine glands are controlled by the pituitary gland but some are stimulated by the nervous system or by chemical substances.

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The main endocrine glands in our body are the pituitary gland, thyroid gland, adrenal gland, pancreas, ovary and testis. The positions of main endocrine glands in the human body are shown below:

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Pituitary Gland

The pituitary gland is a small gland located in the brain and also known as the “mater gland”. It controls the secretion of hormones by the other endocrine glands. The hormones secreted by the pituitary gland are: a) growth hormones b) hormones for the absorption of water c) hormones which stimulate other endocrine glands.

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Thyroid Gland

The thyroid gland is situated in the neck region, in front if the trachea and below the larynx. It secretes the hormone thyroxin which contains the element iodine. The functions of the thyroxin hormone are: a) controlling the metabolic rate b) regulating development especially of the nervous and skeletal tissues.

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Pancreas

The pancreas is situated below the stomach and is connected to the duodenum. It secretes two hormones which are involved in the control of blood sugar or glucose. One of the hormones is insulin. This hormone lowers blood glucose concentration by converting excess glucose in the blood into another substance called glycogen, which is stored in the liver. The other hormone secreted by the pancreas raises blood concentration.

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Adrenal Glands

The adrenal gland occurs in a pair, one on top of each kidney. Adrenal glands secrete many hormones, one of which is adrenaline. Adrenaline is usually secreted during times of excitement, fear or stress. The action of adrenaline is the opposite of insulin. It increases the blood sugar level by changing glycogen into glucose.

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Testes

Testes are part of the male reproductive system. Testes occur in a pair inside the scrotum located outside the male human body. Testes secrete the male sex hormone, testosterone which is responsible for the development of male secondary sexual characteristics during puberty.

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Ovaries

There is pair of ovaries located on either side of the uterus. Like the testes, ovaries are part of the female reproductive system. Ovaries secrete hormones such as oestrogen and progesterone which control female secondary sexual characteristics during puberty. Oestrogen and progesterone also control the menstrual cycle, the ovulation process and the development of the uterus. A summary of the various endocrine glands, hormones and their functions as shown below:

Endocrine glands	Hormones	Functions	Target cells/ organs
Thyroid gland	Thyroxin	*Controls the metabolic rate *Controls normal physical and mental growth and development	All body cells
Pancreas	Insulin	*Controls the blood glucose level *Stimulate the conversion of excess glucose	Muscles and liver
Adrenal glands	Adrenaline	*Controls the blood glucose level by stimulating the conversion of glycogen into glucose to increase the blood glucose level *Prepares the body for state of excitement, for example, increasing the metabolic rate, heartbeat and breathing *Controls the amount of mineral salts in blood	Various cells of the body, heart, respiratory system, blood capillaries, liver and kidneys
Testes	Testosterone	*Controls the development of male secondary sexual characteristics *Controls the production and development of sperms	Male sex organ
Ovaries	Oestrogen, Progesterone	*Control the development of female secondary sexual characteristics *Control the menstrual cycle, ovulation and changes in the thickness of the uterus wall	Female sex organs

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Effects of Hormonal Imbalance on Health

A hormone may act independently or together with other hormones to bring about a response. The production of each hormone must be adequate and there must be a balance between all the hormones in the body. When an endocrine gland produces too little or too much of a particular hormone, it may cause an imbalance and result in harmful effects such as: a) physical and physiological abnormalities b) mental retardation c) diseases. Table below shows the effects of hormonal imbalance:

Hormones	Effects of
Under secretion	Over secretion
Thyroxin	*Myxcedema, a condition where the metabolic rate decreases *Cretinism (related growth) in children *Weight gain *Slows down mental development	*Increases metabolic rate causing acceleration of the heartbeat *Increases blood pressure and body temperature *Increases sweating *Weight loss *Causes goiter
Insulin	*Diabetes mellitus	*Low blood glucose level *May lead to a coma and eventually death if the blood sugar level is too low
Adrenaline	*Muscles become weak *Ability to overcome stress diminishes *Addison’s disease where there is excessive salt and water in the body	*Increases blood pressure *Blood sugar level increases *Salt concentration in blood increases
Testosterone	*Incomplete development of male secondary sexual characteristics	*Men may have female characteristics (feminization)
Progesterone	*Sterility *Irregular menstrual cycle *Miscarriage	*Baby born before the complete gestation period *Failure in uterine contraction during birth of baby

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Info Gallery

Hormones are made up of proteins or steroids. Our body cannot store excess hormones. Excess hormones are brought to the liver to be broken down and excreted through the kidneys.

Hormones such as estrogen and progesterone are used to control menopausal symptoms in Hormone Replacement Therapy (HRT) for women. However, studies have shown that such therapy increases the risk of endometrial cancer.

The pancreas is both an endocrine and exocrine gland. It secretes insulin and it also secretes pancreatic juice which contains enzymes such as lipase and protease.

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Contraceptive pills used to prevent pregnancy contains the female sex hormones oestrogen and progesterone in biotechnology, bacteria are used in genetic engineering to produce insulin to treat diabetic patients.