Biology

Key Unit Competence

To be able to identify the location and function of endocrine glands in the body. 

Learning objectives 

By the end of this unit, I should be able to:

– Define hormones. 

– Explain why hormonal balance is necessary for coordinating the functions in the body. 

– Describe the principle of the negative feedback mechanism by which hormones produce their effects on target cells. 

– Describe the structure and function of the endocrine system. 

– Explain the effects of hormonal imbalances. 

– Compare and contrast the actions of the endocrine and nervous systems. 

– Draw and interpret the flow chart of negative feedback mechanisms. 

– Appreciate the role of hormones in the growth and development of organisms.

Introductory activity 

At a given time, there are certain changes which occur in the body especially during puberty. As girls and boys enter the period of puberty, they start to develop remarkable differences in physical appearance and in their behaviour. 

1. What do you think to be the causes of such changes? 

2. What are some changes which can be observed in boys and not in girls and vice versa?

 3. Which the organs do you think are responsible for producing such changes? 

4. What will be the causes if some of those changes do not appear in a boy or in a girl?

10.1 Structure and function of the endocrine system in humans 

Activity 10.1

By using different books from the school library and a chart showing different endocrine glands, discuss the following: 

1. What are endocrine glands? 

2. Draw and locate the following endocrine glands 

a.  The adrenal glands 

b.  The pancreas 

3. What are the hormones produced by the pancreas and their functions? 

4. Why the pituitary gland was once described as a master gland?

A hormone is an organic substance which is produced in minute quantity by an endocrine gland, transported by blood to other parts or organs of the body where it exerts maximum effects. Such parts of the body or organs are called target organs. The word endocrine means internal secretion and endocrine glands are therefore glands of internal secretion. Since they shed their secretion into the bloodstream, they have no ducts and are hence known as ductless glands. 

Hormones are released into the blood stream as a result of: 

1. Stimulation of the endocrine gland directly by the nervous system e.g. the sympathetic nervous system causes secretion of adrenaline by the adrenal medulla. 

2. The levels of particular metabolites in the blood e.g. glucose levels trigger the release of insulin. 

3. Presence of other hormones called releasing hormones mostly produced in anterior pituitary e.g. TSH stimulates the release of thyroxin by the thyroid gland. 

4. Environmental changes such as high or low temperatures effects activities of the pituitary gland. 

5. Animals’ general mental state does affect the activity of the pituitary.

Once in the bloodstream, the hormones are carried around the body, bringing about responses in various places. Structures that respond to them are called target organs. A hormone is a chemical messenger having the following properties: 

– It travels in the blood 

– It has its effect at a site different from the site where it is produced. The site where it has effect is called the target, while itself is called messenger 

– It fits precisely into receptor molecules in the target like a key in a lock. It is therefore specific for a particular target; 

– It is a small soluble molecule;

It is effective in low concentrations.

Hormones fulfill many functions. These include: 

1. Regulation of growth and development. 

2. Controls homeostasis e.g.in osmoregulation/thermo regulation etc 

3. Regulation of metabolism e.g. digestion storage and utilization of food substances. 

4. Development of the skin coloration. 

5. Enabling the body to withstand shock, tension wounding etc. and to recover from it. 

6. Together with the nervous system it provides for effective responses to all kinds of stimuli both internal and external.  

The endocrine glands include; the pituitary, thyroid, parathyroid, adrenal, and pineal glands (Figure 10.1 below). Taken together, all endocrine glands and hormonesecreting cells constitute the endocrine system. 

a. The pituitary gland 

The pituitary gland which was formerly called the master gland hangs from the base of the brain by a stalk and is enclosed by bone. As shown in figure 10.2, the pituitary gland consists of a hormone-producing glandular portion called anterior pituitary and a neural portion called posterior pituitary, which is an extension of the hypothalamus. The hypothalamus now called the master gland, regulates the hormonal output of the anterior pituitary and synthesizes two hormones that it exports to the posterior pituitary for storage and later release. Most anterior pituitary hormones exhibit a diurnal rhythm of release, which is subject to modification by stimuli influencing the hypothalamus. 

– Growth hormone (GH) or Somatotropic hormone: is a hormone that stimulates growth of all body tissues especially skeletal muscle and bone. GH; mobilizes the use of fats, stimulates protein synthesis, and promotes glucose uptake and metabolism. 

– Thyroid-stimulating hormone (TSH) This hormone causes thyroid glands to secrete thyroxin. The secretion of TSH is controlled by levels of thyroxin in blood. TSH also stimulates growth of thyroid gland. 

– Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release its hormones. ACTH release is triggered by corticotropin-releasing hormone (CRH) and inhibited by rising glucocorticoid levels. 

– The gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) regulate the functions of the gonads in both sexes.  

– Prolactin (PRL) promotes the production of milk in human’s females. Its secretion is triggered by prolactin-releasing hormone (PRH) and inhibited by prolactin-inhibiting hormone (PIH).

The following are the two hormones released from the posterior pituitary gland: 

– Oxytocin: It stimulates powerful contractions of the uterus, which trigger labour and delivery of an infant, and milk ejection in nursing women. Its release is mediated reflexively by the hypothalamus and represents a positive feedback mechanism. 

– Antidiuretic hormone (ADH) stimulates the kidney tubules to reabsorb and conserve water, resulting in small volumes of highly concentrated urine and decreased plasma osmolality.

 b. The hypothalamus 

The hypothalamus plays an important role in integrating the endocrine and nervous systems. The region of the lower brain receives information from nerves throughout the body and from other parts of the brain thus initiates endocrine signals appropriate to environmental conditions. The reason it is called a master gland is that a set of neurosecretory cells in the hypothalamus exerts control over the anterior pituitary by secreting two kinds of hormones into the blood: Releasing hormones which make the anterior pituitary to secrete its hormones and inhibiting hormones that make the anterior pituitary stop secreting hormones. Every anterior pituitary hormone is controlled at least by one releasing hormone and some have both a releasing and an inhibiting hormone.

The posterior pituitary remains attached to the hypothalamus. It stores and releases two hormones that are made by a set of neurosecretory cells in the hypothalamus.

c. Thyroid gland

The thyroid gland is located in the anterior throat. Thyroid follicles store colloid containing thyroglobulin, a glycoprotein from which thyroid hormone is derived. Thyroid hormone (TH) includes thyroxine (T4) and triiodothyronine (T3), which perform the following tasks; 

– Control the basal metabolic rate. 

– Increase the rate of cellular metabolism. Consequently, oxygen use and heat production rise. 

Calcitonin, is another hormone produced by the thyroid gland in response to rising blood calcium levels. Its role is to decrease blood calcium levels by inhibiting bone matrix reabsorption and enhancing calcium deposit in bone.

d. Parathyroid glands

The parathyroid glands are located on the dorsal aspect of the thyroid gland and secrete parathyroid hormone (PTH), which causes an increase in blood calcium levels by; 

– Increasing the rate of calcium reabsorption by the kidney at the expense of phosphate ions.

 – Increasing the rate of calcium absorption from the gut. 

– Causing the release of calcium reserves from the bones.

PTH is antagonistic calcitonin. PTH release is triggered by decreasing blood calcium levels and is inhibited by increasing blood calcium levels.

e. Pancreas 

The pancreas is an organ located in the abdomen close to the stomach and is both an exocrine and an endocrine gland. The endocrine portion (islets of Langerhans) releases insulin and glucagon hormones. Glucagon is released by alpha (α) cells when glucose levels in blood are low. Glucagon stimulates the liver to convert stored glycogen to glucose thus increasing glucose levels. Insulin is released by beta (β) cells of the islets of Langerhans when blood levels of glucose are rising. It increases the rate of glucose uptake and causes the conversion of glucose to glycogen.

f. Gonads  

The ovaries of the female which are located in the pelvic cavity, release two main hormones. Secretion of oestrogen by the ovarian follicles begins at puberty under the influence of FSH. Oestrogen stimulates maturation of the female reproductive system and development of the secondary sex characteristics. Progesterone is released in response to high blood levels of LH. It works with oestrogen in establishing the human menstrual cycle. The testes of the male begin to produce testosterone at puberty in response to LH. Testosterone stimulates the maturation of the male reproductive organs, development of secondary sex characteristics, and the production of sperm by the testes.

g. Adrenal Glands (Suprarenal Glands)

A fresh adrenal gland section shows a bright yellow cortex, making up about 80% of the organ, and a more reddish-grey medulla. The endocrine activities of the adrenal cortex and the adrenal medulla differ both in development and function.

• Adrenal cortex 

Adrenal cortex makes mineralocorticoids (such as aldosterone and cortisol). Cortisol raises blood glucose level whereas aldosterone stimulates the reabsorption of Na+ and excretion of K+ in kidney.

• Adrenal Medulla 

The adrenal medulla makes two hormones epinephrine (adrenaline) (80 %) and norepinephrine (noradrenaline) (20 %). Epinephrine and norepinephrine are released into the bloodstream during stress and they act on the whole organism by preparing it for increased energy use. Both hormones, for instance, activate the liberation of fatty acids from fat depots and liberate glucose from glycogen storage in the liver (producing a rise in the blood sugar level). They raise the blood pressure and stroke volume of the heart and may lead to vasoconstriction in certain defined areas.

h. Other hormone-producing structures 

Many body organs not normally considered endocrine organs contain isolated cell clusters that secrete different hormones. Examples include the; gastrointestinal tract organs (gastrin, secretin, and others), the placenta (hormones of pregnancy such as oestrogen, progesterone, and others) and the kidneys (erythropoietin and renin).

Self-assessment 10.1 

1. What are the hormones produced by the thyroid glands? 

2. What are the functions of the hormones stored and released by the posterior pituitary gland? 

3. By which means are hormones transported in our body?

10.2 Principles of the negative feedback mechanism of hormonal action.

  Activity 10.2  

1. The amount of urine produced varies according to the amount of water consumed. Make a list of events that may occur in the following cases for the human body: 

b. Two days without drinking water 

c. When you have drunk 1 litre of water per day 

d. How can you explain the above observations?

2. Make short notes on what happens to your body if the level of sugars decreases in the blood?

Feedback mechanisms are necessary in the maintenance of homeostatic mechanisms. All homeostatic control mechanisms have at least three interdependent components for the variable being regulated that work together i.e. 

a. The receptor is the sensing component that monitors and responds to changes in the environment. When the receptor senses a stimulus, it sends information to a control center, the component that sets the range at which a variable is maintained. 

b. The control center determines an appropriate response to the stimulus. In most homeostatic mechanisms, the control center is the brain. 

c. An effector, which can be muscles, organs or other structures that receive signals from the control center. After receiving the signal, a change occurs to correct the deviation by either enhancing it with positive feedback or depressing it with negative feedback. 

The homeostatic mechanisms in mammals require information to be transferred between different parts of the body. There are two coordination systems in mammals that control this: the nervous system and the endocrine system. 

– In the nervous system, information in the form of electrical impulses is transmitted along nerve cells (neurons). 

– The endocrine system uses chemical messengers called hormones that travel in the blood, in a form of long-distance cell signalling.

12.2.1 Positive feedback mechanisms

These mechanisms are designed to accelerate or enhance the output created by a stimulus that has already been activated. The positive feedback mechanisms are designed to push levels out of normal levels. To achieve this purpose, a series of events initiates a cascading process that builds to increase the effect of the stimulus. This process can be beneficial but is rarely used by the body due to risks of the acceleration’s becoming uncontrollable. 

Examples include; the accumulation blood platelets which in turn causes blood clotting in response to a break or tear in the lining of blood vessels. The release of oxytocin to intensify the contractions of the uterus that take place during childbirth. Another example of a positive feedback mechanism is the production of milk by a mother for her baby. As the baby suckles, nerve messages from the mammary glands cause the mother’s pituitary gland to secrete a hormone called prolactin. The more the baby suckles, the more prolactin is released, which stimulates further milk production by the mother’s mammary glands. In this case, a negative feedback mechanism would not be helpful because the more the baby nursed, the less milk would be produced. 

12.2.2 Negative feedback mechanisms 

These are mechanisms concerned with keeping changes in the factor within narrow limits. Here, an increase in a factor (input) e.g. hormone levels results in something happening that makes the factor decrease (output).

An example of negative feedback mechanism is regulation of thyroxine levels i.e. The shedding of thyroxine into blood stream is triggered by thyrotropin releasing factor (TRF) produced by the hypothalamus of the brain. TRF passes to the pituitary gland along the blood vessels stimulating the anterior pituitary gland to produce Thyroid stimulating hormones (TSH). TSH then stimulates the thyroid gland to produce thyroxine into blood. A slight excess of thyroxine in blood detected by hypothalamus, inhibits the anterior lobe of the pituitary gland which responds by secreting less TSH. This in turn reduces the activity of the thyroid gland, leading to decrease in the amount of the thyroxine produced. This removes the inhibitory influence on the pituitary so that more thyroid stimulating hormone will be produced again.

The disorders of the endocrine system often involve either the hypo-secretion (hypo means too little or under), inadequate release of a hormone, or the hyper-secretion (hyper means too much or above), excessive release of a hormone. In other cases, the problem is faulty hormone receptors, an inadequate number of receptors, or defects in second-messenger systems. Because hormones are distributed in the blood to target tissues throughout the body, problems associated with endocrine dysfunction may also be widespread.

10.3.1. Pituitary Gland Disorders 

a. Pituitary dwarfism, gigantism, and acromegaly

Several disorders of the anterior pituitary involve human growth hormone. Hyposecretion of human growth hormone during the growth years slows bone growth, and the epiphyseal plates close before normal height is reached. This condition is called pituitary dwarfism. Other organs of the body also fail to grow, and the body proportions are childlike. Treatment requires administration of human growth hormone during childhood, before the epiphyseal plates close. Hypersecretion of human growth hormone during childhood causes gigantism, an abnormal increase in the length of long bones. The person grows to be very tall, but body proportions are about normal. Hypersecretion of human growth hormone during adulthood is called acromegaly. 

The most common abnormality associated with dysfunction of the posterior pituitary is diabetes insipidus. This disorder is due to defects in antidiuretic hormone (ADH) receptors or an inability of the pituitary gland to secrete ADH. A common symptoms of diabetes insipidus are: excretion of large volumes of urine resulting in dehydration and thirst. Bed-wetting is common in afflicted children. Because so much water is lost in the urine, a person with diabetes insipidus may die of dehydration if deprived of water for only one day. Treatment of diabetes insipidus involves the injection of ADH into the body.

10.3.2. Thyroid gland disorders

Thyroid gland disorders affect all major body systems and are among the most common endocrine disorders. Congenital hypothyroidism or the hyposecretion of thyroid hormones that is present at birth has devastating consequences if not treated quickly. Previously termed cretinism, it causes severe mental retardation and stunted bone growth. At birth, the baby typically is normal because lipid-soluble maternal thyroid hormones crossed the placenta during pregnancy and allowed normal development. 

Hypothyroidism during the adult years produces a disorder called myxoedema. An indication of this disorder is oedema (accumulation of interstitial fluid) that causes the facial tissues to swell and look puffy. A person with myxoedema has a slow heart rate, low body temperature, sensitivity to cold, dry hair and skin, muscular weakness, general lethargy, and a tendency to gain weight easily. Because the brain has already reached maturity, mental retardation does not occur, but the person may be less alert. 

The most common form of hyperthyroidism is Graves’ disease which is an autoimmune disorder in which the person produces antibodies that mimic the action of thyroidstimulating hormone (TSH). The antibodies continually stimulate the thyroid gland to grow and produce thyroid hormones. A primary sign is an enlarged thyroid, which may be two to three times its normal size. Graves’ patients often have a peculiar oedema behind the eyes, called exophthalmos, which causes the eyes to protrude. Treatment may include surgical removal of part or all of the thyroid gland (thyroidectomy), the use of radioactive iodine to selectively destroy thyroid tissue, and the use of anti-thyroid drugs to block synthesis of thyroid hormones. A goitre is simply an enlarged thyroid gland. It may be associated with hyperthyroidism, hypothyroidism or by the lack of iodine.

10.3.3. Parathyroid gland disorders 

Parathyroid gland disorders cause the hypoparathyroidism due to the too little parathyroid hormone leading to a deficiency of blood Ca2+, causing neurons and muscle fibres to depolarize and produce action potentials spontaneously. This leads to twitches, spasms, and tetany (maintained contraction) of skeletal muscle. The main cause of hypoparathyroidism is accidental damage to the parathyroid glands or to their blood supply during thyroidectomy surgery. 

Hyperparathyroidism or an elevated level of parathyroid hormone, most often is due to a tumour of one of the parathyroid glands. An elevated level of PTH causes excessive resorption of bone matrix, raising the blood levels of calcium and phosphate ions and causing bones to become soft and easily fractured. High blood calcium level promotes formation of kidney stones. Fatigue, personality changes, and lethargy are also seen in patients with high levels of parathyroid hormone.

10.3.4. Adrenal gland disorders 

a. Cushing’s syndrome 

Hypersecretion of cortisol by the adrenal cortex causes an endocrine disorder known as Cushing’s syndrome. The condition is characterized by breakdown of muscle proteins and redistribution of body fat, resulting in thin arms and legs accompanied by a rounded moon face and buffalo hump on the back. Facial skin is flushed, and the skin covering the abdomen develops stretch marks. The person also bruises easily, and wound healing is very slow. The elevated level of cortisol causes hyperglycaemia, osteoporosis, weakness, hypertension, increased susceptibility to infection, decreased resistance to stress, and mood swings. 

b. Addison’s disease 

Hyposecretion of glucocorticoids and aldosterone causes Addison’s disease (chronic adrenocortical insufficiency). The majority of cases are autoimmune disorders in which antibodies cause adrenal cortex destruction or block binding of ACTH to its receptors. Pathogens, such as the bacterium that causes tuberculosis, also may trigger adrenal cortex destruction. Symptoms, which typically do not appear until 90% of the adrenal cortex has been destroyed, include mental lethargy, anorexia, nausea and vomiting, weight loss, hypoglycemia, and muscular weakness. Loss of aldosterone leads to the elevated potassium and decreased sodium in the blood, low blood pressure, dehydration, decreased cardiac output and even cardiac arrest. 

10.3.5. Pancreas disorders 

The most common endocrine disorder is diabetes mellitus caused by an inability to produce or use insulin.  According to the diabetes atlas of 2018, the prevalence of diabetes in Rwanda is about 3.16% of the population with 1,918 diabetes related deaths per year. Its complications can lead to heart attack, stroke, blindness, kidney failure and lower limb amputation.

Because insulin is unavailable to aid transport of glucose into body cells, blood glucose level is high and glucose is found in the urine, the process known as glycosuria. The cardinal signs of diabetes mellitus are; polyuria (excessive urine production due to an inability of the kidneys to reabsorb water), polydipsia (excessive thirst) and polyphagia (excessive eating).

  Activity 10.4

 Use your knowledge of the nervous system and the endocrine system to answer the questions that follow 

1. Discuss the similarities between the structure and functioning of nervous and hormonal systems. 

2. Discuss the differences between the structure and functioning of nervous and hormonal systems.

10.4 Comparison of hormonal and nervous systems

 Self-assessment 10.3

 1. Which disorders are caused by: 

a. Hypersecretion of GH in children? 

b. Hyposecretion of insulin? c. Hypersecretion of thyroid hormones? 

2.  What are some symptoms of? 

a. Diabetes mellitus? 

b. Grave’s disease?

The nervous and endocrine systems act together to coordinate functions of all our body systems.

A basic similarity between the endocrine system and the nervous system is that both provide means of communication within the body of an organism. Both involve transmission of a message which is triggered by a stimulus and produces a response. Several chemicals function as both neurotransmitters and hormones including norepinephrine. Some hormones such as oxytocin are secreted by neuroendocrine cells; neurons that release their secretions into the blood. The target organs of a hormone are equivalent to nerve’s effectors. 

Similarities

1. Both provide a means of communication and coordination in the body. 

2. In both the information transmitted is triggered by a stimulus and produces a response.

3. Both involve chemical transmission. 

4. Both are controlled by the brain. 

The main differences between the two systems concern the nature of the message. In the endocrine system, the message takes the form of a chemical substance transmitted through the blood stream. In the nervous system it is a discrete-all or none action potential transmitted along a nerve fibre. All other differences arise from this fundamental one. They can be listed as follows: 

– Because of the comparatively high speed at which impulses are transmitted along nerves, nerves responses are generally transmitted more rapidly than hormonal ones. – Since it is conveyed by the bloodstream, there is nothing to stop a hormone being carried to every part of the body. Nervous impulses however are transmitted by particular neurons to specific destinations. 

– As a result, hormones are often widespread, sometimes involving the participation of numerous target organs. In contrast, nervous responses may be much localized, involving perhaps the contractions of only one muscle. 

– Hormonal responses frequently continue over a long period of time. Obvious examples of such long-term responses are growth and metabolism.

 A comparison between nervous and endocrine system is summarized in the table 10.3