Short notes on Endocrine System, Lecture notes of Biology

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ENDOCRINE SYSTEM

ENDOCRINE SYSTEM

• The endocrine system is a system of glands called endocrine glands that release chemical messenger molecules into the bloodstream. The messenger molecules of the endocrine system are called endocrine hormones.
• Other glands of the body, including sweat glands and salivary glands, also secrete substances but not into the bloodstream. They are called exocrine glands.

GLANDS OF ENDOCRINE SYSTEM

• The pituitary gland is located at the base of the brain. It is controlled by the nervous system via the brain structure called the hypothalamus, to which it is connected by a thin stalk. The pituitary gland consists of two lobes, called the anterior (front) lobe and posterior (back) lobe.
• Most of the other endocrine hormones secreted by the pituitary gland control other endocrine glands. This is why the pituitary gland is often referred to as the “master gland” of the endocrine system.

• Steroid hormones such as estrogen are endocrine hormones made of lipids that cross plasma membranes and bind to receptors inside target cells. The hormone- receptor complexes then move into the nucleus where they influence gene expression.
• Non-steroid hormones such as insulin are endocrine hormones made of amino acids that bind to receptors on the surface of target cells. This activates an enzyme in the plasma membrane, and the enzyme controls a second messenger molecule, which influences cell processes.
• Most endocrine hormones are controlled by negative feedback loops in which rising levels of a hormone feedback to stop its own production — and vice-versa. For example, a negative feedback loop controls production of thyroid hormones. The loop includes the hypothalamus, pituitary gland, and thyroid gland.
• Only a few endocrine hormones are controlled by positive feedback loops in which rising levels of a hormone feedback to stimulate continued production of the hormone. Prolactin, the pituitary hormone that stimulates milk production by mammary glands, is controlled by a positive feedback loop. The loop includes the nipples, hypothalamus, pituitary gland, and mammary glands.

PITUITARY GLAND

• The pituitary gland is the master gland of the endocrine system, the system of

glands that secrete hormones into the bloodstream. Endocrine hormones control

virtually all physiological processes. For example, they control growth, sexual

maturation, reproduction, body temperature, blood pressure, and metabolism. The

pituitary gland is considered the master gland of the endocrine system because it

controls the rest of the endocrine system. Many pituitary hormones either promote

or inhibit hormone secretion by other endocrine glands.

STRUCTURE AND FUNCTIONS OF PITUITARY GLAND

ANTERIOR PITUITARY

THYROID GLAND

The thyroid gland is one of the largest endocrine glands in the body. It is located

in the front of the neck below the Adam’s apple (see figure below). The gland is

butterfly shaped and composed of two lobes. The lobes are connected by a narrow

band of thyroid tissue called an isthmus.

FUNCTUONS OF THYROID

• There are two main thyroid hormones produced by the follicles: thyroxine

(T4), which contains four iodide ions and triiodothyronine (T3), which

contains three iodide ions. T3 is much more powerful than T4, but T

makes up about 90 percent of circulating thyroid hormone, and T3 makes

up only about 10 percent. However, most of the T4 is converted to T3 by

target tissues.

• Thyroid hormones increase the rate of metabolism in cells, so cells absorb

more carbohydrates, use more energy, and produce more heat. Thyroid

hormones also increase the rate and force of the heartbeat.

• Calcitonin helps regulate blood calcium levels by stimulating the

movement of calcium into bone. It works in conjunction with parathyroid

hormone to maintain calcium homeostasis.

THYROID DISORDERS

1. HYPERTHYROIDISM

• Hyperthyroidism occurs when the thyroid gland produces

excessive amounts of thyroid hormones. The most common

cause of hyperthyroidism is Graves’ disease.

• Graves’ disease is an autoimmune disorder in which

abnormal antibodies produced by the immune system

stimulate the thyroid to secrete excessive quantities of its

hormones.

• Graves’ disease often results in the formation of an enlarged

thyroid ( goiter ) because of the continued stimulation to

produce more hormones.

• Besides a goiter, other signs and symptoms of

hyperthyroidism may include protruding eyes (see photo

below), heart palpitations, excessive sweating, diarrhea,

weight loss despite increased appetite, muscle weakness, and

unusual sensitivity to heat.

ADRENAL GLANDS

• The adrenal glands are endocrine glands that produce a variety of

hormones.

• Each adrenal gland has two distinct parts, and each part has a

different function, although both parts produce hormones. There

is an outer layer, called the adrenal cortex , which produces

steroid hormones including cortisol. There is also an inner layer,

called the adrenal medulla , which produces non-steroid

hormones including adrenaline.

• The adrenal cortex, or outer layer of the adrenal gland, is divided, in

turn, into three additional layers, called zones (see figure below). Each

zone has distinct enzymes that produce different hormones from the

common precursor molecule cholesterol, which is a lipid.

1. Zona glomerulosa is the outermost layer of the adrenal cortex. It lies immediately

under the outer fibrous capsule that encloses the adrenal gland.

2. Zona fasciculata is the middle layer of the adrenal cortex. It is the largest of the three zones, accounting for nearly 80 percent of the adrenal cortex.
3. Zona reticularis is the innermost layer of the adrenal cortex. It is directly adjacent to the medulla of the adrenal gland. - The adrenal cortex produces steroid hormones called by the

general term corticosteroids , of which there are three types:

mineralocorticoids such as aldosterone, which helps control

electrolyte balance; glucocorticoids such as cortisol, which helps

control the rate of metabolism, suppresses the immune system,

and is the major stress hormone; and androgens such as DHEA ,

which is converted to sex hormones in the gonads.

• The adrenal medulla produces non-steroid catecholamine

hormones including adrenaline and noradrenaline. These

hormones stimulate the fight-or-flight response.

• Disorders of the adrenal glands generally include either

hypersecretion or hyposecretion of adrenal hormones. The cause

may be a problem with the adrenal glands or with the pituitary

gland, which controls adrenal cortex hormone production.

Examples include Cushing’s syndrome , in which there is

hypersecretion of cortisol; and Addison’s disease, in which there

is hyposecretion of cortisol and mineralocorticoids.

DISORDERS OF ADRENAL GLAND

• Hypersecretion of the glucocorticoid hormone cortisol leads to a disorder

named Cushing’s syndrome. The most common cause of Cushing’s

syndrome is a pituitary tumor, which causes excessive production of ACTH

(Adreno-Cortico Tropic Hormone). The disease produces a wide variety of

signs and symptoms, which may include obesity, diabetes, high blood

pressure (hypertension), excessive body hair, osteoporosis, and depression.

• Hyposecretion of the glucocorticoid hormone cortisol leads to a disorder

named Addison’s disease. There may also be hyposecretion of

mineralocorticoids with this disorder. Addison’s disease is generally an

autoimmune disorder, in which the immune system produces abnormal

antibodies that attack cells of the adrenal cortex.