The Endocrine System

UNIVERSITY OF SANTO TOMASFaculty of PharmacyDepartment of Medical Technology

The Endocrine SystemThe body contains two kinds of glands: exocrine and endocrine:

Exocrine glands secrete their products into ducts and the ducts carry the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body.

There glands include sudoriferous (sweat), sebaceous (oil), mucous and digestive glands.

Endocrine glands secrete their products (hormone) into the extracellular space around the secretory cells rather than into ducts. The secretion then diffuses into capillaries and is carried away by the blood. The endocrine glands of the body constitute the endocrine system and include the pituitary, thyroid, parathyroid, adrenal and pineal gland. In addition, several organs of the body contain cells that secrete hormones but are not endocrine glands exclusively. These include the hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestines, skin, heart, and placenta.

The science concerned with the structure and functions of the endocrine glands and the diagnosis and treatment of disorders of the endocrine system is called ENDOCRINOLOGY.

The two physiologic regulatory systems of the body: the endocrine system and the nervous system. The endocrine system consists of ductless glands that produce highly active chemical regulators called hormones. These chemical messengers or messenger molecules (hormones) are secreted into the bloodstream where they act upon all cells throughout the body by interacting with receptors. The nervous system acts locally at tissues or cellular sites. It controls homeostasis through nerve impulses (action potentials) conducted along the axons and neurons. At axon terminals, impulses trigger release of neurotransmistter molecules results in either excitation or inhibition of specific other neurons, muscle fibers or cells, or gland cells. Together thses two systems are coordinated as an interlocking super-system often referred to as the neuro-endocrine system. Certain parts of the nervous system stimulate or inhibit the release of hormones. Hormones, in turn, may promote or inhibit the generation of nerve impulse.

Comparison of the Nervous and Endocrine Regulation of Homeostasis:

CHARACTERISTICS NERVOUS SYSTEM ENDOCRINE SYSTEM1. Mechanism of control Neurotransmitters released in

response to nerve impulses.Hormones delivered to tissues throughout the body by blood.

2. Cells affected Muscle cells, Gland cells, other neurons.

Virtually all body cells.

3. Type of action that results

Muscular contraction or glandular secretion

Changes in metabolic activities

4. Time to onset of action Typically within seconds Seconds to hours or days5. Duration of action Generally briefer Generally longer

Clinical Chemistry 2 – The Endocrine SystemProf. Jacinta B. Cruz

Mary Christelle G. Aquitania (re-typed)

UNIVERSITY OF SANTO TOMASFaculty of PharmacyDepartment of Medical TechnologyFunction of Hormones

1. Regulate the chemical composition and volume of the extracellular fluid.2. Help regulate metabolism and energy balance.3. Help regulate contraction of smooth and cardiac muscle fibers and secretion by glands.4. Help maintain homeostasis despite disruptions such as infection, trauma, emotional stress,

dehydration, starvation, hemorrhage, and temperature extremes.5. Regulate certain activities of the immune system.6. Play a role in the smooth, sequential integration of growth and development.7. Contribute to the basic processes of reproduction, including gamete (oocyte and sperm),

production, fertilization, nourishment of the embryo and fetus, delivery, and nourishment of the newborn.

Circulating hormones or endocrines = hormones that pass into the blood and act on distant target cells.

Local hormones = hormones that act locally without first entering the bloodstream.

a. Paracrines – local hormones that act on neighboring cellsb. Autocrines – local hormones that act on the same cells that secreted them.

The four functional types of hormones:

1. Releasing hormones2. Inhibitory hormones3. Tropic hormones4. Effector hormones

1. Releasing hormones are from the hypothalamus that promotes the secretion of the anterior pituitary hormones.

2. Inhibitory hormones are from the hypothalamus or gastrointestinal tract that suppresses the secretion of particular hormones.

3. Tropic hormones stimulate the growth and activity of other endocrine glands.4. Effector hormones are secreted by all the endocrine glands other than the anterior pituitary and

hypothalamus. The target cells of these type of hormones are the particular nonendocrine tissue cells upon which these hormones exert their metabolic effects.

From the chemical standpoint (structural classes) there are five (5) classes of hormones:

1. Steroids2. Biogenic amines3. Peptides and proteins4. Glycoprotein5. Eicosanoids (fatty acids)



UNIVERSITY OF SANTO TOMASFaculty of PharmacyDepartment of Medical TechnologySteroids

These are lipids derived from cholesterol and synthesized on the smooth endoplasmic reticulum. They are primarily involved in the regulation of sexual development and characteristics. They require transport protein in order to travel through the bloodstream. Examples: aldosterone, cortical, and androgens (male sex hormones) from the adrenal cortex; progesterone (female sex hormones) and estrogen from the ovaries; testosterone from the testes; calcitrol from the kidneys.

Amino Acids

Structurally, these are the simplest hormone molecules. Several are synthesized by modifying amino acid, tyrosine. Tyrosine serves as the precursor for the thyroid hormones: thyroxine (T4) and triiodothyronine (T3) which requires carried protein for transport. Tyrosine can also be directly converted to the adrenal hormones: epinephrine and nonepinephrine (catecholamines). Histamines is synthesized from the amino acid histidine by the mast cells in connective tissues. Serotonin from platelets in the blood and melatonin from the pineal gland are derived from tryptophan.

Peptides and Protein

These hormones consist of 3 – 200 amino acids and are synthesized on rough endoplasmic reticulum. Examples are all hypothalamic releasing and inhibiting hormones (thyrotropin releasing hormones); oxytocin and ADH; insulin, glucagon, somatostatin, and pancreatic polypeptides; parathyroid hormones; calcitonin; gastrin, secretin, cholecystokinin; and anterior pituitary hormones except TSH, FSH, & LH.

Glycoproteins

Amino acid derivatives with attached carbohydrate group. Examples are the TSH, FSH, and LH from the anterior pituitary gland.

Eisonoids

They are derived from a 20-carbon fatty acid called arachidonic acid which is essential fatty acid required as part of a healthy diet. Examples are the prostaglandins and leukotrienes.



UNIVERSITY OF SANTO TOMASFaculty of PharmacyDepartment of Medical TechnologyMechanism of Hormone Action

Hormones act in different ways on their target cells. Lipid soluble hormones, including steroid hormones and thyroid hormones bind to receptors inside the target cells. The lipid-soluble hormone diffuses into the cell. If the cell is a target cell, the hormone will bind to and activate receptors located within the cytosal or nucleus. An activated receptor then alters gene expression: it turns specific genes of the nuclear DNA on or off. The newly formed messenger RNA (mRNA) directs synthesis of specific proteins on the ribosomes. The new proteins alter the cell’s activity and cause typical physiological responses of that hormone.

Water soluble hormone (catecholamines, proteins, and peptides) bind to receptors embedded in the plasma membrane of target cells. The receptors for these water –soluble hormones are plasma membrane integral proteins that protrude into the interstitial fluid. Binding of the hormone (first messender) to its receptors activates G-proteins, which activates adenylate cyclase. Activated adenylate cyclase converts ATP to cyclic AMP (cAMP). Cyclic AMP serves as a second messenger (it translated the presence of the first messenger – the water soluble hormone – into a response inside the cell) to activate protein kinases. Activated protein kinases then phopshorylate other enzymes. The millions of phosphorylated enzyme catalyze reactions that produce physiological responses.

Three hormonal interactions are:

1. Permissive effect2. Synergistic effect3. Antagonistic effect

Permissive effect

The action of some hormones on target cells require a recent or simultaneous exposure to a second hormone, the second hormone is said to have a permissive effect.

Synergistic effect

When the effect of two hormones acting together is greater or more extensive thatn the sum of each hormone acting alone, the hormone are said to have synergistic effect.

Antagonistic effect

When one hormone opposes the action of another hormone, the two hormone are said to have antagonistic effect.

Control of Hormone Secretion

Regulation of secretion is designed to prevent over-secretion or under-section. Hormone secretion is controlled by signals from the nervous system, chemical changes in blood and other hormones. Most



UNIVERSITY OF SANTO TOMASFaculty of PharmacyDepartment of Medical Technologyoften, negative feedback systems decreases the secretory activities of corticotrophic, thyrotrpohs, and gonadotrophs when levels of their target gland hormone rise.



The Endocrine System

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