Endocrinol Pred 01

It is a branch of science that deals with the study of endocrine system. The endocrine system deals with the endocrine glands. The endocrine gland is a ductless gland that releases its secretions directly into the blood stream. These secretions are mainly hormones. Notes: Nervous system

Coordinating system Endocrine system.

The coordinating system regulates all the functions of the body to meat the constantly changing external and internal environments of the body. The nervous system controls acts over seconds or fraction of seconds, while the endocrine system controls acts over minutes, hours or days.

In contrast to the endocrine gland, the exocrine gland releases its secretions into ducts leading into the lumen of other organs (e.g. intestine) or outside of the body.

A hormone is: 1- A chemical substance, 2- Synthesized and secreted by living cells in endocrine glands, 3- It is released directly into blood stream, 4- The blood transports it to the target organ. 5- At the target organ, it relays information regulating a function or body process, 6- Its function is to either stimulate or inhibit certain metabolic activity, 7- It performs its action without taking part in any reaction.

the relation between the secreting cell and the target cell Hormones

are

divided

according

to

into: Hormone-secreting gland cell

1- Endocrine hormone: It is a hormone secreted from an endocrine gland (or a cell) into the blood stream that transports it to its target organ (or cell).

Hormone

Blood

Target cell

Local cell

2- Paracrine hormone: It is a hormone secreted from a cell into the extracellular space (fluid) and affects the neighboring cell.

3- Autocrine hormone: It is a hormone secreted from a cell into the extracellular space (fluid) and affects the same cell that secreted it.

Paracrine hormone Target cell

Local cell

Autocrine hormone

Nerve cell

4Neuroendocrine hormone: It is a hormone secreted from a nerve cell into the blood stream which transports it to the target organ (or cell) to relay metabolic information.

Note:

Nerve cell

Nerve impulse

Neurohormone

Nerve impulse

Neurotransmitter

Blood

Target cell

Nerve cell or other effector cell such as muscle

The neurotransmitter is secreted by a nerve cell in a saynapse to relay a nerve impulse to another nerve cell or muscle.

Chemical structure of the hormones: 1- Protein or peptide hormones: They are made from amino acids, may be short (only 3 amino acids) or long (180 amino acids or more). They may be composed of a linear chain or may contain a ring structure due to bridge formation through disulphide bonds. Some hormones are composed of two chains. The majority of the known hormones are protein hormones. They are soluble in water i.e. hydrophilic. They have short half-life due to their free state in the blood making them available to the action of peptidases.

Chemical

structure

of

the

hormones:

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2- Steroid hormones:

Their precursor is cholesterol. Although steroidproducing endocrine glands synthesize some of their own cholesterol, some major source is the cholesterol ingested with the food. They are insoluble in water i.e. hydrophobic, so, they are easily diffused through cell membrane. In the blood stream, steroid hormones have to be bounded to plasma proteins that are specific for each hormone. The half-life of steroid hormones is long due to their ability to be bound to plasma proteins. They are synthesized only in the adrenal gland, the gonads (testes and ovaries) and the placenta during pregnancy.

Chemical

structure

of

the

hormones:

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3- Tyrosine-derived hormones: They are synthesized from the amino acid, tyrosine.

4- Eicosanoids hormones: They are synthesized from fatty acids derived from the arachidonic acid . Members of this group are prostaglandins, prostacyclins, thromboxanes and leukotriens. They act locally either on the cell that produce them (autocrine) or on the neighboring cell (paracrine). The major physiological roles of this type of hormones are the control of: i- Vascular smooth muscle activity (adjust local blood flow). ii- Blood platelet aggregation. iii- Induce inflammatory (allergic) responses.

Hormone action

1- At the target cell the hormone bind to a specific receptor. 2- Hormone (key) bind to receptor (lock) by mechanism similar to key and lock theory.

Cellular receptors:

Receptors for protein hormones are found in the plasma membrane. Receptors for lipophilic hormones (e.g. steroid hormones & tyrosine-derived hormones) are found in the cytoplasm or nucleus.

Inactivation and excretion: 1- The liver and kidney are the sites of hormones inactivation. 2- The inactivation is an enzymatic process. 3- A small fraction of any hormone (< 1%) is excreted intact in the urine or feces.