Endocrine System 

Chemical Messengers of the Body:

A). Hormones: Specific molecules made by special cells and released into the circulatory system where they travel to target cells causing a specific response.

Hormone Types:

1). Steroid: fat-soluable molecules produced from cholesterol. Sex hormones are examples of these.

2). Hormones derived from amino acids: most frequently derived from tyrosine are small and water soluble. Example epinephrine.

3). Peptide hormones: derived from chains of amino acids, or peptides. May act as signal molecules in nervous and endocrine systems.

B). Pheromones: chemical signals that function between animals of the same species. Small amounts are used to attract mates, act as territorial markers, or act as an alarm substance.

C. Local Regulators: chemical messengers that affect target cells close to their point of secretion. Neurotransmitters, histamine and interleukins.

D. Prostaglandins: Modified fatty acids released into extracellular fluid to function as local regulators. Derived from lipids in the plasma membrane. They work in antagonistic actions. May induce fever and inflammation.


Mechanism Of Hormone Action


A. Steroid Hormones and Gene Expression. These hormones pass through the target cell and into the nucleus where they bind to a receptor protein. Here they activate certain genes.


B. Peptide Hormones and Second Messengers. They are unable to pass through the plasma membrane and have different methods of action. They attach to their receptors in the target cell surface and influence activity within the cell through cytoplasmic intermediates called second messengers.


The two most important messengers are cAMP and inositol triphosphate.

Cyclic AMP: ATP is converted into cAMP after a series of reactions on the plasma membrane following the attachment of the hormone to the membrane. cAMP relays the signal from the membrane to the metabolic machinery of the cytoplasm.

Inositol Triphosphate: Involves the use of Ca+2 that regulates cellular protein activity.


Vertebrate Endocrine System


A. The Hypothalamus and the Pituitary Gland:

The hypothalamus is a region of the lower brain that receives information from the peripheral nerves and the brain and gives off hormones appropriate to environmental conditions.


Two sets of hormones are produces:

1). these are produced and stored in the posterior pituitary

2). releasing factors that regulate the anterior pituitary.


Pituitary Gland: an appendage at the base of the hypothalamus consisting of 2 lobes.

1). Neurohypophysis ( posterior lobe). releases oxytocin and ADH made by the hypothalamus.

2). Adenohypophysis ( anterior lobe). produces several of its own hormones. ACTH, endorphins and enkephalins. Others: GH, Prolactin, FSH, LH, TSH,


B. Thyroid Gland:

Consists of two lobes located on the ventral surface of the trachea. Produces T3 (triiodothyrone) and T4 (thyroxine) derived from the amino acid tyrosine. T3 is usually more reactive than T4. Iodine is the element that is necessary for the production of these hormones. It also produces and secretes the material calcitonin which lowers blood calcium by inhibiting the release from bones. TSH controls the production and release of these thyroid hormones.

C. Parathyroid Glands:

4 small glands embedded in the thyroid gland. Secretes Parathormone which regulated the calcium levels in the body. Vitamin D is needed to function.

D. The Pancreas:

The pancreas produces insulin in the B islets cells, and glucogon in the A islet cells. Both are controlled by the sugar level in the blood.

E. The Adrenal Glands:

Located adjacent to kidneys; each gland is composed of an outer cortex and an inner medulla. The medulla produces epinephrine and norepinephrine from the amino acid tyrosine. Epi is released in times of stress. This is the fight or flight hormone. Noe- is used primarily to sustain blood pressure. Cortex produces Glouccorticoids and mineralocorticoids.

F. Gonads:

Produce estrogens, adrogens and progestines. Controlled by the FSH and LH of the anterior pituitary.

Hormonal control of Calcium Homeostasis:

A negative feedback system involving 2 antagonistic hormones, calcitonin and parathyroid hormone (PTH), maintains the concentration of calcium in the blood.

The narrow range is between 9-11 mg.per 100 ml of blood.

a). A rise of calcium concentration induces the thyroid gland to secrete calcitonin, which lowers the calcium concentration by increasing bone deposition and reducing the calcium uptake in the intestine and the kidneys.

b). When the calcium concentration falls below the recommended level PTH reverses the effects of calcitonin. PTH is secreted by the parathyroid gland.


Hormonal control of blood glucose:

Another negative feedback system involving 2 antagonistic hormones, insulin and glucagon. The ideal glucose level in humans is 90 mg/100 ml of blood. A rise of glucose in the blood triggers the beta cells of the pancreas to produce and release insulin, which acts on the target cells. When blood sugar is too low, the alpha cells of the pancreas respond to release glucagon which causes the break down of glycogen in the liver into glucose. This glucose is released into the blood.