Endocrine System

REVIEW OF ANATOMY AND PHYSIOLOGY OF ENDOCRINE SYSTEM

Endocrine system is a system of ductless glands, which secrete hormones that are pored in the blood stream to be transported to the target cells.
The endocrine system is composed of the following

Pituitary gland
Parathyroid gland
Thyroid gland
Adrenal gland
Pancreas
Tests and ovaries

Hormones secreted by these glands act on the specific target tissue away from their site of secretion. Some hormones are protein in nature while others are not.

They act by interacting with specific cell membrane receptors to stimulate the intra cellular Adenylyl cyclase system (membrane-bound enzyme that catalyzes the conversion of Adenosine triphosphate (ATP)-organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse to Cyclic adenosine monophosphate (cAMP) – messenger used for intracellular signal induction, which in turn forms ATP to stimulate protein synthesis.

Hormones regulate their own production through a feedback (negative feedback mechanism) system where the increase in concentration of the hormone suppresses its own production.

Pituitary gland

The pituitary gland secretes hormones like; (Anterior lobe)

Adrenocorticotrophic hormone (ACTH)
Somatotrophic hormone (STH)/(GH)
Thyroid stimulating hormone (TSH)
Follicle stimulating hormone (FSH)
Luteinizing hormone (LH)’
Melanocyte stimulating hormone (MSH)

The Posterior lobe secretes

Anti diuretic hormone (ADH),
Vasopressin
Oxytocin.

Secretion of the anterior lobe is under the control of Hypothalamus which secretes regulatory hormones.

Growth hormone stimulates muscular and skeletal growth either by regulating synthesis of somatomedins by the liver or by directly stimulating incorporation of amino acids into proteins.
Hypoglycemia is a potent stimulant of growth hormone release; obesity blunts its release.
Excess secretion of growth hormone after epiphyseal fusion produces acromegaly where as before epiphyseal fusion causes gigantism

Image showing hormones produced by the anterior lobe.

Overview of the Endocrine System

The endocrine system consists of glands widely separated from each other with no physical connections . Endocrine glands are groups of secretory cells surrounded by an extensive network of capillaries that facilitates diffusion of hormones (chemical messengers) from the secretory cells into the bloodstream. They are commonly referred to as ductless glands because hormones diffuse directly into the bloodstream. Hormones are then carried in the bloodstream to target tissues and organs that may be quite distant, where they influence cellular growth and metabolism.

Homeostasis of the internal environment is maintained partly by the autonomic nervous system and partly by the endocrine system. The autonomic nervous system is concerned with rapid changes, while endocrine control is mainly involved in slower and more precise adjustments.
Although the hypothalamus is classified as a part of the brain rather than an endocrine gland, it controls the pituitary gland and has an indirect effect on many others.

The ovaries and the testes secrete hormones associated with the reproductive system after puberty. The placenta that develops to nourish the developing fetus during pregnancy also has an endocrine function. In addition there are also other hormones that do not travel to remote target organs but act locally.

Overview of hormones

When a hormone arrives at its target cell, it binds to a specific receptor, where it acts as a switch influencing chemical or metabolic reactions inside the cell.

The level of a hormone in the blood is variable and self-regulating within its normal range. A hormone is released in response to a specific stimulus and usually its action reverses or negates the stimulus through a negative feedback mechanism. This may be controlled either indirectly through the release of hormones by the hypothalamus and the anterior pituitary gland, e.g. steroid and
thyroid hormones, or directly by blood levels of the stimulus, e.g. insulin and glucagon.

The effect of a positive feedback mechanism is amplification of the stimulus and increasing release of the hormone until a particular process is complete and the stimulus ceases, e.g. release of oxytocin during labour