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 Pituitary Gland
The pituitary gland and the hypothalamus act as a unit, regulating the activity of most of the other endocrine
glands. The pituitary gland lies in the hypophyseal fossa of the sphenoid bone below the hypothalamus, to
which it is attached by a stalk. It is the size of a pea, weighs about 500 mg and consists of three distinct
parts that originate from different types of cells. The anterior pituitary (adenohypophysis) is an upgrowth
of glandular epithelium from the pharynx and the posterior pituitary (neurohypophysis) is a downgrowth
of nervous tissue from the brain. There is a network of nerve fibers between the hypothalamus and the
posterior pituitary.
Vasculatures
Arterial blood supply
This is supplied by branches from the internal carotid artery. The anterior lobe is supplied indirectly by
blood that has already passed through a capillary bed in the hypothalamus but the posterior lobe is supplied
directly.
Venous drainage
This comes from both lobes, containing hormones, and leaves the gland in short veins that enter the venous
sinuses between the layers of dura mater.
The Anterior Pituitary
This is supplied indirectly with arterial blood that has already passed through a capillary bed in the
hypothalamus. This network of blood vessels forms part of the pituitary portal system, which transports
blood from the hypothalamus to the anterior pituitary where it enters thin-walled sinusoids that are in close
contact with the secretory cells. As well as providing oxygen and nutrients, this blood transports releasing
and inhibiting hormones secreted by the hypothalamus. These hormones influence secretion and release of
other hormones formed in the anterior pituitary.
Growth Hormone (GH)
This is the most abundant hormone synthesized by the anterior pituitary. It stimulates growth and division
of most body cells but especially those in the bones and skeletal muscles. Body growth in response to the
secretion of GH is evident during childhood and adolescence, and thereafter secretion of GH maintains the
mass of bones and skeletal muscles. It also regulates aspects of metabolism in many organs, e.g., liver,
intestines and pancreas; stimulates protein synthesis, especially tissue growth and repair; promotes
breakdown of fats; and increases blood glucose levels.
Thyroid Stimulating Hormone (TSH)
This hormone is synthesized by the anterior pituitary and its release is stimulated by thyrotrophin releasing
hormone (TRH) from the hypothalamus. It stimulates growth and activity of the thyroid gland, which
secretes the hormones thyroxine (T4) and tri-iodothyronine (T3). Release is lowest in the early evening and
highest during the night. Secretion is regulated by a negative feedback mechanism. When the blood level
of thyroid hormones is high, secretion of TSH is reduced, and vice versa.
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 Pituitary Gland
The pituitary gland and the hypothalamus act as a unit, regulating the activity of most of the other endocrine
glands. The pituitary gland lies in the hypophyseal fossa of the sphenoid bone below the hypothalamus, to
which it is attached by a stalk. It is the size of a pea, weighs about 500 mg and consists of three distinct
parts that originate from different types of cells. The anterior pituitary (adenohypophysis) is an upgrowth
of glandular epithelium from the pharynx and the posterior pituitary (neurohypophysis) is a downgrowth
of nervous tissue from the brain. There is a network of nerve fibers between the hypothalamus and the
posterior pituitary.
Vasculatures
Arterial blood supply
This is supplied by branches from the internal carotid artery. The anterior lobe is supplied indirectly by
blood that has already passed through a capillary bed in the hypothalamus but the posterior lobe is supplied
directly.
Venous drainage
This comes from both lobes, containing hormones, and leaves the gland in short veins that enter the venous
sinuses between the layers of dura mater.
The Anterior Pituitary
This is supplied indirectly with arterial blood that has already passed through a capillary bed in the
hypothalamus. This network of blood vessels forms part of the pituitary portal system, which transports
blood from the hypothalamus to the anterior pituitary where it enters thin-walled sinusoids that are in close
contact with the secretory cells. As well as providing oxygen and nutrients, this blood transports releasing
and inhibiting hormones secreted by the hypothalamus. These hormones influence secretion and release of
other hormones formed in the anterior pituitary.
Growth Hormone (GH)
This is the most abundant hormone synthesized by the anterior pituitary. It stimulates growth and division
of most body cells but especially those in the bones and skeletal muscles. Body growth in response to the
secretion of GH is evident during childhood and adolescence, and thereafter secretion of GH maintains the
mass of bones and skeletal muscles. It also regulates aspects of metabolism in many organs, e.g., liver,
intestines and pancreas; stimulates protein synthesis, especially tissue growth and repair; promotes
breakdown of fats; and increases blood glucose levels.
Thyroid Stimulating Hormone (TSH)
This hormone is synthesized by the anterior pituitary and its release is stimulated by thyrotrophin releasing
hormone (TRH) from the hypothalamus. It stimulates growth and activity of the thyroid gland, which
secretes the hormones thyroxine (T4) and tri-iodothyronine (T3). Release is lowest in the early evening and
highest during the night. Secretion is regulated by a negative feedback mechanism. When the blood level
of thyroid hormones is high, secretion of TSH is reduced, and vice versa.
