Chapter Nine
Chapter Nine
The Endocrine System
The Endocrine System consists of endocrine glands that secrete specific chemicals called hormones into the blood or tissue fluid.
The Endocrine System is one of the body's communication systems and uses these hormones as messengers to help to regulate cellular activity by providing a constant internal environment, i.e. homeostasis.
A hormone is a chemical substance that is generated in one organ and carried by the blood to a target organ where it excites activity.
The Endocrine System works very closely with the nervous system to help control the body. Endocrine glands are ductless and secrete their hormones directly into the bloodstream.
The Endocrine Glands in the body are: Hypothalamus; Pituitary; Pineal; Thyroid; Parathyroids; Adrenals; Pancreas - Islets of Langerhans; Thymus; Ovaries; Testes.
The Hypothalamus and Pituitary Gland act as a unit to regulate activity of each other:
Pituitary has two lobes - anterior lobe and posterior lobe
The Pituitary is attached to the Hypothalamus of the brain by a stalk
This stalk brings blood from the hypothalamus rich in oxygen, nutrients, releasing and inhibiting hormones to the anterior lobe to influence its secretions: 'Pituitary Portal System'
The stalk brings nerves from the hypothalamus to the posterior lobe
The Pituitary is often referred to as the 'Master Gland' because it produces several hormones or releasing factors that influence the secretion of hormones of other endocrine glands.
The Pineal Gland
The Pineal Gland
The Thyroid Gland
A two-lobed gland in the neck in front of the larynx and trachea. Two parathyroid glands lie on the posterior of each
It is controlled by the pituitary gland
The gland contains hormones and colloid, thick sticky semifluid protein
It has three major secretions: T three, tri-iodothyronine, T four, Thyroxine, and Calcitonin
Cells, parafollicular cells, produce the hormone Calcitonin which reduces Calcium levels in the blood. It inhibits calcium reabsorption from bones and renal tubules. An increase in calcium blood levels means an increase in calcitonin levels.
Iodine is essential for formation of T three and T four and has to be ingested.
After formation, T three and T four combine with colloid to form 'Thyroglobulin'
The function of T three and T four are: Physical growth, Mental development, metabolic rate control, Peristalsis, Nervous function.