9. The Endocrine System Flashcards
Describe the physiology of the thyroid
The thyroid gland is located at the base of the throat just inferior of the Adam’s apple. It is a fairly large gland consisting of two lobes joined by a Central mass or isthmus. The thyroid gland makes two hormones: the thyroid hormone and calcitonin.
Internally the thyroid gland is composed of hollow structures called follicles, which store is sticky colloidal material. The thyroid hormone is derived from this colloid.
The thyroid hormone is actually two active iodine containing hormones: thyroxin or T4 and triiodothyronine or T3. Thyroxin is the major hormone secreted by the thyroid follicles.
Most T3 is formed at the target tissues by conversion of T4 to T3. These two hormones are very much alike. Each is constructed from two tyrosine amino acids linked together but T4 has four bound iodine atoms whereas T3 has three.
The thyroid hormone controls the rate at which glucose is oxidized and converted into body heat and chemical energy. All body cells depend on a continuous supply of chemical energy to power their activities for this reason every cell in the body is a target.
This hormone is also important for normal tissue growth and development in particular in the reproductive and nervous system.
Calcitonin decreases blood calcium levels by causing calcium to be deposit in the bones. It acts antagonistically to the parathyroid hormone which is produced by the parathyroid glands.
Calcitonin is made by the so-called parafollicular cells found in the connective tissue between the follicles. It is released directly to the blood in response to increasing levels of blood calcium. Calcitonin production is meager or ceases entirely in adults
Describe the neuroendocrine effects of stress
The fight or flight response is a reaction that occurs in response to a stressful situation such as when people feel threatened physically or emotionally and it’s brought about by the sympathetic nervous system.
One of the organs is stimulate is the adrenal medulla, which pumps its hormones into the bloodstream to enchance and prolong the effects of the neurotransmitters of the sympathetic nervous system.
The adrenal medulla develops from a knot of nervous tissue. When it is stimulated by sympathetic nervous system neurons, its cells release two similar hormones, epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream. Collectively this hormones are called catecholamines.
We can say that the catecholamines increase health rate, blood pressure and blood glucose levels and dilate the small passageways of the lungs. as a result there is more oxygen and glucose in the blood and a faster circulation of blood to the body organs.
In this way the body is better able to deal with a short -term stressor. so they prepare the body to cope with a brief or short-term stressful situation and cause the so-called alarm stage of the stress response.
On the other hand, the glucocorticoids are produced by the adrenal cortex and are important in helping the body to cope with prolonged or continuing stressor, such as dealing with the death of a family member or having a major operation.
They operate mainly during the resistance stages of the stress response. if they are successful in protecting the body the problem will eventually be resolved without lasting damage to the body. otherwise if the stress continues on and on then the adrenal cortex may simply burn out which is usually fatal.
