The Endocrine System
|The Endocrine Glands: (See Figure 17.4)
Endocrine glands secrete chemical messengers into the bloodstream. Through the bloodstream they can have effects on a few or many target organs or other glands. Some of the endocrine glands have already been considered in other contexts or will be shortly. In addition to the endocrine glands per se, the hypothalamus is an important component in regulating endocrine function, and in fact is an endocrine secretor. The hypothalamus controls the anterior pituitary gland (adenohypophysis) and through it the thyroid, gonads, adrenal cortex, and the melanocytes. The hypothalamus actually produces the hormones secreted by the posterior pituitary (neurohypophysis). In addition the hypothalamus acts through the autonomic nervous system to control the adrenal medulla. Other glands such as the parathyroid and pancreas respond directly to humoral stimuli. (See Figure 17.4).
The nature of the connection of the hypothalamus and pituitary is shown by [Figure 17.5] and [Endocrine Control Diagram] The anterior pituitary is also called the adenohypophysis. Adeno means gland and is given to this organ because it actually secretes a group of hormones known as the tropic hormones. These hormones control other glands or act on other tissues. The glands controlled by the tropic hormones are also endocrine glands and represent a second tier gland in the control mechanism. They secrete a second hormone which has actions on specific body tissues or organs and has a feedback effect on the hypothalamus to control its secretion. The hypothalamus controls the adenohypophysis through releasing and/or inhibiting hormones. These hormones either stimulate release of the tropic hormone or inhibit it as part of feedback control.
The posterior pituitary is called the neurohypophysis because the hormones it releases are actually released by neurons arising in the hypothalamus. The posterior pituitary stores these hormones for release on command, again controlled by the hypothalamus.
|The Hormones of the adenohypophysis: See [Hormone Chart]
and (Table 17.1).
The Tropic Hormones:
The gonadotropins FSH and LH (ICSH) are released in response to releasing hormone GnRH. FSH (Follicle Stimulating Hormone) stimulates gametogenesis in both males and females. In females this involves follicle development and the first stage of oogenesis. FSH also stimulates estrogen secretion. In males FSH stimulates spermatogenesis in complex mechanism to be discussed later.
LH (Luteinizing Hormone) causes ovulation and progesterone secretion. In males the same hormone is called ICSH (Interstitial Cell Stimulating Hormone) stimulates interstitial cells in the testes to secrete testosterone.
Thyroid Stimulating Hormone (TSH) is secreted in response to TRH from the hypothalamus. TSH causes the thyroid to secrete its hormones, T4 (thyroxine) and T3.
Adrenal Corticotropic Hormone (ACTH) - stimulates the release of corticosteroids from the adrenal cortex. ACTH is released in response to CRH from the hypothalamus.
The following are not considered tropic hormones proper because they do not control other second tier glands. Instead their effects are on a variety of tissues. Feedback occurs via resulting effects and hormones.
The two hormones below are secreted by the adenohypophysis but are not considered tropic hormones (despite their synonyms) because they don't simulate a second tier gland.
Growth Hormone (GH, somatotropin) - controlled by both releasing and inhibiting hormones, GHRH and GHIH (somatostatin), from the hypothalamus. GH (See Figure 17.6)causes growth and development of the musculoskeletal system and other tissues. It stimulates amino acids to be used for protein synthesis and causes lipolysis to provide fatty acids for catabolism. For these reasons it is sometimes abused to stimulate muscle growth and catabolize fat. Negative feedback results from GH itself and also from mediators called somatomedins (Somatomedin is also known as Insulin-like Growth Factor 1 [IGF-1]) produced by the liver, muscles, and other tissue. Positive feedback is produced by strenuous exercise and energy demanding activities. Childhood hypersecretion of GH causes the excessive growth seen in gigantism, adulthood hypersecretion causes acromegaly, a condition in which the bones are exaggerated in shape. Hyposecretion in childhood causes dwarfism. Abuse of GH can lead to acromegaly and pituitary diabetes caused by the overstimulation of pancreatic beta cells.
Prolactin (PRL) - promotes breast development and milk production. PRL is secreted in response to high estrogen and progesterone levels which occur in pregnancy, and in response to infant suckling. Control is through PRH and PIH from the hypothalamus.
|The hormones from the neurohypophysis are secreted by neurons from the hypothalamus. They
ADH - Anti Diuretic Hormone- as discussed earlier ADH increases reabsorption of water from the kidney's collecting tubules in response to increasing blood osmolarity. Insufficiency of ADH usually results from destruction of cells in the hypothalamus and results in diabetes insipidus, the production of a large volume of dilute urine. It renders the individual unable to concentrate the urine with frequent dehydration.
Oxytocin (OT) - Stimulates uterine smooth muscle contractions in labor, and also triggers milk ejection by the mammary glands. Released by hypothalamic neurons in response to physical and chemical stimuli at the end of pregnancy and by infant suckling. Also used clinically to induce labor.
|Thyroid Hormones: See [Thyroid Function Diagram]
[Thyroid Control Mechanism]
The thyroid gland (Figure 17.8) consists of follicles whose cells secrete the two thyroid hormones, T4 and T3. T4, also called thyroxine or tetraiodothyronine, is the inactive form, while T3, triiodothyronine, is the active hormone. T4 has four iodine atoms while T3 has three. Thyronine is the name given to a dimer of the amino acid tyrosine. T4 is produced in about 20 times as much volume as T3 and both are stored as thyroglobulin colloid in the lumen of the follicles. The follicular cells release the thryroglobulin into the follicle by exocytosis, and also resorb it and release T4 and T3 into the interstitial space to be taken into fenestrated capillaries. T4 and T3 are taken into target cells (muscle and other cells, but NOT the brain, spleen, or gonads) and into the nucleus where T3 activates genes which control cellular metabolism. Target cells convert T4 to T3. These hormones are controlled by TSH from the adenohypophysis in response to TRH from the hypothalamus.
A detailed look at the several causes of low thyroid function, their diagnoses, and
The parafollicular cells secrete calcitonin (a.k.a. thyrocalcitonin). This hormone stimulates deposition of the inorganic calcium into bone in children, and lowers blood calcium. The specific function of calcitonin is to increase osteoblastic activity. It is normally unimportant in bone maintenance in adults, however it is being used clinically to aid patients in reversing osteoporosis. The parafollicular cells are stimulated directly by rising blood calcium levels. But in adults rising calcium levels do not result in increased bone deposition.
|The parathyroid gland (Figure
17.10) consists of small bean-like glands embedded in the posterior portion of
the thyroid. They secrete parathyroid hormone (parathormone, PTH) which is the hormone
responsible for calcium homeostasis. (See Figure 17.11)The parathyroid glands respond to
lowering blood calcium levels by secreting PTH. PTH uses several sources to raise blood
1) first, PTH triggers increased Vit D3 (the active form) formation in the kidney. Vitamin D3 is necessary for calcium absorption from the gut, and increased levels of the vitamin will reduce the amount of calcium which is unabsorbed. This can be significant when calcium is taken in with insufficient Vit D.
2) PTH increases Ca+2 reabsorption from the kidney tubules. This reduces the calcium lost to the urine.
3) The last resort for increasing blood calcium is your bones. PTH increases osteoclastic activity which causes resorption of the bone matrix.
|The Adrenal Cortex: (See Figure 17.12) This is the outer layer of the
adrenal gland. It secretes
a group of hormones known as the corticosteroids. This name reflects their origin and their
chemical structure, based on the cholesterol molecule. They comprise three groups:
1) mineralcorticoids - best known is aldosterone which has already been studied. These are produced mostly by the outer zona glomerulosa layer of the cortex. The mineralcorticoids regulate electrolyte balance by increasing Na+ reabsorption and K+secretion.
2) glucocorticoids - best known is cortisol, also already studied. Cortisol makes glucose available from breakdown of proteins and fats during serve stress and is anti-inflammatory.
The two groups above are released in response to ACTH from the adenohypophysis. This is a stress response mediated by the hypothalamus (See Figures 17.13 and 17.15) which mobilizes the body's resources and raises blood pressure.
3) The third group is the gonadocorticoids, the sex hormones. These are not controlled by ACTH. The adrenal cortex complements the gonads by releasing androgens and estrogens which help regulate bone and muscle development and provide the source of estrogens for women after menopause.
|The Adrenal Medulla - the medulla (center) of the adrenal gland is sympathomimetic, i.e. it complements and enhances the effect of the sympathetic nervous system by secreting epinephrine into the bloodstream. This causes more diverse and prolonged responses than result from sympathetic stimulation by itself. The hypothalamus stimulates the adrenal medulla during "Fight or Flight", exercise, and other short term stress situations. (See Figure 17.15)|
(See Figure 17.16) - the pancreas has both exocrine acini (groups of secretory cells) and endocrine
cells in isolated groups known as Islets of Langerhans. In these islets are two types of cells
which concern us, the alpha and the beta cells. The alpha cells produce glucagon and the beta
cells produce insulin. (See Figure 17.17) You learned the basic functions of those hormones in the last unit. Our
consideration here is the disorders resulting from hyper or hyposecretion of these hormones.
Diabetes Mellitus- literally this means the overproduction of a sweet urine. The urine is sweet because of excess glucose (glucosuria) which results from hyperglycemia. There are two types:
Type I, a.k.a. Insulin Dependent Diabetes Mellitus, IDDM. (Formerly called childhood onset diabetes because typically it surfaces early in life, before the age of 30). In these individuals the beta cells of the Islets of Langerhans have suffered damage, usually due to autoimmunity, childhood disease, exposure to toxins, or congenital damage. As a result the pancreas produces inadequate amounts of insulin, sometimes none. Because of this the individual is unable to maintain normal plasma glucose concentration, and is unable to uptake and use glucose in metabolism. Treatment is by means of insulin administration, either by injection or orally depending on the individual. The amount of insulin must correspond to the amount of carbohydrate intake. Formerly this was difficult and would result in periods of hyperglycemia. With modern insulin pumps the process is much more exact and effective. Complications include: 1) hypoglycemia from overdose of insulin - this results in weakness, sometimes fainting, and, in the extreme, coma, all reversible with administration of glucose. 2) ketoacidosis from fat metabolism when insulin is under-administered. This complication can be life threatening. 3) Hyperglycemia when insulin administration is imprecise. Hyperglycemia damages cells and tissues (see below).
Type II - Non Insulin Dependent Diabetes Mellitus, NIDDM. (Formerly adult onset diabetes because it tends to show up after the age of 30, usually around middle age). This type is caused by insulin-resistant receptors on the target cells. Insulin resistance can be the result of:
a) the result of an inherited mutation.
b) due to abnormal or deficient receptor proteins. This is the result of adaptation to hypersecretion of insulin. Receptors to hormones and other chemical messengers are not stable in number or position. They move in the membrane matrix and they increase or decrease in number (called down regulation) in order to modulate the response, i.e. with the object of maintaining the response within a normal range. When the insulin stimulus increases tremendously, as it does in Type II diabetics, the receptors decrease in number and the receptor proteins may be deficient or abnormal.
|Individuals with NIDDM often start out as hypersecretors of insulin. They may take in large
amounts of carbohydrate and calories and this causes the secretion of insulin to be excessive and nearly
constant. Over the long term this results in the receptor defects mentioned above. The obesity
which usually accompanies Type II diabetes is both the result of the consumption of large
quantities of carbohydrate and fat, and the effect of insulin in directing these excess calories to
Another contributing factor is often the lack of stimulus to cells which use glucose, i.e lack of exercise. Exercise increases the demand for glucose by muscle cells and increases the number of receptors and therefore the efficiency of glucose uptake. This is important in reducing the hyperglycemia which is a perennial part of NIDDM. Exercise is often used, together with a diet low in carbohydrates. Insulin analogs or mimics are also used to stimulate the receptors and make the receptors more efficient. In the later stages of the disease insulin production by the pancreas declines and insulin itself may also be used in treatments.
Keto acidosis is not usually a risk in NIDDM because these individuals do not rely solely on fats for metabolism, even when untreated. However it may occur in NIDDM when insulin secretion has been eliminated by burnout of the beta Islet cells.
|The effects of hyperglycemia. Hyperglycemia is the damaging result of NIDDM. It increases blood osmolarity which causes dehydration of tissues. This interferes with electrolyte and water transport and ultimately transport of nutrients and wastes. Cells and tissues break down. Among the first to show damage are the small vessels in the retina. These can be easily visualized with an ophthalmoscope, and this technique, together with urinalysis, remains one of the most important diagnostic tools for diabetes. Vessels in other tissues break down as well, and this destruction of vasculature leads to hypoxia and ischemia of body tissues including the retina, kidneys, limbs etc.|
|Hypoglycemia - low blood glucose level can result from:
1) hyposecretion of glucagon. The alpha cells may also be damaged and insufficient glucagon secretion will result. This leads to hypoglycemia during the early post-absorptive phase. But this condition is transitory because reversal will occur as the declining blood glucose stimulates the hypothalamus to cause adrenal medullary release of epinephrine. Epinephrine will bring glucose levels back up through glycogenolysis.
2) reactive hypoglycemia. This is a condition often preceding and presaging NIDDM. In individuals said to be "carbohydrate sensitive" the pancreas exhibits an exaggerated response to rising blood glucose after a carbohydrate-rich meal. This will produce hypersecretion of insulin causing the plasma glucose level to plummet. These individuals feel weak and may faint due to the hypoglycemia which results, about an hour after the meal. The usual response is to quickly eat some sugar-rich food, which does reverse the hypoglycemia. But this only compounds the problem over the long term. The solution is to reduce the carbohydrate in the meal, replacing it with protein. And to exercise before or after the meal. This releases epinephrine which helps to keep the plasma glucose level up. Many of us experience hunger about an hour after a sugar-rich meal. But individuals with reactive hypoglycemia this is in the extreme with weakness, shaking, and even blackouts occurring.
|The gonads - secretion of the male and female sex hormones occurs in response to gonadotropin control. We will cover this in the reproductive system. The only item to mention here is that these hormones are important, in addition to their sexual functions, in producing the secondary sex characteristics of bone and muscle growth and maintenance, distribution of fat and body hair, breast development, etc.|
|The pineal gland secretes melatonin in response to the absence of light. This sets the diurnal clock mechanism for determining body rhythms and other activities coordinated with the day-night cycle. Although we can reset the clock, it is sometimes difficult and disturbs other body processes. In the far north lack of light in the winter has been known to cause psychological and physical problems. This can be alleviated with a sunlight-like artificial light stimulation. Jet lag is notorious for upsetting physical and psychological well being. Some people have found that taking melatonin at the time they want to sleep can help to reset their body clock.|