Title: The Hormonal System

Key words: glands, hormones, pituitary, thyroid, pancreas, pineal, testis, ovary, thymus, adrenal

Date: July 2000

Category: 6. The Body

Type: Article

Author: DJE Candlish

 

The Hormonal System

 

Introduction

The hormonal system has a varied range of functions, usually involving slower or longer term processes than the nervous system. These include growth and sexual maturation, menstruation and digestion.

The system consists of a number of ductless glands, which release hormones directly into the bloodstream. These include:

Hormones are chemical messengers which stimulate or suppress activity in their target organs, tissues or metabolic processes. Their effects are slower and more generalised than those of the nervous system. Most hormones are proteins which are synthesised in specialised cells in the gland. Some glands secrete more than one hormone and some hormones affect more than one target organ or process, so the interplay between glands, body systems and hormones can be complex.

While the pituitary gland is considered to be the master gland, the hypothalamus is often called the conductor of the hormonal orchestra, as it co-ordinates the activities of the pituitary and other glands. This indicates the interaction between the nervous and hormonal systems. Negative feedback systems control the levels of each hormone in the blood, to keep a balance between secretion and destruction. Most hormones are destroyed in the liver.

 

The pituitary

This is a pea sized gland located directly below the hypothalamus and connected to it by a thin stalk. It rests on the sphenoid bone at the base of the skull. It has two anatomically and functionally distinct sections, the posterior lobe or neurohypophysis and the anterior lobe or adenohypophysis.

The neurohypophysis

The neurohypophysis is different to most other glands, as it consists of a special type of glial cell. Many of the neurones in the hypothalamus terminate between these cells. Their cell bodies produce the hormones which are carried along axons via the pituitary stalk to fibre ends in the pituitary. Here the hormones are secreted and absorbed in the capillary network. The hormones involved are antidiuretic hormone (ADH) and oxytocin.

Antidiuretic hormone is released in response to any increase in the concentration of the blood. It increases the permeability of kidney tubules to water, so more water is reabsorbed. This makes the blood less concentrated and the urine more concentrated, to avoid the body becoming dehydrated. Once the blood is back to normal concentration, secretion of ADH stops.

Oxytocin is produced towards the end of pregnancy. It stimulates contraction in smooth muscle like that of the uterine wall. The sex hormone progesterone makes smooth muscle insensitive to oxytocin. When progesterone levels fall as delivery begins, oxytocin starts the contractions known as labour.

After the birth, the contractions continue, to expel the placenta (afterbirth). Even then oxytocin continues to be secreted, as it also stimulates milk production. A baby sucking on the nipple increases oxytocin secretion and increases the flow of milk it receives.

The adenohypophysis

There are glandular cells in this part of the pituitary, under the direct control of the hypothalamus. Nerves in the hypothalamus secrete specific releasing factors for each of the seven hormones the adenohypophysis secretes. These are of two types, the first stimulating or regulating the function of other glands (trophic hormones), the second having a direct effect on certain body functions (effector hormones):

 

Trophic hormones:

 

Effector hormones:

Growth hormone or somatotrophin stimulates the growth of all tissues, increasing cell division and cell growth. Melanocyte stimulating hormone is thought to be responsible for the normal intra-uterine colour of the skin. Mammary stimulating hormone stimulates the development of the breasts and lactation after childbirth. Luteotrophic hormone has the same structure as PRH but has a different name to indicate its second function - preserving and increasing secretion from the corpus luteum in early pregnancy.

 

The thyroid gland

The thyroid gland is located below the thyroid cartilage in the neck. It is butterfly shaped, with the lobes of the gland as 'wings'. The glandular tissue is arranged around a number of colloid-filled cavities in a follicular structure with an extensive capillary network. The thyroid produces two hormones, thyronine and calcitonin. Thyronine has two forms - tri-iodothyronine and thyroxine which contain 3 and 4 iodine atoms respectively.

As thyronine is produced, it is stored in the colloid. When TSH is released by the pituitary, it stimulates release of thyronine into the capillaries. When thronine reaches the body tissues, it increases the metabolic rate of cells and stimulates protein synthesis.

Calcitonin is produced in different cells of the thyroid and is secreted directly into the bloodstream. It is released when the calcium (Ca++) concentration in the blood increases. Calcitonin decreases blood Ca++ by depositing calcium in bone, decreasing reabsorption in the kidney and finally by decreasing intestinal absorption. As soon as blood Ca++ levels are normal, calcitonin secretion stops again.

The parathyroid glands

The parathyroid glands are located on the four 'wing tips' of the thyroid. They consist of epithelial tissue surrounded by capillaries and connective tissue. The hormone secreted by the parathyroids is parathyroid hormone (PTH) or parathyrin. It has the opposite effect to calcitonin and is released in response to a drop in blood Ca++ levels.

Between them, calcitonin and PTH keep the Ca++ level of the blood within defined limits, to maintain normal bone metabolism and muscle function.

 

The pancreas

The pancreas secretes enzymes for digestion, but it also contains glandular tissue - the Islets of Langerhans. An 'islet' consists of two hormone producing cell types alpha-cells and beta-cells. The alpha-cells produce glucagon hormone, which stimulates the conversion of glycogen to glucose for energy. Adrenaline (see later) has a similar effect.

The beta-cells produce the hormone insulin. Insulin stimulates the conversion of glucose to glycogen for storage in the muscles and liver. It also accelerates the transport of glucose across cell membranes. As a result, insulin has a strong effect in lowering blood sugar. Between them, glucagon hormone, adrenaline and insulin keep blood glucose level within defined limits, as the parathyroid hormones do with calcium.

The pineal gland

The pineal gland or epiphysis is a small conical gland, almost in the centre of the brain. Its function is not known precisely, but it is thought to secrete melatonin, a hormone that holds back production of sex hormones and so regulates the time of puberty.

The thymus gland

The thymus gland is located behind the sternum or breastbone. It functions until puberty and in adults is only present as a fatty lobe on the heart. Its function is to produce T-lymphocytes which are important in cell-mediated immunology. It also prevents production of antibodies to body proteins or 'self-antigens' during development of the fetus. This stops the embryo destroying itself. The thymus produces thymocytes that become the source of T-lymphocytes. They migrate from the thymus shortly after birth, spreading throughout the reticulo-endothelial system, which consists of bone marrow, lymphatic tissue, spleen and liver.

 

The adrenal glands

These sit over the kidneys, looking rather like a dollop of cream and are encased in the same protective layer of fat as the kidneys. In structure and function the adrenals consist of two separate components, the cortex and the medulla. The cortex is affected by ACTH from the adenohypophysis and the medulla by the sympathetic nervous system.

 

Adrenal cortex

The adrenal cortex produces corticosteroids or corticoids. The three layers in the epithelium of the cortex each produce their own group of these hormones. The outer layer, the zona glomerulosa, is the thinnest layer. It produces mineralocorticoids, which control the mineral balance of the body, especially sodium and potassium in the blood. The most important of these is aldosterone.

The middle layer, the zona fasciculata, produces glucocorticoids, which control glucose balance in the body. The most important, cortisol, has the opposite effect to insulin, as it raises blood glucose. It also slows protein synthesis and damps down inflammatory reactions if these become too severe.

The inner layer, or zona reticularis, produces both male (androgenic) and female (oestrogens) hormones. However, the amounts produced are small in comparison with the ovaries and testes, so their influence on sexual development is minimal.

The medulla

The medulla is at the centre of the adrenal glands. It is derived from nerve tissue and controlled by the sympathetic nervous system. It produces two hormones, adrenaline and noradrenaline. Adrenaline affects the organs of the body via the bloodstream in the same way as the sympathetic system does via nerve fibres. It prepares the body for vigorous activity - the so-called 'flight or fright' reaction, by raising blood sugar for extra energy output and increasing heart rate and muscle tension. It also damps down the digestive process, to divert blood from the stomach to the muscles. Noradrenaline has similar effects and is always secreted with adrenaline.

The testis

The testis is the male organ of reproduction. It produces spermatozoa and also secretes androgens, the male sex hormones. The cells that produce the hormones, the interstitial or Leydig cells, are located in the connective tissue between the spermatic ducts. They secrete testosterone in response to ICSH from the adenohypophysis. Testosterone promotes the male sexual characteristics, such as penis development, growth of pubic and facial hair, deepening of the voice and increased muscle bulk.

The ovary

The ovaries secrete oestrogen and progesterone, the female sex hormones. These are involved in the complex process of menstruation and also control female sexual characteristics such as breast development, growth of pubic hair and body shape.

In its first stage of development in the ovary, the female egg or ova is surrounded by epithelial cells called follicular cells. These secrete oestrogen in response to FSH from the adenohypophysis.

In negative feedback, oestrogen in its turn slows down FSH production and stimulates LH production in the adenohypophysis.

The pre-menstrual stage is controlled by oestrogen. It stimulates uterine development in readiness for the egg during the proliferation stage. At ovulation (release of the egg) oestrogen levels fall due to the effect of LH on the follicles. The follicular cells left behind in the ovary after ovulation swell and fill with a fatty susbstance called lutein, to form the corpus luteum. This continues to produce oestrogen but now also secretes progesterone.

This hormone further prepares the uterine wall for implantation of the egg if it is fertilised. It also stimulates the milk glands in the breasts, which many women find uncomfortable in the second phase of their period. If fertilisation does not take place, the corpus luteum degenerates, stopping production of progesterone and allowing menstruation (removal of the lining of the womb) to take place.

 

These hormones controlling the mentrual cycle interact in a complex way:

(+) (+)

FSH oestrogen è LH progesterone

(-) (-)

(+) stimulates production

(-) inhibits production