The Endocrine System
· The physical human body is made up of a multitude of different kinds of cells but can only grow and develop and function properly if there is co-ordinated interaction between these various kinds of cells. One of the ways in which cells communicate with each other is by chemical signals. The chemical signals are carried in the blood stream to the cells which they act upon by molecules called hormones. These hormones are produced by a number of glands and other body organs which collectively form the endocrine system.
· Hormones can have powerful effects on the body so their production must be in balance for the body to function properly and without adverse effect. Excessive production of hormones may be harmful.
· Some hormones exercise control over the other glands or muscles. Others control or co-ordinate body activities such as growth and development.
· Some glands, e.g. the thyroid gland and the adrenal glands are purely endocrine glands, producing only hormones. Some other glands such as the pancreas produce both external secretions and hormones.
· Some of the glands do not function or produce hormones throughout the complete lifespan of a human. For example, the thymus gland tends to be active only during the early years whilst the reproductive organs of a woman are only active between the onset of puberty and the onset of the menopause.
· The effect of some hormones is short term, whilst the effect of others is long term. For example, the hormone adrenaline has a short term effect whilst the hormone thyroxine creates permanent effects in the body.
· When hormones have performed their functions, they are transported in the blood stream to the liver where they are eventually destroyed.
· The pituitary gland plays an important role as a ‘controller’ or ‘master’ gland. It is divided into three sections, the anterior lobe, the intermediate lobe and the posterior lobe. These secrete hormones which control the activities of particular glands.
· There are two main types of hormones – (Type 1) proteins, peptides, and modified amino acids and (Type 2) steroids. Both types work by finding protein based receptor material in the appropriate part of the body which is able to respond to the presence of the hormone. The binding of the hormones to receptor material initiates a sequence of intracellular signals that may alter the behaviour of the cell or stimulate (or repress) gene expression in the cell nucleus. The mineral calcium is an important facilitator of the interaction between the hormones and receptor material.
Some tables follow as examples to indicate the effect of some of the hormones produced by the body and of their over- or under-production. After these is a longer list (source Wikipedia) of hormones produced within the endocrine system. The key learning objective here is to gain a simple conceptual understanding of hormones in the human endocrine system. Once you understand the concept, you can research further any possible link between hormonal imbalance and health conditions that your clients might have. After the list are diagrams which show the location of the major endocrine glands and diagrams showing the hypothalamus and pituitary glands in the head.
Examples of hormones
ENDOCRINE GLAND : ThyroidHormone produced : Thyroxine (contains iodine)
Trigger for the secretion of hormone : Reception of trigger hormone from pituitary gland
Effects of hormone secretion : The regulation of the metabolic rate of the body, ensuring that normal physical growth and normal mental growth takes place.
Effects of insufficient hormone secretion : Lack of physical and mental development in a child. In extreme cases, cretinism can occur. Later in adult life, thyroxine deficiency can result in an overweight sluggish physical body and simple mindedness.
Effects of excessive hormone secretion : An unusually high metabolic rate. This can result in a person having a lack of attention, difficulty concentrating, irritability, restlessness and nervousness. The physical body might be very thin.
ENDOCRINE GLAND : Pancreas
Hormone produced : Insulin
Trigger for the secretion of hormone : Increased concentration of glucose in the blood.
Effects of hormone secretion : Enables body cells to oxidise glucose to produce energy in the cellular respiratory process. Excessive glucose is converted to glycogen, enabling it to be stored in the liver and the muscles. The overall effect is to enable the body to manage the glucose levels in the blood.
Effects of insufficient hormone secretion : Low insulin levels mean that the transport of glucose across cell membranes is inhibited, and normal cellular respiratory processes are thereby affected. Appetite can be affected adversely. Long term low insulin levels can result in chronic hyperglycemia which is linked to diabetes and resulting medical conditions such as damage to the eyes and nerves and reduced effectiveness of the kidneys. .
Effects of excessive hormone secretion : Blood glucose levels become too low and can result in hypoglycemia. Moderate hypoglycemia can result in a general feeling of not being well due to body functions being affected. Severe hypoglycemia can result in coma or in extreme cases death.
ENDOCRINE GLAND : Adrenal
Hormone produced : Adrenaline
Trigger for the secretion of hormone : A situation of fear, anger, anxiety
Effects of hormone secretion : Adrenaline has the overall effect of supporting a person when facing a fight or flight emergency situation by increasing the amount of energy available through the following. It has the effect of increasing the rate of heartbeat and increasing blood pressure so that oxygen and glucose are carried faster to the muscles. It causes constriction of the blood arteries in the skin so that more blood flows to the muscles. It influences the conversion of stored glycogen to glucose to fuel cell activity. It causes an increase in the metabolic rate so that more energy can be released in the cellular respiration process.
ENDOCRINE GLAND : Testes (testicles) in males
Hormone produced : Testosterone
Trigger for the secretion of hormone : Reception of trigger hormone from pituitary gland
Effects of hormone secretion : It initiates and controls the development of the male sex organs and secondary male sexual characteristics such as deepening of the voice and the growth of facial hair.
Effects of insufficient hormone secretion : The full male sexual characteristics do not develop fully.
Effects of excessive hormone secretion : Excessive masculinity.
ENDOCRINE GLAND : Ovaries in females.
Hormone produced : Oestrogen
Trigger for the secretion of hormone : Reception of trigger hormone from pituitary gland
Effects of hormone secretion : It initiates and controls the development of the female sex organs and secondary female sexual characteristics such as the menstrual cycle.
Effects of insufficient hormone secretion : The full female sexual characteristics do not develop fully.
Effects of excessive hormone secretion : Excessive femininity.
ENDOCRINE GLAND : Ovaries in females.
Hormone produced : Progesterone
Trigger for the secretion of hormone : Reception of trigger hormone from pituitary gland
Effects of hormone secretion : The preparation and enlargement of the wall of the uterus to receive a fertilized egg at the onset of pregnancy and to provide a home for the foetus during the pregnancy. The enlargement of the mammary glands so that milk can be produced.
Effects of insufficient hormone secretion : Risk of failure of the pregnancy as by miscarriage.
Effects of excessive hormone secretion : The process of giving birth might not happen at the correct time and a caesarean section might be needed.
ENDOCRINE GLAND : Pituitary (Anterior Lobe)
Hormone produced : Thyrotropin (Thyroid Stimulating Hormone)
Trigger for the secretion of hormone : Secretion stimulated by the reception of thyrotropin releasing hormone (TRH) from the hypothalamus gland. Secretion inhibited by the reception of somatostatin from the hypothalamus gland.
Effects of hormone secretion : Stimulates the thyroid gland to secret its hormone thyroxine which affects the metabolism within the body.
Effects of insufficient hormone secretion : Inadequate stimulation of the thyroid gland and therefore upon basic body metabolic processes. Depression is sometimes linked to insufficient levels of thyrotropin.
Effects of excessive hormone secretion : Over stimulation of the thyroid gland with the body metabolic processes working in ‘overdrive’ and not at a rate that is matched to body requirements. Resulting symptoms can include low weight combined with a ravenous appetite, hyperactivity, irritability, tiredness and apathy, and problems with body temperature and sensitivity to heat.
ENDOCRINE GLAND : Pituitary (Anterior Lobe)
Hormone produced : Prolactin
Trigger for the secretion of hormone : Secretion stimulated by the reception of thyrotropin releasing hormone (TRH) from the hypothalamus gland.
Effects of hormone secretion : It helps to prepare the breasts of a pregnant woman for the production of milk.
Effects of insufficient hormone secretion : Inadequate levels of milk production.
Effects of excessive hormone secretion : Spontaneous flow of milk from the breasts when a baby is not taking milk.
ENDOCRINE GLAND : Pituitary (Anterior Lobe)
Hormone produced : Somatotropin. Also known as Growth Hormone
Trigger for the secretion of hormone : Secretion stimulated by the reception of Growth Hormone Releasing Hormone (GHRH) from the hypothalamus gland.
Effects of hormone secretion : Growth Hormone binds to receptors on the surface of liver cells which stimulates them to release into the blood stream another hormone called Somatodin. Somatodin acts directly on the ends of the long bones in the body and influences their growth.
Effects of insufficient hormone secretion : Long bone growth is stunted and the long bones are too short for the body, resulting in shortness. In extreme cases, dwarfism can result.
Effects of excessive hormone secretion : Long bone growth is excessive and the long bones are too long for the body. In extreme cases, gigantism can result.
ENDOCRINE GLAND : Pituitary (Posterior Lobe)
Hormone produced : Antidiuretic Hormone (ADH)
Effects of hormone secretion : ADH works on the collecting ducts of the kidneys so that water may be reabsorbed into the blood stream, thereby reducing the volume of urine created. The overall effect is to ensure adequate fluid retention within the body.
Effects of insufficient hormone secretion : Excessive urination. This results in great thirst and the need to drink high volumes of fluids to avoid dehydration.
Effects of excessive hormone secretion : Inadequate urination with subsequent adverse symptoms such as electrolyte disturbance and water intoxication (poisoning). Water intoxication is a disturbance in brain function that occurs when the normal balance of electrolytes in the body is pushed beyond safe limits and in extreme cases can cause death.
The endocrine system and the hormones secreted
· The Hypothalamus produces
o Thyrotropin-releasing hormone (TRH)
o Gonadotropin-releasing hormone (GnRH)
o Growth hormone-releasing hormone (GHRH)
o Corticotropin-releasing hormone (CRH)
o Vasopressin
o Somatostatin (SS; also GHIH, growth hormone-inhibiting hormone)
o Prolactin inhibiting hormone or PIH or Dopamine (DA)
o Prolactin releasing hormone
· The Pineal Body produces
o Melatonin
· The Pituitary Gland (hypophysis) produces
o Anterior pituitary lobe (adenohypophysis)
§ Growth hormone (GH)
§ Prolactin (PRL)
§ Adrenocorticotropic hormone (ACTH, corticotropin)
§ Lipotropin
§ Thyroid-stimulating hormone (TSH, thyrotropin)
§ Follicle-stimulating hormone (FSH)
§ Luteinizing hormone (LH)
o Posterior pituitary lobe (neurohypophysis)
§ Oxytocin
§ Vasopressin (AVP; also ADH, antidiuretic hormone)
o Intermediate pituitary lobe (pars intermedia)
§ Melanocyte-stimulating hormone (MSH)
· The Thyroid Gland produces
o Triiodothyronine (T3), the potent form of thyroid hormone
o Thyroxine (T4), Also known as tetraiodothyronine, it is a less active form of thyroid hormone
o Calcitonin
· The Parathyroid Gland produces
o Parathyroid hormone (PTH)
· The Heart produces
o Atrial-natriuretic peptide (ANP)
o Brain natriuretic peptide (BNP)
o Adenosine
· Striated muscle produces
o Thrombopoietin
· The Skin produces
o Vitamin D3 (calciferol)
· Adipose (or fatty) tissue produces
o Leptin
o Estrogens (mainly Estrone)
· The Stomach produces
o Gastrin
o Ghrelin
o Neuropeptide Y (NPY)
o Secretin
o Somatostatin
o Histamine
o Endothelin
· The Duodenum produces
o Cholecystokinin
· The Liver produces
o Insulin-like growth factor (IGF)
o Angiotensinogen
o Thrombopoietin
· The Pancreas produces
o Insulin
o Glucagon
o Somatostatin
o Pancreatic polypeptide
· The Kidney produces
o Renin
o Erythropoietin (EPO)
o Calcitriol (the active form of vitamin D3)
o Thrombopoietin
· The Adrenal Glands
o Adrenal cortex produces
§ Glucocorticoids (chiefly cortisol)
§ Mineralocorticoids (chiefly aldosterone)
§ Androgens (including DHEA and testosterone)
o Adrenal medulla produces
§ Adrenaline (epinephrine)
§ Noradrenaline (norepinephrine)
§ Dopamine
§ Enkephalin
· The Testes
o Androgens (chiefly testosterone)
o Estradiol
o Inhibin
· The Ovaries
o Progesterone
o Androstenedione
o Estrogens (mainly estradiol)
o Inhibin
· The Placenta (when a woman is pregnant)
o Progesterone
o Estrogens (mainly Estriol)
o Human chorionic gonadotropin (HCG)
o Human placental lactogen (HPL)
o Inhibin
· The Uterus (when a woman is pregnant)
o Prolactin (PRL)
o Relaxin
The Exocrine System
The Exocrine system is another group of body glands and organs that release hormones. One of the differences with the endocrine system is that the hormones are not released directly into the blood stream. The primary exocrine system organs are the salivary glands, stomach, liver, pancreas, mammary glands and sweat glands.