The Endocrine System PDF

# The Endocrine System ## Introduction: All the physiological activities of the body are regulated by two major systems: 1. nervous system 2. endocrine system These systems interact with one another and regulate the body functions. ## Cell-to-cell signaling: It refers to the transfer of infor...

# The Endocrine System ## Introduction: All the physiological activities of the body are regulated by two major systems: 1. nervous system 2. endocrine system These systems interact with one another and regulate the body functions. ## Cell-to-cell signaling: It refers to the transfer of information from one cell to another. It is also called cell signals or intercellular communication. This is completed through chemical substances called chemical messengers. ## Chemical messengers are classified into: 1. endocrine messenger 2. paracrine messenger 3. autocrine messenger 4. neurocrine messenger ### 1- endocrine messenger: are classical hormones ### 2- paracrine messengers: Are chemicals secreted from the control cell to the target cells through the interstitial fluid. ### 3- autocrine messengers: Are chemicals that control the source cells which secrete them. ### 4- neurocrine messengers: They are neurotransmitters and neurohormone. ## Endocrinology - is the study of the endocrine system and the diagnosis and treatment of its dysfunctions. ## Hormone - is a chemical messenger that is secreted into the blood and affects metabolism of target cells. ## Endocrine gland or Endocrine cell - is one that secretes a hormone. ## Endocrine system - is a collective term for all such glands and hormone-secreting cells. ## Examples of endocrine glands: 1. Pituitary gland 2. thyroid gland 3. parathyroid gland 4. adrenal gland 5. pancreas 6. gonads ## Hormones Hormones are synthesized by endocrine glands. Based on chemical nature, hormones are classified into 3 types: 1. steroid hormones 2. protein hormones 3. derivatives of the amino acid ### A- steroid hormones: They are synthesized from cholesterol or its derivatives. They are secreted by adrenal cortex, gonads, and placenta. ### B- protein hormones: Are large or small peptides. They are secreted by pituitary gland, parathyroid glands, pancreas, and placenta. ### C- derivatives of the amino acid: They are 2 types: thyroid and adrenal medullary hormones. They are derived from thyrosin amino acid. ## Duration of action of different hormones: Some hormones are secreted within few seconds after the gland is stimulated and their duration of action is usually 1 to 3 minutes. Ex: epinephrine and norepinephrine Some hormones are stored for several months and they require several weeks for their effect. Ex: thyroid hormones. ## Concentration of hormones in the blood: The quantitative amounts of hormones that are required are very small. It ranges from as little as 1 picogram up to a few microgram/ml of blood. ## Hormonal Action: Hormone does not act directly on target cells. First, it combines with the receptor present on the target cells and forms a Hormone-receptor complex. ## Hormone receptors: Receptors are large proteins in the target cell. Each receptor is specific for one single hormone. So a hormone can act on a cell only if the cell has the specific receptor for this hormone. ## Situation of the hormonal receptors: 3 sites: 1. in the cell membrane: such as receptors of protein and adrenal medullary hormones 2. in the cytoplasm: such as receptors of steroid hormones 3. in the nucleus: such as receptors of thyroid hormones ## Regulation of hormone receptors: The number of receptors increases or decreases according to the hormone secreted. When the hormone is secreted in excess, the number of receptors decreases, and this is called **Down regulation**. During the deficiency of the hormone, the number of receptors increases, and this is called **Upregulation**. ## Diurnal variation (circadian rhythm) of the hormones: The plasma level of some hormones changes physiologically during the 24 hours of the day. **Ex:** - Growth hormone and prolactin: Low level in early morning and high level in evening - Cortison: high level in early morning and low level in evening # The Pituitary Gland Composed of two parts: 1. adenohypophysis (anterior lobe) 3/4 2. neurohypophsis (posterior lobe) 1/4 ## A- anterior pituitary gland (adenohypophysis gland): It is known as the **Master Gland** because it regulates many other endocrine glands. # Regulation of anterior pituitary secretion: by: ## A- Hypothalamus control: By secreting the releasing and inhibitory neurohormones (factors). These hormones are transported through hypothalamo-hypophyseal portal vessels. ## There are 7 hormones secreted from hypothalamus: 1. growth hormone-releasing hormone (GHRH) 2. growth hormone-inhibiting hormone (GHIH) or somatostatin 3. thyrotropic-releasing hormone (TRH) 4. corticotropin-releasing hormone (CRH) 5. prolactin-inhibitory hormone (PIH) 6. prolactin releasing factor (PRF) 7. gonadotropin-releasing hormone (GnRH) ## Hormones secreted by anterior pituitary gland: It secretes 7 hormones: 1. Growth hormone (GH) (somatotropin) 2. Thyroid stimulating hormone (TSH) 3. Adrenocorticotropic hormine (ACTH) 4. follicle stimulating hormone (FSH) 5. luteinizing hormone (LH) 6. Prolactin (PRL) 7. melanocyte stimulating hormone (MSH) ## 1- Growth hormone ### General characteristics: 1. secreted by acidophilic of anterior pituitary gland 2. it is a protein in nature 3. its concentration in plasma of a normal adult is between 1.6 and 3 ng/ml; in children it is about 6 ng/ml 4. in adults the output is 0.5 to 1.0 mg/day 5. its half-life in blood is about 20 minutes 6. it is degraded in the liver and kidney. ### Function: #### 1- on protein metabolism: it stimulates protein synthesis by: - Increasing amino acid transport - Increasing RNA translation - Increasing DNA transcription - Decreasing catabolism of protein - Promoting anabolism of protein #### 2- on fat metabolism: It stimulates adipose tissues to catabolize fats and release free fatty acids and glycerole into the blood. These fatty acids are used for production of energy by the cells. #### 3- on carbohydrate metabolism: - it promotes glycogen synthesis and storage - Decrease in the uptake of glucose by the cells - Decrease in the utilization of glucose for the production of energy #### 4- on bones: It is responsible for the differentiation and development of bone cells and cartilage cells - It increases the growth of the skeleton - It increases both length as well as the thickness of the bones #### 5- on electrolytes: it promotes reabsorption of Na+, K+, Cl-, and Ca++. ## Regulation of GH secretion ### GH secretion is stimulated by: 1. hypoglycemia 2. fasting 3. starvation 4. exercise 5. stress and trauma 6. initial stage of sleep ### GH secretion is inhibited by: 1. hyperglycemia 2. increase in free fatty acids in blood 3. later stage of sleep Secretion of GH is controlled by hypothalamus via 2 hormones: 1. GHRH: it increases the GH secretion 2. GHIH: it decreases the GH secretion # Disorder of GH: ## A- Hyposecretion 1. during childhood: called **dwarfism**: caused by deficiency of GH secretion or an increase in somatomedin- during childhood. These individuals do mature sexually. 2. during adulthood: the persons develops fast aging process **For instance**, a person at the age of 50 looks like 65. - Normally, the average plasma concentration of GH changes during life as follows: - 5-20 years 6 ng/ml - 20-40 years 3 ng/ml - 40-70 years 1.6 ng/ml - Thus, it is clear that part of the aging effects result from diminished GH secretion. ## B- Hypersecretion: 1. **Gigantism (giantism):** caused by over secretion of GH during childhood and characterized by excessive growth of all body tissues, especially the long bones because the epiphyses of long bones have not become closed. The patient becomes hyperglycemic. 2. **Acromegaly** caused by over secretion of GH during adulthood. That is after the epiphyses of the long bones have closed. The person cannot grow taller, instead the soft tissues continue to grow, and the bones can grow in thickness, especially the head, hands, feet, and face. # B- The posterior pituitary gland (lobe) It secretes 2 hormones: 1. Antidiuretic hormone (ADH) (vasopressin) 2. Oxytocin (OT): These 2 hormones are synthesized in the hypothalamus and transported to the posterior pituitary through the nerve fibers and stored in the nerve ending. ## A- ADH (Vasopressin): It is secreted mainly by the supraoptic nucleus of the hypothalamus. ### Function: 2 functions #### 1- Retention of water: It is the main function by acting on kidneys by increasing water reabsorption in the distal convoluted tubule (DCT) and the collecting duct (CD). Therefore ADH lowers urine volume and helps prevent dehydration. ## B-Oxytocin (OT): It is secreted mainly by the paraventricular nucleus of the hypothalamus. ### Function: It acts on mammary glands and uterus. #### 1- in mammary glands: It causes ejection of milk from the mammary gland. It is one of the neuroendocrine reflex. #### 2- On pregnant uterus: At the end of pregnancy, the secretion of OT increases, which causes contraction of uterus muscles during labor. The mechanism is positive feedback mechanism, and the reflex is called neuroendocrine reflex. # Thyroid & Parathyroid glands # 4- Pancreas Human pancreas contains about 1 to 2 million islets called islets of Langerhanse. ## Islets of Langerhanse consist of 4 types of cells: 1. A or alpha (cells (α): secrete **glucagon** hormone 2. B or beta cells (β) : secrete **insulin** hormone 3. D or delta (δ) cells: secrete **somatostatin** hormone 4. F or PP cells: secrete **pancreatic polypeptide** ## 1- Insulin ### Function: It is important for regulation of protein, fat, and carbohydrate. #### 1- on protein metabolism: It helps the synthesis and storage of proteins. It inhibits the utilization of proteins by the cells. #### 2- on fat metabolism: It helps the synthesis and storage of fat in the adipose tissues. #### 3- on carbohydrate metabolism: It is the only hormone in the body that reduces blood glucose level by: - increasing transport and uptake of glucose by the cells - promoting the utilization of glucose in the cell ## Regulation of insulin secretion: It is regulated by 4 factors, but it is mainly regulated by blood glucose level. 1. role of blood glucose level: (negative feedback) - When the blood glucose level is normal (80 to 100 mg/dl), the rate of insulin secretion is low. - When the blood glucose level increases between 100 and 120 mg/dl, the rate of insulin secretion rises rapidly. - When the blood glucose level increases above 200 mg/dl, the rate of insulin secretion rises very rapidly. The turn-off of insulin secretion is almost equally as rapid, occurring within minutes after the reduction in blood glucose concentration back to fasting level. 2. role of proteins: Increase in amino acids in blood causes an increase in insulin secretion. ## 2- Glucagon ### Function Its function is opposite to insulin function. It increases the blood glucose level. #### 1- on carbohydrate metabolism: by - increasing glycogenolysis in the liver - increasing gluconeogenesis # Diabetes mellitus (DM) DM is a metabolic disorder characterized by high blood glucose. ## Classification of DM 2 Types: Type I and Type II ## 1- Type I DM **Causes:** degeneration or destruction of β cells by autoimmune disease. It usually occurs before 40 years of age. The patient requires insulin injections, so it is called insulin-dependent diabetes mellitus (IDDM). - There are two types of DM. They are: - insulin-dependent diabetes mellitus (IDDM), also called Type I or Type 1 **Causes:** It is a genetically autoimmune disease. The immune system destroys the β cells. - It occurs suddenly before the age of 15-20 years. **Complications:** 1. weight loss 2. heart disease 3. vascular disease 4. gangrene 5. loss of vision 6. severe kidney problem 7. blood acidosis which leads to death **Treatment:** regular insulin injections to prevent death. When it occurs during childhood, it is called **juvenile diabetes**. ## 2- Type II DM: It is an insulin resistance due to a failure of the insulin receptor to respond to insulin. So the body is unable to use insulin. - About 90% of diabetic patients have Type II DM. It usually occurs after 40 years. - In most cases, it is controlled by orally hypoglycemic drugs. Only a few require insulin injection. Type II is called non-insulin dependent diabetes mellitus (NIDDM). **Causes:** 1. Genetically disorders 2. Lifestyle changes, such as bad eating habits 3. stress **It is controlled by:** 1. diet 2. exercise 3. weight loss 4. antidiabetic drugs, such as glyburide (DiaBeta). It stimulates β cells to secrete insulin.