Endocrine Glands: Medical Physiology Lecture Notes PDF

Endocrine glands By Sally Anwar Sayed Assistant Professor of Medical Physiology Faculty of Medicine - Assiut University Objectives: By the end of the study the students should be able to understand: 1. Endocrine glands and their hormones. 2. Functions of the hormones...

Endocrine glands By Sally Anwar Sayed Assistant Professor of Medical Physiology Faculty of Medicine - Assiut University Objectives: By the end of the study the students should be able to understand: 1. Endocrine glands and their hormones. 2. Functions of the hormones secreted by the different glands 3. Different endocrine disorders Hormones are chemical messengers secreted into the circulation by ductless glands. Along with the nervous system, hormones provide the means by which different portions of the body can communicate. Important endocrine glands of the body:  Pituitary gland  Thyroid gland  Parathyroid glands  Adrenal glands  Pancreas (Islets of Langerhan)  Testes and ovaries Pituitary gland Pituitary gland, locates at the base of the brain just below the hypothalamus. The pituitary is connected to the hypothalamus by the hypophysial or pituitary stalk. The pituitary gland exerts a major control over endocrine functions, therefore it is known as the master gland. Physiologically, the gland is divided into: 1) The anterior pituitary (adenohypophysis). 2) The posterior pituitary (neurohypophysis). 3) The pars intermedia, a small part and lies between these two parts. Anterior Pituitary Hormones The anterior pituitary gland (adenohypophysis) secrets six hormones: 1) Growth hormone (GH), also called somatotropin. 2) Thyroid-stimulating hormone (TSH) which stimulate the formation of thyroid hormones. 3) Adrenocorticotropic hormone (ACTH), which stimulate the formation of adrenocortical hormones. 4) Gonadotropic hormones (GnH), which include both luteinizing hormone (LH) and follicle-stimulating hormone (FSH). 5) Prolactin (PRL). Control of the anterior pituitary secretion I- The hypothalamic hormones: The secretion of anterior pituitary hormones is controlled by seven hypothalamic polypeptide hormones called hypothalamic hormones (factors). Depending on their actions, hypothalamic hormones are called releasing or inhibitory hormones. 1) Growth-releasing hormone (GRH), which stimulates release of GH, and growth-inhibitory hormone (GIH), also called somatostatin(SS), which inhibits release of GH. 2) Thyrotropin-releasing hormone (TRH), which stimulates release of TSH. 3) Corticotrpin-releasing hormone (CRH), which stimulates release of ACTH. 4) Gonadotropin-releasing hormone (GnRH), which stimulates release of GnH (LH and FSH). 5) Prolactin inhibitory hormone (PIH), which tonically inhibits PRL secretion and prolactin- releasing hormone (PRH) which stimulate PRL release during pregnancy. Growth hormone (GH) Growth hormone ( GH), also called somatotropic hormone or somatotropin, a peptide hormone exerts its effects on all tissues of the body. Physiologic functions: I. Effect on soft tissues → Growth hormone promotes growth of soft tissues by: 1. Increasing the number of cells (hyperplasia) by Stimulation of cell division. 2. Increasing the size of the cells (hypertrophy) by increasing synthesis of protein in most cells of the body. II. Effect on bone tissues → Growth hormone stimulates growth of bone through: 1- Increase in the length of long bone: Before puberty, new chondrocytes (cartilage cells) are formed by the ossification center in the epiphyseal plates, thus elongating the shaft and the bone growth in length. 2. Increase in the thickness of long bone:  After puberty, growth hormone strongly stimulates osteoblasts, therefore, the rate of deposition of new bone is greater than that of absorption. Bones can continue to become thicker throughout life. III. Metabolic effects: 1. Effect on carbohydrate metabolism:  Decrease in glucose uptake by muscle.  Enhance glucose synthesis in the liver. These actions lead to: Increased blood glucose level. So, GH has diabetogenic effect with insulin resistance. III. Metabolic effects: 2. Effect on fat metabolism: a) Increase in blood fatty acids. b) Stimulation of lipolysis. c) Inhibition of lipogenesis. Factors that affect growth hormone secretion Factors that stimulate secretion: I. Hypoglycemia VI. Exercise II. Low blood free Fatty A VII. Trauma III. Early period of sleep VIII. GRH IV. Fasting and starvation IX. Testosterone V. Excitement X. Estrogen Factors that inhibit secretion: I. Hyperglycemia V. Obesity II. High blood free fatty A VI. Somatomedins III. Late period of sleep iv. VII. Somatostatin (GIH, SS) IV. Aging Dwarfism Definition: Dwarfism is a disease characterized by retarded growth due to growth hormone deficiency before puberty. Physiologic effects: 1. Retarded growth. 2. Normal mental development. 3. They are sexually mature and fertile. Gigantism Definition: Gigantism is a syndrome caused by excess secretion of the growth hormone, commonly due to tumor, before puberty. III. Metabolic effects: Physiologic effects: There is a rapid growth in high of the long bones, so that the person becomes a giant - up to 240 cm tall. Acromegaly Definition: Acromegaly is a disease caused by excess secretion of growth hormone after puberty, commonly due to tumor. Physiologic effects: Bone thickening is most marked in the bones of the extremities and elongated face with protruded forehead, supraorbital ridges, and lower jaw and increased nose size. The fingers become thickened. Posterior Pituitary Hormones The hypothalamus contains two groups of neurosecretory neuron: 1. Supraopticnuclei, form antidiuretic hormone (ADH), also known as arginine vasopressin. 2. Paraventricular nuclei, form oxytocin. The synthesized hormones transported by the hypothalamic- hypophysial tract to the posterior pituitary, where they are stored. Posterior Pituitary Hormones Antidiuretic Hormone (Arginine Vasopressin) Physiologic functions: 1. Water reabsorption: The single most important effect of ADH is to conserve body water by reducing the loss of water in urine, and the hormone was named for this effect. The site of action of ADH is the distal convoluted tubules and collecting ducts of the kidney. 2. Vasoconstriction: High concentrations of ADH cause constriction of arterioles, which leads to increased arterial pressure (so called vasopressin). In healthy humans, ADH plays only a minor role in regulating blood pressure. Oxytocin hormone Physiologic functions: 1. Effect on the pregnant uterus: At the end of gestation, oxytocin is released during labor to enhance uterine smooth muscle contraction to facilitate labor. 2. Effect on the breast: Oxytocin stimulates milk ejection (milk letdown) during lactation through milk ejection reflex.