13 The Endocrine System 1

The Endocrine System Produces Hormones Is the ‘other' control system Endocrine control is slower than nervous system Endocrine glands: ductless organs that release hormones into blood Hormones Chemical messengers secreted by endocrine glands Circulate in the bloodstream Act on specific cells in the body (target cells) that have the appropriate hormone receptor

Hypothalamus Pituitary gland Thyroid gland Pancreas Adrenal glands - Anterior lobe - Posterior lobe Hypothalamus Releasing hormones Inhibiting hormones Pineal gland Melatonin Pituitary gland Anterior lobe Adrenocorticotropic hormone (ACTH) Thyroid-stimulating hormone (TSH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Prolactin Growth hormone Posterior lobe (stores) Antidiuretic hormone (ADH) Oxytocin Thyroid gland Thyroid hormones (T3, T4) Parathyroid glands Parathyroid hormone (PTH) Thymus gland Thymosin Thymopoietin Heart Atrial natriuretic hormone Stomach Gastrin Pancreas Glucagon Insulin Adrenal glands Cortex Cortisol Aldosterone Medulla Epinephrine Norepinephrine Kidneys Erythropoietin Renin Intestines Secretin Cholecystokinin Ovaries (Female) • Estrogen • Progesterone Testes (Male) Testosterone Figure 13.1

Types of hormones Steroid hormones Structurally related to cholesterol Lipid soluble Receptors inside the cell Activate specific genes to produce specific proteins Slow acting (min. to hrs.) Examples: sex hormones, steroid hormones Nonsteroid hormones Made of amino acids, generally a protein Water soluble receptors on cell membrane fast acting (secs. to mins.) Examples: insulin, thyroid hormones

Hormones Participate in Negative Feedback Loops Many hormones participate to maintain internal homeostasis Hormone levels regulated by: negative feedback: deviations detected and counteracted Hormone regulation mostly by negative feedback loops example, insulin, thyroid hormones positive feedback: amplifies original change Example oxytocin release and child birth

Oxytocin is transported by blood to mammary glands. Figure 13.6 Neuroendocrine cell 3 Release of oxytocin when stimulated. Hypothalamus Anterior pituitary 4 Spinal cord Oxytocin is transported by blood to mammary glands. 2 Sensory and spinal nerves carry impulses to the hypothalamus. Posterior pituitary Milk ejected Figure 13.6 The control of oxytocin secretion by nursing. Oxytocin 1 Nursing stimulates nerve receptors in nipple. 6

The Hypothalamus and the Pituitary Gland Homeostatic control center of the brain Produces two hormones of its own (ADH, oxytocin) controls hormone secretions of the pituitary gland (by inhibiting & releasing hormones Pituitary gland “Master” gland Secretes six different hormones most of which regulate other endocrine organs Two lobes: posterior and anterior

Posterior Pituitary Stores ADH and Oxytocin Posterior pituitary stores & releases: Antidiuretic hormone (ADH) Conserves water in kidneys Regulates water balance in body Oxytocin Causes uterine contractions during labor and milk ejection through a reflex

The Anterior Pituitary Produces Six Key Hormones ACTH Stimulates adrenal cortex to release its hormones TSH (thyroid-stimulating hormone) promoting release of thyroid hormones FSH and LH Stimulate growth and function of ovaries and testes Prolactin Stimulates milk production Growth hormone Major effects on bone, muscle Most of its growth effects occur during childhood

Hypersecretion of growth hormone in childhood Pituitary dwarfism Figure 13.8 Gigantism Hypersecretion of growth hormone in childhood Pituitary dwarfism Decreased secretion of growth hormone Figure 13.8 Effect of growth hormone on body growth. Acromegaly Excessive growth hormone secretion in adults 10

The Pancreas Secretes Glucagon & Insulin Pancreatic hormones are involved in regulating blood glucose levels Alpha cells: secrete glucagon Beta cells: secrete insulin Glucagon Raises blood glucose by breakdown of glycogen to glucose in liver Insulin Lowers blood sugar by promoting uptake of glucose by cells in muscle and adipose tissue Promotes storage of glycogen, fat & protein formation

Disorders of the Endocrine System Diabetes mellitus Type 1: insulin-dependent diabetes 5–10% of all cases Inadequate insulin Treated with insulin injections Type 2: non-insulin-dependent diabetes 90–95% of all cases Insulin resistance

The Adrenal Glands consist of the cortex and medulla Adrenal cortex: outer layer of adrenal gland; produces: Cortisol- increases blood levels of glucose, fatty acids & glucose For stress response Aldosterone- promotes sodium, water retention by the kidney Adrenal medulla: inner layer of adrenal gland; releases Epinephrine (adrenaline) Norepinephrine (noradrenaline)

Adrenal medulla Hormones: Neuroendocrine organ Secretion stimulated by Perceived or real threat Brain Activation of sympathetic nerves Adrenal medulla cortex Norepinephrine and epinephrine Target cells Adrenal medulla Neuroendocrine organ Secretion stimulated by Sympathetic nervous system Hormones: Epinephrine & norepinephrine enhance function of sympathetic nervous system (fight-or-flight response) Figure 13.11

Thyroid located just below larynx in neck Figure 13.12 Thyroid located just below larynx in neck Parathyroid glands embedded in back of thyroid Opening to trachea Larynx Thyroid gland Parathyroids Trachea Figure 13.12 The thyroid and parathyroid glands. Anterior (front) Posterior (back) T4 and T3 Increase the metabolic rate 15

Disorders of the Endocrine System Hypothyroidism: underactive thyroid gland Children: cretinism slowed body growth, untreated can cause mental retardation Adults: lethargy, weight gain, cold intolerance due to low metabolism Hyperthyroidism: overactive thyroid gland Increased metabolism, hyperactivity, nervousness, agitation, weight loss, heat intolerance Graves’ disease: autoimmune form of hyperthyroidism

Parathyroid Hormone (PTH) Controls Blood Calcium Levels Secretion In response to lowered blood calcium levels Action Removes calcium from bones Increases absorption of calcium from food in the digestive tract Causes kidneys to retain calcium Combined effects increase blood calcium levels Major regulator of blood calcium concentration in adults

Table 13.2 Table 13.2 Hormones of endocrine glands other than the hypothalamus and pituitary. 18