Endocrine System (part 1) & General Adaptation Syndrome Keri Muma Bio 6

What is the Endocrine System? Composed of glands that secrete hormones Responsible for continuous processes such as growth, cell metabolism, & reproduction

What do Hormones Do? Hormones are chemical substances that travel through the blood to a target cell Target cells must have specific receptors to which the hormone binds These receptors may be intracellular or located on the plasma membrane They trigger a change in cellular activity

Target cell activation depends on three factors: Blood levels of the hormone Relative number of receptors on the target cell Up-regulation – target cells form more receptors in response to the hormone Down-regulation – target cells lose receptors in response to the hormone The affinity of those receptors for the hormone Target Cell Activation

Hormones are synthesized and released in response to humoral, neural, and hormonal stimuli Control of Hormone Release

Humoral stimuli - secretion of hormones in response to changing blood levels of ions and nutrients Examples: Concentration of calcium ions in the blood and the release of PTH Concentration of blood glucose and the release of insulin

Control of Hormone Release Neural stimuli – nerve fibers stimulate hormone release Preganglionic sympathetic fibers stimulate the adrenal medulla to secrete catecholamines Figure 16.4b

Hormonal stimuli – release of hormones in response to hormones produced by other endocrine organs The anterior pituitary gland secretes hormones that stimulate other endocrine glands Control of Hormone Release

Pituitary gland – two-lobed endocrine gland that secretes nine major hormones Neurohypophysis – posterior lobe, made up of neural tissue Adenohypophysis – anterior lobe, made up of glandular tissue Pituitary Gland

The posterior lobe is a down growth of hypothalamic neural tissue and has a neural connection with the hypothalamus Nuclei of the hypothalamus synthesize oxytocin and antidiuretic hormone (ADH) These hormones are transported to the posterior pituitary where they are stored and released Hypothalamus-Pituitary-Axis

ADH (or Vasopressin) Triggers increased water reabsorption in the kidney tubules Release of ADH is stimulated by: Increase in osmolarity detected by the hypothalamus Decrease in blood pressure and volume Antidiuretic Hormone (ADH)

The anterior lobe of the pituitary and the hypothalamus are connected by the hypophyseal portal system Hypothalamus-Pituitary-Axis

Regulatory hormones from the hypothalamus control the release of hormones from the anterior pituitary Releasing hormones stimulate the release of hormones Inhibiting hormones shut off release of hormones Hypothalamus-Pituitary-Axis

Tropic hormones – regulates the release of hormones from other endocrine glands Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH ) Growth hormone (GH) Additional non-tropic hormones released by the anterior pituitary Prolactin (PRL) Melanocyte-stimulating hormone (MSH) Anterior Pituitary Hormones

Targets most cells, especially bone and skeletal muscle when young Growth promoting actions – stimulates bone and soft tissue growth Increases protein synthesis, cell growth, and cell division Metabolic actions: Mobilizes fat stores and increases fatty acid levels in the blood Increases blood glucose Through glycogenolysis – breakdown of glycogen Inhibits skeletal muscle uptake of glucose Growth Hormone (GH)

Growth Hormone

Anterior Pituitary Gland Adrenocorticotropic hormone (ACTH) Targets the adrenal cortex Stimulates the release of glucocorticoids

Adrenal glands – paired, pyramid- shaped organs atop the kidneys Structurally and functionally, they are two glands in one Adrenal medulla – nervous tissue that acts as part of the sympathetic NS Adrenal cortex – glandular tissue derived from embryonic mesoderm Adrenal Glands

Synthesizes and releases steroid hormones called corticosteroids Zona reticularis – gonadocorticoids (chiefly androgens) Zona fasciculata – glucocorticoids (chiefly cortisol) Zona glomerulosa – mineralocorticoids (chiefly aldosterone) Adrenal Cortex

Glucocorticoids (Cortisol) - helps the body resist stress by: Increasing blood glucose levels Cortisol has the following effects : Promotes gluconeogenesis - formation of glucose from non-carbohydrates Enhances lipolysis Triggers protein catabolism in skeletal muscle Suppresses the immune system Adrenal Cortex

Cortisol

Mineralocorticoids – regulate the electrolyte concentrations of extracellular fluids Aldosterone – regulates blood pressure, blood volume, and Na+ and K+ levels in the blood Stimulates reabsorption of Na + by the kidneys Causes secretion of K+ Adrenal Cortex

Aldosterone secretion is stimulated by: Low blood Na + Rising blood levels of K + Decreasing blood volume or pressure Stress Mineralocorticoids

Plasma concentration of sodium and potassium – directly influences the zona glomerulosa cells ACTH – causes small increase in aldosterone during stress Renin-angiotensin mechanism – kidneys release renin in response to decreased BP, stimulates aldosterone release Atrial natriuretic peptide (ANP) – produced by the heart in response to increased pressure, inhibits the release of aldosterone The Four Mechanisms of Aldosterone Secretion

Renin-Angiotensin-Aldosterone Pathway Is triggered when the kidneys release renin Renin converts the plasma protein angiotensinogen into angiotensin I Angiotensin I is converted into angiotensin II by ACE (angiotensin converting enzyme) Angiotensin II: Causes systemic arteriole vasoconstriction Stimulates the adrenal cortex to release aldosterone Results in increased blood pressure and volume

Figure Mechanisms of Aldosterone Secretion

Got Stress? Stress = any factor that disrupts our natural balance (homeostasis) Stressor = anything that causes us to adjust Can be emotional, physical, environmental Immediate response to a threat vs. Prolonged exposure to stress How does stress affect you?

General Adaptation Syndrome Stress stimulates the hypothalamus to initiate a chain of reactions that results in general adaptation syndrome Definition: “the sum of all non-specific systemic reactions of the body to long continued exposure to stress” (Hans Selye)

General Adaptation Syndrome Phases of G.A.S. Alarm reaction – mobilize resources Resistance reaction – cope with stress Exhaustion – deplete reserves

Alarm Reaction Immediate response to stress triggers the sympathetic nervous system “fight or flight” through the hypothalamus Mobilizes the body for immediate physical activity Short-lived

Figure Stress and the Adrenal Gland

Resistance Reaction If stress persists longer than a few hours then the resistance reaction is initiated Prepares the body for long term protection, slow to start but longer lasting The hypothalamus triggers the pituitary gland to secrete hormones that will allow the body to continue to survive the stress until homeostasis is returned

Resistance Reaction Overall goal is to: Maintain blood pressure and volume Increase ATP production Prevent water loss Prevent inflammation from causing tissue damage Maintained by ADH, aldosterone, cortisol, growth hormone, and thyroid hormones

Hypothalamus Posterior Pituitary Anterior Pituitary Sympathetic response ACTHGHTSH Adrenal Cortex Liver Thyroid Mineralocorticoids Aldosterone Glucocorticoids Cortisol T3, T4 Lipolysis Glycogenolysis Cell metabolism Catabolism of protein & fats Stimulates glyconeogenesis Anti-inflammatory & Immunosuppresent Retention of Na+ Secretion of K+ / H+ ADH Kidneys Water retention Renin – Angiotensin Stress

Prolonged exposure to stress hormones: Depresses cartilage and bone formation Depresses the immune system Promote changes in cardiovascular, neural, muscular and gastrointestinal function (usually due to hypokalemia – potassium deficiency) Cardiac arrhythmias Muscle wasting Fatigue, concentration loss, irritability Excessive Levels of Stress Hormones

Exhaustion Phase If the resistance reaction fails to overcome the stress eventually the body reserves are exhausted and the resistance reaction cannot be sustained The adrenal cortex cannot produce aldosterone and cortisol “Link between the breakdown of the hormonal adaptation mechanism and fatal diseases” Hans Selye Results in illness or death Cancer, heart disease, depression, hypertension, diabetes

Some Stress is Good! We all need a little stress Norepinephrine which is a neurotransmitter released during stress plays a role in: Creating and retrieving memories Improves moods (feel good chemical) How well are you “managing” your stress?