Endocrine.

• The anterior pituitary gland secretes growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, and melanocyte-stimulating hormone.
• • Endocrine System: any cell with a receptor for the hormone
  • Response Time:
  • Nervous System: rapid; minutes
  • Endocrine System: slow; minutes to hours
  • General Functions of the Endocrine System:
  • Regulating development, growth, and metabolism
  • Maintaining homeostasis of blood composition and volume
  • Controlling digestive processes
  • Controlling reproductive activities
• Endocrine System:

  • Composed of endocrine glands that synthesize and secrete hormones
  • Hormones are chemical messengers released into the blood
  • Hormones are transported to target cells by being released into interstitial fluid, entering the blood, and binding to target cells' receptors
  • Nervous System vs. Endocrine System:
  • Communication Method:
  • Nervous System: nerve signal causes neurotransmitter release into the synapse
  • Endocrine System: secretes hormone into blood and transported throughout the body
  • Target cells:
  • Nervous System: neurons, muscles, and glands
• Endocrine Glands:

  • Function to release hormones into the blood
  • Consist of epithelial cells within a connective tissue framework
  • Major endocrine organs include pituitary, pineal, thyroid, parathyroid, and adrenal glands
  • Hormone release is regulated by endocrine reflexes triggered by hormonal, humoral, or nervous stimuli
  • Stimulation of Hormone Synthesis and Release:
  • Hormonal stimulation: release of a hormone in response to another hormone
  • Humoral stimulation: release of a hormone in response to changes in blood nutrients or ions
• • Nervous system stimulation: release of a hormone in response to stimulation by the Nervous System
• Categories of Circulating Hormones:

  • Steroids: lipid-soluble molecules synthesized from cholesterol (e.g., estrogen, testosterone)
  • Biogenic Amines: water-soluble molecules derived from modified amino acids (e.g., norepinephrine, melatonin)
  • Proteins: water-soluble molecules consisting of amino acids (e.g., insulin, growth hormone)
  • Local Hormones:
  • Signaling molecules that do not circulate in the blood
  • Types include autocrine (acting on the same cell) and paracrine (acting on neighboring cells)
• • Eicosanoids are the primary local hormones, regulating cellular changes through autocrine and paracrine stimulation
• Hormone Transport in the Blood:

  • Lipid-soluble hormones require carrier proteins for transport through the body
  • Carrier proteins bind to and transport specific lipid-soluble hormones, protecting them from destruction
  • Water-soluble hormones travel freely in the blood and do not require carrier proteins
  • Factors Influencing Hormone Concentration:
  • Hormone concentration depends on synthesis and elimination rates
  • Balance between synthesis and elimination determines blood concentration
• Factors that influence hormone concentration:

  • The concentration within the blood increases when the rate of synthesis and release increases
  • The concentration within the blood decreases when the rate of synthesis and release decreases
  • Hormone elimination:
  • The faster the rate of elimination, the lower the hormone concentration in the blood
  • The slower the rate of elimination, the higher the hormone concentration in the blood
  • Enzymatic degradation (liver cells)
  • Excretion through the kidney
  • Uptake by the target cell
• Half-Life of Hormones:

  • Time necessary to reduce a hormone's concentration to half of its original level
  • Depends on how efficiently it is eliminated
  • Hormones with short half-life must be secreted frequently to maintain normal concentration
  • Water-soluble hormones generally have short half-life
  • Lipid-soluble hormones (i.e., Steroid hormones) generally have a long half-life
  • Carrier proteins protect lipid-soluble hormones
  • For example, testosterone has a half-life of 12 days
• Lipid-Soluble Hormone Physiology:

  • Lipid-soluble hormones are lipophilic and can diffuse across the plasma membrane
  • Hormone-receptor complex is formed when the hormone binds to the intracellular receptor
  • Hormone-response element (HRE) is a DNA sequence where the hormone-receptor complex binds
• Water-Soluble Hormone Physiology:

  • Signal transduction pathway involves a series of biochemical events across the plasma membrane
  • First messenger is the hormone that binds to the receptor
  • Second messenger is formed when the first messenger binds to a specific receptor
  • G protein binds guanine nucleotides (GDP and GTP) and can change states when certain hormones are bound
  • Adenylate cyclase is a plasma membrane protein that increases the formation of the second messenger (cAMP)
• Target Cells:

  • A cell's response to a hormone varies and can display differing numbers of receptors
  • Up-regulation increases the number of receptors toward a specific hormone, increasing sensitivity
  • Down-regulation decreases the number of receptors toward a hormone, decreasing sensitivity
  • Different hormones can interact synergistically, permissively, or antagonistically
• Anatomic Relationship:

  • The hypothalamus controls the pituitary gland, which controls several endocrine organs
  • Pituitary gland is the master gland of the endocrine system
  • Posterior pituitary is neural tissue and stores and releases oxytocin and antidiuretic hormone (ADH)
  • Posterior pituitary does not create any hormones
• Neurosecretory cells in the hypothalamus produce Oxytocin (OT) and Antidiuretic Hormone (ADH)
• Oxytocin (OT) functions:

  • Smooth Muscle: Uterine contractions
  • Mammary gland: Milk ejection
  • Emotional bonding
• Antidiuretic Hormone (ADH) functions:

  • Kidneys: decrease urine output
  • Hypothalamus: thirst center activation (drink water)
  • Blood Vessels: vasoconstriction
• Anatomic relationship between the Hypothalamus and the Posterior Pituitary Gland:

  • Hormones created by the Hypothalamus are released by the Posterior Pituitary Gland
• Primary hormones released from the Anterior Pituitary Gland:

  • Thyroid-stimulating hormone (TSH)
  • Prolactin (PRL)
  • Follicle-stimulating hormone (FSH)
  • Luteinizing Hormone (LH)
  • Adrenocorticotropic hormone (ACTH)
  • Growth Hormone (GH)
• Regulation of Growth Hormone (GH):

  • Controlled through hormonal stimulation from the Hypothalamus (GHRH and GHIH)
  • Stimuli for GH release include age, time of day, nutrient levels, stress, and exercise
• Effects of Growth Hormone (GH):

  • Stimulates protein synthesis, cell division, and cell differentiation
  • Stimulates release of insulin-like growth factors (IGF) from the liver
  • Stimulates various metabolic processes such as breakdown of glycogen and triglycerides
• Thyroid Gland Anatomy:

  • Largest endocrine structure located inferior to the thyroid cartilage of the larynx and anterior to the trachea
  • Consists of right and left lobes connected by the isthmus
• Thyroid Hormones:

  • T3 (triiodothyronine) and T4 (tetraiodothyronine) are collectively known as thyroxine
  • Thyroglobulin (TGB) is a glycoprotein precursor of TH that binds with Iodine
• Thyroid Hormone Synthesis, Storage, and Release:

  • Follicular cells synthesize and release TH into the blood
  • TH regulation involves the Hypothalamic-Pituitary-Thyroid axis
• Effects of Thyroid Hormone:

  • Increases metabolic rate, protein synthesis, and various metabolic processes in target cells
  • Regulates functions in neurons, tissues, adipose cells, lungs, and heart
• Thyroid Hormone Deficiency:

  • Hyperthyroidism results from excessive TH production
  • Hypothyroidism results from decreased TH production
• Calcitonin Hormone:

  • Synthesized and released from parafollicular cells of the thyroid gland
  • Acts to decrease blood calcium levels in response to high blood calcium or stress
• Parathyroid Hormone:

  • Synthesized and released from parafollicular cells of the thyroid gland
  • Stimulus for release is high blood calcium or stress from exercise
  • Acts to decrease blood calcium levels by:
  • Inhibiting osteoclast activity
  • Stimulating kidneys to increase excretion of calcium in urine
• Adrenal Gland Anatomy:

  • Located on the superior surface of each kidney
  • Retroperitoneal; embedded in fat and fascia
  • Two regions:
  • Adrenal medulla:
  • Forms the inner core of each adrenal gland
  • Red-brown color due to extensive blood vessels
  • Releases epinephrine and norepinephrine with sympathetic stimulation
  • Adrenal cortex:
  • Synthesizes more than 25 corticosteroids
  • Yellow color due to lipids within cells
  • Three regions producing different steroid hormones: zona glomerulosa, zona fasciculata, and the inner zona reticularis
• Hormones of the Adrenal Cortex:

  • Zona Glomerulosa (outer cortical layer):
  • Mineralocorticoids: hormones that regulate electrolyte levels
  • Aldosterone fosters Na+ retention and K+ secretion
  • Zona Fasciculata (middle cortical layer):
  • Glucocorticoids: hormones that regulate blood sugar
  • Cortisol increases blood sugar
  • Zona Reticular (inner cortical layer):
  • Gonadocorticoids: sex hormones
  • Androgens are male sex hormones made by adrenals
  • Converted to estrogen in females
  • Amount produced by adrenals is less than amount from testes
• • Increased cortisol levels inhibit CRH release from the Hypothalamus
  • Inhibit ACTH secretion from the Anterior Pituitary Gland
• Cortisol Hormone:

  • Release regulated by hypothalamic-pituitary-adrenal axis
  • Stimuli: Stress, late stages of sleep, and low levels of cortisol stimulate the hypothalamus to release CRH
  • Cortisol and corticosterone increase nutrient levels in the blood
  • Cortisol causes target cells to increase blood nutrient levels:
  • Liver: Glycogenolysis and Gluconeogenesis
  • Adipose: Lipolysis and decreased lipogenesis
  • All cells (except hepatocytes): protein catabolism to amino acids and a decrease in glucose uptake (sparing it for the brain)
  • Negative Feedback mechanism:
• Glucagon Hormone:

  • Stimulus: Raising low blood glucose levels with glucagon
  • Glucagon causes target cells to release stored nutrients into the blood:
  • Liver: Glycogenolysis and Gluconeogenesis
  • Adipose: Lipolysis
  • Negative Feedback mechanism:
  • As blood glucose levels increase, glucagon secretion is inhibited
• Insulin Hormone:

  • Pancreatic hormones help maintain blood glucose
  • Stimulus: Lowering high blood glucose levels with insulin
  • Insulin causes target cells to:
  • Liver (hepatocytes): stimulates Glycogenesis
  • Adipose (adipocytes): stimulated Lipogenesis
  • All cells (especially muscle): nutrient uptake protein anabolism
  • Most cells: increased glucose uptake
  • Negative Feedback:
  • As blood glucose levels decrease, Insulin secretion is inhibited
• Melatonin Hormone:

  • Pineal Gland
  • Stimulus: Nighttime
  • Net Effect:
  • Causes drowsiness
  • Regulates circadian rhythm and has effects on mood
• • Net Effect: Angiotensin II increases blood pressure by constricting blood vessels, decreasing urine output, and activating the thirst center
• Other Hormones:

  • Thymus:
  • Hormone secreted: thymic hormones
  • Net Effect: participate in T-lymphocyte maturation
  • Heart:
  • Hormone secreted: atrial natriuretic peptide (ANP)
  • Net Effect: decreases blood pressure by increasing urine output and blood vessel dilation in kidneys
  • Kidneys:
  • Hormone secreted: Erythropoietin (EPO)
  • Net Effect: stimulates red bone marrow to create red blood cells
  • Liver:
  • Hormone secreted: Angiotensinogen (inactive hormone) and insulin-like growth factors
• Aging and the Endocrine System:

  • Endocrine changes with aging
  • Secretory activity wanes with age
  • Reduces efficiency of endocrine system functions
  • Decreased levels of normal hormones
  • For example, decreased levels of GH and sex hormones
  • Reduced GH levels leading to loss of weight and body mass in elderly