1. Endocrine System
Section 1, Chapter 13

2. Introduction
The endocrine system assists the nervous system with communication and control of the body
Characteristics of endocrine glands
They are ductless
Endocrine glands secrete hormones
Hormones are carried to distant target cells through the bloodstream
Hormones only act on cells (target cells) that possess receptors sensitive to the hormone – highly specific action.

3. Other signals Exocrine glands Have ducts
Secrete chemicals directly onto a surface
Examples: sweat glands, mucous cells
Paracrine signals
Chemicals that affect only nearby cells
Example: prostaglandins secreted with semen stimulate muscular contractions within female reproductive organs
Autocrine signals
Chemicals that affects only the cells that produced it.
Example: T-cells secrete interleukins (IL), which stimulate the proliferation of the T-cells (monoclonal population)
Neuroendocrine
Nervous tissue that secretes hormones
Example: hormone secretion from the hypothalamus.

4. Endocrine vs. Nervous Tissue
The endocrine and nervous systems communicate using chemical signals
Neurons release neurotransmitters into a synapse affecting postsynaptic cells
Endocrine glands release hormones into the bloodstream to specific target cell receptors
Figure 13.2 Chemical communication. (a) neurons release neurotransmitters onto synapses, affecting postsynaptic cells. (b) Glands release hormones into the bloodstream. Blood carries hormone molecules throughout the body, but only target cells respond.

5. Endocrine vs. Nervous Tissue
Nervous System
Endocrine System
Cell…………………………………. Neuron Glandular Epithelium
Signal………………………………. neurotransmitter hormone
Specificity of action…………. receptors on postsynaptic cell receptors on target tissues
Speed of onset…………… <second seconds to hours
Duration of action……………. usually very brief may be brief or last for days

6. Chemistry of Hormones Chemically, hormones are either:
Steroid or steroid-like hormones
Biogenic Amines
Peptide hormones

7. Steroid Hormones Properties
Steroid hormones are derived from cholesterol
They are composed of hydrophobic lipids (insoluble in water)
Include
Estrogen
Testosterone
Androgens (weak sex hormones)
Aldosterone
Corticoids (hormones secreted from the adrenal cortex)

8. Biogenic Amines Properties Amines are synthesized from amino acids
Include
Norepinephrine
Epinephrine
Melatonin
Thyroid hormones (these are also hydrophobic, or water insoluble)

9. Peptide Hormones Properties
Composed of long chains of amino acids (polypeptides)
Include
Hypothalamic hormones
Pituitary hormones
Pancreatic hormones
Gastrointestinal hormones

10. Water Solubility & Membrane Permeability
Steroid + Thyroid Hormones
Are hydrophobic – transported in the plasma attached to proteins
Cell membrane permeable – due to their hydrophobic properties, these hormones readily cross the phospholipid bilayer of the cell membrane.
All other Hormones
Are hydrophilic– freely dissolved in plasma
Cell membrane impermeable – these hormones do not cross the cell membrane, and must rely on 2nd messengers to relay a signal into target cells.
2nd messenger – molecule that relays and amplifies a hormone signal into the cell.

11. Actions of steroid hormones
A steroid hormone crosses the cell membrane
Hormone combines with a protein receptor in the nucleus
The hormone-receptor complex activates transcription of a specific DNA region
The mRNA leave the nucleus into the cytoplasm
The mRNA is translated into a protein.

12. Actions of Non-steroid hormones
A non-steroid hormone reaches the target cell,
The hormone binds to a membrane receptor
Binding to the receptor activates an enzyme in the cell membrane (adenlyate cyclase)
Adenlyate cyclase converts ATP into cyclic adinosine monophosphate (cAMP)
cAMP is a second messenger that promotes a series of reactions leading to the cellular changes associated with the hormone’s action.

13. Control of Hormonal Secretions
Hormone secretion is generally controlled in three ways:
Negative Feedback
Hormone Deactivation
Up/Down Regulation

14. Negative Feedback
The endocrine gland, or system controlling it senses the concentration of the hormone that gland secretes.
When the level of a specific hormone drops below needed levels, the endocrine gland is stimulated to secrete more hormone.
As the hormone concentration reaches the needed level, stimulation of that endocrine gland is reduced, and production of that hormone is reduced.
Figure Hormone secretion is under negative feedback.

15. Negative Feedback Indicates negative feedback inhibition.
Figure 13.8 Examples of endocrine system control. (a) one way the hypothalamus controls the anterior pituitary, which in turn controls other glands (b) the nervous system controls some glands directly, and (c) some glands respond directly to changes in the internal environment.
Figure As a result of negative feedback, hormone concentration s remain relatively stable, although they may fluctuate slightly above and below average concentrations.

16. Hormone Deactivation
Half-life: measures the time for half of the hormone molecules to be removed from plasma
Time Hormone Concentration
0 minutes 100%
10 minutes 50%
20 minutes 25%
30 minutes 12.5%
Example of half-life: a hormone with a half-life of 10 minutes, decreases in concentration by half every 10 minutes.
Hormones are continually secreted in the urine, and broken down by enzymes, primarily in the liver.

17. Up/Down Regulation
Up-regulation increases the number of receptors on the target cell
Up regulation increases a cell’s sensitivity to a hormone
Down-regulation decreases the number of receptors on target cells.
Down regulation decreases a cell’s sensitivity to a hormone

18. Endocrine System
Section 2, Chapter 13

19. Pituitary Gland (Hypophysis)
Location: Lies at the base of the brain in the sella turcica, connected to hypothalamus by a pituitary stalk (infundibulum)
2 Lobes:
Anterior pituitary (adenohypophysis) Posterior pituitary(neurohypophysis)

20. Control of Pituitary Gland
Anterior Pituitary Gland
Posterior Pituitary Gland
Releasing hormones secreted from hypothalamus regulates the anterior lobe.
Nerve impulses from hypothalamus regulate the posterior lobe.

21. Anterior Pituitary Gland
Hypophyseal Portal System –
Releasing hormones secreted by the hypothalamus are conveyed to the anterior gland through Hypophyseal portal veins.
Releasing hormones act on specific target cells within the anterior pituitary gland
In response, the pituitary gland secretes tropic hormones that travel throughout the body acting on distant target cells.
Tropic hormone = hormones that have other endocrine glands as their target

22. Example of hypophyseal pathway
Releasing Hormone:
Thyroid releasing Hormone (TRH)
secreted from hypothalamus
Tropic Hormone:
Thyroid Stimulating Hormone (TSH)
is secreted from the anterior pituitary
Target Cells:
Thyroid Hormone (Thyroxine) is secreted from thyroid glands

23. Anterior Pituitary Hormones
Growth Hormone (somatotropin)
Hypothalamic Control of GH:
Growth Hormone Releasing Hormone (GHRH): promotes GH secretions
Somatostatin: inhibits GH secretion
Target Cells:
Epithelial and Connective Tissue
Adipose Tissue
Liver
Actions of GH:
Promotes cell growth and division, especially in skeletal muscles and chondrocytes
Promotes breakdown and use of fat for energy
Liver: promotes breakdown of glycogen for energy

24. Growth Hormone Disorders
Gigantism
Results from oversecretion of GH in childhood
Usually caused by a tumor of the pituitary gland
Hypopituitary Dwarfism
Insufficient GH during development
GH therapy may treat condition if administered before the epiphyseal plates ossify

25. Anterior Pituitary Hormones
2. Thyroid Stimulating Hormone (thyrotropin)
Hypothalamic Control of TSH: Thyroid Releasing Hormone
Target Cells: Thyroid Gland
Actions: TSH promotes secretions of thyroid hormones
(T3 & T4)

26. Thyroid Hormones and Negative Feedback
Under normal conditions, T3 and T4 inhibit further secretions of TRH and TSH
Iodine obtained from the diet is essential for thyroid hormone (T3 & T4) synthesis

27. An Iodine deficiency prevents the formation of Thyroid Hormones.
TRH & TSH continually stimulate the thyroid gland without inhibition.
Goiter = enlarged thyroid gland

28. Anterior Pituitary Hormones
3. Prolactin (mammotropin)
Hypothalamic Control of PRL:
Prolactin Releasing Factor: promotes secretion of prolactin
Prolacting Release Inhibiting Hormone: inhibits PRL secretion
Target Cells: Mammary Glands
Actions: Prolactin promotes milk production

29. Anterior Pituitary Hormones
3. Adrenocorticotropic Hormone (ACTH)
Hypothalamic Control of ACTH:
Corticotropin Releasing Hormone
Target Cells: Adrenal Cortex
Actions: ACTH promotes secretions of hormones from the adrenal cortex (e.g. cortisol)

30. Anterior Pituitary Hormones
Follicle Stimulating Hormone (FSH) Luteinizing Hormone (LH)
4 & 5 = gonadotropes
Hypothalamic Control:
Gonadotropin Releasing Hormone (GRH)
Target Cells: Gonads
Male: testes Female: Ovaries
Actions of gonadotropes:
Follicle Stimulating Hormone:
Female = promotes development of ovarian follicles
Male = promotes development of sperm
Luteinizing Hormone:
Female = promotes the secretion of estrogens and progesterone
Male = promotes the production of testosterone

31. Figure Hormones released from the hypothalamus, the corresponding hormones released from the anterior lobe of the pituitary gland, and their target organs.

32. Posterior Pituitary Gland
Structurally consists of neurosecretory cells
Hormones are produced by the hypothalamus, then released from the posterior pituitary gland.

33. Posterior Pituitary Hormones
Antidiuretic Hormone (ADH)
(also called vasopressin)
Target Cells: Kidneys & Blood Vessels
Actions of ADH depend the receptors to which it binds
V1 receptors
Located within blood vessels
ADH, in high concentrations promotes vasoconstriction
May prevent a drop in blood pressure with profuse bleeding
V2 receptors
Located within tubules of kidneys
ADH promotes water reabsorption at the kidneys, and thus decreases water loss.
Alcohol inhibits ADH secretion, which explains its role as a diuretic.

34. Posterior Pituitary Hormones
2. Oxytocin
Actions of Oxytocin
Females:
stimulates smooth muscle contractions in the uterus during delivery
Promotes ejection of milk from mammary glands
Males: Function is unknown

36. End of Section 2, Chapter 13.

37. The Peripheral Endocrine Glands
Endocrine System
Section 3, Chapter 13
The Peripheral Endocrine Glands

38. Thyroid Gland
Location: The thyroid gland is located just inferior to the larynx.
Structure:
It consists of two lateral lobes connected by an isthmus
Contains several follicles.

39. Thyroid Gland Follicles
Follicles consists of simple cuboidal epithelium & a colloid center
Follicular Cells: produce T3 & T4
Coloid: contains Thyroglobulin, which is a storage form of thyroid hormones.
Extrafollicular Cells: produce Calcitonin
Follicular Cells take up thyroglobulin by endocytosis, then release the thyroid hormones into the bloodstream.

40. Thyroid Hormones
Target Cells: T3 & T4 affect many cells throughout the body.
Actions of T3 & T4: Raise Metabolic Rate
Increase rate of carbohydrate catabolism
Enhance protein synthesis
Promotes the breakdown and use of lipids
T3 & T4 are major factors in determining the basal metabolic rate (BMR)
BMR = calories required to sustain life

41. Thyroid Hormones
Follicular cells require iodine salts (iodide) to produce T3 and T4.
T3 & T4 are hydrophobic molecules (insoluble in water)
Nearly 75% of thyroid hormones are attached to thyroid binding globulins.
Only the small amounts of the unbound hormones act on target cells.

42. Transport of Thyroid Hormones
T4 accounts for 95% of circulating Thyroid Hormone, But…
T3 is physiologically more active.
T3 is 5 times as potent as T4
T3 also has a 50-fold higher “free” concentration in the plasma (see figure below).

43. Thyroid Disorders Hypothyroidism – insufficient T3 & T4
During infancy – results in intellectual disability, stunted growth, abnormal bone formation (cretinism)
During adulthood – low metabolic weight, sluggishness, poor appetite, and sensitivity to cold
Infantile hypothyroidism
Hyperthyroidism – excess T3 & T4
Results in high metabolic rate, hyperactivity, weight loss, sensitivity to heat, and exophthamia (protruding eyes)
Grave’s Disease
Autoimmune Disorder: Antibodies target the thyroid gland and mimic TSH. Thyroid antibodies overstimulate thyroid gland, resulting in hyperthyroidism.
Grave’s disease may cause exophthalmia

44. Calcitonin Extrafollicular cells (C-cells) secrete Calcitonin
Calcitonin lowers blood calcium concentrations.
Actions of Calcitonin
Stimulates Osteoblast activity – increases bone deposition
Inhibits osteoclast activity – reduces bone resorption
Promotes the excreting of calcium from the kidneys
Major Source of Control: elevated blood calcium ion concentration

45. Parathyroid Glands Location:
4 small parathyroid glands are located on the posterior aspect of the thyroid gland
Hormone: PTH (parathyroid hormone)
One parathyroid gland surrounded by thyroid follicles.

46. Parathyroid Hormone (PTH)
Parathyroid Hormone elevates blood calcium levels.
Actions of PTH:
Stimulates Osteoclast activity – increases bone resorption
Inhibits osteoblast activity – reduces bone deposition
Promotes calcium reabsorption from the kidneys.
PTH also promotes the activation of Vitamin D, which enhances calcium absorption from the small intestine.
Major Source of Control: Inadequate blood calcium ion concentration

47. Figure Parathyroid Hormone (PTH) stimulates bone to release Calcium (Ca2+) and the kidneys to conserve calcium. It indirectly stimulates the intestine to absorb calcium. The resulting increase in blood calcium concentration inhibits secretion of PTH by negative feedback.

48. Calcitonin and PTH have opposing effects on the levels of calcium ions in circulation. Both work together to maintain calcium homeostasis.

49. Adrenal Glands
Location: The adrenal glands are located on the superior aspect of the kidneys.
Structure:
Adrenal glands are pyramid shaped organs that consist of two parts
Adrenal Medulla = secretions controlled by sympathetic nerve fibers Adrenal Cortex = Under hormonal control

50. Hormones of the Adrenal Medulla
Nerve fibers control secretions: Hormones of the adrenal medulla are under control by the sympathetic division (fight or flight) of the ANS.
Hormones: Norepinephrine (noradrenalin) & Epinephrine (adrenalin)
Both are classified as catecholamines.
Actions: Effects are similar to sympathetic nerve fibers, but longer lasting.
Increases heart rate and force of contraction
Increases blood pressure
Increases metabolic rate
Increases blood glucose levels (primarily epinephrine)
Decreases digestion

51. Beta Blockers
Epinephrine & Norepinephrine exert their effects by binding to Beta (ß) adrenergic receptors in heart and walls of the blood vessels.
Beta blockers bind to ß-receptors, thus obstructing the binding of catecholamines.
Hence beta blockers reduce sympathetic influences of the heart and blood vessels.
Therefore, beta blockers decrease heart rate, contractility, and reduce blood pressure.

52. Hormones of the Adrenal Cortex
3 Layers of the adrenal cortex secrete over 30 types of steroid hormones.
Hormones
Aldosterone – produced in zona glomerulosa
Cortisol – produced in zona fasciculata
Androgens – produced in zona reticularis

53. Hormones of the Adrenal Cortex
Aldosterone (mineralocorticoid)
regulates Na+ and K+ concentrations
regulates blood pressure
Actions
Aldosterone causes the kidneys to reabsorb Na+ and to excrete K+
Aldosterone indirectly raises blood pressure:
Increased Na+ reabsorption increases water reabsorption by osmosis.
Controls of Aldosterone Secretion
Low blood pressure stimulates aldosterone secretion
(renin-angiotensin-aldosterone pathway)
Elevated blood K+ concentration promotes aldosterone secretion
Low Na+ has only a slight effect on aldosterone secretion.

54. Renin-Angiontensin-Aldosterone System
ACE Inhibitors block the actions of ACE, and thus lower blood pressure.

55. Hormones of the Adrenal Cortex
2. Cortisol (glucocorticoid)
Its primary effect is to build up and conserve blood glucose supplies
Its actions keep blood glucose levels constant between meals.
Actions
Promotes gluconeogenesis in the liver
gluconeogenesis = glucose synthesis from non-carbohydrates
Promotes the release and used of fatty acids from adipose for energy.
Using fatty acids for energy allows glucose to be conserved.
Inhibits protein synthesis: amino acids used in gluconeogenesis

56. Hormones of the Adrenal Cortex
3. Androgens
Supplement the sex hormones secreted from the gonads.
Androgens may be converted into testosterone and estrogens.

57. The Pancreas
Structure & Location: The pancreas is located posterior to the stomach, attached to the duodenum.
The pancreas has both digestive and endocrine functions.
Pancreatic Islets (Islets of Langerhans) = endocrine cells
Digestion cells
(we’ll discuss these with the digestive system)

58. Cells of the Pancreatic Islets
3 distinct type of cells secrete 3 hormones:
Alpha Cells – secrete glucagon
Beta Cells – secrete insulin
Delta Cells – secrete somatostatin
Pancreatic hormones regulate the storage, use, and release of fuels (glucose).

59. Pancreatic Hormones 1. Glucagon
Overall Effect: During fasting, when blood glucose levels drop, glucagon elevates blood glucose levels
Actions of Glucagon:
Stimulates glycogenolysis in the liver (breakdown of glycogen into glucose)
Glucagon also promotes gluconeogenesis
Glucagon also stimulates the breakdown of fats into glycerol and fatty acids.
Glycerol is used in gluconeogenesis
Fatty Acids are metabolized for energy

60. Liver Gluconeogenesis Glycogenolysis Amino acids glycerol glycogen
glucose
glucose
Glucagon secretions elevates blood glucose concentrations.
Gluconeogenesis converts noncarbohydrates, such as amino acids and glycerol, into glucose.
Glycogenolysis breaks down large glycogen molecules into glucose.

61. Pancreatic Hormones 2. Insulin 3. Somatostatin
Overall Effect: Following a meal, when blood carbohydrate levels are high, insulin removes excess glucose from the blood.
Actions of Insulin:
Stimulates glycogenesis in the liver (formation of glycogen from glucose).
It inhibits gluconeogenesis.
Insulin promotes glucose uptake in adipose tissue, skeletal muscles, and cardiac muscle.
3. Somatostatin
Overall Effect: Helps regulate glucose metabolism by inhibiting the secretion of glucagon and insulin.

62. Hormonal Control of Glucose
Insulin and glucagon function together to stabilize blood glucose concentration. Negative feedback responding to blood glucose concentration controls the levels of both hormones.

63. Diabetes Mellitus Type I Diabetes Mellitus (juvenile)
Autoimmune disease – immune system destroys beta cells, resulting in the loss of insulin production.
Without insulin, blood glucose cannot be taken up and used for energy.
Glucose accumulates in the blood and urine = hyperglycemia.
Type II Diabetes Mellitus (adult onset)
Receptors on target cells wear down and become insensitive to insulin.
Target cells resist glucose uptake, even in the presence of insulin.
Insulin levels must be higher than normal just to maintain normal glucose concentrations.

64. Other Endocrine Glands
Pineal Gland
Located posterior to thalamus.
The pineal gland secretes melatonin, which regulates circadian rhythms (sleep/wake cycle)
Melatonin secretions are greatest in dark. Light inhibits secretions.
Thymus Gland
Secretes thymosins
Promotes development of certain lymphocytes
Important in role of immunity

65. Other Endocrine Glands
Reproductive Organs
Ovaries produce estrogens and progesterone
Testes produce testosterone
Placenta produces estrogens, progesterone, and gonadotropin
Other organs: digestive glands, heart, and kidney
End of Section 3, Chapter 13