The Endocrine System

Name the 2 components of the endocrine system

• Endocrine glands

• Hormone-secreting cells

Name and describe the 3 types of hormones

• Endocrine - travels to a distant target cell via the blood stream

• Paracrine - travels a short distance to act on a nearby target cell

• Autocrine - acts on the secretory cell it was released by

Name and describe the 2 types of hormones based on chemistry

• Water Soluble - peptide/protein hormones that dissolve in water and can freely travel in the blood, however cannot freely pass the lipid cell membranes.

• Lipid Soluble - steroid hormones derived from cholesterol that cannot dissolve in water, therefore travel through the blood via a transport protein, however can freely pass through lipid cell membranes.

Name and describe the 2 types of receptors

• Surface Receptors - on the surface of target cell membranes, typically activated by water soluble hormones that cannot diffuse through the cell membrane

• Internal Receptors - within the nucleus or cytosol of the target cell, typically activated by lipid-soluble hormones that can diffuse through the cell membrane

Describe what occurs when lipid-soluble hormones bind to nuclear receptors.

The hormone can bind to DNA and alter gene expression, therefore altering patterns of protein synthesis and cell function, stimulating a physiological response.

Describe the location and function of the hypothalamus.

A region located in the center of the brain that links the endocrine and nervous systems together. Therefore regulating many functions via receiving and transmitting neural signals for the release of hormones.

Describe the location and structure of the pituitary gland.

An endocrine gland surrounded by the sphenoid bone and attached to the hypothalamus via the infundibulum. It is separated into 2 portions, the anterior and posterior pituitary glands.

Describe the role of the posterior pituitary gland.

Contains the axons of neurosecretory cells of the hypothalamus which releases oxytocin and ADH into the capillary plexus of the posterior pituitary. These hormones are then released into the bloodstream via the hypophysel vein.

Describe the 2 functions of oxytocin

• The stretch of the cervix during childbirth transmits signals to the hypothalamus that triggers the release of oxytocin, increasing uterine wall contractions. Via a positive feedback loop, the cervix continues to stretch, transmitting further signals and releasing more oxytocin until the baby is born.

• After childbirth oxytocin induces milk ejection from the mammary glands.

Describe the 3 functions of ADH

• Stimulates water reabsorption from urine in the kidneys into the bloodstream, reducing water loss

• Stimulates reduction of sweating by the sweat glands to reduce water loss

• Stimulates vasoconstriction to increase blood pressure

Explain the regulation of ADH release

ADH release is regulated via osmoreceptors detecting solute concentration in the blood:

• High activity of osmoreceptors occurs when there is high solute concentration, stimulating ADH release.

• Low activity of osmoreceptors occurs when there is low solute concentration, reducing ADH release.

Describe the role of the anterior pituitary gland.

Contains the hypophyseal portal system in which neurosecretory cells of the hypothalamus secrete releasing or inhibiting hormones into the primary plexus within the median eminence. These hormones travel to the secondary plexus via the hypophyseal portal veins, stimulating specific anterior pituitary cells to release different hormones into the bloodstream via the hypophyseal vein.

Explain how hormones work when travelling throughout the body.

Hormones travel throughout the entire body via the bloodstream, however only act on certain organs and cells that contain their specific receptors.

Briefly explain how Insulin-like Growth Factor-1 (IGF-1) is produced.

• Hypothalamus - growth releasing hormone (GRH)

• Somatotrophs - growth hormone (GH)

• Liver and bones - IGF-1

Briefly explain how cortisol and DHEA are produced.

• Hypothalamus - corticotropin releasing hormone (CRH)

• Corticotrophs - adrenocorticotropic hormone (ACTH)

• Adrenal Glands - cortisol and DHEA

Briefly explain how testosterone, oestrogen and progesterone are produced.

• Hypothalamus - gonadotropin releasing hormone (GnRH)

• Gonadotrophs - follicle stimulating hormone (FSH) and leutinising hormone (LH)

• Testes and Ovaries - testosterone, oestrogen and progesterone

Briefly explain how thyroid hormones are produced.

• Hypothalamus - thyrotropin-releasing hormone (TRH)

• Thyrotrophs - thyroid stimulating hormone (TSH)

• Thyroid - T3 and T4

Briefly explain how milk is produced in the mammary glands.

• Hypothalamus - prolactin releasing hormone (PRH)

• Lactotrophs - prolactin (PRL)

• Mammary glands - lactation

Name the hypothalamic inhibiting hormone for the releasing hormone of PRH

Dopamine

Explain the structure of growth hormone (GH).

Growth hormone is a water-soluble peptide hormone made up of 191 amino acids.

Explain in detail, the process of growth hormone release and regulation

1. Growth hormone releasing hormone (GHRH) is produced and secreted by neurosecretory cells of the hypothalamus into the primary plexus in the median eminence

2. GHRH travels to the secondary plexus in the anterior pituitary via hypophyseal portal veins, stimulating the release of growth hormone (GH) from somatotrophs

3. GH is released into the bloodstream via the hypophyseal vein, travelling to the liver and bones to stimulate release of insulin-like growth factor-1 (IGF-1)

4. Via negative feedback loops, GH and IGF-1 act on the hypothalamus to inhibit further release of GHRH and stimulate the release of somatostatin which inhibits GH release.

5. Additionally IGF-1 acts on the somatotrophs, inhibiting further release of GH.

Describe other factors that affect growth hormone (GH) release.

• Onset of sleep stimulate surge of GH release while REM sleep reduces GH release

• Stress and exercise stimulate GH release

• Increase in glucose and free fatty acids reduces GH helease

Explain the roles of GH and IGF-1

• Both GH and IGF-1 increase protein synthesis and cell division, therefore increasing bone and soft tissue growth

• IGF-1 increases cartilage tissue growth

• GH increases blood glucose levels

Explain what occurs when someone has a GH deficiency

Dwarfism occurs when there is damage to the pituitary gland, resulting in low GH production.

Explain what occurs in childhood and adulthood when there is excessive GH

• In childhood, gigantism occurs when there is a benign tumour in the pituitary gland before epiphyseal plate closure. Therefore there is excessive production of GH that results in abnormally tall stature and rapid growth in children.

• In adulthood, acremogaly occurs when there is a benign tumour tumour in the pituitary gland after epiphyseal plate closure. Therefore there is excessive production of GH that results in lengthening of the jaw, large hands and feet.

Describe the structure and location of the adrenal glands.

2 triangular shaped organs that sit superior to the kidneys. They are a few cm in size and compose of the adrenal cortex and adrenal medulla.

How do the adrenal glands receive blood supply?

Via 3 main arteries; superior, middle and inferior arteries.

Explain the location, structure and function of the adrenal medulla.

The inner portion of the adrenal glands that primarily consist of chromaffin cells which synthesize and release catecholamines.

Describe catecholamines

Catecholamines (80% adrenaline and 20% noradrenaline) are released by increased sympathetic NS activity. They bind to and activate alpha and beta catecholaminergic receptors throughout the body to increase metabolism, HR, BP, vasoconstriction and bronchiole dilation.

Identify and explain the 3 parts of the adrenal cortex.

1. Zona glomerulosa - outermost portion containing cells organised in spherical clusters and arched columns, secreting mineralocorticoids, eg. Aldosterone

2. Zona fasciculata - middle portion containing cells organised in long vertical columns, secreting glucocorticoids, eg. Cortisol

3. Zona reticularis - innermost portion containing cells organised in branches, secreting androgens, eg. DHEA

Name the 2 factors that increase aldosterone production.

• Increased angiotensin ll (ANG ll) levels

• Increase potassium ion levels in extracellular fluid

Explain in detail, how aldosterone increases blood volume and blood pressure.

1. Aldosterone is lipid-soluble and binds to intracellular receptors in the cytosol within cells of the nephron.

2. The aldosterone-receptor complex binds to DNA in the nucleus, influencing gene transcription.

3. This increases protein synthesis and the formation of sodium and potassium channels and transporters.

4. This increases sodium reabsorption into the blood from filtrate and increases potassium excretion into filtrate

5. Increasing sodium and therefore water retention, increases blood volume and therefore blood pressure.

Name other factors that affect cortisol release.

• Time of day (morning = high, night = low)

• Stress (short term = brief spike, long term = consistently high)

• Low blood glucose

• Inflammation

• Intense exercise

Explain in detail the effects of cortisol and how they occurs 

1. Cortisol is lipid soluble and binds to intracellular receptors within cytosol of all nucleated cells 

2. The cortisol-receptor complex travels to the nucleus, binding to DNA and influencing gene transcription and therefore proteins synthesis and cell function. 

3. Physiological responses include immunosuppression, include glucose production in the liver as well as increased protein and triglyceride breakdown for the formation of glucose. 

Explain hypercortisolism/Cushing syndrome

Excessive amounts of cortisol produced in the body over a prolonged time period. Symptoms include weight gain in the upper body, red cheeks, fat pads between shoulders and thin skin, resulting in easy bruising and stretch marks.

Explain Addison’s disease

Cortisol and aldosterone deficiency over a prolonged time period. Symptoms include fatigue, weight loss, low BP, salt cravings, hypoglycaemia and increased skin pigmentation.

Explain the role of DHEA in producing sex hormones

DHEA is a precursor to oestrogen and testosterone. However only a small amount are produced in the adrenal glands, oestrogen is primarily produced in the ovaries and testosterone is primarily produced in the testes.

Explain the role of DHEA in puberty

DHEA plays a role in growth of axillary and pubic hair as well as prepubertal growth.

Explain the additional possible roles of DHEA

• Aging - DHEA levels decrease with age

• Immune System - DHEA counters some effects of cortisol

• Cognition and Memory - acts on receptors of cognition-related brain areas

Explain the location of the thyroid in relation to the larynx and trachea

• The thyroid is caudal (inferior and posterior) to the larynx

• The thyroid is anterior to the trachea

Explain the blood supply to and from the thyroid

• The thyroid is supplied by the superior and inferior thyroid arteries

• The thyroid drains blood into the superior, middle and inferior thyroid veins

Name these structures in the neck

A - superior thyroid vein

B - superior thyroid artery

C - larynx

D - thyroid gland

E - middle thyroid vein

F - inferior thyroid artery

G - trachea

H - inferior thyroid vein

Name these structures of the thyroid

A - basement membrane

B - thyroid follicle

C - C/parafollicular cell

D - colloid

E - follicular cell

F - capillary

Explain the secretion of thyroid hormone

• Thyroid hormone with attached thyroglobulin is transported into follicular cells via pinocytosis in vesicles

• Vesicles merge with lysosomes, digesting thyroglobulin and releasing thyroid hormone

• Thyroid hormone diffuses into the bloodstream and binds to thyroxine-binding globulin (TBG).

Explain the proportions of T4 and T3 produced

More T4 is produced than T3, however T3 is more potent and therefore T4 is converted to T3 by target tissues.

State the additional factors that affect thyroid hormone regulation

• Cold temperature

• Stress

• Low blood glucose

• Inflammation/infection

• Pregnancy

Compare the effects of thyroid hormone on adults and children

In adults:

• Increases base metabolic rate

• Increases glucose production, HR and VP

• Decreases blood cholesterol levels

In children:

• Required for normal growth and development

• Required for neural development

State the 3 ways the PTH increases blood calcium levels

• Increases bone breakdown, releasing calcium and phosphate into the blood

• Increases calcium reabsorption into the bloods and phosphate secretion into urine within the kidneys

• Increases production of calcitriol from vitamin D. Calcitriol travels to GI tract and increases reasborption of dietary calcium

Explain the function of parafollicular cells

Synthesizes and secretes calcitonin which has opposing effects of PTH.

Explain the location and structure of the pancreas

Located within the abdominal cavity, within the posterior to the stomach. The pancreas is divided into 3 main regions:

• Head - located within the curvature of the duodenum 

• Body - extends horizontally from the head to the left abdomen 

• Tail - narrow tapering from the body towards the spleen

Explain the 2 functions of the pancreas and correlating structure

• 98% of the pancreas is made of acini, clusters of exocrine cells that synthesize and secrete bicarbonate and digestive enzymes into the GI tract

• 2% of the pancreas is made of Islets of Langerhans, clusters of endocrine cells that synthesise hormones, regulating blood sugar

Name and explain the 4 types of cells within the Islets of Langerhans

• Alpha Cells - makes up 17%, produces glucagon

• Beta Cells - makes up 75%, produces insulin and amylin

• Delta Cells - makes up 5%, produces somatostatin

• F Cells - makes up 3%, produces pancreatic polypeptides

Explain the structure of insulin and glucagon

• Insulin - peptide containing 51 amino acids into 2 strands held together by disulphide bridges

• Glucagon - peptide containing 29 amino acids

Explain how insulin reduced blood glucose levels

• Increases glycogenesis, uptake and storage of glucose into the liver and other body cells as glycogen

• Increases glycolysis, breakdown of glucose for energy

Explain how glucagon increase blood glucose levels

• Increases glycogenolysis, breakdown of glycogen in the liver and other body cells into glucose, released into the bloodstream

• Increases glucogenesis, production of glucose from lactic acid and amino acids

• Increases ketogenesis, production of ketone bodies from fatty acids as an energy source when there is insufficient glucose

Explain diabetes and symptoms

Chronically raised blood glucose levels. Symptoms include:

• Increased urination

• Increased thirst

• Increased hunger

Name the diseases that diabetes increases the risk of

• Cardiovascular diseases

• Kidney diseases

• Eye disease

• Neurological dysfunction

Explain Type l Diabetes

An autoimmune disease that destroys pancreatic beta cells and therefore no insulin can be produced.

State the population with the largest contribution to Type l Diabetes

95% of people with Type l Diabetes are children

Explain ketoacidosis and its relation to Type l Diabetes

For those with Type l Diabetes, there is not enough insulin to stimulate glycogenesis and glucose is not used as energy. Therefore the body undergoes ketogenesis, converting fatty acids to ketones and relying on these ketones for energy. However this results in ketoacidosis, in which high levels of ketones in the blood dangerously increases blood acidity. 

Explain Type ll Diabetes

Chronically raised insulin levels due to the body’s resistance to the effects of insulin or the underproduction of insulin.