Endocrine System Part 2

Specific endocrine organs

Glands in the brain

Hypothalamus

Master gland

•The hypothalamus provides highest level of endocrine control

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Pituitary Gland (Hypophysis)

Located in the sella turcica of the sphenoid bone

•Hangs inferior to hypothalamus

•Connected by infundibulum

•Releases nine important peptide hormones

•Hormones bind to membrane receptors

•Use cAMP as second messenger

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Histologically distinct

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The Hypophyseal Portal System

•Median eminence, a swelling near attachment of infundibulum

•Where hypothalamic neurons release regulatory factors into interstitial fluids through fenestrated capillaries


* A portal system links two capillary beds. This ensures that the hormones travel to the adjacent pituitary without first going through the circulatory system.

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Neuroendocrine reflexes - Pathways include both neural and endocrine components

Complex Commands Issued by changing:

•Amount of hormone secreted

•Pattern of hormone release

•Hypothalamic and pituitary hormones can be released in sudden bursts

•Frequency changes response of target cells

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The hypothalamus controls the anterior pituitary by two classes of regulatory hormones

1. Releasing hormones (RH)

•Stimulate synthesis and secretion of one or more hormones at anterior lobe


2. Inhibiting hormones (IH)

•Prevent synthesis and secretion of hormones from the anterior lobe

•Rate of secretion is controlled by negative feedback

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Negative Feedback Loops - Control the rate of secretion

Three examples:

1. Regulation when multiple organs are involved

When the desired concentration of the end hormone is reached, this level acts as a negative feedback to inhibit further production of the releasing hormone.
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2. Variations of the typical pattern

Prolactin regulation
In this negative feedback example, PRL (Prolactin), a hormone that stimulates mammary glands for breast feeding is produced in the anterior pituitary. When the levels of PRL are adequate, PIF (Prolactin Inhibiting Factor) is secreted to reduce PRL production and (PRF) Prolactin Releasing Factor is inhibited.
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3. Variations of the pattern

Growth hormone (GH)
When sufficient levels of GH are achieved:
1. Tissues are directly stimulated by the hormone (eg epithelia)
2. The liver produces somatomedins that in turn stimulates growth in tissues (eg bones)

The levels of somatomedins inhibit GH-RH (growth hormone-releasing hormone) and stimulate GH-IH (growth hormone inhibiting hormone)
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The Posterior Lobe of the Pituitary Gland (neurohypophysis )

Contains unmyelinated axons of hypothalamic neurons

Supraoptic and paraventricular nuclei manufacture:

•Antidiuretic hormone (ADH)

kidneys: reabsorption of water to increase blood volume and pressure

•Oxytocin (OXT):

(f) uterus and mammary glands - labor contractions and milk ejection

(m) prostate gland - contractions of ductus deferens and prostate gland

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Pineal Gland

•Lies in posterior portion of roof of third ventricle

•Contains pinealocytes (cells of the pineal)

•Synthesize hormone melatonin

  1. Inhibits reproductive functions
  2. Protects against damage by free radicals
  3. Influences circadian rhythms
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Thyroid gland

Lies inferior to thyroid cartilage of larynx

Consists of two lobes connected by narrow isthmus
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Thyroid follicles

•Hollow spheres lined by cuboidal epithelium, these cells surround follicle cavity that contains viscous colloid

The follicles are surrounded by network of capillaries that:

•Deliver nutrients and regulatory hormones

•Accept secretory products and metabolic wastes

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Functions of Thyroid Hormones

•Affect most cells in body

•Enter target cells by transport system


•Bind to receptors in:

1.Cytoplasm

2.Surfaces of mitochondria

3.Nucleus

•In children, essential to normal development of:

Skeletal, muscular, and nervous systems

Calorigenic Effect

•Cell consumes more energy resulting in increased heat generation

•Is responsible for strong, immediate, and short-lived increase in rate of cellular metabolism

Hormone carriers

Thyroid-binding Globulins (TBGs)

•Plasma proteins that bind about 75 percent of T4 and 70 percent of T3 entering the bloodstream

Transthyretin (thyroid-binding prealbumin – TBPA) and albumin

•Bind most of the remaining thyroid hormones

•About 0.3 percent of T3 and 0.03 percent of T4 are unbound

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Effects of Thyroid Hormones on Peripheral Tissues

1.Elevates rates of oxygen consumption and energy consumption; in children, may cause a rise in body temperature

2.Increases heart rate and force of contraction; generally results in a rise in blood pressure

3.Increases sensitivity to sympathetic stimulation

4.Maintains normal sensitivity of respiratory centers to changes in oxygen and carbon dioxide concentrations

5.Stimulates red blood cell formation and thus enhances oxygen delivery

6.Stimulates activity in other endocrine tissues

7.Accelerates turnover of minerals in bone

The C Cells of the Thyroid Gland and Calcitonin

•C (clear) cells also called parafollicular cells

•Produce calcitonin (CT)

•Helps regulate concentrations of Ca2+ in body fluids

1.Inhibits osteoclasts, which slows the rate of Ca2+ release from bone

2.Stimulates Ca2+ excretion by the kidneys


reduces Ca2* blood levels

Four Parathyroid Glands

Embedded in the posterior surface of the thyroid gland

Altogether, the four glands weigh 1.6 g

Parathyroid Hormone (PTH) or parathormone is produced by parathyroid (chief) cells in response to low concentrations of Ca2+

•Antagonist for calcitonin


increases blood Ca2+ levels

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Negative feedback of throid secretion

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Low TSH levels leads to:

inactive thyroid follicles so that neither synthesis nor secretion occurs
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Three Effects of PTH


1.It stimulates osteoclasts and inhibits osteoblasts

•Accelerates mineral turnover and releases Ca2+ from bone

•Reduces rate of calcium deposition in bone

2.It enhances reabsorption of Ca2+ at kidneys, reducing urinary losses

3.It stimulates formation and secretion of calcitriol by the kidneys

•Effects complement or enhance PTH

•Also enhances Ca2+, PO43- absorption by digestive tract

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Adrenal glands

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Lie along superior border of each kidney and can be subdivided into:


Superficial adrenal cortex

Stores lipids, especially cholesterol and fatty acids

Manufactures steroid hormones (corticosteroids)


Inner adrenal medulla

Secretory activities controlled by sympathetic division of ANS

Produces epinephrine (adrenaline) and norepinephrine

Metabolic changes persist for several minutes

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The three regions of the adrenal cortex

1. Zona glomerulosa -Outer region of adrenal cortex

Produces mineralocorticoids For example, aldosterone

Aldosterone stimulates conservation of sodium ions and elimination of potassium ions

Increases sensitivity of salt receptors in taste buds

•Secretion responds to:

•Drop in blood Na+, blood volume, or blood pressure

•Rise in blood K+ concentration

*acts to increase Na+ levels and reduce K+ levels

2. Zona fasciculata

•Produces glucocorticoids, for example, cortisol (hydrocortisone) with corticosterone


Glucocorticoids

•Accelerate glucose synthesis and glycogen formation

•Show anti-inflammatory effects

•Inhibit activities of white blood cells and other components of immune system

Liver converts cortisol to cortisone

•Secretion regulated by negative feedback

•Has inhibitory effect on production of:

•Corticotropin-releasing hormone (CRH) in hypothalamus

•ACTH in adenohypophysis (anterior pituitary)

3. Zona reticularis

•Network of endocrine cells

•Forms narrow band bordering each adrenal medulla

•Produces androgens under stimulation by ACTH

The Adrenal Medulla

Contains two types of secretory cells

•One produces epinephrine (adrenaline): 75 to 80 percent of medullary secretions

•The other produces norepinephrine (noradrenaline): 20 to 25 percent of medullary secretions

What do Epinephrine and Norepinephrine do?


Neural activation of the adrenal medullae has the following effects:

In skeletal muscles, epinephrine and norepinephrine trigger mobilization of glycogen reserves to accelerate the breakdown of glucose to provide ATP for muscular strength and endurance


In adipose tissue, stored fats are broken down into fatty acids released into the bloodstream for other tissues to use for ATP production


In the liver, glycogen molecules are broken down and the resulting glucose molecules are released into the bloodstream

Primarily for use by neural tissue, which cannot shift to fatty acid metabolism


In the heart, the stimulation of beta 1 receptors triggers an increase in the rate and force of cardiac muscle contraction

Pancreas

•Lies between:

•Inferior border of stomach

•And proximal portion of small intestine

•Contains exocrine and endocrine cells

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Exocrine Pancreas

Consists of clusters of gland cells called pancreatic acini and their attached ducts

Takes up roughly 99 percent of pancreatic volume

Gland and duct cells secrete alkaline, enzyme-rich fluid

That reaches the lumen of the digestive tract through a network of secretory

Endocrine Pancreas

Consists of cells that form clusters known as pancreatic islets, or islets of Langerhans

1. Alpha cells produce glucagon

2. Beta cells produce insulin

3. Delta cells produce peptide hormone identical
to GH–IH

4. F cells secrete pancreatic polypeptide (PP)

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Blood sugar regulation

•Blood Glucose Levels

•When levels rise: Beta cells secrete insulin, stimulating transport of glucose across plasma membranes

•When levels decline: Alpha cells release glucagon, stimulating glucose release by liver

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Insulin

Is a peptide hormone released by beta cells

•Affects target cells

•Accelerates glucose uptake

•Accelerates glucose utilization and enhances ATP production

•Stimulates glycogen formation

•Stimulates amino acid absorption and protein synthesis

•Stimulates triglyceride formation in adipose tissue

Glucagon

Released by alpha cells

•Mobilizes energy reserves

•Affects target cells

•Stimulates breakdown of glycogen in skeletal muscle and liver cells

•Stimulates breakdown of triglycerides in adipose tissue

•Stimulates production of glucose in liver (gluconeogenesis)

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Diabetes Mellitus

Type 1 (insulin dependent) diabetes is characterized by inadequate insulin production by the pancreatic beta cells

  • Persons with type 1 diabetes require insulin to live and usually require multiple injections daily, or continuous infusion through an insulin pump or other device


  • This form of diabetes accounts for only around
    5–10 percent of cases; it often develops in childhood

Type 2 (non-insulin dependent) diabetes is the most common form of diabetes mellitus

  • Most people with this form of diabetes produce normal amounts of insulin, at least initially, but their tissues do not respond properly, a condition known as insulin resistance
  • Type 2 diabetes is associated with obesity
  • Weight loss through diet and exercise can be an effective treatment

Complications of untreated, or poorly managed diabetes mellitus

•Kidney degeneration

•Retinal damage

•Early heart attacks

•Peripheral nerve problems

•Peripheral tissue damage

Many Organs of Other Body Systems Have Secondary Endocrine Functions

•Intestines (digestive system)

•Kidneys (urinary system)

•Heart (cardiovascular system)

•Thymus (lymphatic system and immunity)

•Gonads (reproductive system)

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The Hormonal Responses to Stress

•General Adaptation Syndrome (GAS)

•Also called stress response

•How body responds to stress-causing factors

•Is divided into three phases

1. Alarm phase

2. Resistance phase

3. Exhaustion phase

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