The endocrine system

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Endocrine/exocrine + targeting the signal

Hormones are released directly into the blood from ENDOCRINE GLANDS.

- These are ductless glands - consist of cells that produce and release a hormone into the blood capillaries running through the gland.

ENDOcrine glands - Release/SECRETE HORMONES directly into the BLOOD.

EXOcrine glands - Do NOT release hormones. Have a small  tube/DUCT that carries their SECRETION to another place. 

Hormones = Molecules that are released by endocrine glands directly into the blood.

- They act as MESSENGERS carrying a signal from the endocrine gland to a SPECIFIC target organ or tissue. 

Cells receiving a hormone signal must possess a COMPLEENTARY RECEPTOR on their plasma membrane. The hormone BINDS to the receptor. 

TARGET CELLS = Cells possessing specific receptors on their cell surface membrane. 

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Types of hormones and action of adrenaline

There are 2 types of hormones:

  • PROTEIN/PEPTIDE hormones and deritatives of AMINO ACIDS (e.g. adreanline, insulin, glucagon). 
  • STEROID hormones (e.g. the sex hormones).

Adrenaline action:

  • Adrenaline is unable to enter the target cell so it must cause an effect inside the cell without entering the cell itself.
  • Adrenaline RECEPTOR on the outside of the cell surface membrane has a COMPLEMENTARY shape to the adrenaline molecule's shape.
  • The receptor is associated with an enzyme on the inner surface of the cell surface membrane.

This enzyme is called ADENYL CYCLASE.

Adenyl cyclase is an ENZYME associated with the RECEPTOR for many hormones including ADRENALINE. It is found on the INSIDE of the CELL SURFACE MEMBRANE.

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Adrenaline action continued and the function of ad

  • Adrenaline in the blood BINDS to its specific receptor on the cell surface membrane.
  • The adrenaline molecule is called the 1st MESSENGER
  • When it binds to the receptor, it ACTIVATES the enzyme ADENYL CYCLASE. 
  • The adenyl cycalse converts ATP to CYCLIC AMP (cAMP). 
  • cAMP is the 2nd MESSENGER inside the cell.
  • The cAMP can then cause an effect inside the cell by activating enzyme ACTION. 


- Adrenal glands are found anterior to the kidneys. Each can be divided into a MEDULLA region and a CORTEX region.

  •  Medulla is found in the CENTRE of the gland. 
  • Cells in the MEDULLA manufacture and RELEASE the hormone ADRENALINE in response to STRESS such as pain/shock. 
  • The effect of adrenaline is to PREPARE the body for activity. 
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Effects of adrenaline and the adrenal cortex

  • RELAX smooth muscle in the BRONCHIOLES. 
  • INCREASE STROKE volume of the heart.
  • INCREASE heart RATE.
  • STIMULATE conversion of GLYCOGEN to GLUCOSE.
  • INHIBIT action of the GUT.
  • Cause body HAIR to ERECT.

Adrenal CORTEX:

- Uses CHOLESTEROL to produce certain STEROID HORMONES.

  • MINERALOCORTICOIDS (e.g. aldosterone) help to CONTROL concentrations of SODIUM and POTASSIUM in the blood.
  • GLUCOCORTICOIDS (e.g. cortisol) help to CONTROL the METABOLISM of CARBOHYDRATES and PROTEINS in the liver.
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