Hormones

An endocrine gland secretes its product directly into the blood and/or lymph. Together, the endocrine glands form the endocrine system. 

An exocrine gland has a duct to take the gland's secretion to its site of action, for example salivary glands with ducts to the buccal (mouth) cavity. 

The product of an endocrine gland is a hormone. 

• A molecule that acts as a chemical messenger.
• Travels in the blood and lymph all over the body. 
• Causes a specific response in specific target cells or tissues. 
• May be a steroid, such as oestrogen, progesterone or testorene. 
• May be a protein, such as insulin or glucagon.
• May be derived from an amino acid, like thyroxine and adrenaline.

Hormones act in two different ways when they reach a target tissue.

• Steroids and thyroxine pass through the plasma membrane and bind with receptors in the nucleus. The hormone-receptor complex binds with DNA and regulates transcription. 
• Protein hormones and…