Endocrine gland: A gland that secretes hormones directly into the blood. Endocrine glands have no ducts.
Exocrine gland: A gland that secretes molecules into a duct that carries the molecules to where they are used.
Hormone: A molecule released into the blood which acts as a chemcial messenger.
Target tissue: A group of cells that have receptors embedded in the plasma membrane that are complementary in shape to a specific hormone molecule. Only these cells will respond to that specific hormone.
The first messenger is the hormone that transmits a message around the body, e.g. adrenaline. The second messenger, e.g. cAMP transmits a signal inside the cell. The cAMP acts by activating enzymes.
Adrenaline has different effects on different tissues because they have different types of adrenaline receptors causing cAMP concentrations to increase or decrease. cAMP may activate different enzymes in different target cells. Also, the second messenger may be different, causing different effects.
The adrenal glands have two distinct regions - the cortex region and the medulla region. The adrenal medulla releases adrenaline, which:
- Relaxes smooth muscle
- Increases the stroke volume of the heart
- Increases heart rate
- Causes general vasocontriction - raising blood pressure
- Stimulates the conversion to glycogen to glucose - glycogenolysis
- Dilates the pupils
- Increases mental awareness
- Inhibits the action of the gut - peristalsis
- Causes body hair to erect
The adrenal cortex manufactures and releases corticosteroid hormonds which are made from cholesterol.
- Mineralocorticoids help control the concentrations of Na and K in the blood
- Glucocorticoids help control the metabolism of carbohydrates and proteins in the liver
The exocrine cells of the pancrease secrete digestive enzymes into the pancreatic duct, which transports them to the small intestine (duodenum). These cells make up the majority of the pancreas.
The exocrine cells are found in the islets of Langerhans and consist of alpha and beta cells. The alpha cells manufacture and secrete glucagon, whereas the beta cells manufacture and secrete insulin. They are involved in the regulation of blood glucose levels.
If blood glucose concentration drops too low:
- Detected by alpha cells which inhibit insulin production
- They secrete glucagon into the blood
- Glucagon binds to receptrs on hepatocytes and causes:
Glycogenolysis - conversion of glycogen to glucose
More fatty acids are used in respiration