The Hormonal System and Glands

What is the hormonal system?

The hormonal system sends information as chemical signals. It's made up of glands, called endocrine glands, and hormones. Endocrine glands are groups of cells that are specialised to secrete hormones, e.g. the pancreas secretes insulin. Hormones are 'chemical messengers'. Many hormones are proteins or peptides, e.g. insulin. 

Hormones are secreted when an endocrine gland is stimulated. Glands can be stimulated by a change in concentration of a specific substance. They can also be stimulated by electrical impulses. 

Hormonal communication

Hormonal diffuse directly into the blood, then they are taken around the body by the circulatory system. They diffuse out of the blood all over the body but each hormone will only bind to specific receptors for that hormone, found on the membrane of some cells, called target cells. Tissue that contains target cells is called target tissue. The hormones trigger a response in the target cells (the effectors). 

Example: 

Stimulus- Low bloody glucose concentration.

Receptors- Receptors on the pancreas cells detect the low blood glucose concentration.

Hormone- the pancreas releases the hormone glucagon into the blood.

Effectors- target cells in the liver detect glucagon and convert glycogen into glucose.

Response- Glucose is released into the blood, so glucose concentration increases

Glands

There are two types of gland- endocrine and exocrine.

Endocrine glands secrete hormones directly into the bloody. 

Exocrine glands are groups of cells specialised to secrete chemicals through ducts into cavities or onto the surface of the body, e.g. sweat glands secrete sweat onto the skin surface. They usually secrete enzymes, e.g. digestive glands secrete digestive enzymes into the gut. Some organs have exocrine tissue and endocrine tissue, so act as both types of gland. 

For example, the pancreas is both an exocrine and endocrine gland. 

Action of hormones

A hormone is called a first messenger because it carries the chemical message the first part of the way, from the endocrine gland to the recpetor on the target cells. When a hormone binds to its receptor it activates an enzyme in the cell membrane. The enzyme catalyses the production of a molecule inside the cell called a signalling molecule- this molecule signlas to other parts of the cell to change how the cell works. 

The signalling molecule is called a second messenger because it carries the chemical message the second part of the way, from the receptor to other parts of the cell. Second messengers activate a cascade inside the cell. 

Adrenaline

Adrenaline is a hormone that's secreted from your adrenal glands. It's secreted when there is a low concentration of glucose in the blood, when you are stressed and when you are exercising. Adrenaline gets the body ready for action by making more glucose available for muscles to respire, e.g. by activating glycogenolysis, which is the breakdown of glycogen to glucose.

Adrenaline is a first messenger. It binds to specific receptors in the cell membrane of many cells, e.g. liver cells. When adrenaline binds it activates an enzyme in the membrane called adentlate cyclase. Activated adentlate cyclase catalyses the production of a second messenger called cyclic AMP (cAMP). cAMP activates a cascade, e.g. a cascade of enzyme reactions make more glucose available to the cell. 

Adrenal Glands

The adrenal glands are endocrine glands that are found just above your kidneys. Each adrenal gland has an outer part called the cortex and an inner part called the medulla. The cortex and the medulla have different functions:

• The cortex secretes steroid hormones, e.g. it secretes glucocorticoids such as cortisol when you are stressed. 
• the medulla secretes catecholamine hormones which is a modified amino acid, e.g. it secretes adrenaline when your are stressed.   

The Pancreas

The pancreas is an endocrine and an exocrine gland that is found below the stomach.

Endocrine function

The areas of endocrine tissues are called the Islets of Langerhans. They are found in clusters around blood capillaries and they secrete hormones directly into the blood. They are made up of two types of cells, Alpha cells and beta cells. Alpha cells screte a hormone called glucagon and beta cells secrete a hormone called insulin. These both help to control blood glucose concentrations.

Exocrine function

Most of the pancreas is exocrine tissue. The exocrine tissues are called acinar cells. They are found in clusters around the pantreatic duct- a duct that goes to the duodenum. The acinar cells secrete digestive enzymes into the pancreatic duct. The enzymes digest food in the duodenum, e.g. amylase breaks down starch to glucose.