The endocrine system and adrenal glands

​​In a nutshell

The endocrine system oversees hormonal release throughout the body in response to stimuli and this keeps the internal environment constant. The different areas of the endocrine system control different functions. Hormones interact with specific receptors which activate secondary messengers. 

Endocrine system

The endocrine system is made up of endocrine glands which are groups of specialised cells that release hormones into the blood stream.

Example

The pancreas secretes insulin into the bloodstream.

Hormones

Hormones exert their effects by interacting with receptors present on target cells. Only cells expressing the specific receptors are affected by the hormone. The interaction between hormones and receptors on the membrane of target cells leads to the production of a secondary messenger.

When a hormone interacts with receptors it activates an intracellular enzyme that produce a molecule. This molecule is known as a secondary messenger. Secondary messengers stimulate a cascade reaction inside the cell. 

The cascade reaction is a chain of reactions which result in the activation of intracellular enzymes that catalyse reactions. Hormones work in negative feedback systems. 

Example

Insulin is released by the pancreas when blood glucose concentration is too high. Insulin interacts with receptors presented on the membrane of liver cells. The interaction stimulates the production of cAMP which is the secondary messenger. cAMP induces a cascade reaction which activate enzymes that convert glucose into glucagon, therefore, lowering the blood glucose concentration. 

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Adrenal glands

The adrenal glands are located above the kidneys. There are two parts of the adrenal glands which produce different responses. The outer section of the adrenal glands is called the cortex and the inner section is called the medulla.

Cortex

The cortex releases steroid hormones. 

Example

The cortex of the adrenal glands releases cortisol and aldosterone during periods of stress.

Hormones released by the cortex contribute to long and short term responses to stress. They induce the conversion of glucose from proteins and fats increasing the amount of energy provided to the muscles and brain through respiration. 

Cortex hormones also supress the immune system and increase blood pressure and volume by increasing the uptake of sodium ions and water by the kidneys.

Medulla

The medulla releases catecholamine hormones.

Example

The medulla of the adrenal glands releases adrenaline and noradrenaline during periods of stress.

Hormones released by the medulla contribute to short term responses to stress. They trigger faster breathing and increase heart rate.

Medulla hormones induce the conversion of glucose from glycogen which provides energy through respiration. They also cause blood vessel constriction which directs blood flow towards the muscles and brain.