A Level Biology:
The Endocrine System

The Endocrine System


Humans have two complementary control systems that they can use to respond to their environment: The Nervous system and the Endocrine (or hormonal) system. 


[It is important that you are able to compare and contrast the nervous system and endocrine system]


Lets begin will a diagram of the endocrine system and a table showing the hormones, target organ and function:


[insert diagram of endocrine system w/ table]

This diagram with corresponding table shows some of the main endocrine glands and their hormones. The hormones marked with a * are ones that you’ll need to know in more detail, and will be covered in detail later.


Hormones are secreted by glands into the blood stream. 


There are two kinds of glands:


1. Exocrine glands. Exocrine glands secrete solutions to the outside, or to body cavities, usually through ducts (tubes). e.g. sweat glands, tear glands, mammary glands, digestive glands.


2. Endocrine glands. Endocrine glands do not have ducts but secrete chemicals directly into the tissue fluid, and subsequently these ‘chemicals’  diffuse into the blood stream. The hormone secreting glands are all endocrine glands, e.g. thyroid gland, pituitary gland, and adrenal gland.


Once a hormone has been secreted by its gland, it diffuses into the blood stream and is carried around the body to all organs. However, specific hormones only affect certain ‘target organs’, which can respond to it. 


Target organs have specific receptor molecules in their cells to which the hormone binds. The receptor molecules are proteins and they form specific hormone-receptor complexes, in very much the same way enzyme-substrate complexes are formed.


Cells without the specific receptor will just ignore a hormone.

The hormone - receptor complex can affect almost any aspect of a cell’s function, including metabolism, transport, protein synthesis, cell division or cell death.


There are three different ways in which a hormone can affect cell function: - 


1. Some hormones affect the permeability of the cell membrane. They bind to a receptor on the membrane, which then activates a transporter, so substances can enter or leave the cell. (E.g. acetylcholine stimulates sodium uptake.)

2. Some hormones release a “second messenger” inside the cell. They bind to a receptor on the membrane, which then activates an enzyme in the membrane, which catalyses the production of a chemical in the cytoplasm, which affects various aspects of the cell. (E.g. adrenaline stimulates glycogen breakdown.)


3. The steroid hormones are lipid-soluble so can easily pass through membranes by lipid diffusion. They diffuse to the nucleus, where they bind to a receptor, which activates protein synthesis. (E.g. testosterone stimulates spermatogenesis.)


[insert images showing example of 1, 2 and 3 above]


In most cases, hormones do not enter into cells, rather the effect of a hormone is determined not by the hormone itself, but by a receptor on/in the target cell. Thus, the same hormone can have different effects in different target cells!


As stated above it is important that you are able to compare and contrast the endocrine system and nervous system. Moreover, you must understand that these systems work closely together, e.g. endocrine glands are usually controlled by the nervous system, and a response to a stimulus often involves both systems.


Table comparing Nervous and Endocrine (hormone) systems:

[insert table]




What is Paracrine Signalling?


Paracrine signalling is communication between close cells using chemicals called local chemical mediators.


These chemicals are released by cells into the surrounding tissue fluid, but not into the blood. Thus they only have a local effect on the cells surrounding their release, in contrast to hormones. Like hormones and neurotransmitters they bind to receptors on the surface of the target cells to cause an effect. Two such local mediators are prostaglandins and histamine, which we’ll consider next.


What are Prostaglandins?


Prostaglandins are lipid molecules. 


Prostaglandins are lipid molecules that are produced by cells in almost every tissue in the body, targeting local smooth muscle cells and endothelial (lining cells). There are over a dozen different prostaglandins known, causing a number of effects, especially the inflammatory response to injury and infection. Prostaglandins cause vasodilation by stimulating smooth muscle cells in the walls of local arterioles to relax, increasing blood flow to the area (so the area turns red). Prostaglandins stimulate the blood clotting process (so wounds are sealed) and Prostaglandins stimulate the pain (receptors) neurones


Did you know that Anti-inflammatory drugs, e.g. aspirin and ibuprofen, reduce inflammation and associated pain by inhibiting a key enzyme in the synthesis of prostaglandins.


What is Histamine? 


You’re most likely aware that histamines are also involved in the inflammatory response, e.g. inflammation following stings from insects or nettles but what is histamine?


Histamine is compound made from the amino acid histidine, which is stored in granules in mast cells (found in connective tissue, especially in the skin). 


When stimulated by the immune system or by injury, mast cells release histamine into the surrounding tissue fluid. Here, Histamine stimulates vasodilation of nearby arterioles, loosening of nearby capillary walls, which in turn causes capillaries to leak. The leaking of capillaries allows blood plasma and leukocytes (white blood cells) to reach the site injury. 


Histamine also causes broncho-constriction of the airways (the bronchi). In fact, sometimes too much histamine can be released, which results in an “allergic reaction” specifically this is an extreme inflammatory response due to too much histamine. As such, antihistamine drugs inhibit the release of histamine and so are used to counter allergic reactions.


Note: The action of neurotransmitters at synapses can also be described as paracrine signalling, since these chemicals only have a local effect and are not released into the blood.