Introduction

Homeostasis is the maintenance of a steady state in the body despite changes in the external environment. The steady state is the optimum level for the body functions.

For example the core temperature of the human body remains at about 37°C despite fluctuations in the surrounding air temperature.

For example the blood glucose level remain about 1g per litre despite fasting or eating a meal rich in sugars.

Other examples of homeostatically controlled variables include: blood concentration (osmoregulation), blood pH (about pH 7.35), blood O₂ and CO₂ levels

• Sensors to detect changes in the internal environment.

• A comparator which fixes the set point of the system (e.g. body temperature). The set point will be the optimum condition under which the system operates.

• Effectors which bring the system back to the set point.

• Feedback control. Negative feedback stops the system over compensating (going too far).

• A communication system to link the different parts together.

These should consist of the following components:

In animals there are two communication systems

Hormones are organic substances produced in small quantities in one part of an organism (an endocrine gland), transported by the blood system to a target organ or tissue where it has a profound effect.

The endocrine system therefore produces chemical signals. Each hormone is different and they travel relatively quickly through the blood stream all over the body. Their effects may be very slow (e.g. growth hormone over years) or very fast (e.g. adrenaline which acts in seconds).

The nervous system is composed of excitable cells called neurones (also neurons). Neurones, characteristically, have long thin extensions which carry electrical nerve impulses. This electrical signal needs to be converted into a chemical signal (a neurotransmitter) so that it can pass from nerve cell to nerve cell.

All nerve impulses look the same. So the nervous system sends signals along nerves to specific parts of the body. The nerve impulses travel very quickly and affect their target tissues in milliseconds.

• A Central Nervous System (CNS) made of the brain and spinal cord
• And peripheral nerves connecting it to sensors and effectors.