II. Introduction

Most living organisms are complex and multicellular. It is necessary to have an efficient transport system to facilitate movement of useful substances such as nutrients and oxygen gas round the body. At the same time, toxic metabolic wastes such as urea and carbon dioxide need to be removed quickly.

Simple organisms such as the fish has ‘single circulatory system’ while complex organisms such as amphibians, reptiles and man have ‘double circulatory system’.

Above: Single blood circulatory system in a typical fish. For a single blood circulatory system, blood flow between heart and lungs and blood flow between heart and the other parts of the body are not separated.

Above: Most mammals have a double circulatory system which consists of the pulmonary blood circulatory system made up of the lungs and the heart as well as the systemic blood circulatory system made up of the heart and other parts of the body.

Circulatory system in man typically consists of three main components. They are:

1. Blood: An important fluid tissue that serves vital transport and protective functions.

2. Blood vessels: Interconnecting structures that send blood round the body.

3. Heart: Muscular pump that sends blood round the body.

Check out the following links to find out the following:

- Two main circulations that make up the human blood circulatory system. (URL 1)

- ‘Double blood circulatory system’ found in human beings. (URL 2)

References

URL 1:

http://library.thinkquest.org/C0115080/?c=circ_sys

URL 2:

http://en.wikipedia.org/wiki/Double_circulatory_system