Circulation

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Open Circulatory Systems - In insects and other small animals, blood circulates in large open spaces. The heart pumps blood into cavities surrounding the organs and the substances diffuse between the blood and the cells.

Closed Circulatory Systems - Many animals have closed circulatory systems in which blood is enclosed in blood vessels. It delivers substances via diffusion across capillary walls, into the cells.

Within a closed circulatory system, there is a single circulatory system and a double circulatory system...

- Single circulatory system

Heart -----> Lungs/Gills etc. -----> Capillaries -----> Heart

- Double circulatory system

It features two circulations, a pulmonary circulation (to the lungs) and a systemic circulation (to the body).

Heart -----> Lungs -----> Heart -----> Capillaries -----> Heart

In a closed circulatory system a liquid the particles it contains are transported in one direction due to a difference in pressure - this is MASS FLOW. The fluid, called plasma is mainly water and contains dissolved substances such as oxygen and CO2 etc. The blood also contains red blood cells and white blood cells as well as platelets.

Water is a useful medium because...

- It is a polar molecule (unevenly distributed electrical charge) with hydrogen being slightly positive and oxygen is slightly negative. This means are formed between molecules and water is therefore a liquid at room temp.

- Many chemicals disolve in water due to its dipole nature. These dissolved substances can then be easily transported around the body therfore.

- It takes a long time to warm up or cool down due to the hydrogen bonds needing a lot of energy to be broken. This means organisms dont have a rapid change of internal temperature.