Our bodies contain about 60% water. There can be no life as we know it without water, because most biochemical reaction occur in water. water has some very unique properties that make it very important as a biological molecule. A molecule of water is made up of an oxygen atom join to two hydrogen atoms. A molecule like this, which has different areas of positive and negative charges is called polor.

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Water 2

Water molecules are attracted to each other. The positive area of the hydrogen in a water molecule attracts to the negative charge of the oxygen atom of another water molecule. These forces of attraction are hydrogen bonds. Water molecules therefore stick together.


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Water 3

Water molecules are also attracted to other polar molecules. Water molecules form a shell around ions and many other molecules that have a slight charge on their surfaces, causing them to dissolve. This makes it a very good solvent for ions and other polar molecules such as sugars, amino acids ect. This makes water a good transport medium.

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Blood Plasma

Blood plasma is the liquid part of the blood. Its exact composition varies depending on which part of the body it is taken from. However its main components are water plus: Proteins such as fibrinogen used in blood clotting Ions such as sodium , calcium ect. These maintain the osmotic balance of the blood Hormones such as insulin and oestrogen. Dissolved food substances such as amino acids, glucose and glycerol. Oxygen Waste products such as urea and carbon Heat blood plasma transports heat around the body.

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when the blood clotting protein fibrinogen  has been removed from the blood plasma the solution which remains is called serum.

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tisuue fluid

Tissue fluid is formed when blood passes through capillaries. Capillary walls are permeable to everything except most blood cells and large plasma proteins. At the arterial end of the capillary, the blood is under enough pressure for the smaller components of blood plasma to be squeezed out. This allows the exchange of materials between blood and tissues.


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Some tissue fluid returns to the blood capillaries at the venule end. The rest of the tissue fluid drains into blind ended lymphatic capillaries. Once the fluid is inside these vessels it is called lymph.

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