I begin my journey in a capillary, the movement in here is very slow due to an apparent low pressure, and the walls are only one cell thick to allow substances to diffuse out of the blood

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I begin my journey in a capillary, the movement in here is very slow due to an apparent low pressure, and the walls are only one cell thick to allow substances to diffuse out of the blood e.g. oxygen  and are very thin. In front and behind me I can see other red blood cells like myself. They have no nucleus to allow more space to carry oxygen, and are small biconcave disc- shaped cells filled with a red pigment called haemoglobin.

 As I squeeze through the capillary walls, I see in the distance a cauliflower-shaped sac, as I got closer it was revealed to me that this ‘bunch of grapes’ was an alveoli, as I looked round there were thousands of them branching off from the trachea – the ‘flexible hose of a vacuum’.  The windpipe as it is more commonly called is held open by rings of cartilage, and the alveoli are situated at the end of each bronchiole – a narrow tube. The alveoli have a high concentration of oxygen. The haemoglobin now associates with the oxygen that is diffusing through the thin moist walls to form a loosely bonded compound called oxyhaemoglobin.

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 I know travel via the capillaries through the soft and spongy lungs, to the pulmonary vein, as I travel along the vein at low pressure, I feel myself being dragged backwards slightly but I was soon stopped by a valve, a valve prevents the back flow of blood. I flowed through the vein, right into the left atrium of the heart. I felt something contract and push me through the bicuspid valve into the left ventricle, the valve then snaps shut. The left ventricle then contracts, forcing me along with the others into the aorta; the semi-lunar valve fills with ...

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