- Simple unicellular organisms rely on diffusion because they are too small to have a circulatory system.
- Multicellular organisms need a transport system so that all of the cells can get the nutrients that they need in and expel the waste.
Composition of blood:
- Plasma - the liquid part of the blood which is made mainly out of water. it carries hormones, urea, carbon dioxide, nutrients and blood cells around the body. it also distrubutes heat around the body. Makes up abput 90% of the blood.
- Red Blood Cells (RBC) - approx. 1 million RBC in one drop of blood. Made in the bone marrow. Carry O2 around the body, from the lungs to all the cells in the body. Biconcave disc shape. No nucleus. Haemoglobin - a red pigment that contains iron & picks up oxygen.
- White Blood Cells (WBC) - do not contain Haemoglobin. Colourless adn all have a nucleus. Not as many of them as RBC. MAde in the bone marrow as well. they…