Question

Why do human beings need a transportation or circulatory system?

Answer 1

Glucose and oxygen needed for respiration, amino acids and other essential foodmolecules for cell-building and growth, and the removal of poisonous waste products are all the basic things cells need. These must be transported from outside the organism into the cells. For a unicellular organism such as amoeba, nutrients and oxygen can diuse directly into the cell from its external environment, and waste substances can diuse directly out. This is because the surface area of the amoeba is very large in relative to the volume of the inside of its cell. !o, its surface area to volume ratio is large. "nother example of a specieswith a large surface area to volume ratio is #atworm. Their body is typically #attened, it is much wider than it is tall, thus increases the surface area to volume ratio. $n contrast to unicellular organisms such as an amoeba, larger organisms such as humans, are made up of billions of cells, often organise into specialised organs and tissues. Their surface area to volume ratio is small% therefore substances need to travel a long distance from the outside to reach all body cells. !imple diusion is not e&cient enough as it is not fast enough for nutrients and oxygen to reach the inner cells of the body to sustain the processes of life. This is why complex multicellular organisms have evolved specialised systems to get food and oxygen into their bodies and to remove waste. They also have an internal transport system which carries substances to every cell in the body, delivering oxygen and nutrients and ta'ing away waste (uic'ly to prevent them from damaging the cells. $n humans, this transport system is the heart and circulatory system and the blood #ows through it. !ubstances are transported in the #ow of the #uid with a mechanism for moving it around the body to reach all body cells and are delivered over short distances from the transport system to individual cells by processes such as diusion, osmosis and active transport

$n addition to the circulatory system, the human body has the lymphatic system. To cope with the portion of blood plasma, also called extracellular #uid, lea'ing out of capillaries and surround tissue cells, #uid is able to drain into the system of tubes that increase with si)e from the tiny lymph capillaries to lymph vessels to lymph ducts. The extracellular #uid contains oxygen, glucose, amino acids andother nutrients needed by tissue cells. The #uid eventually collects together into two large collecting ducts, the thoracic duct and the right lymphatic duct, and is drained bac' into the blood at the right and left subclavian veins prior to the blood returning to the heart at the right atrium. The lymphatic vessels are punctuated at intervals by lymph nodes that remove foreign materials such as infectious microorganisms. The movement of the #uid is aided by the force of themuscles attached to the s'eleton. There are also semilunar valves that restrict the bac' #ow of #uid.