In simple organisms such as unicellular and one-or two-layer organisms, materials can be transferred among cells by simple processes of diffusion.
However, in large organisms, a system is needed for the distribution and collection of materials.
This is because diffusion does not occur fast enough to carry the large volumes of materials necessary through the greater distances required.
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Substances carried by circulatory systems in the human body include:
Some of the substances distributed to the body cells are products of the digestive system.
These materials meet individual cell requirements for energy, growth, repair, synthesis of new materials, and storage for later use.
Oxygen is obtained by the blood through the process of external respiration in the lungs.
Oxygen is then transported to the individual body cells, where it is used in metabolic oxidation.
This provides energy for production of ATP (adenosine triphosphate), which is necessary for carrying on the life processes of the body.
Some substances are collected from the body cells for elimination.
These include carbon dioxide, nitrogenous wastes, and other potentially harmful substances that are carried to organs like the lungs, liver, or kidneys for elimination from the body.
Hormones are the products of endocrine glands.
Hormones and other control substances are distributed throughout the body by circulatory systems.
The tissues or organs affected by these substances are usually called target organs.
In turn, substances released by the target organs often affect the original endocrine gland. This results in a feedback system.
Continuous renewal and removal of fluids - Secretory processes continuously renew the various fluid systems of the human body.
At the same time, the volume of fluid in each system is kept at a constant level through the removal of excess fluids.
Should the removal processes be interrupted, the volume of fluid will increase. The resulting increase in pressure can have serious consequences. Depending on the system involved, the consequences might include
deafness, hydrocephalus, or pulmonary edema.