In this article we will discuss about the internal transport of fluids in animals.
The cells need a constant supply of water, oxygen, nutrients, minerals, vitamins and other materials and removal of nitrogenous wastes, carbon dioxide and other products of metabolism. The cells of the metazoans are bathed in an interstitial fluid.
The systems which carry materials from the sites of intake or sites of origin in the cells to the interstitial fluid or other parts of the body, respectively, are collectively called internal transport system. Materials from one to other part within the body are carried only through fluid media.
Very small animals as the protozoon and certain metazoans in which the cells are exposed to water current, as in sponges and some cnidarians, do not need a transport system. With the progress of evolution and enlargement of body size, the problem of transporting materials from one to another part of the body became complex and a well-knit transport system developed.
The fluids transporting materials within the body are plasma water in blood, lymph fluid, trans-cellular water (fluid secreted by cells in the coelom and digestive system) and interstitial fluid. Flow of liquids through body cavities or a circulatory system that carries different materials is known as bulk flow.
The degree of perfection of fluid flow depends on the size of the animal and its metabolic rate, larger the body size or higher the metabolism, greater is the perfection of bulk flow.
Bulk flow helps in quick transport of oxygen, glucose, amino acids, minerals and other materials needed in the metabolism of cells, from the site of intake to the interstitial fluid, from where the materials enter the cells through diffusion across the plasma membrane.
Conversely, the metabolic products of cells — carbon dioxide, nitrogenous wastes and other by-products diffuse out of the cells and bulk flow carry them quickly to their sites of disposal or sites of actions (hormones).
Most cells cannot withstand large changes in their immediate environment. Addition or removal of materials by different organs from the blood and interstitial fluid occurs in a regular way to maintain the steady state of the fluids. The maintenance of a constant internal environment is known as homeostasis.
Internal transport in animals involve moving the materials from the site of intake to interstitial fluid or from interstitial fluid to specific organs and from the organs to the site of expulsion. This is effected by a continuous movement of haemolymph or blood in the body in definite channels.