It took many millions of years of evolutionary development, before any complex organisms. Their cells acquired narrow specialization: neurons, the cells skeletal and smooth muscles, and glandular epithelial cells, blood cells, and so on habitat blood cells, plasma, and for all other cells of this environment is intercellular fluid. Between the blood and the cells of the organs of vertebrate no direct contact: the blood gives brought her substance interstitial fluid, and the cell itself takes away all she needed to do, and as much as it requires. From the capillaries, through their one-layer wall and into the intercellular environment permeates a lot of different substances or under pressure, either by diffusion. Oxygen, the most wanted of all the nutrients, goes from red blood cells through the capillary wall under the laws of the gas diffusion, i.e. the concentration gradient. The more oxygen is consumed by the cell, the lower is the concentration (usually say, voltage) in the intercellular space next door to the cage-the consumer and the faster oxygen diffuses into this zone, having the oxygen deficit. However, if blood will bring oxygen in excess, the cell will not use this extra for her gas it consumes just as much oxygen as demand oxidative reactions associated with cell metabolism. If oxygen in the intercellular space is small, the cell can still sniff them, because under normal conditions, a very significant part of oxygen is not used tissues, and this part goes with venous blood flow. But if the voltage of oxygen near the cells very little time, if it's in periechontology space cells (mitochondria rule oxidative activity) falls below 5 mm Hg, i.e. 30 times in comparison with atmospheric, the cell cannot continue normal operation. However, it is in these conditions, you may some time to live in debt, in anaerobic mode. Later, when the oxygen supply will be improved, the cell will demand shortfalls, i.e. non-completely oxidized products of vital activity of oxidized to the desired extent. Many believe that the opening of the "oxidative of life organisms is one of the greatest achievements of physiology after the Harvey. Did Antoine Lavoisier (1743-1794), which determined breath as slow oxidation to produce heat.
Products of cellular metabolism are also in the intercellular fluid, and then either move into the blood, either together with the intercellular liquid fall into the lymph vessels, whose network, merging into venous bed at the base of the subclavian vein. Those substances that penetrate from capillary blood in the tissue or back, can make this movement only when a particular line hydrostatic, osmotic and oncotic pressure in the capillary blood and tissue fluid. The hydrostatic pressure in the blood end of the capillary - about 25-30 mm Hg, and in the end, venous parts - on 10-15 mm below. So there is a movement of fluid from the blood into the tissue at the beginning of the capillary, and at the end of it - on the contrary. This is the classic image of the exchange of fluids between blood and tissue. Today, it seems a little simplistic, but in principle such representations close to the truth.
When cells are working intensively, their oxygen request repeatedly increases. In this situation normal blood flow is unable to satisfy the need of tissue (for example, twitch muscle) in oxygen, although it is timely to bring waste cellular metabolism, in particular carbon dioxide, to deliver the nutrients necessary for work number. In this situation, the most precious oxygen, and for the sake of delivery it is necessary to strengthen the bloodstream. It would be a terrible waste to increase the blood flow in all vessels: it grows only in the zone, which is responsible for the oxygenation of the hard working body. So, the blood flow to the working muscles can be 70-100 times more than at rest. As it happens, we will tell later.

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