The first living things on Earth, single-celled micro-organisms<span> or microbes lacking a cell nucleus or cell membrane known as prokaryotes, seem to have </span>first<span> appeared on </span>Earth<span> almost four billion years ago, just a few hundred million years after the formation of the </span>Earth<span> itself.</span>
The ultimate source of energy (for most ecosystems) is the sun<span>. The ultimate fate of energy in ecosystems is for it to be lost as heat. Energy and nutrients are passed from organism to organism through the food chain as one organism eats another.</span>
During bleeding, both formed elements (platelets, white blood cells, red blood cells) and plasma are lost from the circulatory system. They are lost proportionally, so initially there is no change in hematocrit.
Hematocrit is the percentage of the blood volume made up of elements (Hct = cell volume/blood volume). During dehydration, only water and electrolytes are lost, and the number of cells remains constant - the same number of cells in a smaller volume leads to an increase in hematocrit. When the body tries to restore blood volume, the first thing to recirculate is water from the ECF and this increases the amount of water without increasing the amount of red blood cells, so the compensatory mechanism causes the hematocrit to fall.
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