Homeostasis is the control of the internal environment so that conditions inside the body stay roughly constant, even when the outside world changes. Body temperature and the water content of the blood are two of the conditions the body controls.
Temperature matters because of enzymes. Enzymes are proteins, and each has an active site that fits its substrate. Near 37 °C most human enzymes work at their optimum rate. If the body gets too cold, the molecules move more slowly, so enzyme-controlled reactions become slower. If the body gets too hot, the enzyme loses its shape and the active site no longer fits. The enzyme is denatured and the reaction stops. This control of body temperature is called thermoregulation.
Water content matters because animal cells have no cell wall. This control is called osmoregulation. If the blood becomes too dilute, water moves into cells by osmosis, so they swell and may burst. If the blood becomes too concentrated, water moves out of cells by osmosis, so they shrink and cannot work properly.