How is that the volume of blood is kept constant in the human body?
The number of red blood cells only affects the hematocrit levels in the body (the concentration of red blood cells in the blood) it doesn’t affect the total volume. If the hypothalmus detects a drop in blood pressure (blood volume), then it will release antidiuretic hormone, which causes the kidneys to retain more water and prevent further loss. When we drink, the water is absorbed in the intestines and enters the blood stream increasing the volume. If the volume is too high, the kidneys will remove some of the water (see glomerular filtration as well a reabsorption in the kidneys) but also some of the water will be forced into the extracellular space to decrease the blood pressure. This will be taken up by the lymphatic system over time and then return to the blood stream, which means that the kidneys are the only source of true removal, extracellular space is only temporary.
