You may have heard that humans are made partly of seawater, but that isn't exactly accurate. True, the average adult body is 60 percent water, and that water is almost as salty as ocean water – but not quite, and the small difference in salinity makes a big difference. Drinking seawater or any kind of salt water increases the salinity of the blood. That actually draws water out of the cells, which ultimately shrivel and die, and the person drinking the water can die of dehydration. The mechanism responsible for this is osmosis.

Osmosis is a phenomenon you can easily study at home. Dissolve 1/2 cup of salt in a quart of water and put a carrot into the container. After a day or two, the carrot will shrivel up. Pickle makers use salt water for dehydration by soaking cucumbers, carrots and other vegetables in brine. The dehydration is caused by osmosis, and it's pretty much what happens to the cells in the body when you drink salty water.

The reason that osmosis occurs is that the cell walls are semi-permeable membranes. They allow water molecules to pass through, but not large solute molecules or charged ones like the sodium and chlorine ions created when salt dissolves. Water migrates across the barrier to equalize the solute concentration on both sides. This migration is what is known as osmosis. The more salt there is in the blood stream, the greater the osmotic pressure, and the faster the cells lose water. They end up looking like the shriveled up carrot. Consequently, after drinking salt water, your body may be full of water, but you feel thirstier than you did before.

The cells in your body will dehydrate if you drink even mildly salty water, but the dehydration might not be enough to kill you. However, you'll put stress on your kidneys, and they may become diseased or perhaps even stop functioning altogether if you drink the salty water often enough.

The kidney damage is also caused by osmosis. As the blood passes through the kidney for purification, excess water passes through a semi-permeable membrane into a collection channel inside the kidney. The solute concentration in the chamber is normally higher than it is in the blood. If the blood has a high salt concentration, however, water won't pass through the barrier, and the blood won't get purified. This puts pressure on the kidneys and creates an abnormally high level of proteins in the blood. This raises blood pressure and can also damage other organs, such as the heart and liver.

The fluids in the human bodies contain sodium chloride and other salts, which is why tears are salty. The concentration is about one-third the concentration of salt in seawater. Excess sodium is bad for the body, and the kidneys excrete it in urine. Researchers studying a simulated space flight noted that the body also regulates sodium concentration by retaining and expelling water in weekly and even monthly cycles. This suggests that regularly ingesting salt is more likely to cause health problems like heart disease than doing it once or twice.

When your body has too much salt, you feel thirsty, and when you feel thirsty, you should drink plain water. This lowers the salt concentration in your blood and protects your kidneys and heart, as well as every cell in your body. On the other hand, the body also loses sodium through sweat, and it needs a certain amount for proper metabolism. This is why athletes sometimes take salt tablets.