Osmosis is a Matter of Life and Death
All substances have a tendency to undergo diffusion. In diffusion, the random movement of individual molecules results in a mass movement from regions of high concentration to regions of lower concentration. For example, if you open a bottle of perfume indoors, the odor molecules gradually diffuse outward from the bottle to spread throughout the room. The perfume would eventually stop diffusing when the concentrations are equal in all parts of the room.Water, like any other substance, can also undergo diffusion, and in the case of water this is referred to as osmosis. Because water is such an important molecule in living things (making of about 70% of the human body), changes in the amount of water in our tissues can have a severe impact on all aspects of physiology.
To understand how osmosis works, it helps to think of the individual water molecules. A solution that contains very few dissolved molecules (referred to as "solutes") is considered to have a very high concentration of water, because water makes up a high proportion of the molecules present. Conversely, a solution that contains a high concentration of solutes is considered to have a relatively low concentration of water molecules.
Living cells are surrounded by a membrane that many substances cannot penetrate. An example of such a substance is salt. If salt is sprinkled on a slug (a cruel experiment, which is definitely no fun in real life), the salt increases the solute concentration in the water that lies on the animal's skin. The salt (represented by the orange particles in the illustration) cannot penetrate the skin in significant amounts. However, the difference in the concentration of water between the inside and outside of the cells results in osmosis. Water (represented by the blue particles and arrows) rapidly leaves the animal's tissues, leading to death.
Humans are are much better protected against water loss, because our skin cells are covered with a protein called keratin. However, osmosis is still a life-and-death matter for us in other ways. For example, the functioning of our kidneys relies heavily on osmosis. Osmosis can be harmful to humans as well; excessive salt in the diet is thought to be one factor promoting high blood pressure, because the salt goes into the fluids surrounding your cells and draws water out by osmosis.