A hyperosmotic solution is defined by its higher concentration of solutes, such as salt or sugar, when compared to another solution. This comparison is made across a semipermeable membrane, a type of barrier that allows certain molecules to pass through while blocking others. The term originates from the Greek words ‘hyper,’ meaning excess, and ‘osmos,’ meaning push, which alludes to the increased pressure a hyperosmotic solution exerts.
The Process of Osmosis
Osmosis is the passive movement of a solvent, most commonly water, across a semipermeable membrane. This movement is driven by a difference in solute concentration between the two sides of the membrane. Water molecules naturally move from an area where solutes are less concentrated to an area where they are more concentrated.
Imagine a container divided by a fine screen that allows only water to pass. On one side is freshwater, and on the other is saltwater. The salt particles are the solutes, and the water is the solvent. Water will move from the freshwater side into the saltwater side in an attempt to equalize the concentrations.
This creates three possible scenarios. A solution with a lower solute concentration is called hypotonic. When two solutions have equal solute concentrations, they are referred to as isotonic, and there is no net movement of water between them.
Effects on Living Cells
Living cells, which are enclosed by semipermeable membranes, are affected by the osmolarity of their surrounding environment. When an animal cell, such as a red blood cell, is placed in a hyperosmotic solution like very salty water, the concentration of solutes outside the cell is higher than inside. This causes water to move out of the cell and into the surrounding solution. As the cell loses water, it shrinks and shrivels, a process known as crenation.
Plant cells react differently due to their rigid cell wall. When a plant cell is in a hyperosmotic solution, water still moves out of the cell’s large central vacuole. This causes the cell’s internal contents, the protoplast, to shrink and pull away from the cell wall. This process is called plasmolysis. The cell wall provides structural support, preventing the entire cell from collapsing, but the plant will wilt as its cells become flaccid.
Biological and Environmental Examples
Organisms have developed adaptations to manage life in hyperosmotic conditions. Saltwater fish, for instance, live in an environment—the ocean—that is more concentrated with salt than their own body fluids. To counteract constant water loss from their bodies into the sea, these fish must drink large amounts of seawater. Drinking salt water, however, introduces excess salt that must be expelled. They accomplish this using specialized cells in their gills, called ionocytes, which actively pump salt ions out of their bloodstream and back into the ocean. Additionally, their kidneys are adapted to produce small volumes of highly concentrated urine to conserve water.
The human body also utilizes hyperosmotic gradients for its functions, most notably in the kidneys. A structure responsible for this is the loop of Henle, part of the kidney’s filtering unit, the nephron. The loop of Henle creates a hyperosmotic environment deep within the kidney’s medulla. It achieves this by actively pumping salts into the surrounding tissue, making it very salty. This salty, hyperosmotic medulla then draws water out of the collecting ducts, concentrating the urine before it is excreted.
Medical Applications and Implications
Hyperosmotic solutions are used for several medical treatments. Hyperosmotic laxatives, for example, work by introducing poorly absorbed substances like polyethylene glycol or lactulose into the colon. These agents create a hyperosmotic environment within the gut, which draws water from the surrounding tissues into the bowel. This influx of water softens the stool and stimulates bowel movements.
Another significant medical use is the administration of hyperosmotic agents like mannitol to treat cerebral edema, or swelling in the brain. When given intravenously, mannitol increases the osmolarity of the blood, making it hyperosmotic compared to the fluid in the brain tissue. This osmotic gradient pulls excess fluid out of the brain and into the bloodstream, where it can be filtered out by the kidneys, thus reducing dangerous intracranial pressure.
The concept is also relevant in disease states, such as Hyperosmolar Hyperglycemic State (HHS), a serious complication of type 2 diabetes. In HHS, extremely high blood glucose levels make the blood dangerously hyperosmotic. This severe hyperosmolarity pulls massive amounts of water out of the body’s cells, including brain cells, leading to profound dehydration and neurological symptoms.