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Osmosis is the process by which water passes passively across the plasma membrane.

The classic experiment to help explain osmosis involves taking a vessel such as a beaker and dividing it into two using a semi-permeable material such as cellophane. Into one side of the beaker a solution of sugared water is added, and to the other side pure water is added. If the experiment is left at room temperature for an hour or so then the pure water will gradually move across the selectively permeable cellophane into the side of the beaker containing the sugared water, and the water level on this side of the beaker will begin to rise (Figure 1.6). The cellophane is referred to as being selectively permeable since it has pores that are just large enough to allow the water molecules to pass through but too small to allow the larger sugar molecules through. All human plasma membranes are selectively permeable and behave like the cellophane in this experiment.

Figure 1.6 The process of osmosis

Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://openstax.org/books/anatomy-and-physiology/pages/preface

Of all the mechanisms of membrane transport, osmosis causes most confusion among students. The reason much of this confusion arises is because there are two common definitions provided for osmosis in textbooks. Although these definitions are worded differently, they are effectively saying the same thing.

Osmosis can be defined as:

The movement of water from a region of low-solute concentration to a region of high-solute concentration across a selectively (semi-) permeable membrane.

Osmosis is also frequently defined as:

The movement of water from a region of high water concentration to a region of low water concentration across a selectively (semi-) permeable membrane.

While both definitions are accurate, the second definition is preferable since it highlights that osmosis is actually the diffusion of water through a selectively permeable membrane.

A nice, simple rule to help remember osmosis is that ‘water follows solutes’, or in plain English, ‘water follows sugar, salt or other dissolved material’.

Knowledge of osmosis is essential for nurses to understand how the kidneys function and to understand water balance. Now that you have an understanding of osmosis and diffusion, read through the therapeutic clinical application to develop your understanding of how this knowledge can be applied to a patient with significant kidney disease.

Therapeutic clinical application

Patients with renal failure may undergo peritoneal dialysis in which a catheter is implanted into the abdomen and glucose-rich fluid (known as the dialysate) is infused via the catheter into the peritoneal cavity. The peritoneum acts as a selectively permeable membrane through which excess fluid (and electrolytes and waste products) are drawn out of the blood and into the dialysis fluid. Peritoneal dialysis may be continuous ambulatory peritoneal dialysis (CAPD) in which the dialysate is infused into the abdomen and retained there for approximately eight hours before being allowed to drain. The process is then repeated two or three times a day. Alternatively, automated peritoneal dialysis (APD) may be used in which a machine is used to cycle the fluid into and out of the abdomen. This is usually done overnight.