Blood, like the rest of the body, is mostly water. Many chemicals are dissolved in water, each making its own unique contribution to the fluid’s chemical character. Water has its own unique chemical properties that make it well-suited to this task. Water moves around the body in many ways, but on a cellular scale, water moves by osmosis.

Why this is important…Water is vital to life. When we get dehydrated our health declines quickly. Our health depends on a proper water balance, so understanding how and why water moves is crucial to maintaining proper health.

Foundations

Water is the most abundant molecule in the body and plays a dominant role in the chemical environment of all body fluids. The human body is about 50-60% water, most of which is held inside cells (Figure 6-5). Since blood is mostly water and flows near every cell of the body, the composition of blood reflects what is happening in cells. For example, if the cells are low on water (dehydrated) then the blood is low on water.

Most of the fluid we take in comes from food and drink. Over half of the water we lose each day is through urine, which is the major regulator of your body’s total water content. The rest of the water we lose – through skin, exhaled breath, feces, and sweat – we have less control over.

Key terms

Osmosis – process of moving water through a selectively-permeable membrane

Osmolarity – measure of how concentrated particles are in a solution

Aquaporin – protein channel in cell membranes that allows water to pass through freely

Chart breaking down how much fluid is found in the body and where.

FIGURE 6-5. Fluid compartments of the body. About 60% of the body is made of watery fluids. Most of that is locked in cells (intracellular fluid), while the rest (extracellular fluid) is divided between the fluid around cells (interstitial) and the blood plasma. ("Fluid composition of the body" by Alan Sved and David Walsh is licensed under CC BY 4.0)

Details

You feel thirsty when your body lacks water. What your brain actually senses is that your blood is getting too salty. A more technical way of saying that is your blood has a higher osmolarity – it has a higher concentration of particles dissolved in it. This is easily solved by drinking water, but the water still needs to be distributed around the body.

There are a few ways to move fluids around the body. The method we use depends on how much and how far those fluids need to move.

Muscular pumps move the highest volumes the fastest and farthest. They also require the most energy. Examples include the heart and digestive system.
Filtration moves fluids through microscopic gaps with high pressure. The energy to push fluids comes from muscular pumps. An example is blood plasma leaking out of capillaries.
Cellular drinking (pinocytosis) is used by cells to move microscopic droplets of fluids into cells. This requires energy from the cell to suck in its outer membrane enough to slurp up some of the water from outside the cell. Examples include intestinal cells absorbing nutrients and kidneys processing fluids. We will return to pinocytosis later.
Osmosis happens in every cell of the body and requires no energy. Water moves through cell membranes following the highest concentration of solutes. Water does not move far or fast by osmosis, because it is only happening across thin cell membranes.

Let’s follow a sip of water all the way to the body cells. It involves most of the processes above. Water is moved down the digestive tract by muscles, starting with a swallow. It crosses into the bloodstream by osmosis becoming part of the plasma. It is then pumped around the body by the heart. It leaks into cellular spaces by filtration, squeezing out of the capillaries. Cells take up the water they need by osmosis. Any extra is either returned to the vein side of the capillary by osmosis or is taken up by lymph vessels (see Chapter 12).

Water moves mostly by osmosis at a cellular level. Osmosis is enormously important since it affects every cell in the body! The basic principle is that water moves toward areas of higher solutes (the particles dissolved in water). The phrase “water follows salt” is a good starting point to understand this. But let’s dig a little deeper.

Osmosis is very similar to diffusion. In fact, it is diffusion, but it only applies to water moving across a membrane. With diffusion, the main message was that substances tend to spread out. (The more technical way of stating this is that they move from areas of high concentration to low concentration.) Water does the same thing, but since it is the most abundant molecule, that looks different (Figure 6-6).

Osmosis demonstration in beakers. Water moves through a semipermeable membrane from the side with low solute concentration to the higher concentration.

FIGURE 6-6. Osmosis. Fluid in a beaker is separated by a semipermeable membrane (representing the cell membrane) and only water can move through. Left: at the beginning, the right half of the beaker has a greater concentration of solutes. Right: after some time, water has moved through the membrane and the solute concentration is now equalized, raising the water level on the right half. ("Osmosis" by OpenStax College is licensed under CC BY 3.0)

How does water move through the cell membrane? With all of our previous discussion about polar and nonpolar molecules not being able to mix, you might be confused to hear that water can move through cell membranes. Since water is a very small molecule, some of it is able to slip between the phospholipids and pass right through the membrane. The passage of water can be sped up a lot by adding special water channels (aquaporins) to the cell membrane. Cells can even adjust the number of water channels in the membrane in order to become more or less permeable to water.

VIDEO 6c: Osmosis

After watching this video, you should be able to:

describe what drives the process of osmosis
explain how water moves through cell membranes
predict the direction of water movement in different scenarios
describe several applications of osmosis in the human body

To determine where water will move, we compare the fluid inside the cell to the fluid outside. Water tends to move toward the area with a greater solute concentration. Solutes come in many forms – electrolytes, sugars, amino acids, proteins – anything that dissolves in water. The measure of dissolved solutes is called osmolarity. If we drink water to rehydrate cells, we can assume that the blood has brought lots of fresh water to the cells so it probably has a lower solute concentration (osmolarity) than the cell. In this case, water would enter the cell, hydrating it and causing it to swell.

Stop & Think #8: Normally the large intestine absorbs lots of water through osmosis, hydrating the blood. One treatment for constipation (when feces is drier) is to take an “osmotic laxative.” How do you think an osmotic laxative works?

Cells must have the right water content to stay healthy. Normally this works just fine if we are drinking enough water and our kidneys are working normally. Illnesses that affect the solutes in the blood can seriously harm cells because these solutes draw water toward them and away from cells. Receiving fluids directly into their veins – an intravenous fluid or I.V. fluid – can also affect water balance around cells. For example, refer to Figure 6-7 to see how different fluids can impact red blood cells.

One way of thinking about the concentration of solutes in a fluid is to compare it to the solute concentration of the cell – tonicity. Using the cell as a reference, determine if the concentration of solutes around the cell are more or less concentrated than inside the cell. This will determine which way the water moves or if it moves at all. This is illustrated in Figure 6-7 and terms that apply to this are summarized here:

Hypotonic solutions are less concentrated than inside the cell. Water moves into the cell.
Isotonic solutions are the same concentration as inside the cell. Water moves in and out equally – no net change.
Hypertonic solutions are more concentrated than inside the cell. Water moves out of the cell.

Stop & Think #9: Explain why your eyes sting if you open them underwater. This can happen in freshwater (hypotonic) and saltwater (hypertonic).

Osmosis demonstration in beakers. Water moves through a semipermeable membrane from the side with low solute concentration to the higher concentration.

FIGURE 6-7. Tonicity. Red blood cells are affected by the tonicity of their environment. ("Osmotic pressure on red blood cells" by Mariana Ruiz Villarreal, LadyofHats is in the Public Domain, CC0)

As you can see from Figure 6-7, the tonicity of a solution has a significant impact on cells. An extremely hypotonic solution (like pure water) can cause cells to burst because they take in so much water. In the blood, protein and sodium concentrations have the greatest influence on the tonicity of the plasma. The concentration should be equivalent to a 0.9% salt solution. This is referred to as “normal saline,” and is commonly the base mixture for all intravenous solutions.

Stop & Think #10: If solutes, such as proteins, accumulated in the spaces around cells how might this affect the water balance of that region?

High salt levels in the blood can also contribute to high blood pressure. You may have already made this connection if you know someone who has high blood pressure. They are often advised to avoid salty foods, because the more salt that makes its way into the bloodstream, the more water follows it. This results in a greater volume of blood. The blood pushes harder against the walls of the arteries and veins causing higher blood pressure.

Stop & Think #11: In people with diabetes mellitus, blood glucose levels are usually higher than normal. Explain why this might make the person thirsty, even if they have enough water in their body already.

Learning objectives

Describe the process of osmosis and predict the direction of water movement across a membrane
Predict how cells respond to hypertonic, isotonic and hypotonic solutions

Stop & Think Questions Explained

8. Osmotic laxatives contain salts that are not usually absorbed by the small intestine. Since they remain in the undigested food, they retain more water causing feces to retain more water. This makes it easier for a person to poop, since the feces is less dried out.

9. Freshwater or saltwater can both sting the eyes because they are not isotonic; therefore, they cause water to enter or leave the tissues of the eye. This is sensed by the nerves in the eye and registers as pain or discomfort.

10. Accumulating proteins around cells would attract water, both from cells and the blood. This buildup of fluids would cause edema.

11. Our brains sense our overall water balance by reading the osmolarity of the blood, which is a measure of how many solutes are dissolved in it. Since there is more sugar than normal, the brain thinks water levels are low. (I wouldn’t expect you to know the answer to that one based on the reading. I asked to get you to speculate a little.)