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Human kidney cross-section (Mohammed Haneefa Nizamudeen, iStockphoto)
Human kidney cross-section (Mohammed Haneefa Nizamudeen, iStockphoto)
Learn how and where osmosis takes place in the digestive system and excretory system and the role of osmosis in kidney dialysis.
Imagine playing basketball with your friends on a hot summer day. By the end of the game, you feel thirsty. You decide to drink some water. But have you ever wondered how your body absorbs it? It happens because of osmosis. We will look at how osmosis happens and why it is important for our bodies. What is a semipermeable membrane?Before we jump into osmosis, we need to understand some important things about cells. The cells in our bodies are surrounded by a wall-like structure called a cell membrane. This membrane is special because only water and very small molecules can pass through it. We use the word semipermeable to describe the ability to only let certain things pass through a membrane.  Why is this membrane important? It’s because water must pass through semipermeable membranes to travel from one place in our body to another. What are solutes and solvents?Osmosis is when water molecules travel from a place with low solute concentration to a place with high solute concentration. To understand this better, we need to talk about solutes and solvents. A solute is a chemical that can dissolve in a solvent. Chemicals that can do this are called soluble. When you dissolve one or more solutes into a solvent, you get a solution. Sugar and salt are both chemicals that are soluble in water.
What is concentration?You may have heard some products described as concentrated - like laundry detergent or orange juice. Concentrated refers to the amount of solute compared to the amount of solvent in a solution. When there is a lot of solute compared to solvent, a solution is said to be concentrated. When there is a small amount of solute compared to solvent, then a solution is said to be dilute.
When solutions of different osmolarities are separated by a membrane that lets water but not solutes pass through, water will move from the side with lower osmolarity (high concentration of water) to the side with higher osmolarity (lower concentration of water). Moving water from place to place is what allows plants and animals to keep their levels of water and of nutrients in balance or equilibrium. In living things, all processes involved in maintaining conditions necessary for survival is called homeostasis. It’s kind of like the story of Goldilocks and the Three Bears. Organisms like to keep everything not too hot, not too cold, but just right! Osmosis and our Gastro-intestinal SystemSo now that you know more about osmosis, it’s time to talk about your gastro-intestinal (GI) system - the parts of your body that deals with food and drink. When you eat food or drink water, it travels from your mouth, down your esophagus and into your stomach. In the stomach, the food is broken into tiny pieces that are mixed with stomach liquids. This mush of food and stomach liquids is called chyme. The chyme travels into the small intestine. This is where osmosis takes place. The chyme has a higher concentration than the epithelial cells that line your intestines. So, in order to reach homeostasis, water moves into these cells through their semipermeable membranes, taking small nutrients along with it. Near the epithelial cells are capillaries. The water and nutrients move through the cells of the capillaries and into the bloodstream.
Chyme passes from the stomach into the small intestines. Within the small intestine are folds of tissue called villi. Water passes through the epithelial cells on the villi and into capillaries which carry blood. Osmosis and our KidneysThe water in your blood then travels to your kidneys. Kidneys are some of the most complex parts of the body, and they use osmosis as well. Kidneys are made up of two parts - the cortex and medulla. The cortex is the outer part and the medulla is the inner part of the kidney. The kidneys are made up of groups of cells called renal pyramids. Each pyramid contains little units called nephrons. Nephrons look like a bunch of tubes connected to each other. Nephrons are important because they help filter waste out of your blood and put it into your urine. The largest part of each nephron is inside the medulla. The environment of the medulla has a higher osmolarity than the inside of the nephron. You know what that means - osmosis time! Water travels from inside the nephron tubes, through a semipermeable membrane, out into the medulla. Eventually, concentrated urine is left in the nephron. The urine travels through the ureter to the bladder. DialysisSo you can see that the kidneys have a vital role in your body. But what happens if one of your kidneys can’t do its job anymore? This is when doctors have to use dialysis to help. A dialysis machine also uses a semipermeable membrane. It works in a similar way to a nephron. Blood is pumped next to a membrane that has dialysis fluid on the other side. Because of osmosis, the water in the blood, and very small molecules of waste, move across the membrane into the dialysis fluid. Eventually the dialysis fluid will remove all of the waste materials it can from the blood. That’s why dialysis can be life-saving for people who don’t have healthy kidneys.
Have you ever noticed how your fingers become wrinkly after sitting in the bath too long? You might think this is because osmosis is causing water to leave your finger cells and into the bath water. That’s what doctors used to think. In the 1930s, Dr. T. Lewis and Dr. G.W. Pickering noted that people with nerve damage in their fingers did not experience this wrinkling. If your fingers only wrinkled because of osmosis, then nerve damage would not change it at all! Thus, it was discovered that osmosis is not responsible for the wrinkling, but it’s actually due to our sympathetic nerves. Sympathetic nerves are a special type of nerves that help with vasoconstriction, which is the narrowing of the blood vessels. The nerves make the skin on our fingers wrinkle. The wrinkles act like threads on tires. This allows us to get a better grip on things in water. Our ancestors may have benefited from wrinkly feet to good footing in wet areas. Let’s summarize what we have learned:
I admit, that was quite a bit of information! Maybe it’s time to drink some water and eat a snack!
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What does osmosis do in a cell?
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