It would be great if we could breathe underwater, wouldn't it? One key idea for understanding why we can't is surface area. A good way to think about surface area is to think about painting. Let's say that you were going to paint a smooth ball. Now let's say that the ball were the same size, but covered with big bumps. It would take more time and paint to cover it because there's now more surface area.

When we need to move matter or energy, surface area is an important idea. For example, if you are in cold air, you can curl up tight to decrease your surface area and hold onto your body heat.

Sometimes you want more surface area. In your lungs, oxygen has to move from the air in your lungs into the blood that's in capillaries in your lungs. (Capillaries are just tiny blood tubes that connect veins and arteries.) If the inside of your lungs were smooth, you couldn't move enough oxygen because there would be a lot less surface area where capillaries would be near the air. Instead, the inside of your lungs is made up of millions of little pouches called alveoli. Each pouch is open to the tubes that bring air into your lungs, and the walls are covered with capillaries. This creates lots of surface area. It's plenty to get enough oxygen from the air.

Air is about 20% oxygen, but water has a lot less oxygen in it. How much depends on things like temperature (warm water hold less oxygen),how much oxygen is getting mixed in from the air, produced by photosynthesis, or used up. So our lungs just don't have as much surface area as they would need to get oxygen from water.

Fish need less oxygen than we do because they are "cold-blooded," what scientists call "exothermic," meaning they get their heat from outside their bodies. We spend a lot of energy making heat, so we need more oxygen. Fish gill look like millions of tiny fingers arranged on curved bars. These provide a lot of surface area of getting oxygen out of the water. These work well when they are in water, which supports these delicate, finger-like things. Unfortunately for fish, when they are out of the water, all of these fingers collapse against each other, so there is very little surface area, that's why fish out of water suffocate.

Here's a picture of fish gills:
fish_gills

Can you think of other places that surface area is important? (Hint:think about absorbtion.) Can you think of ways that surface area would be important for marine mammals like whales?

In order for the cells in our bodies to live, they need Oxygen.The way we get oxygen into our blood and then into our cells, is through our lungs. We we breathe in, our lungs fill up with air. In our lungs are tiny Avioli, that have millions of little blood capillaries right at the surface. Oxygen can then pass into the capillaries, and into our blood stream. The blood is then pumped into the heart and around our body.

Even though there is some oxygen dissolved in water, or lungs are not designed to filter out this oxygen like a fish's gills are. Instead, our lungs fill up with water which is heavy, and we can't breath in anymore.

There isn't very much air dissolved in water and our lungs can't take that air out of the water anyway, we'd drown with that much water in our lungs.

There are two reasons: (1) the content of oxygen is very low compared to that of air because oxygen isn't very soluble in water. You need specialized organs like gills to get enough oxygen out of water to survive on it. (2) Water is much denser than air, and we don't have the strength to force it in and out of our lungs the way we can air, or if we could we would be exhausted very quickly and be unable to breathe that way either.

Humans use lungs to breathe - our lungs are specialized to exchange oxygen and carbon dioxide. Blood coming into the lungs carries carbon dioxide (in the form of bicarbonate) bound to a molecule called hemoglobin - when it reaches the cells that are exposed to the air (the alveoli), oxygen diffuses through the thin membrane, bumps off the carbon dioxide and binds the hemoglobin. The oxygenated blood is then carried throughout the body. The lungs are specialized for gas exchange from air and rely on pressure changes to ventilate (this comes from musculature in the thoracic cavity). In water, there is usually plenty of dissolved oxygen, but our lungs cannot access it - the design is wrong. Not only is it impossible to generate the pressure changes to exchange water in and out of the lungs, but the physical exchange of the gases will not work as the hemoglobin binding is dependent on the diffusion rate of the gases. Another way to address your question is to ask why some other animals CAN breathe underwater. Whales and other cetaceans (aquatic mammals) can simply hold their breath for a very long time and their body tissues slowly use and exchange oxygen that was inhaled right before the dive. Fish and amphibians are a totally different story. They do not have lungs like mammals. Fish have specialized organs called gills that can extract oxygen from water as it passes over the surface of the tissue. Many amphibians can simply exchange gases through their skin.