Capillary Fluid Exchange

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  • 0:00 Capillaries
  • 1:06 Fluid Exchange
  • 2:11 Arteriole End of the…
  • 2:44 Middle of the Capillary Bed
  • 3:09 Venule End of the…
  • 3:56 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

How do we get nutrients from blood into the rest of our bodies? The answer is through capillary fluid exchange, and in this lesson we'll discuss what this process is and how it works. Then, test your understanding with a brief quiz.


I'm going to give you a quick test on geography. If I'm in the Atlantic Ocean but I want to get to the Pacific, what's the best way to get there? The answer: the Panama Canal, a narrow channel that connects the oceans. When transporting people and products long distances, shortcuts like the Panama Canal are really important. Well, our bodies have a similar sort of system for transporting blood. We've got arteries, blood vessels that transport blood away from the heart, and we've got veins, blood vessels that transport blood back to the heart. But how does blood get from one type of blood vessel to the other? Just like the Panama Canal connects oceans, the body uses capillaries, very small blood vessels to transport blood from arteries to veins. Through small holes in the vessel walls, capillaries distribute the nutrients and byproducts of blood throughout the body, making these tiny blood vessels something of huge importance.

Fluid Exchange

So how exactly does this all work? Blood passes in and out of capillaries by diffusion, the movement of molecules from areas of high pressure to low pressure. Actually, there are two different pressures at work here, and each influences fluid exchange differently. First, you've got the hydrostatic pressure within the capillary vessel. Basically, this is blood pressure, or the pressure created by blood circulating through blood vessels. The other factor is osmotic pressure, which is a constant pressure needed to keep blood from diffusing. Blood pressure changes and is the actual pressure in the vessels, but osmotic pressure is a mathematical constant, a stable pressure that blood is often trying to reach. These two forces interact in a network of capillaries in between arteries and veins called the capillary bed. These beds are found in major muscles, tissues, and anywhere that needs nutrients from the blood.

Arteriole End of the Capillary Bed

Ok, let's watch how fluid exchange works across the capillary bed. Since blood comes from the heart in arteries, this is the side where we'll start. As blood begins to move into capillaries from the arteries, the small size of the blood vessel increases blood pressure. Since blood pressure is higher than osmotic pressure, fluids containing oxygen and other nutrients are diffused out of the capillary and into the surrounding body tissue. That's how the oxygen and nutrients in blood makes it into our muscles and organs.

Middle of the Capillary Bed

As we get into the middle of the capillary bed, enough fluid has been diffused that the blood pressure levels out, and equals osmotic pressure. Since the pressure is equal inside the vessel and outside in the tissues, fluid passes freely between them. Any nutrients still in the capillary flow into the tissues, and gases like carbon dioxide begin to travel from the tissues into the capillary.

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