The Body's Defense System: Internal & External Defenses

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  • 0:02 Why the Body Needs to…
  • 0:31 External Defense Systems
  • 3:02 Internal Defense Systems
  • 4:35 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson examines external and internal defense systems. Learn how your body defends itself both internally and externally with mucus, saliva, stomach acid, antibodies and more.

Why the Body Needs to Defend Itself

Germs are everywhere! You're surrounded by all kinds of creepy crawlies, from bacteria to viruses to fungi. I'm not saying this to turn you into a germophobe. In fact, most of them are perfectly harmless, but there are some that can wreak havoc on your body. And it's because of those few, the ones that cause you to get sick, that our bodies have a sophisticated set of defense systems both inside and out. Let's talk about how these defense systems work.

External Defense Systems

The first lines of defense for your body are the external defense systems. These are the parts of your body designed to protect you from harmful outside agents by preventing them from getting into your body in the first place. Sometimes this can be a little confusing because, scientifically, there are several places you might think of as internal that are considered by biologists to be external. Your skin is external, so that's an easy one. But did you know that your stomach is external, too? Nasty stuff can be eaten by you and pass through your entire digestive system without anything bad ever entering your system. So your digestive pathways are actually external. External defense systems include your skin, tears, mucus, saliva, stomach acid, cilia (small hairs) and helpful bacteria in your bowel.

Your skin provides a physical block to stop bacteria and viruses from finding their way straight into your blood. It forms part of the integumentary system, which also includes your hair and nails, which trap dirt. Skin doesn't always work perfectly because you can get cuts that allow pathogens to find a way in, but on the whole, it's a pretty good block. It must be because the skin is usually teeming with bacteria. If you do end up getting cut and bleeding, cells called platelets rush to the sight of the cut and cause blood to clot into harder lumps. This stops the bleeding fast and protects your body.

Tears in your eyes and mucus in your nose and throat are also tasked with trapping these dangerous organisms if they try to make it into your body. This is important because some of the most common ways people get colds is by touching their eyes with dirty hands or breathing pathogens into their bodies. The cilia, or small hairs in your windpipe, also make this less likely by gradually pushing organisms back up towards your mouth and nose where they can be removed from the body.

If organisms can't be trapped, it's possible they can be killed with saliva or stomach acid. Saliva contains enzymes that break down the cell walls of many bacteria and stomach acid is highly corrosive and kills much of what remains. In a similar way, helpful bacteria in the lower gut can take care of much of what remains by outcompeting pathogens and making it harder for the dangerous stuff to establish itself.

Internal Defense Systems

Your internal defense systems kick in when the external defenses fail and something bad finds its way into your body. This could be a virus or bacteria, for example. Once it reaches this stage, it's called an antigen. Once there, antigens can multiply at a staggering rate, so it's important your body has a way to react. Pretty much the entire internal defense system is called the immune system, which involves the work of white blood cells flowing around your veins and arteries all the time.

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