Now we're going to talk about systems that perform gas, nutrient, and waste exchange. What these systems have in common is they exchange material between the outside of the body and the blood. Before we go on, I just want to think about what we mean when we say "inside" and "outside" the body. You probably think that when you inhale something or eat something, it goes into your body. I want to think about things a little bit differently.
To be inside your body, something has to cross into the tissue of your body, has to be in a tissue or the blood or the cells of your body. So, for example, if you eat a penny, that penny is going to go into your digestive tract and it'll go down and through your body and it'll come out the other side, but it has no chance of ever getting into the tissues of your body. It has no chance of being in the blood or the cells. For our purposes, that penny was never really in your body. It was just kind of traveling through the middle of you.
So how do things get in and out of those tissues? Well, the first way is the respiratory system. The respiratory system consists of tubes that lead to the very important lungs. The lungs are the site of gas exchange, and gases, chiefly oxygen (O2) and carbon dioxide (CO2), are involved. Oxygen is the major requirement for cellular respiration and carbon dioxide is the major waste product.
Importantly, anything that's in different concentration in the air, in the blood, and soluble in both will be exchanged in the lungs. So, for example, that's why a breathalyzer test works. If a person has alcohol in their blood, while alcohol is soluble in both the blood and the air, it's going to be at a higher concentration in the blood. So when they exhale, it's going to dissolve into the air, come out, and be registered by a breathalyzer test. With that in mind, we'll move on to the digestive system.
The digestive system consists of the mouth, the esophagus, the stomach, the small intestines, and the large intestines. That’s the tube that just goes through you from one end to the other, and then you have some accessory organs, like your liver, pancreas, gallbladder, and salivary glands. The function of the digestive system is to obtain water and nutrients from food. The first half of that tube is all about breaking that food up, crushing it, ripping it, getting it wet.
We add acids and enzymes to it to break it down to its component molecules, and the second half is all about absorbing it into the blood. Now you'll note I didn't say removing waste as a major function of the digestive system. The waste of the digestive system that comes out in your feces is really more like the penny. You eat a lot of food that your body just doesn't use, so it travels through your body, but it never has a chance to get into your tissues or your blood.
When we think of getting rid of waste, we want to think of waste generated in the tissues. Very little waste is actually put into the digestive system. The majority of it is just stuff that passes through. So how does your body get rid of waste? Well, the first way was the respiratory system, but another way is the urinary system.
The urinary system consists of your kidneys, the ureters, which are tubes that connect the kidneys to the bladder, and your urethra, which is the tube that connects the bladder to the outside world. The urinary system removes waste and excess water from the blood. The waste is largely urea, a nitrogenous waste that gives urine its characteristic smell and color, but plenty of other waste will be removed as well. Now in terms of water, your kidneys are responsible for keeping a constant water level in your body. If you drink a big glass of water, that water is going to be absorbed in the digestive system.
It'll enter the blood. Now you have too much water in your blood. The kidneys of the urinary system will take that water out, put it in the urine, and you can pass it out of your body. Alright. With that, 3 more systems down. I'll see you in the next video.