In this video, we will describe the processes by which waste products are removed from the human body. Then we’ll try some practice questions together, and finally we’ll review what we’ve learned. So let’s not waste any more time and go ahead and get started.
Homeostasis is what scientists call a constant internal environment or state of balance within the body. Removing excess, unnecessary, and dangerous materials from the body is an important part of maintaining that homeostasis. It allows all our systems to function normally. And disruptions to homeostasis can cause illness and death.
The excretory system consists of the organs in our body that serve the function of removing waste. The primary excretory organs are the lungs, the skin, the kidneys, and the large intestine.
The large intestine is sometimes referred to as the colon. And it’s the organ responsible for the removal of solid and insoluble wastes in the form of feces.
The lungs are organs responsible for the removal of gaseous waste from the body. A process called cellular respiration is occurring constantly in almost all of our cells. Cellular respiration breaks down oxygen and glucose, producing water and carbon dioxide. This process releases energy, which is stored in a molecule called ATP that you may recall our cells use to power almost all of our life functions. The excess carbon dioxide diffuses out of our cells and is carried by the blood to the lungs where it’s removed when we exhale. Some water is also removed from our bodies when we exhale as it evaporates from the moist surfaces within our lungs.
Our kidneys are excretory organs responsible for the production of urine. Urine is the primary method for removing liquid or soluble wastes from the body. Urine contains water, ions, also called salts or minerals, urea, and other waste chemicals that are filtered from the blood. Urea is a byproduct of the breakdown of amino acids. When our bodies convert amino acids into other molecules, a toxic chemical called ammonia is produced. Ammonia is then converted by the liver into a chemical called urea, which is much safer for our bodies. And that urea is filtered from our blood by the kidneys to be removed in our urine.
Our skin removes waste from the body in the form of sweat. This diagram shows a cross section of our skin. Sweat is excreted from special glands in the middle layer of our skin called sweat glands. The primary function of sweat is to regulate body temperature. Sweat is also one of the ways our bodies remove excess water, ions, and urea.
You may not think of water as a waste product since we definitely need it to survive. But too much water in the body can increase blood pressure and damage our cells. Don’t worry about it too much though. Our skin and our kidneys work together very efficiently to maintain the perfect balance of water in our bodies at all times. Surprisingly, we’re almost always sweating even when we don’t notice it. That sweat just usually evaporates as quickly as it’s produced.
Now that we’re familiar with what waste is, where it comes from, and how it’s removed from our bodies, let’s try a few practice questions together.
Which of the following substances are waste products produced by the human body? (A) Carbon dioxide, (B) urea, (C) glucose, (D) hemoglobin, or (E) oxygen.
If we read this question carefully, we’ll notice that it says, “substances are,” plural, meaning that there’s likely more than one correct response to this question. And when we talk about waste products, we’re talking about excess and unnecessary substances in the human body.
So in order to answer this question, first, we’ll recall the role that each of these substances plays in the human body. And then we’ll be able to easily identify the ones that are waste products. Let’s start by recalling that carbon dioxide is a byproduct of a process known as cellular respiration. During this process, oxygen from the air that we breathe and glucose from the food that we eat are converted into carbon dioxide and water. This releases chemical energy that our cells store in a molecule called ATP. The excess carbon dioxide is released from our bodies through our lungs when we exhale. So it sounds like carbon dioxide is indeed a waste product.
But let’s keep moving since we know that more than one answer might be right. Well, we get the oxygen that we need for cellular respiration from the air around us when we inhale. Oxygen is not produced by the human body, and oxygen is not a waste product. And glucose is a sugar that we get mostly from the food that we eat. We need glucose for cellular respiration. And it is also not a waste product produced by the human body.
When our bodies metabolize amino acids, a toxic chemical called ammonia is produced. This ammonia is converted into a chemical called urea by the liver, which is much safer for our bodies. And then this urea is filtered from the blood by our kidneys, which remove it in the form of urine. So urea is indeed a waste product.
Our last answer choice, hemoglobin, is an iron-rich molecule that’s found within our red blood cells. And it helps these cells to transport oxygen throughout our bodies. So hemoglobin is not a waste product produced by the human body.
So the substances that are waste products are carbon dioxide and urea.
How about another practice question?
Which of the following is not a method of removing water from the body? (A) Shedding skin, (B) sweating, (C) urinating, or (D) exhaling.
This question is asking us to select the response that is not a method of removing water from the body. So we can expect that three of the choices will be methods of removing water. And we’re looking for the one that is not. So we’ll start by identifying what is being removed by each of these methods described. And at the end, we’ll select the answer choice that does not remove any water.
Shedding skin is what happens to the outer layer of our skin cells at the end of their life cycle. These cells can be rubbed off by clothing and furniture, washed off when we bathe, or simply fall off without us noticing. Sweating is an excretory process carried out by the sweat glands in our skin. Our sweat consists mostly of water, but it also contains urea, salts, and many other chemicals. Urinating is the process of releasing urine from the bladder. Urine is filtered from the blood by the kidneys. It’s mostly water, and it contains urea, salts, and other soluble wastes produced by our body. Exhaling is how our lungs remove gaseous wastes from our bodies. We mostly exhale carbon dioxide, but some water also evaporates from the moist surfaces within our lungs. So sweating, urinating, and exhaling are all different methods of removing water from the body, but shedding skin is not.
Let’s try one last practice question together.
Which of the following correctly describes the correlation between the rate of respiration and carbon dioxide concentration? (A) The more cells respire, the lower the carbon dioxide concentration in the blood. (B) The more cells respire, the higher the carbon dioxide concentration in the blood. (C) There is no correlation between the rate of respiration and the carbon dioxide concentration in the blood.
This question is asking us to look for a correlation, which is another word for relationship. And we’re looking for the relationship between the rate of cellular respiration and the concentration of carbon dioxide, also written as CO2, in the bloodstream.
When we talk about relationships, I immediately think graphs. So first, we’ll sketch out three little graphs, one for each answer. And the 𝑥-axis will represent the rate of cellular respiration, while the 𝑦-axis represents the carbon dioxide concentration.
Our first choice states that the more cells respire, the lower the carbon dioxide concentration in the blood. And we’d expect a graph of this relationship to look something like this. Choice (B) states that the more cells respire, the higher the carbon dioxide concentration in the blood. And we can expect a graph of this relationship to look something like this. Our last answer choice states that there’s no correlation between the rate of respiration and the carbon dioxide concentration. So a graph of this relationship might look something like this.
Let’s start by recalling that cellular respiration is the process by which our cells convert oxygen and glucose into water and carbon dioxide, which releases energy that our cells store as ATP. But what does this all have to do with our blood?
Well, our lungs take in oxygen from the atmosphere, which I represented here in blue. And then our bloodstream carries that oxygen to all the cells of the body that use it to carry out cellular respiration. As a byproduct of cellular respiration, our cells generate carbon dioxide, which I’m using black to show here. And then, once again, our blood carries that carbon dioxide back to the lungs where it’s removed when we exhale. So if there’s an increase in the cellular respiration occurring within our cells, we’d expect to see an increase in the carbon dioxide being produced as a result. And since our blood carries that carbon dioxide to the lungs to be removed, we’d also expect to see an increase in carbon dioxide concentration there. And we’d expect the graph of this information to look something like this.
So the correlation can be described as the more cells respire, the higher the carbon dioxide concentration in the blood.
Let’s wrap up our lesson by taking a moment to review what we’ve learned. Our excretory organs do the important job of removing waste from the human body. The major excretory organs include the lungs, which remove gaseous waste in the form of carbon dioxide; the skin, which removes waste in the form of sweat; the kidneys, which remove waste by filtering it from the bloodstream; and the large intestines, which remove waste in the form of feces.