Video Transcript
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.
