Lesson Video: Kidney Structure Biology • Second Year of Secondary School

Video Transcript

In this video, we will learn how to describe the macroscopic and microscopic structure of the kidney. We’ll learn about the role of the kidney in the excretory system, and then we’ll learn about the process of the formation of urine. After that, we’ll try a practice question, and then we’ll end by summarizing what we’ve learned.

The kidneys are one of the major organs of the excretory system. The excretory system is the organ system that deals with the removal of waste from our bodies. The lungs function within the excretory system by removing carbon dioxide when we exhale. The skin functions within the excretory system by removing excess salt in sweat. The large intestine functions within the excretory system by removing digestive waste products as feces. And the kidneys function within the excretory system by removing nitrogen containing waste products and urine. When we metabolize proteins, our cells generate nitrogen containing wastes in the form of ammonia. But ammonia is very toxic, so it’s converted by the liver into a molecule that’s safer for ourselves called urea. And urea is removed from the bloodstream by our kidneys in the form of urine.

Next, let’s take a closer look at the function of our urinary system. Our urinary system includes the kidneys, which are dark red fist-sized organs that filter the blood and form urine, the ureters, which carry urine from the kidney to the bladder, the urinary bladder, which stores urine until it’s ready to be released from the body, and the urethra, which carries urine out of the body from the bladder. Ureter and urethra are too commonly confused terms. An easy way to remember the difference is that there are two e’s in ureter and there two ureters, one for each kidney. In addition to safely removing nitrogen-containing waste, the urinary system helps the body to maintain internal balance, also referred to as homeostasis, by helping to regulate fluid levels, ion levels, and pH levels in the body.

Next, let’s take a closer look at the internal structure of the kidney. Here we have a cross-sectional diagram of one of your two kidneys. Blood enters the kidneys through the renal arteries. “Renal” is a word that means dealing with the kidneys. After blood has been filtered, it leaves the kidneys through the renal veins. Around the kidney is a tough lining, known as the renal capsule. Kidney tissue is very soft and delicate, and the capsule helps to protect it and hold it together. The outer layer of the renal tissue is called the renal cortex. “Cortex” is a Latin word that means bark, like on a tree, and it’s often used to describe the outer layer of different types of tissue.

Within the cortex are teardrop-shaped sections called the renal medulla. “Medulla” is a Latin word that means pith, referring to the soft inner tissue of the stem of a plant. Medulla is a word that’s often used to describe an inner layer of tissue. Here, in the middle of the kidney is the place where urine collects before it leaves the kidney through the ureters, and it’s called the renal pelvis. All of these are parts that make up the kidney. But how does the kidney actually make urine?

To understand that, we’ll have to zoom in and investigate the nephron. The nephron is the functional unit of the kidney. That means that the nephron is the part of the kidney that actually does the job of making urine. Nephrons are microscopic, and each kidney contains about a million of these tiny structures, which is great because our kidneys filter about 140 liters of blood every day, removing about one and a half liters of urine.

The production of urine occurs in three steps: filtration, selective reabsorption, and then urine formation. The diagram of the nephron can be complicated to look at, so I’m adding this extremely simplified version here so that we can see things a little more clearly. First, blood from the renal arteries enters a ball of capillaries called the glomerulus. The capillaries of the glomerulus are especially porous, and almost all of the small molecules filter out of the blood, leaving behind the cells and the proteins. This includes water, ions, nutrients like glucose, and waste products like urea. The glomerulus is surrounded by a structure called the glomerular capsule. This part is also known as a Bowman’s capsule. The capsule absorbs the filtrate that has left the blood, and this step is known as filtration.

Next, the filtrate flows through a tiny coiled and twisted tube called the nephric tubule. The first part of this tube is called the proximal convoluted tubule. “Proximal” is a word that means close or near, and “convoluted” means twisted. Much of what has been filtered from the blood in the filtrate is actually still useful and necessary to the body. In the proximal convoluted tubule, things like ions and glucose are reabsorbed from the filtrate back into the bloodstream. Most of the nephron is located in the cortex, but there’s a loop of the tubule that dips into the medulla. This is called the magilary loop, or the loop of Henle.

In this loop, most of the water from the filtrate is reabsorbed into the bloodstream. The process of removing useful materials from the filtrate and returning them to the bloodstream is called selective reabsorption. The process of efficiently removing waste from our blood is kind of like cleaning out your pockets. It’s hard to reach in there and just pick out the trash. So, instead, you remove everything from your pocket. Then you just put back the things you actually wanna keep, and you throw the trash away. In the same way, selective reabsorption returns the useful substances to our bloodstream while allowing the waste products to leave the body. After the loop of Henle, the filtrate enters the distal convoluted tubule. “Distal” is a word that means far, so this is the last or the second twisted tube.

At this point, whatever remains in the tubule is officially considered urine. This urine is a mixture of water, salts, urea, and other waste products. The distal convoluted tubule empties into the collecting duct. These ducts join together into larger and larger vessels until they reach the renal pelvis to leave the kidney through the ureter. This final step is called urine formation. The freshly cleansed blood leaves the kidney and travels back to the heart through the renal veins. Students often think that urea is what gives urine its characteristic yellow color. But the pigments in urine are actually the result of the breakdown of red blood cells and bile. Now that we’ve learned about the structure and the function of the kidneys, let’s try a practice question.

The diagram provided shows the macroscopic outline of a human kidney. What structure in the kidney is represented by label one? What structure in the kidney is represented by label two? What structure in the kidney is represented by label three?

The question provides us with a diagram that represents a human kidney. One part shows us the outer view, and the other part is a cross-sectional diagram showing us the inner structure. And they’ve asked us to identify the parts indicated by the numbered labels. So in order to answer this question, we’ll go ahead and label all the parts of the kidney, and then we’ll be able to easily recall our answers. First, the kidney has a tough outer coating, referred to as the renal capsule. Kidney tissue is very soft and delicate, and the capsule helps to protect and support it. Inside the kidney, there are two different types of tissues: the cortex, which is the outer layer of tissue, and the medulla, which are these teardrop-shaped inner sections.

“Cortex” is a Latin word that means bark like on a tree, and “medulla” is the Latin word for the soft inner plant tissue of the stem. In anatomy, cortex and medulla are often used to refer to the outer and inner layers of different types of tissue. The urine produced by the kidneys collects in the center in a section known as the renal pelvis. And the urine is transported from the kidneys to the urinary bladder by the ureter. The blood vessels that supply the kidneys are the renal artery, which carries blood from the heart to the kidneys, and the renal vein, which carries blood from the kidneys back to the heart. So now we recalled all the information we need to answer our questions. The structure represented by label one is the cortex. The structure represented by label two is the medulla. And the structure represented by label three is the renal artery.

How about another practice question?

About 180 liters of plasma are filtered by the kidney every day. However, the average urine excreted per day is about 1.5 liters only. Which of the following is responsible for that?

This question is pointing out that the amount of plasma filtered by the kidneys is much greater than the amount of urine that we excrete every day. And it’s asking us to correctly identify the part of the urine production process which is responsible for this notable difference. Well, let’s start by recalling that the kidney filters waste from our blood, which is then removed from our body in the form of urine. The nephron is the functional unit of the kidney. It’s a microscopic structure responsible for urine production. And this process takes place in three steps. The first step is filtration, in which almost all of the small molecules are filtered out of the bloodstream and into the tubule. The filtrate includes water, ions, nutrients, and waste products such as urea.

The second step in this process is called selective reabsorption. During this step, the nutrients, most of the water, and any of the ions that your body needs are returned to the blood stream from the filtrate. In the last step, anything that’s left in the tubule is considered to be urine. And several nephric tubules will empty into one collecting duct. The collecting ducts join into larger and larger vessels until they converge in the renal pelvis, where the urine leaves the kidney through the ureter. So let’s return to our answer choices and figure out which of these is responsible for the volume of filtrate being much larger than the volume of urine.

Choice (A) is, the filtration process taking place in the nephric tubule. Well, the filtration process is actually responsible for generating the filtrate. And it occurs in a structure called the glomerulus and not the nephric tubule. Our next choice is, the reabsorption process taking place in the nephric tubule. Well, the reabsorption step is where most of the water leaves the filtrate and reenters the bloodstream. And this does occur in the nephric tubule, so this seems correct, but let’s keep going.

The filtration process taking place in the collecting duct. Well, filtration occurs in the glomerulus, not in the collecting duct. And our last choice, the reabsorption process taking place in the collecting duct. Well, reabsorption occurs in the nephric tubule. The collecting duct just collects urine. So our correct answer is, the reabsorption process taking place in the nephric tubule.

Let’s try one last practice question together.

Which of the following exist in the kidney in a number less than the number of Bowman’s capsules? (A) Glomeruli, (B) loops of Henle, (C) nephric tubules, or (D) collecting ducts.

In order to answer this question, we must first recall that the Bowman’s capsule is a part of the nephron. So we’re looking for the answer choice that will have less of than the number of nephrons in the kidney. So we’ll start by diagramming and labeling a nephron, and then we’ll be able to choose the correct response. So here’s our simplified diagram of a nephron. Our nephron consists of the glomerulus, a ball of porous blood vessels that release filtrate, the Bowman’s capsule which collects that filtrate. The Bowman’s capsule empties the filtrate into the nephric tubule. This coiled twisted tube makes up most of the nephron. One of the features of the nephric tubule is the loop of Henle, where most of the water is reabsorbed from the filtrate back into the bloodstream. And several nephrons will connect to one collecting duct.

The collecting ducts collect urine, and they join into larger and larger vessels until they reach the renal pelvis, where the urine exits the kidney through the ureter. Which of the following will exist in the kidney in a number less than the number of nephrons? Well, the glomerulus, the loop of Henle, and the nephric tubule are all different parts of a nephron. In the kidney, we should have the exact same number of each. But one collecting duct collects urine from multiple nephrons. So in the kidney, there will be fewer collecting ducts than Bowman’s capsules.

Well, let’s wrap up our lesson now by taking a moment to review what we’ve learned. In this video, we learned about the structure of the kidney, the structure of the nephron, which is the functional unit of the kidney, and how filtration and selective reabsorption are key steps in the production of urine and the removal of waste from our bloodstream.