In this explainer, we will learn how to describe the macroscopic and microscopic structures of the kidney.
The kidneys are incredibly hardworking organs. They serve the function of filtering waste from the blood and making it into urine. At any point in time, about of your body’s blood volume is located within the kidneys. They filter a total of about 180–200 litres of blood each day and produce an average of about 1.5 litres of urine. The specialized structure of the kidney is what allows it to carry out this essential function.
Key Term: Kidneys
The kidneys are fist-sized, bean-shaped organs located near the spine that filter the blood to make urine.
The kidneys are one of the major organs of our excretory system. The excretory system is the organ system that deals with the removal of various types of metabolic waste. This system, as shown in Figure 1, includes the kidneys, lungs, skin, and liver. The kidneys are also the primary organ of what we often refer to as the urinary system. This organ system is specifically responsible for removing urine from the body. The urinary system includes the kidneys, ureters, urinary bladder, and urethra.
The kidneys are two fist-sized organs located on either side of the spine near the base of the ribs and behind the peritoneum. In contrast to humans, lower vertebrates have long, thin kidneys that are situated on the sides of the vertebral column. The kidneys function to filter waste from the blood and make urine. The ureters connect the kidneys to the urinary bladder. The urinary bladder, also commonly just called the bladder, is an expandable muscular sac-like organ that sits in the lower pelvis and stores urine until it is ready to be removed from the body. A sphincter muscle controls the expulsion of urine, and when we urinate, urine passes from the bladder to the external environment through a tube called the urethra.
Urine is a liquid waste product made in the kidneys that consists of water, urea, ions, and other types of soluble waste.
Example 1: Recalling the Organs of the Urinary System from Their Functions
What organ in the human body does the ureter connect the kidney to?
- Small intestine
- Large intestine
The urinary system is the organ system responsible for the production and removal of urine from the body. Urine is a type of liquid waste composed of water, ions, and soluble waste products, such as urea. Urine is made in the kidneys. The kidneys filter the blood, removing the waste and balancing the ions and fluids. Anything removed from the blood by the kidneys is considered to be urine.
The kidneys work together with the other organs of the urinary system to remove this urine from the body. The ureters connect the kidneys to the urinary bladder, which stores the urine, and this urine is then excreted through the urethra, which is connected to the urinary bladder. After urine is made by the kidneys, it is pumped down the ureters into the bladder to be stored. During urination, urine from the bladder is released from the body through the urethra.
The organ that the ureter connects the kidney to is the bladder.
Urine gets its name from the chemical urea (carbamide). Urea is a waste product created as a result of the breakdown of amino acids, the building blocks of proteins, in the body. This breakdown of amino acids creates a toxic chemical called ammonia. The liver converts this ammonia into urea, which is much safer for the body. This urea is then filtered from the blood by the kidneys and expelled from the body in the urine. You can see these processes illustrated in Figure 2. Because each molecule of urea contains atoms of nitrogen, as shown in Figure 3, it is referred to as a nitrogen-containing waste.
Definition: Urea (Carbamide)
Urea is a form of nitrogen-containing waste produced as an end product from the metabolism of the components of proteins.
Removal of waste products like urea is an important part of how our bodies maintain a constant, normal internal environment, a process that is also called homeostasis. In addition to safely removing nitrogen-containing waste products, the kidneys function to maintain homeostasis by aiding in the regulation of fluid levels, ion levels, and pH levels in the blood. Disruption of homeostasis can quickly lead to illness and sometimes death, which is why the kidneys must work so hard and constantly.
Homeostasis is the maintenance and regulation of a constant, normal internal environment that allows the biological processes to function properly within an organism.
Example 2: Identifying the Waste Substance Excreted by the Kidneys
The kidneys filter out waste products from the blood. In what substance are these waste products excreted from the body?
- Carbon dioxide
The kidneys are organs within the urinary system. Their function is to filter the blood and remove waste in the form of urine. Urine is composed of water, ions, and soluble waste products, such as urea. Waste is what we call the products of chemical reactions in the body that are not needed or are harmful to the body. The process of removing waste from the body is called excretion.
Urea is a waste product that results from the breakdown of amino acids. Amino acids are small molecules joined together to make proteins. When amino acids are broken down in the body, one of the products made is a toxic chemical called ammonia. Ammonia is converted into urea in the liver, which is much safer for our bodies. However, if urea builds up in the body, it could also become toxic. Urea is constantly filtered out from the blood by the kidneys and removed in urine. Urine has its name from the urea it contains.
Therefore, the substance described is urine.
Let’s take a closer look at the different parts of the kidney and how they work together to filter the blood and form urine.
The primary function of the kidneys is to filter the blood and convert waste into urine. Blood enters each of our kidneys through the renal arteries, which come from the aorta, and leaves through the renal veins, which filter into the posterior vena cava. The word renal means “referring to the kidneys.”
Key Term: Renal
Renal is a word that means “referring to the kidneys.”
The kidneys are soft, delicate organs. They are protected by the bones of the ribs and are surrounded and supported by a tough membrane known as the renal capsule, as shown in Figure 4. The renal capsule surrounds a layer of tissue called the renal cortex. Within the cortex are teardrop-shaped sections of tissue called the renal medulla. At the center of the kidney is the renal pelvis, which connects the kidney to the bladder through the ureter.
Spanning the renal cortex and medulla are millions of microscopic structures called nephrons, as shown in Figure 5. These nephrons, a series of tubules and capillaries, are the functional units of the kidney that make the urine. Each nephron connects to a collecting duct, which connects to the renal pelvis and ureter to deliver urine to the bladder. The production of urine by the nephron occurs in three steps: glomerular filtration, selective reabsorption, and urine formation (secretion).
Example 3: Recalling the Name of the Component of the Kidney That Makes Urine
The kidney contains thousands of small tubules that filter blood to form urine. What is the scientific term given to these tubules?
The kidney is the organ responsible for the production of urine. The kidney makes urine by filtering the blood and removing waste. This job is carried out by thousands of microscopic structures within the kidney called nephrons. Each nephron is made of a tiny twisted tubule surrounded by capillaries. Nephrons empty their contents, the urine they make, into larger tubes called the collecting ducts. These ducts carry the urine to the ureters to remove it from the kidney.
Using this information, we can conclude that the scientific name given to the tubules that filter the blood to form urine is nephrons.
Now, let’s describe the steps of urine formation. The first step is the filtration step, which is summarized in Figure 6. Blood from the renal arteries enters a ball of capillaries called the glomerulus. The capillaries of the glomerulus are especially porous, and almost everything is filtered out of the blood except for cells and large proteins like albumin. This includes water, ions, nutrients (e.g., glucose), and waste products (e.g., urea). This mixture is called the filtrate. The glomerulus is surrounded by a structure called the glomerular capsule, also called the Bowman capsule, which is found in the kidney cortex and functions to absorb the filtrate that has left the blood. The Bowman capsule funnels the filtrate into the nephron.
Key Term: Filtration
Filtration is the first step in urine production. During filtration, almost everything, except for cells and large proteins, is filtered out of the blood and absorbed by the Bowman capsule.
The second step is selective reabsorption, as shown in Figure 7 below. The long, twisted renal tubule of the nephron is surrounded by more capillaries, which are responsible for reabsorbing materials that the body needs. Much of the filtrate is actually still useful and necessary to the body. During selective reabsorption, things like ions, water, and glucose are reabsorbed from the filtrate and transported back into the bloodstream.
The first part of the nephron is called the proximal convoluted tubule, or the first coiled tubule. The word proximal means “close” or “near,” and the word convoluted means “twisted” or “tangled.” In the proximal convoluted tubule, glucose, some water, and ions are reabsorbed from the filtrate from the Bowman capsule into the bloodstream through the capillaries surrounding the proximal convoluted tubule.
The next section of the nephron is called the medullary loop, or the loop of Henle. This loop dips from the cortex into the renal medulla. In this section of the nephron, most of the remaining water from the filtrate is reabsorbed into the bloodstream and the ion content of the blood is carefully balanced to form concentrated urine.
Key Term: Selective Reabsorption
Selective reabsorption is the second step in urine production. During selective reabsorption, any materials in the filtrate that are useful to the body are reabsorbed into the bloodstream.
The last step is urine formation. After leaving the loop of Henle, the filtrate enters the distal, or far, convoluted tubule, also called the second coiled tubule. At this point, whatever remains in the tubule is officially considered urine. Urine typically contains urea as mentioned, some inorganic salts (e.g., sodium), also called ions, and water. Glucose and albumin are not normally found in the urine and may indicate health problems if discovered.
The distal convoluted tubules of many nephrons connect into larger and larger collecting ducts until they reach the renal pelvis in the kidney. The urine is emptied into the renal pelvis to leave the kidney through the ureter. The freshly cleansed blood leaves the kidneys and travels back toward the heart through the renal veins. The steps of urine production in a nephron are shown in Figure 8.
Key Term: Urine Formation
Urine formation is the last step in urine production. After filtration and selective reabsorption, anything that remains in the tubule is considered urine and transferred to the urinary bladder.
Example 4: Recalling the Functions of the Parts of a Nephron
The diagram provided shows a simplified outline of the first part of the nephron.
- As blood moves through the glomerulus, small molecules pass through into the nephron, and large molecules (like proteins) remain in the blood. What is this process called?
- As the filtrate passes through the proximal convoluted tubule, useful substances (like glucose) are taken back into the blood. What is this process called?
The diagram shows us some of the parts of a nephron. We are shown the glomerulus, the Bowman capsule, and the proximal convoluted tubule. The nephron is the structure within the kidney that is responsible for the production of urine.
Urine production occurs in three steps. The first step, filtration, occurs in the Bowman capsule. During filtration, almost all of the small molecules are filtered out from the blood passing through the glomerulus. This includes water, ions, and soluble waste products, but not cells and larger proteins. The Bowman capsule funnels this filtrate into the proximal convoluted tubule. This is the first part of the long, twisted renal tubule of the nephron.
Therefore, the process that occurs in the glomerulus is known as filtration.
After filtration, in the proximal convoluted tubule, the process of selective reabsorption begins. Selective reabsorption is the process where the useful or necessary materials removed in the filtrate are reabsorbed from the tubule back into the bloodstream within the capillaries. This includes substances like water, some ions, and nutrients, like glucose. As the filtrate continues through the rest of the tubule, selective reabsorption continues and the contents of the blood are carefully balanced. At the end, anything left in the tubule is emptied into the collecting duct and removed in the form of urine.
Therefore, the process that occurs in the proximal convoluted tubule is selective reabsorption.
The function of the kidney is to make urine. Urine production is carried out by hundreds of thousands of microscopic structures within the kidney called the nephrons. Our kidneys make urine by filtering our blood, which is important to the maintenance of homeostasis within the body. The failure of our kidneys to properly carry out their function can rapidly lead to a disruption in homeostasis, which may result in illness and death.
Let’s summarize what we have learned about the structure of the kidney from this explainer.
- The function of the kidneys is to remove waste as urine.
- The structure of the kidney includes the cortex, medulla, renal artery, renal vein, and ureter.
- The functional unit of the kidneys is a microscopic structure called the nephron.
- Each nephron consists of a renal tubule surrounded by capillaries.
- The steps of urine production in the nephron are glomerular filtration, selective reabsorption, and urine formation (secretion).