The urinary system rids the body of waste materials and controls the volume and composition of body fluids. Highly specialized cells in the kidneys are essential to these processes. Your goals for learning are: To review the anatomy of the urinary system, particularly the kidney. To examine the vascular and tubular portions of the nephron. To compare and contrast the specialized cells of the tubular epithelium. To review the unique structure of the juxtaglomerular apparatus. Here's what you need to know. The structural characteristics of epithelial cells. The urinary system is composed of paired kidneys and ureters, the urinary bladder, and the urethra. Urine is produced in the kidneys and then drains through the ureters to the urinary bladder, where the urine is stored. Urine is eliminated from the body through the urethra. Now let’s take a closer look at a kidney’s surrounding structures. Here we see that each bean-shaped kidney is embedded in the perirenal fat capsule. The kidneys are retroperitoneal, lying against the dorsal body wall in the upper abdomen. An adrenal gland which is part of the endocrine system lies on top of each kidney. Several structures enter or exit the concave surface of the kidney at the renal hilum, including the ureter and a renal vein which drains into the inferior vena cava. Let’s learn more about the kidney’s blood supply and see the underlying renal artery, after the left renal vein is removed. When the renal vein is removed and the kidney is shown in frontal section, you can see the deeper renal artery and its connection to the abdominal aorta. Branching from the renal artery are the segmental and interlobar arteries. Together these vessels provide the kidneys with a rich blood supply under high pressure that allows them to continuously filter and cleanse the blood. Now, let us take a closer look at a left kidney’s internal structure. Internally, the human kidney is composed of three distinct regions: The first region is the outermost layer called the renal cortex. It contains about one million nephrons, the filtering units that form urine. There are two types of nephrons: cortical nephrons and juxtamedullary nephrons. Cortical nephrons lie largely with the cortex but their nephron loops dip into the medulla. Juxtamedullary nephrons have glomeruli that lie at the border of the cortex and the medulla and their nephron loops dip deeply into the medulla. The second region of the kidney is the middle layer called the renal medulla in which you can see the triangular renal pyramids. These pyramids look striated because of parallel bundles of ducts carrying urine from the nephrons. The areas between pyramids are the renal columns. They are extensions of the cortex that provide a route for the passage of blood vessels and nerves to and from the outer cortex. The third region of the kidney includes the funnel-shaped renal pelvis, lying within the renal sinus. The renal pelvis collects urine from the pyramids and conveys it into the ureter for passage to the urinary bladder. Let’s take a look at the structure of a nephron in greater detail. Here you see a juxtamedullary nephron. The nephron is the structural and functional unit of the kidneys. It consists of a specialized tubular structure and closely associated blood vessels. Let’s trace the blood flow to and from a nephron. Recall that blood entering the kidney through the renal artery flows first into the segmental arteries. From there, it enters the interlobar arteries, the arcuate arteries, the small cortical radiate arteries and the still smaller afferent arterioles which empty into a capillary bed called the glomerulus. Leading away from the glomerulus is the efferent arteriole. Notice that the afferent arteriole is larger in diameter than the efferent arteriole. Blood passes from the efferent arteriole into the peritubular capillaries and vasa recta. From there blood drains into the cortical radiate vein, flows into the arcuate vein and enters the Interlobar vein eventually reaching the renal vein. Let’s see the nephron’s tubular structure. The tubular structure of the nephron is relatively complex. Let’s simplify the diagram further. The expanded cup-shaped end of the tubule surrounding the glomerulus is called the Glomerular, or Bowman's capsule. Water and solutes pass from the blood into the Glomerular capsule and then flow into the proximal convoluted tubule, or PCT. After many loops and convolutions, the tubule straightens out and fluid flows down the descending, or thin segment, of the nephron loop into the medullary region and then up the ascending, or thick segment, back into the cortical region. From the nephron loop, the fluid then enters the twists and turns of the distal convoluted tubule, or DCT, eventually emptying into a cortical collecting duct. This duct extends into the medulla, forming the medullary collecting duct, which carries the urine through the tubules of the renal pyramids to the renal pelvis. The next few parts of this lesson will follow the flow of fluid through the nephron and discuss the unique characteristics of the specialized cells in each segment. Let’s start with the Glomerular capsule to examine this initial segment in greater detail. The glomerulus with its larger incoming afferent arteriole and smaller outgoing efferent arteriole is nested within the Glomerular capsule, something like a fist thrust into a balloon. Together, these structures are called the renal corpuscle. The visceral layer of the Glomerular capsule is made up of specialized cells called podocytes, which surround the permeable capillaries. Between the visceral and parietal layers of the capsule lies the capsular space which collects the fluid and solutes being filtered from the blood. Let’s take a look at an enlarged view of a glomerular capillary. Here we see a glomerular capillary in longitudinal section.The endothelial lining shows small openings called fenestrations, which allow for the passage of water and solutes such as ions and small molecules. Let’s see a view of the endothelium from outside. Now as the capillary endothelium is completed on the screen, you can see the fenestrations more clearly. Let’s now see a basement membrane. The porous basement membrane encloses the capillary endothelium. Surrounding the basement membrane is a layer of podocytes. These cells have large leg-like extensions, which in turn have small fringe-like extensions called pedicels. Pedicels from adjacent areas interdigitate loosely to form spaces called filtration slits. Substances being filtered must pass first through the fenestrations, then through the basement membrane and finally through the filtration slits and into the capsular space. Together the capillary endothelium, basement membrane, and podocytes make up the filtration membrane. Let’s take a look at a photomicrograph of a glomerular capillary. Here is a photomicrograph of a podocyte overlying a capillary. Extending from the podocyte cell body are leg-like extensions containing the fringe-like pedicels. Notice how these extensions and pedicels wrap around the capillary and interdigitate to form the filtration slits. This photomicrograph shows a cross-section of the filtration membrane. Starting from the top, you see a large podocyte with its nucleus and pedicels. The wide areas are portions of the capsular space. Gaps between the pedicels are the filtration slits. Next you see the basement membrane of the capillary endothelium which separates the podocyte above from the capillary with its fenestrations below. You can also see portions of two red blood cells. Let’s now see an animation of filtration. Notice that the filtration membrane permits the escape of small molecules while preventing large molecules, particularly proteins, from leaving the bloodstream and passing through into the capsular space. Continuing our tour of the nephron, we will now look at the proximal convoluted tubule on the small nephron to see the cells of that region. The simple cuboidal cells of the proximal convoluted tubule are called brush border cells because of their numerous microvilli which project into the lumen of the tubule. These microvilli greatly expand the surface area of the luminal membrane adapting it well for the process of reabsorption. Tight junctions between adjacent cells permit passage of water but limit the escape of large molecules from the tubular lumen into the interstitial space. The highly folded basolateral membrane of the cells contains numerous integral proteins involved in passive or active transport of substances between the intracellular and interstitial spaces. Numerous mitochondria provide the ATP necessary for these active transport processes. The key feature of these cells is that they are highly permeable to water and many solutes. Now let’s look at the thin segment of the nephron loop on the small nephron to see the cells of that region. The cells of the thin segment of the descending nephron loop are simple squamous epithelial cells. These cells lack brush borders, which reduces their surface area for reabsorption. In addition, although these cells continue to be permeable to water, they possess relatively few integral proteins that function as active transport molecules for reabsorbing solutes from the filtrate. Consequently the key feature of these cells is that they are highly permeable to water but not to solutes. Now let’s look at the thick segment of the ascending nephron loop and the distal convoluted tubule on the small nephron to see the cells of that region. The epithelia of the thick ascending nephron loop and the distal convoluted tubule are similar. They are composed of cuboidal cells but they have several structural differences compared to the cells of the proximal convoluted tubule. For example these cells have fewer and smaller microvilli projecting into the lumen. In addition, the cells of the ascending limb are covered by a glycoprotein layer which along with tighter tight junctions greatly restricts the diffusion of water. The basolateral membrane is similar to that of the PCT, containing many integral proteins and closely associated mitochondria for passive and active membrane transport processes. The key feature of the cells of the ascending limb is that they are highly permeable to solutes, particularly sodium chloride but not to water. The cells of the DCT are more permeable to water than those of the ascending limb. This photomicrograph shows a cross section of a glomerulus surrounded by a glomerular capsule. It also shows several proximal convoluted tubules and a single distal convoluted tubule. The microvilli in the lumen of the proximal convoluted tubules appear fuzzy because they do not stand up well to the slide preparation process. Notice the much clearer open lumen of the DCT which is less obstructed because it has fewer microvilli. Now we will look at the DCT to see a specialized region called the juxtaglomerular apparatus. As the thick ascending nephron loop transitions into the early distal convoluted tubule, the tubule runs adjacent to the afferent and efferent arterioles. Where the cells of the arterioles and of the thick ascending nephron loop are in contact with each other, they form the monitoring structure called the juxtaglomerular apparatus. The modified smooth muscle cells of the arterioles, mainly the afferent arteriole, in this area are called granular cells. These enlarged cells serve as baroreceptors sensitive to blood pressure within the arterioles. Cells of the thick ascending segment in contact with the arterioles form the macula densa. These cells monitor and respond to changes in the sodium chloride concentration of the filtrate in the tubule. Now let’s look into the cortical collecting duct on the small nephron to see the cells of this region. The cuboidal cells of the cortical collecting duct fall into two distinct functional types, principal cells and intercalated cells. The more numerous principal cells have few microvilli and basolateral folds. These specialized cells respond to certain hormones that regulate the cell’s permeability to water and solutes, specifically sodium and potassium ions. When the acidity of the body increases, the intercalated cells secrete hydrogen ions into the urine to restore the acid base balance of the body. The key feature of principal cells is that their permeability to water and solutes is physiologically regulated by hormones. The key feature of intercalated cells is their secretion of hydrogen ions for acid base balancing. Now let’s take a look at the medullary collecting duct on the small nephron. Principal cells of the medullary collecting duct are mostly cuboidal in shape. The luminal and basolateral membranes are relatively smooth and the cells possess few mitochondria. The permeability of these cells to water and urea is hormonally regulated as the fluid passes through this region. The key feature of these cells is their hormonally regulated permeability to water and urea. These photomicrographs show a longitudinal section and a cross-section of collecting ducts. Notice that the ducts are composed of cuboidal cells. Also notice that the lumen of the collecting duct, shown in cross-section, is much larger than the lumens of the adjacent thick ascending tubules. This reflects the volume of fluid the collecting ducts contain as they gather the fluid from many nephrons. The longitudinal section here shows two ducts joining to form a larger duct. Here is a summary of what we've covered The urinary system is composed of the kidneys, ureters, urinary bladder, and urethra. The kidney is composed of three regions: the renal cortex, medulla and pelvis. The functional unit of the kidney, the nephron, is composed of a tubular portion and associated blood vessels. Each region of the tubular portion of the nephron depends on the unique features of its epithelial cells to carry out its function.