13.2 Microscopic Anatomy of the Kidney: Anatomy of the Nephron

As mentioned in the previous section, the nephron is the functional unit of the kidney. It is functionally compartmentalized: it is a long tubule with different parts that have distinct functions. Generally speaking, nephrons rid the blood of toxins and balance the constituents of the plasma to homeostatic set points. 

• Glomerular capsule (also called Bowman’s capsule)
• Proximal convoluted tubule (PCT)
• Loop of Henle (also called the nephron loop)
• Distal convoluted tubule (DCT)
• Collecting duct

As previewed in the previous section, each nephron has blood vessels associated with it. The nephron functions that are crucial to maintaining homeostasis of plasma constituents—filtration, reabsorption, and secretion—involve exchanges between the nephron tubules and the associated vessels. A general explanation of filtration, reabsorption, and secretion is below, and detailed explanations of these processes are presented in later sections of this chapter.The glomerulus and glomerular capsule together form the renal corpuscle. Blood is filtered by the glomerulus to produce a fluid called filtrate. The portion of the nephron tubule that surrounds the glomerulus and catches the filtrate is the glomerular capsule (also called Bowman’s capsule). Filtrate entering the glomerular capsule travels to the proximal convoluted tubule (PCT), loop of Henle (nephron loop) and distal convoluted tubule (DCT), in this order, before exiting the nephron into a common collecting duct shared by many nephrons. Though all nephron glomeruli are in the cortex, some nephrons have short loops of Henle that do not dip far beyond the cortex. These nephrons are called cortical nephrons. About 15% of nephrons have very long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons (Figure 13.2.2). Blood exits the glomerulus into the efferent arteriole (Figure 13.2.1). The efferent arteriole then forms a second capillary network around the tubule, called the peritubular capillaries. For juxtamedullary nephrons, the portion of the capillary that follows the loop of Henle deep into the medulla is called the vasa recta. As the glomerular filtrate progresses through the tubule, these capillary networks recover most of the solutes and water, and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, this is an example of a portal system (portals systems are also seen in the hypothalamus-pituitary axis and in the hepatic portion of the digestive system).

 

Microanatomy of the Nephron

Renal Corpuscle

As discussed earlier, the renal corpuscle consists the glomerulus and the glomerular capsule. The glomerulus is a high pressured, fenestrated capillary with large holes (fenestrations) between the endothelial cells. The glomerular capsule captures the filtrate created by the glomerulus and directs this filtrate to the PCT. The outermost part of glomerular capsule is a simple squamous epithelium. It transitions over the glomerulus as uniquely shaped cells (podocytes) with finger-like arms (pedicels) that cover the glomerular capillaries (Figure 13.2.3). A thin basement membrane lies between the glomerular endothelium and the podocytes. The pedicels interdigitate to form filtration slits, leaving small gaps that form a sieve. As blood passes through the glomerulus, 10% to 20% of the plasma filters out of the fenestrations, through the basement membrane and between these sieve-like fingers to be captured by the glomerular capsule and funneled to the PCT. These features comprise the filtration membrane.

The filtration membrane prevents passage of blood cells, large proteins, and most negatively charged particles but allows most other constituents through. These substances cross readily if they are less than 4 nm in size and most pass freely up to 8 nm in size. Negatively charged particles have difficulty leaving the blood because the proteins associated with the filtration membrane are negatively charged, so they tend to repel negatively charged substances and allow positively charged substances to pass more readily. Asa result, the filtrate that does not contain cells or large proteins, and has a slight predominance of positively charged substances.

Proximal Convoluted Tubule (PCT)

Filtered fluid collected by the glomerular capsule enters into the PCT. Simple cuboidal cells form this tubule with prominent microvilli on the luminal surface, forming a brush border. These microvilli create a large surface area to maximize the reabsorption from and secretion of solutes into the filtrate. Reabsorption and secretion are the main functions of the PCT.

Loop of Henle

The descending and ascending limbs of the loop of Henle (sometimes referred to as the nephron loop) are continuations of the same tubule. They run adjacent and parallel to each other after making a hairpin turn at the deepest point of their descent. The descending limb of the loop of Henle consists of an initial short, thick portion and long, thin portion, whereas the ascending loop consists of an initial short, thin portion followed by a long, thick portion. The descending and ascending thick portions consist of simple cuboidal epithelium. The descending and ascending thin portions consists of simple squamous epithelium. These distinct histologies result in different permeabilities for solutes and water in the different portions of the loop. Reabsorption is the main function of the loop of Henle.

Distal Convoluted Tubule (DCT)

The DCT, like the PCT, is formed by simple cuboidal epithelium, but it is shorter than the PCT. These cells are not as active as those in the PCT and there are fewer microvilli on the apical surface. However, the DCT still carries out reabsorption and secretion of select substances according to the levels of certain hormones.

Collecting Ducts

The collecting ducts are continuous with the nephron but not technically part of it. In fact, each duct collects filtrate from several nephrons for final modification. Collecting ducts merge as they descend deeper in the medulla to form about 30 terminal ducts that empty at a papilla. They are lined with simple cuboidal epithelium. Cells called principal cells, found in the collecting duct and in the late portion of the DCT, enable water reabsorption in response to the hormone ADH. ADH is discussed later in this chapter.

Juxtaglomerular Apparatus (JGA)

Image Descriptions

Figure 13.2.1. This detailed anatomical illustration shows the structure of a nephron, the functional unit of the kidney, along with its associated blood supply. At the top, the glomerular capsule (Bowman’s capsule) encloses the glomerulus, a ball of capillaries fed by the afferent arteriole. The filtrate then flows through the proximal convoluted tubule (shown in pale pink), which has a twisted, coiled appearance. The tubule descends into the loop of the nephron (loop of Henle), shown in peachy-orange with a characteristic hairpin bend that dips below the cortex-medulla junction (marked by a horizontal dashed line). The tubule then ascends and becomes the distal convoluted tubule at the top right, eventually draining into the collecting duct, a straight tube extending downward that ultimately carries urine into the renal pelvis. The blood supply is illustrated with the interlobular artery (red) branching from larger vessels on the left, and the interlobular vein (blue) collecting blood via venules. The peritubular capillary network (shown in red) extensively surrounds the tubular structures, allowing for reabsorption and secretion between the blood and the filtrate. This diagram effectively demonstrates the intimate relationship between the nephron’s tubular components and its vascular supply throughout both the renal cortex and medulla. [Return to Figure 13.2.1]

Figure 13.2.2. This anatomical diagram illustrates two types of nephrons in the kidney: a juxtamedullary nephron on the left and a cortical nephron on the right, shown against a pink background representing kidney tissue divided into the cortex (upper region) and medulla (lower region). The juxtamedullary nephron features Bowman’s capsule with its glomerulus at the top, followed by a long loop of Henle that extends deep into the medulla. The cortical nephron, positioned to the right, also shows Bowman’s capsule and glomerulus but has a much shorter loop of Henle that barely penetrates the medulla. Both nephrons display the proximal convoluted tubule (the coiled section near the top), the descending and ascending limbs of the loop of Henle, the distal convoluted tubule, and connecting to a shared collecting duct that runs vertically through both cortex and medulla. The diagram uses purple/maroon coloring for the nephron structures and labels key components including “Loop of Henle” at the bottom. This illustration effectively demonstrates the structural differences between the two nephron types, particularly the varying lengths of their loops of Henle, which relates to their different roles in urine concentration. [Return to Figure 13.2.2]

Figure 13.2.3. This anatomical illustration consists of two parts showing the glomerular capillary structure in the kidney. Image (a) on the left displays a cross-sectional view of the glomerulus, showing the intricate network of capillaries (depicted in red) surrounded by the glomerular capsule (Bowman’s capsule, shown in beige/tan). The capillaries form a complex, intertwined ball-like structure with numerous loops and branches filling the capsular space. Image (b) on the right provides a magnified view of a single glomerular capillary, illustrated as an orange-red tubular structure containing multiple cell bodies (shown as dark red irregular shapes within the capillary wall). The diagram labels the “filtration slits” as narrow spaces between specialized cells, the “cell bodies” of podocytes that wrap around the capillary and identifies the structure as a “capillary.” This detailed view demonstrates how blood filtration occurs through the specialized cells and filtration slits that allow plasma to pass into the capsular space while retaining blood cells and large proteins. [Return to Figure 13.2.3]

Figure 13.2.4. This anatomical illustration shows a three-dimensional cross-sectional view of a capillary and surrounding tissue. The diagram depicts a tubular blood vessel with its internal structure visible in the foreground and extending into the background. The capillary wall is composed of an inner endothelium layer, shown as flattened cells with visible nuclei (depicted in purple) and characterized by tight junctions between adjacent endothelial cells that seal the spaces between them. Small openings called fenestrations are indicated at the bottom of the vessel, representing pores in the endothelial lining. Surrounding the endothelium is the basement membrane, shown as a thin bluish layer that provides structural support. The tissue beyond the capillary is illustrated with a translucent quality, showing cellular components (brown dots representing nuclei) and blood vessels (thin red lines) in the surrounding connective tissue. This diagram effectively demonstrates the microscopic architecture of capillaries and their relationship to adjacent tissues, which is essential for understanding nutrient and gas exchange between blood and tissues. [Return to Figure 13.2.4]

Figure 13.2.5. This figure consists of two parts illustrating the juxtaglomerular apparatus of the kidney nephron. Image (a) on the left is a detailed anatomical diagram showing a cross-section of the glomerulus and surrounding structures. The central glomerulus appears as a ball of tan-colored capillaries enclosed within Bowman’s capsule (shown in beige with a brown outer layer). Key labeled structures include the podocyte (specialized cells wrapping around capillaries at the top), the afferent arteriole (entering from below with a gray arrow), the macula densa (a specialized region of green-colored cells where the distal tubule contacts the arteriole), juxtaglomerular cells (green cells in the arteriole wall), renal nerve fibers (shown in yellow), and the proximal convoluted tubule (extending to the right). Image (b) on the right is a histological photomicrograph stained in pink and purple hues, showing the actual tissue structure with labels pointing to the macula densa (darker staining region of tightly packed cells), glomerulus (cluster of capillaries in the center), proximal convoluted tubule (with prominent brush border), distal convoluted tubule, and basement membrane. The microscopy image provides real-world visualization of the structures depicted schematically in the diagram, demonstrating the close anatomical relationship between the distal tubule and the glomerular arteriole that forms the juxtaglomerular apparatus, which plays a crucial role in regulating blood pressure and kidney function. [Return to Figure 13.2.5]

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