13.2 Microscopic Anatomy of the Kidney: Anatomy of the Nephron

Leslie Bach; Nour Al-muhtasib; Leslie King; Nicole Thometz

Chapter 13. The Urinary System

13.2 Microscopic Anatomy of the Kidney: Anatomy of the Nephron

Learning Objectives

By the end of this section, you will be able to:

list the main parts of the nephron tubule and describe the order in which filtrate passes through them;
compare and contrast cortical nephrons and juxtamedullary nephrons;
describe the components of and function of the renal corpuscle;
describe the structure of the filtration membrane;
describe the structure and function of the juxtaglomerular apparatus; and
describe the histology of and functional significance of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting ducts.

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.

Figure 13.2.1 shows the main parts of a nephron:

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).

This image shows the blood vessels and the direction of blood flow in the nephron. — **Figure 13.2.1 – Nephron Components:** The nephron tubules consist of the glomerular capsule (Bowman’s capsule), proximal convoluted tubule (PCT), nephron loop (loop of Henle), distal convoluted tubule, and collecting duct.

This image illustrates the difference in position of cortical and juxtamedullary nephrons. The loop of Henle of a juxtamedullary nephron extends deep into the renal medulla, while the majority of a cortical nephron is in the renal cortex. — **Figure 13.2.2 – Cortical and Juxtamedullary Nephrons:** The loop of Henle of a juxtamedullary nephron extends deep into the renal medulla, while the majority of a cortical nephron is in the renal cortex. Approximately 15% of nephrons in the kidneys are juxtamedullary nephrons, and the remaining 85% are cortical nephrons. (credit: Holly Fischer, CC BY 3.0 in Wikimedia Commons)

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 left panel of this figure shows an image of a podocyte. The right panel shows a tube-like structure that illustrates the filtration slits and the cell bodies. — **Figure 13.2.3 – Podocytes:** Podocytes interdigitate with structures called pedicels and filter substances into the glomerular capsule. In (a), the large cell body of a podocyte can be seen at the top right corner, with branches extending from the cell body. The smallest finger-like extensions are the pedicels. The interdigitation of podocyte branches to form filtration slits is seen in (b).

The top panel of this figure shows a tube-like structure with the basement membrane and other parts labeled. — **Figure 13.2.4 – Fenestrated Capillary:** Fenestrations allow many substances to leave the blood based primarily on size.

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)

Lying just outside the glomerular capsule and the glomerulus is the juxtaglomerular apparatus (JGA) (Figure 13.2.5). The JGA is comprised of the macula densa, juxtaglomerular cells, and mesangial cells. Here we focus on the macula densa and juxtaglomerular cells.

At the juncture where the afferent and efferent arterioles enter and leave Bowman’s capsule, the initial part of the distal convoluted tubule (DCT) comes into direct contact with the afferent and efferent arterioles. The wall of the DCT at that point forms a part of the JGA known as the macula densa. This cluster of cuboidal epithelial cells monitors the composition of filtrate flowing through the DCT. In response to the concentration of Na⁺ in the filtrate flowing past them, these cells release paracrine signals. They also have single, nonmotile cilia that respond to the rate of filtrate flow in the tubule. The paracrine signals released in response to changes in flow rate and Na⁺ concentration are adenosine triphosphate (ATP) and adenosine.

A second function of the macula densa cells is to regulate renin release from the juxtaglomerular cells (granular cells) of the afferent arteriole.

The top panel of this image shows the cross section of the juxtaglomerular apparatus. The major parts are labeled. — **Figure 13.2.5 – Juxtaglomerular Apparatus and Glomerulus:** (a) The JGA allows specialized cells to monitor the composition of the fluid in the DCT and adjust the glomerular filtration rate. (b) This micrograph shows the glomerulus and surrounding structures (LM × 1540). (credit: Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Juxtaglomerular (JG) cells, or granular cells, are modified smooth muscle cells lining the afferent arteriole that can contract or relax in response to ATP or adenosine released by the macula densa. Such contraction and relaxation regulate blood flow to the glomerulus. Juxtaglomerular cells also produce renin which initiates a cascade of events to control systemic blood pressure, to be discussed later.

Section Review

The functional unit of the kidney, the nephron, consists of the renal corpuscle, PCT, loop of Henle, and DCT. Cortical nephrons have short loops of Henle, whereas juxtamedullary nephrons have long loops of Henle extending into the medulla. About 15% of nephrons are juxtamedullary. The glomerulus is a capillary bed that filters blood principally based on particle size. A filtration membrane is formed by the fused basement membranes of the podocytes and the glomerular capillary endothelial cells. The formed filtrate enters the glomerular capsule followed by the PCT.

Specialized cells in the juxtaglomerular apparatus (JGA) produce paracrine signals to regulate blood flow and filtration rates of the glomerulus. Other JGA cells produce the enzyme renin, which plays a central role in blood pressure regulation. The filtrate enters the PCT where reabsorption and secretion of several substances occur. The descending and ascending limbs of the loop of Henle consist of thick and thin segments. Reabsorption and secretion continue in the DCT but to a lesser extent than in the PCT. Each collecting duct collects urine from several nephrons and functions to fine tune water reabsorption.

Review Questions

Critical Thinking Questions

Glossary

brush border: formed by microvilli on the surface of certain cuboidal cells; in the kidney it is found in the PCT; increases surface area for absorption in the kidney

fenestrations: small pores in a cell, allowing rapid filtration based on size

filtration slits: spaces formed by pedicels of podocytes; along with fenestrations, allow filtration to occur

filtrate: a plasma-like liquid (lacking cells and most proteins) formed during filtration

juxtaglomerular apparatus (JGA): located at the juncture of the DCT and the afferent and efferent arterioles of the glomerulus; plays a role in the regulation of renal blood flow and GFR

juxtaglomerular cells: modified smooth muscle cells of the afferent arteriole; secrete renin in response to a drop in blood pressure; also called granular cells

macula densa: cells found in the part of the DCT forming the JGA; sense Na⁺ concentration in the forming urine

pedicels: finger-like projections of podocytes surrounding glomerular capillaries; interdigitate to form a filtration membrane

podocytes: cells forming finger-like processes; form the visceral layer of Bowman’s capsule; pedicels of the podocytes interdigitate to form a filtration membrane

principal cell: cell type found in the collecting duct and in the late portion of the DCT; under the control of ADH, are responsible for water reabsorption

Glossary Flashcards

This work, Human Physiology, is adapted from Anatomy & Physiology by OpenStax, licensed under CC BY. This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images from Anatomy & Physiology by OpenStax are licensed under CC BY except where otherwise noted.

Access the original for free at OpenStax.

Report an Error

Did you find an error, typo, broken link, or other problem in the text? Please follow this link to the error reporting form to submit an error report to the authors.

License

Icon for the Creative Commons Attribution-ShareAlike 4.0 International License

Human Physiology Copyright © by Leslie Bach, Nour Al-muhtasib, Leslie King, and Nicole Thometz is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.