22.3: Microscopic Anatomy of the Kidney
- Page ID
- 63509
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Describe the structure of the filtration membrane
- Explain the major structures and subdivisions of the renal corpuscles and renal tubules
- Describe the histology of the proximal convoluted tubule, nephron loop, distal convoluted tubule, and collecting duct
The renal structures that conduct the essential work of the kidney cannot be seen by the naked eye. Only a light or electron microscope can reveal these structures. Even then, serial sections and computer reconstruction are necessary to give us a comprehensive view of the functional anatomy of the nephron and its associated blood vessels.
Nephrons: The Functional Unit
Nephrons (Figure \(\PageIndex{1}\)) take a simple filtrate of the blood and modify it into urine. Many changes take place in the different parts of the nephron before urine is created for disposal. The term forming urine will be used hereafter to describe the filtrate as it is modified into true urine. The principle task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three principle functions—filtration, reabsorption, and secretion. They also have additional secondary functions that exert control in three areas: blood pressure (via production of renin), red blood cell production (via the hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, the active form of vitamin D).
Different parts of the nephron are found in different regions. The renal cortex is easily distinguished from the renal medulla by the presence of renal corpuscles. These easily visualized balls contain the glomerular capillaries and the glomerular capsule. Around the corpuscles, the arrangement of the tubules in the cortex appears to be random. That is because the convoluted tubules twist, turn, and wrap around themselves and the renal corpuscles. Filtrate leaves the glomerular capillaries and enters the capsular space and then flows into the proximal convoluted tubule.
- Proximal convoluted tubules (PCT) are the larger tubules in the region composed of simple cuboidal epithelium. The cells of the PCT are larger and have abundant microvilli making up a brush border.
- The distal convoluted tubule is also composed of simple cuboidal epithelium. It has a smaller diameter and smaller cells than the PCT and the cells have few microvilli.
The renal medulla lacks renal corpuscles, so visually it only contains long tubules that run mostly parallel to each other. The two regions of the tubule found here both have two types of epithelial tissue in their walls.
The nephron loop is a part of the nephron that has thin and thick limbs. The thin descending nephron loop and the thin ascending nephron loop will be made up of simple squamous epithelium. The thick ascending nephron loop is made of simple cuboidal epithelium that lacks a brush border.
- The collecting duct (technically not part of the nephron - see below) starts as a simple cuboidal epithelium and transitions to become a simple columnar epithelium as it nears the renal papilla.
Renal Corpuscle
As discussed earlier, the renal corpuscle consists of a tuft of capillaries called the glomerulus that is largely surrounded by the glomerular (Bowman's) capsule. The glomerulus is a high-pressure capillary bed between afferent and efferent arterioles. The glomerular capsule surrounds the glomerulus to form a lumen, and captures and directs this filtrate to the PCT. The outermost part of the glomerular capsule, the parietal layer, is a simple squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule. Here, the cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like arms (pedicels) to cover the glomerular capillaries (Figure \(\PageIndex{2}\)). These projections interdigitate to form filtration slits, leaving small gaps between the digits to form a sieve. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters between these sieve-like fingers to be captured by the glomerular capsule and funneled to the PCT. Where the fenestrae (windows) in the glomerular capillaries match the spaces between the podocyte “fingers,” the only thing separating the capillary lumen and the lumen of the glomerular capsule is their shared basement membrane (Figure \(\PageIndex{3}\)). These three features (fenestrated endothelium, basement membrane, and pedicels) comprise what is known as the filtration membrane. This membrane permits very rapid movement of filtrate from capillary to capsule.
The fenestrations prevent filtration of blood cells or large proteins, but allow 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. An additional factor affecting the ability of substances to cross this barrier is their electric charge. The proteins associated with these pores are negatively charged, so they tend to repel negatively charged substances and allow positively charged substances to pass more readily. The basement membrane prevents filtration of medium-to-large proteins such as globulins. Overall, filtration is regulated by fenestrations in capillary endothelial cells, podocytes with filtration slits, membrane charge, and the basement membrane between capillary cells. The result is the creation of a filtrate that does not contain cells or large proteins, and has a slight predominance of positively charged substances.
Proximal Convoluted Tubule
Filtered fluid collected by the glomerular capsule enters into the proximal convoluted tubule. It is called convoluted due to its contorted path. The simple cuboidal cells forming this tubule with prominent microvilli on the luminal surface, forming a brush border. These microvilli create a large surface area to maximize the reabsorption and secretion of solutes (Na+, Cl–, glucose, etc.), the most essential function of this portion of the nephron. These cells actively transport ions across their membranes so they possess a large number of mitochondria in order to produce sufficient ATP and abundant rough endoplasmic reticulum (rER) and Golgi apparatus to produce the protein transporters. Because of the large number of organelles, these cells are larger than a typical cuboidal epithelial cell and their cytoplasm stains more readily (Figure \(\PageIndex{4}\)).
Nephron Loop
The nephron tubule continues and becomes the descending and ascending portions of the nephron loop (of Henle). These limbs run adjacent and parallel to each other as they make a hairpin turn at the deepest point of their descent. The descending limb consists of an initial short, thick portion and long, thin portion. The ascending loop consists of an initial short, thin portion followed by a long, thick portion.
- The descending thick portion consists of simple cuboidal epithelium similar to that of the PCT.
- The descending and ascending thin portions consists of simple squamous epithelium (Figure \(\PageIndex{4}\)).
- The ascending thick portion consists of simple cuboidal epithelium similar to the DCT.
These variations in cell type are important, since different portions of the loop have different permeabilities for solutes and water. The thin portions exchange salts and water by passive diffusion, while the thick portions utilize active transport.
Distal Convoluted Tubule
The distal convoluted tubule, like the PCT, is very contorted and formed by simple cuboidal epithelium. These cells are not as active in reabsorption and secretion as those in the PCT, thus less surface are is needed. Accordingly, the DCT is shorter than the PCT and its cells have fewer microvilli on the apical surface. However, these cells must also pump ions against their concentration gradient, so you will find of large numbers of mitochondria, rER, and Golgi apparatus, although fewer than in the PCT. The functional difference results in an alteration of the structure, which can be seen on a slide. The DCT has cuboidal epithelial cells that are smaller and lighter staining than the PCT and their luminal border is less fuzzy and cleaner appearing (Figure \(\PageIndex{4}\)).
Collecting Duct System
Connecting tubules are the initial segment of the collecting duct system. Located in the cortex they area continuous with the nephron but not technically part of it in humans, as recent research (Sarkany & Kovacs, 2021) has determined their embryonic origin is shared with the ureters and not with the kidney proper. Each connecting tubule connects a single nephron to a collecting duct and each collecting duct gathers filtrate from several nephrons for final modification. Collecting ducts start in the cortex and shortly enter the medulla. As they descend deeper in the medulla they merge to form about 30 terminal ducts, which empty at a papilla into the minor calyx.
Collecting ducts initially are lined with simple cuboidal epithelial cells (Figure \(\PageIndex{4}\)) with receptors for ADH. When stimulated by ADH, these cells will insert aquaporin channel proteins into their membranes, which as their name suggests, allow water to pass from the duct lumen through the cells and into the interstitial spaces to be recovered by the vasa recta (Figure \(\PageIndex{1}\)). This process allows for the recovery of large amounts of water from the filtrate back into the blood. In the absence of ADH, these channels are not inserted, resulting in the excretion of water in the form of dilute urine. Deeper in the medulla, simple columnar epithelial cells become more numerous and increase in size. These cells primarily function in acid-base balance through the reabsorption of bicarbonate (HCO3-) and secretion of hydrogen ions (H+).
Juxtaglomerular Apparatus
Lying just outside the glomerular capsule and the glomerulus is the juxtaglomerular apparatus (JGA). At the juncture where the afferent and efferent arterioles enter and leave the glomerular capsule capsule, the initial part of the distal convoluted tubule (DCT) comes into direct contact with the 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 fluid composition of fluid flowing through the DCT (Figure \(\PageIndex{5}\)).
A second cell type in this apparatus is the juxtaglomerular cell. This is a modified, smooth muscle cell 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. A second function of the macula densa cells is to regulate renin release from the juxtaglomerular cells of the afferent arteriole. Renin and hormones such as antidiuretic hormone (ADH) and aldosterone regulate blood pressure by adjusting the amount of water retained by the kidneys.
Renal Medulla
The renal medulla is deep to the cortex. It contains several parallel tubular structures that are involved in the retention of water and ions. These structures, some of which were describe above (Figure \(\PageIndex{4}\)), include:
- Collecting ducts. Composed of lightly stained simple cuboidal cells with clear borders.
- Ascending thick limb of nephron loop.Darker staining cells with a less clear border.
- Thin segments of nephron loop. Simple squamosal cells whose nuclei are observed to protrude into the lumen. (No observable blood cells; if blood cells are seen this is a capillary!)
- Vasa recta. Capillaries containing numerous blood cells. (Not highlighted on slide).
Concept Review
The functional unit of the kidney, the nephron, consists of the renal corpuscle, PCT, loop of Henle, and DCT. The glomerulus is a capillary bed that filters blood principally based on particle size. The filtrate is captured by Bowman’s capsule and directed to the PCT. A filtration membrane is formed by the fused basement membranes of the podocytes and the capillary endothelial cells that they embrace. Contractile mesangial cells further perform a role in regulating the rate at which the blood is filtered. Specialized cells in the 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 absorption and secretion of several substances occur. The descending and ascending limbs of the loop of Henle consist of thick and thin segments. Absorption and secretion continue in the DCT but to a lesser extent than in the PCT. Each collecting duct collects forming urine from several nephrons and responds to the posterior pituitary hormone ADH by inserting aquaporin water channels into the cell membrane to fine tune water recovery.
Review Questions
Q. Blood filtrate is captured in the lumen of the ________.
A. glomerulus
B. glomerular capsule
C. calyces
D. renal papillae
- Answer
-
Answer: B
Q. Which is least associated with the renal corpuscle?
A. podocyte
B. fenestration
C. cuboidal epithelium
D. pedicels
- Answer
-
Answer: C
Q. Which is least associated with the juxtaglomerular apparatus?
A. afferent arteriole
B. distal convoluted tubule
C. proximal convoluted tubule
D. macula densa
- Answer
-
Answer: C
Critical Thinking Questions
Q. Why is it abnormal to have blood in urine if the woman is not menstruating?
- Answer
-
A. Red blood cells are too big to pass through the fenestrations from the glomerulus into the glomerular capsule.
Q. What are the major structures comprising the filtration membrane?
- Answer
-
A. The major structures comprising the filtration membrane are fenestrations and filtration slits.
Glossary
- aquaporin
- protein-forming water channels through the lipid bilayer of the cell; allows water to cross; activation in the collecting ducts is under the control of ADH
- 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 windows through a cell, allowing rapid filtration based on size; formed in such a way as to allow substances to cross through a cell without mixing with cell contents
- filtration slits
- formed by pedicels of podocytes; substances filter between the pedicels based on size
- forming urine
- filtrate undergoing modifications through secretion and reabsorption before true urine is produced
- 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 cell
- modified smooth muscle cells of the afferent arteriole; secretes renin in response to a drop in blood pressure
- 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
- renin
- enzyme produced by juxtaglomerular cells in response to decreased blood pressure or sympathetic nervous activity; catalyzes the conversion of angiotensinogen into angiotensin I
Contributors and Attributions
- Sarkany B, Kovacs G. Connecting tubules develop from the tip of the ureteric bud in the human kidney. Histochem Cell Biol. 2021 Dec;156(6):555-560. doi: 10.1007/s00418-021-02033-5. Epub 2021 Sep 23. PMID: 34554322.
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology