22.2: Overview of the Kidney
- Page ID
- 63508
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \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 external structure and the locations of the kidney
- Identify the major internal divisions and structures of the kidney
- Identify the major blood vessels associated with the kidney and trace the path of blood through the kidney
- Name the regions in a nephron
- Trace the flow of fluid from blood through the kidney tubules
The kidneys lie laterally to the spine in the retroperitoneal space between the parietal peritoneum and the posterior abdominal wall, well protected by muscle, fat, and ribs. Each kidney weighs about 125–175 g in males and 115–155 g in females. They are about 11–14 cm in length, 6 cm wide, and 4 cm thick. This makes them roughly the size of your fist, with the male kidney typically a bit larger than the female kidney. The kidneys are well vascularized, receiving about 25 percent of the cardiac output at rest.
There have never been sufficient kidney donations to provide a kidney to each person needing one. Watch this video Printing A Human Kidney to learn about the TED (Technology, Entertainment, Design) Conference held in March 2011. In this video, Dr. Anthony Atala discusses a cutting-edge technique in which a new kidney is “printed.” The successful utilization of this technology is still several years in the future, but imagine a time when you can print a replacement organ or tissue on demand.
External Anatomy
The left kidney is located at about the level of the T12 to L3 vertebrae, whereas the right is slightly lower due to slight displacement by the liver. Superior portions of the kidneys are somewhat protected by the eleventh and twelfth ribs (Figure \(\PageIndex{1}\)). They are directly covered by a fibrous capsule, composed of dense irregular connective tissue that helps to hold their shape, and are surrounded by a shock-absorbing layer of adipose tissue called the renal fat pad. This adipose tissues is, in turn, encompassed by a tough renal fascia. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the posterior abdominal wall.
Renal Hilum
The renal hilum (Figure \(\PageIndex{2A}\)) is the medial-facing, concave entry and exit site for structures servicing the kidneys: blood vessels, nerves, lymphatics, and ureters. The renal arteries branch directly from the abdominal aorta, whereas the renal veins return blood directly to the inferior vena cava. The artery, vein, and ureters are arranged in an anterior-to-posterior order.
Internal Anatomy
A frontal section through the kidney reveals a thin outer region called the renal cortex and an inner region called the medulla (Figure \(\PageIndex{2A}\)). The renal columns are connective tissue extensions that pass through the medulla to separate the most characteristic features of the medulla: the renal pyramids. Each of the 8-18 lobes within the human kidney contain a pyramid and the cortex immediately superficial to it, and these can be delineated by following the column radiations to the surface of the kidney. The renal columns also provide a supportive framework for vessels that enter and exit the cortex.
Each renal lobe contains microscopic tubules, called nephrons, that filter the blood and produce urine. This urine is gathered by collecting ducts passing from the cortex, through the pyramid and terminating in the renal papilla. The renal papilla is the apex of the pyramid, located at the boundary between the renal medulla and the minor calyx of the renal collecting system (Figure \(\PageIndex{2B}\)). A minor calyx surrounds the renal papilla of each pyramid and collects urine from that pyramid. Several minor calyces converge to form a major calyx and the major calyces merge to form the renal pelvis. The smooth muscle in the renal pelvis funnels urine via peristalsis out of the kidney and into the ureter.
A. Major regions of the kidney 
B. Urine collecting structures from each renal lobe
C. Dissected Pig KidneyBlood Vessels
The renal artery enters the kidney medially, first divides into segmental arteries moving toward the medulla, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex (Figure \(\PageIndex{3}\)). The interlobar arteries, in turn, branch into arcuate arteries lying between the renal cortex and renal medulla. Arcuate arteries will branch further to form interlobular or cortical radiate arteries that extends into the cortex, and then branch into afferent arterioles. Afferent arterioles lead blood to the beginning of the nephron.
Nephrons are the “functional units” of the kidney; they cleanse the blood and balance its plasma constituents. The afferent arterioles have higher blood pressure than typical arterioles, and thus have more muscle tissue in their walls. Each afferent arteriole leads to a tuft of fenestrated capillaries about 200 µm in diameter, called the glomerulus (Figure \(\PageIndex{3}\)). At their distal end the glomerular capillaries form a second arteriole, the efferent arteriole that will next form another capillary network around the more distal portions of the nephron tubule, the peritubular capillaries (in the cortex) and vasa recta (in the medulla). As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water and return them to the circulation. Blood from the second capillary beds then empty into venules. These merge to form interlobular veins, then arcuate veins, interlobar veins, and segmental veins, which finally merge to become the renal vein. (Figure \(\PageIndex{3}\)).
Nephrons
Nephrons are microscopic structures to make urine, thus they are the "functional" units in the kidneys. Each kidney has over 1 million nephrons. Figure \(\PageIndex{4}\) and Figure \(\PageIndex{5}\) show two nephrons and the blood vessels running around them. Each nephron starts in the cortex with the renal corpuscle, which has the glomerular capsule surrounding the glomerulus (capillary bed). The glomerular capsule continues as the proximal convoluted tubule (PCT) which then straightens out and enters the renal medulla as the descending limb of the nephron loop. At various depths within the medulla the nephron loops make a U-turn to become the ascending limb. The nephron loop is also called the loop of Henle. The loop will enter the cortex and become the distal convoluted tubule (DCT), the end of the nephron. The content inside, soon to be urine, will continue from the DCT to a collecting duct.
In a dissected kidney identifying the cortex and medulla is easy, as they have different colors and patterns (Figure \(\PageIndex{2C}\)). Different parts of the nephrons are found in the two regions. All of the renal corpuscles as well as both the proximal convoluted tubules (PCTs) and distal convoluted tubules are found in the cortex. Some nephrons have a short nephron loop that does not dip much beyond the cortex. These nephrons are called cortical nephrons. About 15 percent of nephrons have long nephron loops that extend deep into the medulla and are called juxtamedullary nephrons. Figure \(\PageIndex{4}\) shows two types of nephrons, cortical and juxtamedullary nephrons, named according to their locations. Details of nephron structure will be discussed in the next section, Section 22.3.
Figure \(\PageIndex{5}\): Nephron Structure. Blood in the glomerular capillaries is filtered into the glomerular capsule. This filtrate is modified as it flows through the nephron regions (in order - proximal convoluted tubule, nephron loop, distal convoluted tubule) and the collecting duct. (Image credit: "Nephron Structure" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original "Slagter - Drawing Kidney macroscopic and microscopic anatomy - no labels" by Ron Slagter.)
Concept Review
As noted previously, the structure of the kidney is divided into two principle regions—the peripheral rim of cortex and the central medulla. The two kidneys receive about 25 percent of cardiac output. They are protected in the retroperitoneal space by the renal fat pad and overlying ribs and muscle. Ureters, blood vessels, lymph vessels, and nerves enter and leave at the renal hilum. The renal arteries arise directly from the aorta, and the renal veins drain directly into the inferior vena cava. Kidney function is derived from the actions of about 1.3 million nephrons per kidney; these are the “functional units.” A capillary bed, the glomerulus, filters blood and the filtrate is captured by Bowman’s capsule. A portal system is formed when the blood flows through a second capillary bed surrounding the proximal and distal convoluted tubules and the loop of Henle. Most water and solutes are recovered by this second capillary bed. This filtrate is processed and finally gathered by collecting ducts that drain into the minor calyces, which merge to form major calyces; the filtrate then proceeds to the renal pelvis and finally the ureters.
Review Questions
Query \(\PageIndex{1}\)
Critical Thinking Questions
Query \(\PageIndex{2}\)
Query \(\PageIndex{3}\)
Glossary
Query \(\PageIndex{4}\)
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology