UNIT 1: RENAL ANATOMY
UNIT OBJECTIVES:
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Identify and state primary functions of the macroscopic structures of the kidney.
Describe the structure and state general function of each portion of the nephron.
Trace renal blood circulation and its role in renal function
ENGAGE – RECALL
THE URINARY SYSTEM
What is the first thing that comes to mind when you recall this organ system based on your
background in basic anatomy and physiology?
The urinary system disposes most metabolic wastes
produced by the body. It performs this function by removing
these wastes from blood and excreting them into urine.
Disposal of wastes through the release of urine is not the
only purpose of the urinary system. It also helps regulate
blood composition, pH, volume, and pressure; maintains
blood osmolarity; and produces hormones.
The urinary system consists of kidneys, ureter,
urinary bladder and urethra. However, most of you might
remember the kidney as it is the main organ responsible in
urine formation.
Figure 1.1: The Urinary System
EXPLORE - UNLOCKING CONCEPTS
ANATOMY OF THE KIDNEY
Kidneys are paired, reddish, bean-shaped organs located retroperitoneally (lies behind
the peritoneum in the abdominal cavity), with the right kidney being slightly lower because the
liver occupies considerable space superior to it. It is about 10 – 12 cm in length, 5 – 7 cm
wide and 3 cm thick with an average mass of 135 – 150 g. If we liken it to an ordinary object,
it is about the size of a bar soap. Each kidney has a characteristic indentation at the concave
side called renal hilum from which the ureter, blood vessels and nerves emerge.
There are three (3) layers of tissue covering each kidney (as shown in Figure 1.2). The
renal fascia is the superficial layer of the kidney composed of dense irregular connective
tissue that anchors each kidney to the surrounding organs. Beneath the renal fascia is the
adipose capsule made up of fatty tissue that protects the kidney. The innermost layer is called
the renal capsule made up of dense irregular tissue that maintains shape of the kidney.
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Figure 1.2: Transverse Section of the Abdomen
Cross-sectionally, there are two distinct areas of the kidney that can be observed: the
renal cortex and renal medulla. The renal cortex is the superficial layer with a granular
appearance when observed macroscopically and light red in Figure 1.3. The outer cortex is
considered as the exclusive site for the plasma filtration process since all of the glomeruli are
located here.
The renal medulla is the dark red inner layer of the kidney consisting of renal tissue. A
portion of the renal cortex that extends into the renal medulla are called renal columns
dividing the medulla into renal pyramids. Anatomically, these structures can be grouped as
renal lobe consisting of a renal pyramid, the overlying renal cortex and half of the adjacent
renal column. The renal parenchyma is the functional portion of the kidney comprised of the
renal cortex and renal medulla only.
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Figure 1.3: Cross section of the Kidney
The apex of each renal pyramid is called renal papilla that drains urine into a cavity
called renal calyx. Minor calyces that receive urine from the renal papillae joins together,
forming two to three major calyces that drains the urine to the funnel-shaped renal pelvis.
Eventually, the renal pelvis narrows and joins the ureter to facilitated excretion of urine.
Can you visualize the path of urine drainage using parts mentioned in this section?
EXPLAIN – LET’S GET INTO DETAILS
NEPHRON AS A FUNCTIONAL UNIT
There are approximately 1.3 million nephrons in each kidney that facilitate urine
formation. Majority (85%) of these nephrons are cortical nephrons located in the renal cortex
which primarily removes waste and reabsorb nutrients. Juxtamedullary nephrons are located
closer to the medulla and primarily concentrates urine. Structurally, juxtamedullary nephrons
have a longer loop of Henle that extend into the renal medulla.
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Figure 1.4: Two Types of Nephrons
The nephron is divided into three (3) regions, namely; renal corpuscle, renal tubule and
collecting duct.
A. Renal Corpuscle
The renal corpuscle is comprised of the glomerulus and Bowman’s capsule. The
glomerulus is a network of fenestrated capillaries surrounded by a thin epithelial layer called
the Bowman’s capsule from which the renal tubules originate. Blood is filtered by the
glomerulus and the initial low molecular weight plasma ultrafiltrate is collected in Bowman’s
space which then flows into the renal tubules.
The renal corpuscle has four distinct structures that facilitate filtration: (a)the
mesangium, consisting of mesangial cells of the capillaries; (b) fenestrated endothelial cells
of the capillaries; (c) podocytes or visceral epithelial cells of the capillaries and; (d) a threelayered basement membrane between podocytes and the mesangium.
Mesangial cells in the mesangium are contractile components capable of removing
entrapped macromolecules in from the filtration barrier via phagocytosis and
pinocytosis. Its contractility contributes in controlling blood flow in the glomerulus.
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Figure 1.5: Glomerular Filtration Barrier
Fenestrated capillaries have large open pores (50 to 100 nm) and a negativelycharged coating that repels anionic (negatively charged) molecules which confers
solute selectivity during filtration.
Podocytes are foot-like structures that completely cover the glomerular capillaries
with their finger-like projections that interdigitate with each other forming snakelike
channels called filtration slits. These slits are lined with an extracellular structure
called slit diaphragm that filters albumin from the plasma. Macromolecules that are
unable to proceed through the slit are rapidly phagocytized by podocytes to prevent
occlusion of the filtration barrier.
The basement membrane separates the epithelium of the urinary space from the
endothelium of the glomerular capillaries made up of three (layers): lamina rara,
lamina densa and lamina rara externa. These three layers contributes specifically to
the permeability of the glomerular filtration barrier due to heparan sulfate that confers
a negative charge in the structure.
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Figure 1.6: Renal Corpuscle and Juxtaglomerular Apparatus
Situated in the vascular pole of the glomerulus is the juxtaglomerular apparatus
comprised of juxtaglomerular cells of the afferent and efferent arterioles, extraglomerular
mesangial cells and specialized cells in the distal convoluted tubule called macula densa. This
apparatus, particularly the macula densa, detects and responds to changes in glomerular
pressure. In response, the juxtaglomerular cells in the arterioles release renin as a regulatory
substance.
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B. Renal Tubules
The renal tubule is comprised of the proximal tubule (convoluted and straight), Loop of
Henle (ascending and descending) and distal tubule (convoluted and straight). The
epithelium that lines the renal tubules changes throughout the distinct areas of the
nephron.
The proximal convoluted tubule (PCT) begins at
the glomerulus and is lined with simple cuboidal to low
columnar epithelium that interdigitate with each other
to increase surface area. The cells of the epithelium also
have microvilli to facilitate reabsorption and also
numerous mitochondria necessary for active transport
of various solutes. This tubule eventually straightens to
become straight proximal tubule then turns downward
into the medulla to become the Loop of Henle.
The Loop of Henle has distinct anatomical areas
which includes the thin descending limb, thin ascending
limb and thick ascending limb. The thin descending and
ascending limb of the loop of Henle have simple
squamous epithelium but differs in permeability due to
the lack of interdigitation between epithelial cells along
the descending limb. The thick ascending limb has
simple columnar to low columnar epithelium that also
interdigitate similar to those found in PCT. Upon
reentering the cortex, it becomes the straight distal
tubule which eventually becomes the distal convoluted
tubule (DCT).
The distal convoluted tubule is predominantly lined with
simple cuboidal epithelium. The latter portion is also
lined with simple cuboidal epithelium (without
interdigitations).
Figure 1.7: Renal Tubular Epithelium
C.
Collecting ducts
The collecting ducts serve as the site for final urine
concentration. It is lined with simple cuboidal epithelium
without interdigitations, principal cells and intercalated
cells. Principal cells are receptors for the anti-diuretic
hormone (ADH) and aldosterone. Intercalated cells are
involved in the regulation of blood pH. In the presence
of antidiuretic hormone, the spaces between the cells
dilate, making it more permeable to water.
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ELABORATE – CONNECTING CONCEPT
RENAL BLOOD CIRCULATION
In order to regulate the internal environment of the body, the kidneys require a rich
blood supply. Each kidney is supplied by a single renal artery that originates from the aorta.
The renal artery successively subdivides and forms a distinct vascular arrangement that is
suitable for the function of the kidney. In fact, it is the only organ in which an arteriole
subdivides into a capillary bed and becomes an arteriole again. Aside from that, renal
arterioles also terminal arteries that supply blood to specific areas and do not interconnect.
Figure 1.8: Renal Blood flow in relation to the Nephron
Approximately 20-25 % of the blood that leaves the left ventricle of the heart enters the kidneys
via the renal arteries. In normal adult, the blood passes through the kidneys at a rate of about 1200
mL per minute (or 600 mL/min/kidney) and renal plasma flow of about 600-700 ml/min.
The pathway of blood in the kidney begins in the renal artery down to the segmental artery,
interlobar artery, arcuate artery, interlobular artery and afferent arteriole. The afferent arteriole
supplies blood to the glomerulus of each nephron and branches into a capillary bed. This capillary
network eventually becomes the efferent arteriole which leaves the glomerulus and branches into
fine capillary plexus called peritubular capillaries. The vascular arrangement of peritubular capillaries
depends on the type of nephron. Cortical nephrons have a shorter loop of Henle and therefore has
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peritubular capillaries that wrap around its tubules entirely. Meanwhile, juxtamedullary nephrons have
a longer loop of Henle, hence, the peritubular capillaries divide into a series of long U-shaped vessels
called vasa recta which descend deep into the medulla to cover the Loop of Henle.
From the peritubular capillaries, blood flows into the interlobular vein then to the arcuate vein,
interlobar vein and renal vein, back into the circulation.
Renin plays a major role in regulating blood flow. In response to decreased arterial pressure
and decreased blood flow into the kidneys, the juxtaglomerular cells in the arterioles release renin
which facilitates conversion of angiotensinogen to Angiotensin I that eventually becomes
Angiotensin II. Angiotensin II regulates renal blood flow by dilating the afferent arterioles and
constricting the efferent arterioles and trigger release of aldosterone and ADH/Vasopressin.
NOTE:
1. Since the vasa recta are located adjacent to the
ascending and descending loop of Henle in
juxtamedullary nephrons, it is in this area where
major exchanges of water and salts take place
between the blood and the medullary
interstation. This exchange maintains the
osmotic gradient (salt concentration) in the
medulla, which is necessary for renal
concentration.
2. The proximity of the renal tubules and
peritubular capillaries allow processing and
exchange of solutes between the ultrafiltrate of
plasma (tubular lumen) and blood (capillary
lumen) throughout the nephron.
3. The varying sizes of these arterioles help to
create the hydrostatic pressure important for
glomerular
filtration
and
to
maintain
consistency of glomerular capillary pressure
and renal blood flow within the glomerulus.
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EVALUATE – ACTIVITY ALERT!
PATHWAY OF URINE FORMATION
This activity is equivalent to 5 points.
INSTRUCTIONS: Given the following anatomical structures, trace the pathway of urine
production by the kidneys, designating “1” for the first step and “15” for the last step.
_____ Ascending loop of Henle
_____ Bowman’s capsule
_____ Collecting ducts
_____ Descending loop of Henle
_____ Distal convoluted tubules
_____ Glomerulus
_____ Major calyx
_____ Minor calyx
_____ Renal Papillae
_____ Renal pelvis
_____ Ureter
_____ Urethra
_____ Urethral orifice
_____ Urinary bladder
_____ Proximal Convoluted Tubules
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