Internal Anatomy of the Kidneys

A frontal section through the kidney reveals two distinct regions:

a superficial, light red area called the renal cortex and a deep, darker reddish-brown inner region called
the renal medulla. The renal medulla consists of several cone-shaped renal pyramids. The
base (wider end) of each pyramid faces the renal cortex, and its
apex (narrower end), called a renal papilla, points toward the
renal hilum. The renal cortex is the smooth-textured area extending from the renal capsule to the bases
of the renal pyramids and into the spaces between them. It is divided into an outer cortical
zone and an inner juxtamedullary zone. Those portions of the renal cortex that extend between renal
pyramids are called renal columns. A renal lobe consists of a
renal pyramid, its overlying area of renal cortex, and one-half of
each adjacent renal column.
Together, the renal cortex and renal pyramids of the renal
medulla constitute the parenchyma or functional portion of the kidney. Within the parenchyma are the
functional units of the kidney—about 1 million microscopic
structures called nephrons. Filtrate formed by the nephrons drains into large papillary ducts, which
extend through the renal papillae of the pyramids. The papillary
ducts drain into cuplike structures called minor and major calyces. Each
kidney has 8 to 18 minor calyces and 2 or 3 major calyces. A
minor calyx receives urine from the papillary ducts of one renal
papilla and delivers it to a major calyx. Once the filtrate enters the
calyces it becomes urine because no further reabsorption can
occur. The reason for this is that the simple epithelium of the
nephron and ducts becomes transitional epithelium in the calyces.
From the major calyces, urine drains into a single large cavity
called the renal pelvis and then out through the
ureter to the urinary bladder.
The hilum expands into a cavity within the kidney called the
renal sinus, which contains part of the renal pelvis, the calyces,
and branches of the renal blood vessels and nerves. Adipose
tissue helps stabilize the position of these structures in the renal
sinus.
The Nephron
Parts of a Nephron
Nephrons are the functional units of the kidneys.
Each nephron consists of two parts: a renal corpuscle, where blood plasma is filtered,
and a renal tubule into which the filtered fluid passes. The
two components of a renal corpuscle are the glomerulus (capillary network) and the glomerular
(Bowman’s) capsule, a double-walled epithelial cup that surrounds the
glomerular capillaries. Blood plasma is filtered in the glomerular
capsule, and then the filtered fluid passes into the renal tubule,
which has three main sections. In the order that fluid passes through
them, the renal tubule consists of a (1) proximal convoluted tubule or PCT, (2) loop of Henle (nephron
loop), and (3) distal convoluted tubule or DCT. The renal corpuscle
and both convoluted tubules lie within the renal cortex; the loop
of Henle extends into the renal medulla, makes a hairpin turn, and
then returns to the renal cortex. The distal convoluted tubules of several nephrons empty into a
single collecting duct. Collecting ducts then unite and converge
into several hundred large papillary ducts, which
drain into the minor calyces. The collecting ducts and papillary
ducts extend from the renal cortex through the renal medulla to
the renal pelvis. So one kidney has about 1 million nephrons, but
a much smaller number of collecting ducts and even fewer papillary ducts.
In a nephron, the loop of Henle connects the proximal and distal
convoluted tubules. The first part of the loop of Henle dips into the
renal medulla, where it is called the descending limb of the loop
of Henle. It then makes that hairpin turn and returns
to the renal cortex as the ascending limb of the loop of Henle.

Types of a Nephron
About 80–85% of the nephrons are cortical nephrons. Their renal corpuscles lie in the outer portion of
the renal cortex, and they have short loops of Henle that lie mainly in the cortex
and penetrate only into the outer region of the renal medulla. The short loops of Henle receive their blood
supply from peritubular capillaries that arise from efferent arterioles.
The other 15–20% of the nephrons are juxtamedullary nephrons. Their renal corpuscles lie
deep in the cortex, close to the medulla, and they have a long loop of Henle that extends into the deepest
region of the medulla. Long loops of Henle receive their blood supply from peritubular capillaries and
from the vasa recta that arise from efferent arterioles. In addition, the ascending limb of the loop of Henle
of juxtamedullary nephrons consists of two portions: a thin ascending limb followed by a thick
ascending limb. The lumen of the thin ascending limb is the same as in other areas of the
renal tubule; it is only the epithelium that is thinner. Nephrons with
long loops of Henle enable the kidneys to excrete very dilute or
very concentrated urine.
Fig: Cortical nephron
 Fig: Juxtamedullary nephron

Fig: The nephron tubules and associated blood vessels.

Formation of Urine
Urine formation is a blood cleansing function. Normally,
about 1,300 mL of blood (26% of cardiac output) enters
the kidneys. Kidneys excrete the unwanted substances
along with water from the blood as urine. Normal urinary
output is 1 L/day to 1.5 L/day.

The processes of urine formation are divided

into these steps:

1. Glomerular filtration,
2. Tubular reabsorption, and
3. Tubular secretion.

Fig: Events of Urine formation.

1. GLOMERULAR FILTRATION
Glomerular filtration is the process by which the blood is
filtered while passing through the glomerular capillaries
by filtration membrane. It is the first process of urine
formation. The structure of filtration membrane is well
suited for filtration.
Filtration Membrane
Filtration membrane is formed by three layers:
i. Glomerular capillary membrane
ii. Basement membrane
iii. Visceral layer of Bowman capsule.
i. Glomerular capillary membrane
Glomerular capillary membrane is formed by single
layer of endothelial cells, which are attached to the
basement membrane. The capillary membrane has
many pores called fenestrae or filtration pores with a
diameter of 0.1 µ.

ii. Basement membrane

Basement membrane of glomerular capillaries and
the basement membrane of visceral layer of Bowman
capsule fuse together. The fused basement membrane
separates the endothelium of glomerular capillary and
the epithelium of visceral layer of Bowman capsule.

iii. Visceral layer of Bowman capsule

This layer is formed by a single layer of flattened epithelial cells resting on a basement membrane. Each
cell is connected with the basement membrane by
cytoplasmic extensions called pedicles or feet. Epithelial
cells with pedicles are called podocytes (Refer to Fig.
49.4). Pedicles interdigitate leaving small cleft-like
spaces in between. The cleft-like space is called slit
pore or filtration slit. Filtration takes place through
these slit pores.

Process of Glomerular Filtration

When blood passes through glomerular capillaries,
the plasma is filtered into the Bowman capsule. All the
substances of plasma are filtered except the plasma
proteins. The filtered fluid is called glomerular filtrate.
Glomerular filtration is called ultrafiltration because even
the minute particles are filtered. But, the plasma proteins
are not filtered due to their large molecular size. The
protein molecules are larger than the slit pores present
in the endothelium of capillaries. Thus, the glomerular
filtrate contains all the substances present in plasma
except the plasma proteins.

2. TUBULAR REABSORPTION
Tubular reabsorption is the process by which water and
other substances are transported from renal tubules
back to the blood. When the glomerular filtrate flows
through the tubular portion of nephron, both quantitative
and qualitative changes occur. Large quantity of water
(more than 99%), electrolytes and other substances
are reabsorbed by the tubular epithelial cells. The
reabsorbed substances move into the interstitial fluid
of renal medulla. And, from here, the substances move
into the blood in peritubular capillaries.
Since the substances are taken back into the blood
from the glomerular filtrate, the entire process is called
tubular reabsorption. Tubular reabsorption is known as selective reabsorption
because the tubular cells reabsorb only the substances
necessary for the body. Essential substances such
as glucose, amino acids and vitamins are completely
reabsorbed from renal tubule.

3. TUBULAR SECRETION
Tubular secretion is a second way by which substances are removed from blood and actively transported into the
tubular fluid. It is also called tubular excretion. As filtered fluid flows through the renal
tubules and collecting ducts, the renal tubule and duct cells
secrete other materials into the fluid. Hydrogen ions, potassium ions, creatinine, and drugs such as penicillin are
some of the substances that are moved by active transport from the blood into the distal convoluted tubule. In the
end, urine contains (1) substances that have undergone glomerular filtration
but have not been reabsorbed, and (2) substances that have undergone tubular secretion.

GLOMERULAR FILTRATION RATE

Glomerular filtration rate (GFR) is defined as the total
quantity of filtrate formed in all the nephrons of both the
kidneys in the given unit of time.
Normal GFR is 125 mL/minute or about 180 L/day.

PRESSURES DETERMINING FILTRATION

Pressures, which determine the GFR are:
1. Glomerular capillary pressure
2. Colloidal osmotic pressure in the glomeruli
3. Hydrostatic pressure in the Bowman capsule.
These pressures determine the GFR by either
favoring or opposing the filtration.
1. Glomerular Capillary Pressure
Glomerular capillary pressure is the pressure exerted
by the blood in glomerular capillaries. It is about 60 mm
Hg and, varies between 45 and 70mm Hg. Glomerular
capillary pressure is the highest capillary pressure in the
body. This pressure favors glomerular filtration.
2. Colloidal Osmotic Pressure
It is the pressure exerted by plasma proteins in the
glomeruli. The plasma proteins are not filtered through
the glomerular capillaries and remain in the glomerular
capillaries. These proteins develop the colloidal
osmotic pressure, which is about 25 mm Hg. It opposes
glomerular filtration.
3. Hydrostatic Pressure in Bowman Capsule
It is the pressure exerted by the filtrate in Bowman
capsule. It is also called capsular pressure. It is about
15 mm Hg. It also opposes glomerular filtration.