Kidney (part-2)

Two poles of kidney

The upper pole is broad and is in close contact with the corresponding suprarenal gland. The lower pole is pointed.

Two surfaces

The anterior surface is said to be irregular and the posterior surface flat, but it is often difficult to recognize the anterior and posterior aspects of the kidney by

looking at the surfaces. The proper way to do this is to examine the structures present in the hilum as described below.

Two borders

The lateral border is convex. The medial border is concave. Its middle part shows a depression, the hilus or hilum.

Hilum

The following structures are seen in the hilum from anterior to posterior side.

L The renal vein

2 The renal artery, and

3 The renal pelvis, which is the expanded upper end of the ureter.

Examination of these structures enables the anterior and posterior aspects of the kidney to be distinguished  fromeach other. As the pelvis is continuous inferiorly with the ureter, the superior and inferior poles of the kidney can also be distinguished by examining the hilum.

So it is possible to determine the side to which a kidney belongs by examining the structures in the hilum.

Commonly, one of the branches of the renal artery enters the hilus behind the renal pelvis, and a tributary of the  renal vein may be found in the same plane.

Relations of the Kidney

The kidneys are retroperitoneal organs and are only partly covered by peritoneum anteriorly.

Relations common to the the Two Kidneys

I The upper pole of each kidney is related to the corresponding suprarenal gland. The lower poles lie about 2.5 cm above the iliac crests.

2 The medial border of each kidney is related to:

a. The suprarenal gland, above the hilus, and

b. To the ureter below the hilus.

3 Posterior relations: The posterior surfaces of both kidneys are related to the following.

a. Diaphragm

b. Medial and lateral arcuate ligaments

c. Psoas major

d. Quadratus lumborum

e. Transversus abdominis

f. Subcostal vessels; and

g. Subcostal, iliohypogastric and ilioinguinal nerves.

In addition, the right kidney is related to twelfth rib, and the left kidney to eleventh and twelfth ribs.

4 The structures related to the hilum have been described earlier.

Other relations of the Right kidney

Anterior relations

1. Right suprarenal gland

2 Liver

3 Second part of duodenum

4 Hepatic flexure of colon

5 Small intestine.

Out of these the hepatic and intestinal surfaces are covered by peritoneum.

The lateral border of the right kidney is related to

the right lobe of the liver and to the hepatic flexure of the colon.

Other relations of the Left Kidney

Anterior relations

1 Left suprarenal gland

2 Spleen

3 Stomach

4 Pancreas

5 Splenic vessels

6 Splenic flexure and descending colon

7 Jejunum.

Out of these the gastric, splenic and jejunal surfaces are covered by peritoneum.

The lateral border of the left kidney is related to the  spleen and to the descending colon.

CAPSUTES OR COVERINGS OF KIDNEY

The Fibrous Capsule

This is a thin membrane which closely invests the kidney and lines the renal sinus. Normally it can be easily stripped off from the kidney, but in certain

diseases it becomes adherent and cannot be stripped.

Perirenal or Perinephric Fat

This is a layer of adipose tissue lying outside the fibrous capsule. It is thickest at the borders of the kidney and

fills up the extra space in lhe renal sinus.

Renal Fascia

The perirenal fascia was originally described as being made up of two separate layers.

Posterior layer was called fascia of Zuckerkandall and anterior layer as fascia of Gerota.

These two fasciae fused laterally to form lateral conal fascia. According to this view, lateral conal fascia continued anterolaterally behind colon to blend with

parietal peritoneum.

But lately it has been researched that the fascia is not made up of fused fasciae, but of a single multi-laminated structure which is fused posteromedially with muscular fasciae of psoas major and quadratus

lumborum muscles.

The fascia then extends anteromedially behind the kidney as bilaminated sheet, which divide at a variable

point into thin layer which courses around the front of kidney as anterior perirenal fascia and a thicker posterior layer which continues anterolaterally as the

lateral conal fascia.

It was believed earlier that above the suprarenal gland the anterior and posterior perirenal fasciae fuse with each other and then get fused to the diaphragmatic

fascia, but research presently demonstrates that superior aspect of perirenal space is "open" and is in  continuity to the bare area of liver on the right side and

with subphrenic extraperitoneal space on the left side.

On the right side at the level of upper pole of kidney, anterior fascia blends with inferior coronary layer and  bare area of liver.

On the left side, anterior layer fuses with gastro-phrenic ligament.

Posterior layer on both right and left sides fuses with fasciae of muscles of posterior abdominalwall, i.e. psoas major and quadratus lumborum as well as with fascia

on the inferior aspect of thoracoabdominal diaphragm.

Medially the anterior layer is continuous from one to the other kidney and the posterior layer is attached either side of vertebra.Below both the layer extend along the ureter and fuse with iliac fascia.

Pararenal or Paranehpric (Fat)

It consists of a variable amount of fat lying outside the  renal fascia. It is more abundant posteriorly and towards the lower pole of the kidney. It fills up the

paravertebral gutter and forms a cushion for the kidney.

Structure

Naked eye examination of a coronal section of the kidney shows:
1 An outer, reddish brown cortex.
2 An inner, pale medulla.
3 A space, the renal sinus.
The renal medulla is made up of about 10 conical masses, called the renal pyramids. Their apices form the renal papillae which indent the minor calyces.
The renal cortex is divisible into two parts.
a. Cortical arches or cortical lobules, which form caps over the bases of the pyramids.
b. Renal columns, which dip in between the
pyramids.Each pyramid along with the overlying cortical arch forms a lobe of the kidney.The renal sinus is a space that extends into the kidney from the hilus. It contains:
a. Branches of the renal artery.
b. Tributaries of the renal vein.
c. The renal pelvis. The pelvis divides into 2 to 3 major calyces, and these in their turn divide into7 to 13 minor calyces. Each minor calyx (kalyx =cup of a flower) ends in an expansion which is indented by one to three renal papillae.

Blood supply of the Kidney

Arterial supply

The abdominal aorta gives off many branches, including the renal arteries. The renal arteries branch off perpendicular to the abdominal aorta, travelling posterior to the renal veins, nerves and the pancreas.

Renal artery - ventral view

Subsequent to branching from the aorta, the renal artery enters the kidney at the hilum, where it divides into anterior and posterior branches. The posterior division goes on to supply the posterior region of the kidney, while the anterior branch divides further to produce apical, anterior superior, anterior inferior and inferior segmental arteries; each supplying their respective segments.

Anterior branch of renal artery (ventral view)

At the level of the minor calyces, the branches of the anterior renal arteries further divides into interlobar arteries that course around the borders of the medullary pyramids. At the base of the pyramids, these arteries are referred to as arcuate arteries.

Interlobar arteries of kidney - ventral view

Finally, the arteries enter the nephrons (functional units of the kidneys) as the interlobular arteries, where afferent arterioles bring blood to the glomerulus to be filtered. It should be noted that these arteries neither anastomose nor have accompanying veins.

Interlobular arteries of kidney - ventral view

Capillary network

As the afferent arterioles enter the glomerulus, they form an intricate network of communicating capillaries. The capillaries are lined by a unique fenestrated epithelium (each space being around 70 – 100 nm wide). The fenestration allows selective passage of smaller particles into the renal tubules and keeps larger blood cells in the vessels.

Afferent glomerular arteriole of kidney (histological slide)

The capillaries leave the glomerulus as efferent arterioles, after which they form capillary beds around the nephron’s loop of Henle. In cortical nephrons (loop of Henle does not extend deep into the medulla), the capillary beds are called peritubular capillaries; while in juxtamedullary nephrons (loop of Henle extends deep into the medulla), the capillary beds are called vasa recta. These capillary beds facilitate blood pressure regulation and ionic homeostasis both passively and under hormonal influence.

Vasa recta of kidney (histological slide)

Venous drainage

As the capillaries leave the nephron, they condense to form interlobular veins. Similar to the branches of the renal arteries, the interlobular veins become arcuate veins at the base of the medullary pyramids, then interlobar veins. About five or six interlobar veins join together to form each renal vein. Unlike the branches of the renal arteries, the tributaries of the renal vein communicate with each other.

Renal vein - ventral view

Lymphatic drainage

Superficial lymphatic vessels form a plexus under the renal capsule (thin layer covering the kidneys) known as the subcapsular lymphatic plexus. They, along with medullary lymph vessels, communicate with cortical lymph vessels that travel alongside interlobular, arcuate and interlobar arteries. The renal lymphatics then drain directly to the lumbar lymph trunks (which then drain to the thoracic duct and cisterna chyli) and to para-aortic nodes, including precaval, lumbar and postcaval nodes.

Nerve Supply

The kidney is supplied by the renal plexus, an off shoot of the coeliac plexus. It contains sympathetic (T10-L1)fibres which are chiefly vasomotor. The afferent nerves of the kidney belong to segments T10 to T12.

HISIOLOGY

The cortex of kidney shows cut sections of glomeruli,many sections of proximal convoluted tubule, some sections of distal convoluted tubule and few collecting ducts.

Section through the pyramid of the medulla shows light staining collecting ducts, sections of loop of Henle,thick and thin segments of descending and ascending limbs, capillaries and connective tissue.