Kidney Anatomy

Internal structure of the kidneys seen on a dissected specimen

INTRODUCTION

The closely packed structure and numerous functions of the kidney illustrate the beautiful workmanship of our creator. It not only applies to the kidney but to each and every part of our body.

Synonyms

The kidney are also called renes from which we have the derivative renal and nephros from which we have the terms nephron, nephritis, etc

Definition

Kidneys are a pair of excretory organs located on the posterior abdominal wall, one on each side of the vertebral column, behind the peritoneum. They remove waste products of metabolism and excess of water and salts from the blood, and maintain its pH.

Location

Location of the kidneys

The kidneys occupy the epigastric, hypochondriac. lumbar and umbilical regions. Vertically they extend from the upper border of twelfth thoracic vertebra to the center of the body of third lumbar vertebra. The right kidney is slightly lower than the left, and the left kidney is a little nearer to the median plane than the right.

The transpyloric plane passes through the upper part of the hilus of the right kidney, and through the lower part of the hilus of the left kidney.

Shape, Size, Weight and Orientation

Position of the kidneys from anterior aspect

Each kidney is about 11 cm long, 6 cm broad, and 3 cm thick. The left kidney is a little longer and narrower than the right kidney. On an average the kidney weighs 150 g in males and 135 g in females. The kidneys are reddish brown in color.

The long axis of the kidney is directed downwards and laterally, so that the upper poles are nearer to the median plane than the lower poles. The transverse axis is directed laterally and backwards.

In the foetus the kidney is lobulated and is made up of about 12 lobules. After birth the lobules gradually fuse, so that in adults the kidney is uniformly smooth. However, the evidence of foetal lobulation may persist.

EXTERNAL FEATURES

Each kidney is bean-shaped. It has upper and lower poles, medial and lateral borders, and anterior and posterior surfaces.

Two poles

The upper polé is broad and is in close contact with the respective 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 side to posterior side.

1 The renal vein

2 The renal artery

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 from each 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 kidne 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 KIDNEYS

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

Relations Common to the Two Kidneys 

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

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 

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

Posterior 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

Vertical section through the posterior abdominal wall showing the relationship of the pleura to the kidney

 Anterlor 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. 

CAPSULES OR COVERINGS OF KIDNEY 

Transverse section through the lumbar region showing the capsules of the 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 the extra space in the renal sinus. 

Renal Fascia 

The perirenal fascia was originally described as being made Posterior layer was called fascia of Zuckerkandal 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. of two separate layers. dn The fascia then extends anteromedially behind the kidney as bilaminated sheet, which divides 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 abdominal wall, 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 Paranephric Body (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 into 7 to 13 minor calyces. Each minor calyx (kalyx = cup of A flower) ends in an expansion which is intended by one of three renal papillae.

Structure of Uriniferous Tubule 

Each kidney is composed of one to three million uriniferous tubules. Each tubule consists of two parts which are embryologically distinct from each other. These are as follows: 

 The excretory part, called the nephron, which elaborates urine. Nephron is the functional unit of the kidney, and comprises: 

a. Renal corpuscle or malpighian corpuscle (for filtration of substances from the plasma) made up of glomerulus (Latin ball) a tuft of capillaries and Bowman's capsule.

b. Renal tubule (for selective resorption of substances from the glomerular filtrate) made up of the proximal convoluted tubule, loop of Henle with its descending and ascending limbs, and the distal convoluted tubule.

   The collecting part begins as a junctional tubule from the distal convoluted tubule. Many tubules unite together to form the ducts of Bellini which open into the minor calyces through the renal papillae. Juxtaglomerular apparatus is formed at the vascular pole glomerulus which is intimately related to its own ascending limb of the Henle's loop near the distal convoluted tubule. The apparatus consists of:

a. Macula densa, formed become altered cells of the distal convoluted tubule. 

b. Juxtaglomerular cells, formed by the epithelioid cells in the media of the afferent arteriole. 

c. Some agranular cells between macula densa and the glomerulus proper. 

VASCULAR SEGMENTS

A coronal section through the kidney showing the naked eye structure including the blood supply of the kidney

 The renal artery gives 5 segmental branches, 4 from its anterior division and one from its posterior division.

      The segments are apical, upper, middle and lower on anterior aspect. On posterior aspect segments seen are posterior and parts of apical and lower segments. 

Blood Supply of Kidney 

The blood supply of kidney shown belowt: 

 Abdominal aorta at L2 level - renal artery-5 segmental arteries-each segmental artery-lobar artery-interlobar artery-arcuate artery-interlobular artery-afferent arteriol-glomerulus-efferent arteriols-peritubular plexus-interlobular vein-arcuate vein-interlobar vein-lobar vein-segmental vein-5 segmental veins-renal vein- (IVC) inferior vena cava

Lymphatic Drainage

 The lymphatics of the kidney drain into the lateral aortic nodes located at the level of origin of the renal arteries (L2). 

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.

EXPOSURE OF THE KIDNEY FROM BEHIND

 In exposing the kidney from behind, the following layers have to be reflected one by one: 

1 Skin 

2 Superficial fascia 

3 Posterior layer of thoracolumbar (lumbar) fascia with latissimus dorsi and serratus posterior inferior 

4 Erector spinae, which can be removed for convenience 

5 Middle layer of thoracolumbar fascia

6 Quadratus lumborum 

7 Anterior layer of thoracolumbar lumbar fascia in which the related nerves are embedded. 

HISTOLOGY

Histology of the kidney

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