Synonyms

1. Helps in purification of blood by:
   • Ultrafiltration
   • Removal of metabolic waste products
2. Conserves body fluid and electrolytes and maintains their balance.
3. Functions as an endocrine organ by producing:
   • Erythropoietin → helps in formation of RBCs.
   • Renin → converts angiotensinogen to angiotensin, which helps in blood pressure regulation.
   • Vitamin D3
   • Kidney enzyme 1-hydroxylase converts 25-hydroxy vitamin D3, produced in liver, to active 1,25-dihydroxy vitamin D3.

Size

Weight

Lobulation

• In the newborn, the kidney is lobulated.
• In adults, its surface is smooth due to fusion of lobules.
• In the foetus:
  • Kidney is lobulated.
  • It is made up of about 12 lobules.
• After birth:
  • Lobules gradually fuse.
  • Adult kidney becomes uniformly smooth.
  • Evidence of foetal lobulation may persist.

Regions Occupied

Vertebral Level

• Kidneys lie opposite T12 to L3 vertebrae.
• Right kidney is slightly lower than the left.
• Left kidney is a little nearer to the median plane than the right.

Transpyloric Plane

Each kidney is bean-shaped and has:

Poles

Surfaces

Borders

Hilum

Anatomical Position

Hold the kidney in such a way that:

1. Broad upper pole faces upward and slightly medially.
2. Convex, irregular surface faces anteriorly.
3. Flat posterior surface faces posteriorly.
4. Concave border with hilum faces medially and is directed towards the opposite side of the kidney.

Side Determination

Observe the structures at the hilum.

Long Axis

Transverse Axis

Movements During Respiration

Peritoneal Relations

Anterior Relations

Posterior Relations of Kidneys

1. 11th rib
   • Only to the left kidney.
2. 12th rib
3. Muscles
   • Diaphragm
   • Quadratus lumborum
   • Psoas major
   • Transverse abdominis
4. Nerves emerging from lateral border of psoas
   • Subcostal
   • Iliohypogastric (L1)
   • Ilioinguinal (L1)
5. Subcostal vessels

From within outward, kidney is surrounded by four coverings:

1. Fibrous Capsule

• Thin membrane.
• Closely invests the kidney.
• Lines the renal sinus.
• Normally, it can be easily stripped off from the kidney.

2. Perirenal / Perinephric Fat

• Layer of adipose tissue lying outside the fibrous capsule.
• Thickest at the borders of the kidney.
• Fills the extra space in the renal sinus.

3. Renal Fascia

• Dense, elastic connective tissue sheath.
• Covers:

• Kidney
• Suprarenal gland
• Perirenal fat

4. Pararenal / Paranephric Body (Fat)

• Consists of a variable amount of fat lying outside the renal fascia.
• More abundant:
• Posteriorly
• Towards the lower pole of kidney
• Fills the paravertebral gutter.
• Forms a cushion for the kidney.

Attachments of Renal Fascia

Laterally

• Both layers fuse.
• Continue behind the colon with parietal peritoneum.

Medially

• Anterior layer covers the kidney and renal vessels.
• Continues with the similar layer of opposite renal fascia after crossing the aorta and inferior vena cava.
• Posterior layer is attached to the bodies of lumbar vertebrae.

Posteriorly

• Posterior layer covers the posterior surface of kidney.
• Fuses with fascia covering psoas major.

Superiorly

• The two layers of fascia do not fuse with each other.
• Anterior layer:
  • Fuses with inferior layer of coronary ligament on the right.
  • Fuses with gastrosplenic ligament on the left.
• Superior part of posterior layer fuses with fascia covering:
  • Psoas major
  • Quadratus lumborum
  • Inferior fascia

Inferiorly

Superior Continuity

• Superiorly, the peritoneal space is open and continuous with:
  • Connective tissue at bare area of liver on the right.
  • Subdiaphragmatic connective tissue on the left.

A longitudinal section of kidney on naked-eye examination shows three zones:

1. Renal Cortex

Reddish-brown outer zone of kidney.

a. Cortical Arches / Cortical Lobules

Form a cap over the bases of renal pyramids.

b. Renal Columns

Extensions of renal cortex between renal pyramids.

2. Renal Medulla

• Pale inner zone.
• Consists of 5–10 conical masses called renal pyramids.
• Each renal pyramid has:
• Peripheral base
• Apex
• Apices converge to the renal sinus.
• Papillae are the projecting apices of renal pyramids into the minor calyces.

3. Renal Sinus

• Cavity of kidney.
• Continuous with the hilum of kidney.
• Contains:
1. Renal pelvis
2. Minor calyces
3. Major calyces
4. Renal vessels
5. Lymphatics
6. Nerves
7. Fat

Renal Pelvis

Major Calyces

• Renal pelvis divides into 2–3 major calyces.
• Kalyx = cup of a flower.

Minor Calyces

• 7–13 small cavities.
• They unite to form major calyces.
• The dilated outer part of minor calyces receives the renal papilla.

Medullary Rays of Ferrin

• Striations of the renal cortex.
• Extensions of medulla into the cortex.
• Contain:
• Straight tubules
• Collecting ducts

Cortical Labyrinth

Region of cortex between adjacent medullary rays.

Area Cribrosa

• Perforated area at the tip of each renal papilla.
• Perforations are due to collecting duct openings.

Kidney Lobe

• Each pyramid with the surrounding cortical tissue forms a lobe of kidney.
• Each kidney contains 8–18 lobes.
• Lobulation is a prominent feature of foetal kidneys.
• In adult kidneys, lobes are indistinct.

Kidney Lobule

• Similar to the secretory unit of exocrine glands.
• Collecting ducts of medullary ray + surrounding cortical tissue constitute the kidney lobule.

The juxtaglomerular apparatus consists of:

1. Macula Densa

• Cells of the distal convoluted tubule.
• These cells are crowded and overlapping.
• Hence, they are called macula densa.

2. Juxtaglomerular Cells

• Modified smooth muscle cells of the afferent arteriole.
• Processes form a lace-like network.
• Secrete renin.

3. Lacis Cells

• Also called extraglomerular mesangial cells.
• Modified mesangial cells near the juxtaglomerular cells.
• Show receptors for angiotensin II.
• Probably regulate glomerular filtration rate.

Blood Supply of Kidney

• Each kidney is supplied by a renal artery arising from the abdominal aorta.
• Usually, there is a single renal artery for each kidney.
• In 30% individuals, an accessory renal artery is present.

Renal Vein

• Each kidney is drained by a renal vein into the inferior vena cava.
• Left renal vein:
  • Is longer.
  • Crosses anterior to the aorta.
  • Receives blood from the left gonadal vein.

Lymphatic Drainage

Nerve Supply

• Kidney is supplied by the renal plexus.
• Renal plexus is an offshoot of the coeliac plexus.
• It contains sympathetic fibres from T10–L1.
• These fibres are chiefly vasomotor.
• Afferent nerves of the kidney belong to T10–T12 spinal segments.

1. Each kidney is supplied by the renal artery.
   • Lobar arteries
   • Interlobar arteries
2. Interlobar arteries divide to form arcuate arteries along the base of pyramids.
3. Arcuate arteries form interlobular arteries.
   • These traverse the cortex towards the capsule.
   • Interlobular arteries give afferent arterioles.
4. Afferent arterioles give rise to a network of glomerular capillaries.
   • These unite to form efferent arterioles.
5. Efferent arterioles form a second set of capillaries called the peritubular capillary plexus.
6. Efferent arterioles form the juxtamedullary glomeruli and vasa recta and help in the countercurrent exchange system.
7. Peritubular capillaries in the cortex drain into:
   • Interlobular veins
   • Arcuate veins
   • Interlobar veins
   • Finally renal veins
8. Vasa recta are associated with the juxtamedullary glomeruli.

Blood Supply — Arterial Sequence

Venous Drainage Sequence

Capsule

Renal Lobe