The Urinary System and Kidneys

• The urinary system comprises two kidneys, their excretory tracts, the bladder, and the urethra.
• The primary function of the kidney, a glandular organ, involves urine secretion.
• The kidneys are located in the lumbar retroperitoneal region on either side of the spinal column.
• These vital organs perform functions essential for sustaining health, including blood filtration, waste elimination, regulation of water and electrolyte balance, blood pressure control, hormone production, and maintenance of acid-base balance.

General points

• Two kidneys exist, situated retroperitoneally on each side of the vertebral column, extending from T11 to the L2-L3 intervertebral disc.
• Each kidney is obliquely oriented downwards and outwards.
• The right kidney sits lower than the left.

Shape

• The kidney is shaped like a bean, featuring:
• A convex lateral border and a concave medial border, with the hilum in the middle.
• Two surfaces: dorsal and ventral.
• Two extremities: proximal and distal.

Color and Consistency

• The kidneys are firm and reddish-brown.

Dimensions

• The kidneys weigh approximately 140g in men and 125g in women.
• They measure 12 cm in length, 6 cm in width, and 3 cm in thickness.

Renal Fascia and Fixation

• The kidney and adrenal gland are enclosed by the renal fascia, forming a closed renal lodge.

Renal Fascia

• The renal fascia results from the condensation of the kidney's adipose capsule, aiding in fixation and protection.
• It consists of two layers that fix to the edges of the hilum and extend into the vascular sheath of the aorta and inferior vena cava.
• The posterior layer adheres to the diaphragmatic fascia and separates from the fascia of the quadratus lumborum and psoas muscles via the pararenal adipose body.
• An inter-adrenorenal layer separates the adrenal gland from the kidney.
• The kidney is surrounded by fatty connective tissue within its adipose capsule.
• It is held in its lodge by abdominal pressure, which depends on the tone of the abdominal wall.

Relationships

Dorsal Aspect

• Corresponds to the standard surgical approach to the kidney.
• It comprises thoracic and lumbar segments.

Thoracic Segment

• The kidney relates to the diaphragm (at the lateral arcuate ligament), the costodiaphragmatic recess, and the 11th and 12th ribs on the left, and the 12th rib on the right.

Lumbar Segment

• The kidney relates to the pararenal adipose body, the fascia of the psoas and quadratus lumborum muscles, and the thoracolumbar fascia with the transverse abdominis muscle.

Ventral Aspect

• The right kidney is situated lower and relates to:
  • The right colic angle at its anterior part.
  • The descending part of the duodenum along its medial border and the root of the transverse mesocolon.
  • The inferior surface of the right lobe of the liver.
• The left kidney presents three segments:
  • The upper segment relates to the spleen, the tail of the pancreas, and the posterior aspect of the stomach.
  • The middle segment relates to the root of the transverse mesocolon and the left end of the transverse colon.
  • The lower segment relates to the loops of the small intestine.

Lateral Border

• Covered by the posterior parietal peritoneum forming the parietocolic gutter on both sides. The lateral border of the right kidney relates to the liver.
• The lateral border of the left kidney relates to the spleen in its upper half and the descending colon in its lower half.

Medial Border

• The middle part corresponds to the renal hilum.
  • The hilum marks the entry point into the renal sinus, a longitudinal cleft about 4 cm long and 1 cm wide.
  • The supra-hilar part relates to the adrenal gland.
  • The infra-hilar part runs along the ureter.

Extremities

• The proximal extremity relates to the adrenal gland.
• The distal extremity is approximately 4 cm from the iliac crest on the right and 5 cm on the left.

Vascularization

Arteries

• The right and left renal arteries, collateral branches of the abdominal aorta originate from its lateral aspect at the L1 level.
• The interlobar arterial distribution is terminal, dividing the kidney into superior, inferior, anterosuperior, anteroinferior, and posterior segments for conservative surgery.
• The renal artery splits into ventral and dorsal branches near the hilum.
  • The ventral branch crosses the anterior aspect of the renal pelvis, providing a prepyelic ramification that gives rise to arteries for the superior, anterosuperior, anteroinferior, and inferior segments.
  • The dorsal branch goes around the superior border of the renal pelvis, along the posterior border of the renal hilum, and provides a retropyelic ramification for the posterior segment.
• Each segmental or lobar artery branches into interlobar arteries, which further divide into arcuate arteries above the renal pyramids.
• Arising from the arcuate and interlobar arteries are interlobular arterioles that lead to glomerular arteries.
• The glomerular arteries form the glomeruli of Malpighi through capillarization.

Adipose Capsule Arteries

• Originate from the renal artery, adrenal arteries, and testicular or ovarian artery; sometimes from the right and left colic, lumbar, and aortic arteries.

Veins

• Each renal vein, right and left, represents the final venous collector of a kidney, draining into the inferior vena cava at the L2 level.
• Each renal vein begins from the union of segmental veins at the renal hilum.
• A segmental vein collects interlobar veins that drain arcuate veins at the base of the renal pyramids.
• Interlobar and arcuate veins receive straight and interlobular veins.
• These collect intralobular veins and stellate venules located under the renal capsule.

Lymphatics

• Originate from two plexuses:
  • The intrarenal lymphatic plexus drains into the latero-aortic and latero-caval nodes.
  • The capsular lymphatic plexus, in the fibrous and adipose capsules of the kidney, drains into the intrarenal collectors at the hilum level.

Innervation

• Arises from the renal plexus, formed by nerve fibers from the celiac and aortico-renal ganglia.

General Structure

• The kidney consists of renal parenchyma surrounded by a capsule, indented by the renal sinus. Its structure is visible on a frontal section.

Fibrous capsule

• Composed of elastic and smooth muscle fibers applied to the renal parenchyma.

Renal Parenchyma

• The parenchyma consists of two parts: an external cortex and an internal medulla.

Medulla

• Composed of conical masses known as renal pyramids (or pyramids of Malpighi), numbering 8–10, with their apices towards the papillae.

Cortex

• Reddish-brown and granular, surrounding the renal pyramids and penetrating between them. It has several parts:
  • The cortical substance between the renal pyramids is known as the renal columns (columns of Bertin).
  • The part surrounding the renal pyramids has two parts: the radiate part (pyramids of Ferrein) and the convoluted part (the labyrinth).
  • The pyramids of Ferrein face the pyramids of Malpighi (approximately 500 per renal pyramid).
  • The labyrinth lies between the pyramids of Ferrein and contains the glomeruli.
  • Each pyramid of Malpighi corresponds to a renal lobe, and each pyramid of Ferrein corresponds to a lobule.

Renal Sinus

• A hollow cavity opening through the renal hilum, containing renal vessels, calices, and the renal pelvis.