Kidneys

The kidneys are retroperitoneal organs that lie on the ventral surface of the quadratus lumborum muscle and lateral to the psoas muscle and vertebral column. They are immediately covered by a fibrous capsule called the renal capsule (or true capsule) and further surrounded by perirenal fascia (or false capsule). The perirenal fascia defines the perirenal space that contains the kidney, adrenal gland, ureter, gonadal artery and vein, and perirenal fat.

Any fat located outside the perirenal space is called pararenal fat. At the concave medial margin of each kidney is a vertical cleft called the renal hilum, where the following anatomic structures are arranged in an anterior-to-posterior direction:

  1. Renal veins (most anterior)

  2. Renal artery

  3. Renal pelvis (most posterior)

The renal hilum is continuous with a space called the renal sinus that contains:

  1. Renal pelvis

  2. Major and minor calyces

  3. Renal blood vessels, nerves, lymphatics

  4. Fat.

The right and left kidneys can be differentiated according to the following categories:

inside kidney.jpg

Internal Anatomy of the Kidney

Blood Supply:

  1. The abdominal aorta branches between lumbar vertebral level L1-L2 into the right renal artery and left renal artery.

  2. The longer right renal artery passes posterior to the inferior vena cava on its path to the right kidney.

  3. Near the renal hilum, each renal artery divides into an anterior division and a posterior division.

Anterior and posterior divisions of the renal arteries:

  1. The anterior segments of the kidney are supplied by the anterior division of the renal artery.

  2. The posterior segments of the kidney are supplied by the posterior division of the renal artery.

    - These segmental arteries are end arteries and are distributed to various segments of the kidney.

Interlobar arteries:

  1. The interlobar arteries branch into the arcuate arteries, which travel along the base of the renal pyramids at the corticomedullary junction.

Arcuate arteries:

  1. The arcuate arteries branch into interlobular arteries, which travel through the cortex toward the capsule.

Afferent arterioles:

  1. Each afferent arteriole forms a capillary bed called the renal glomerulus, which is drained by an efferent arteriole.

Efferent arterioles:

  1. The efferent arteriole of renal glomeruli form cortical and midcortical nephrons branches into a cortical peritubular capillary bed.

  2. The efferent arteriole of a renal glomeruli from juxtamedullary nephrons branches into 12 to 25 descending vasa recta, which are long, straight capillaries that run to varying depths of the medulla.

  3. The ends of the descending vasa recta give rise to a medullar peritubular capillary bed.

Venous Drainage:

  1. The kidneys are drained by the interlobular veins.

  2. The arcuate veins drain into the interlobal veins, which anastomose and converge to form several renal veins that unite as the renal vein.

Innervation:

The kidney is innervated by the renal plexus, which is intimately associated with the renal artery. The renal plexus contains only sympathetic components. There is no (or at least very minimal) parasympathetic innervation of the kidney.

Sympathetic innervation is provided by the aorticorenal ganglion.

Sensory innervation is provided by the least thoracic splanchnic nerve, first lumbar splanchnic nerve, and second lumbar splanchnic nerve and relay pain sensation from the kidney to T12-L2 spinal cord segments within the central nervous system. Pain associated with kidney pathology may be referred over the T12-L2 dermatomes (ie lumbar region, inguinal region and anteriosuperior thigh).

 

Clinical Notes:

  1. Renal pain is typically referred to the flanks and the anterior abdominal wall following the T12 dermatome.

  2. Nephrotosis is when the kidney drops due to inadequate renal fascia support.

  3. Renal vein entrapment syndrome occurs when the left renal vein gets compressed and results in left flank pain, hematuria, proteinuria, nausea, vomiting, and testicular pain. This syndrome occurs because the left renal vein lies in an acute angle that is formed by the superior mesenteric artery and aorta and a downward traction of the superior mesenteric artery may compress the left renal vein.

  4. Kidney stones are passed down from the ureter and they cause severe intermittent pain (ureteric colic) as they are passed. Loin to groin paid is when pain is referred to the T11-T12 dermatomes as the visceral pain afferents from the ureters travel with the sympathetic nerves of T11-T12. Complications from kidney stones include hematuria, infection, urinary obstruction, and eventual renal failure. A lithotripsy procedure sends shockwaves to break the kidney stones into smaller fragments that can pass through the ureter with more ease.