Kidney

Human body has two kidneys, which are the prime organs of the urinary system and function to excrete most of the waste products of metabolism. The waste products are formed into urine, which leaves the kidney by passing down the ureters to the urinary bladder, from where it is expelled out of the body through urethra. In addition to their excretory function, the kidneys are the chief controllers of the water and electrolyte balance as well as the acid base balance within the human body.

Location:

Kidneys lie behind the peritoneum (retroperitoneal) high up on the posterior abdominal wall on either side of the vertebral column. In their common location, they are largely under cover of the costal margin. The left kidney lies approximately at vertebral level T12 to L3 and the right kidney slightly lower.

The right kidney lies slightly lower than the left kidney, because of the larger size of the right lobe of liver. Both kidneys move up and down with the movements of the diaphragm during respiration.

Coverings:

Each kidney has the following four coverings on its exterior.

  • Fibrous capsule: It surrounds the kidney and is closely applied to its outer surface.
  • Perirenal fat: It surrounds the fibrous capsule.
  • Renal fascia: It is a condensation of connective tissue and lies outside the perirenal fat. In addition to the kidney, it also encloses the suprarenal gland. The renal fascia is continuous with the fascia transversalis.
  • Pararenal fat: It is found often in large quantity and lies external to the renal fascia. If forms part of the retroperitoneal fat.

External structure:

Each kidney is reddish-brown, bean shaped organ and is about 4 inches long and 2 inches wide. It has a medial concave surface and a lateral convex surface.

On the medial concave border of each kidney, there is a vertical slit called hilum, which is bounded by thick lips of the renal substance. Inside the kidney, the hilum extends into a large cavity called renal sinus. All structures that enter or exit the kidney pass through the hilum. From front backwards, these structures are:

  • Renal vein
  • Two branches of renal artery
  • Ureter
  • Third branch of renal artery
  • Lymph vessels
  • Sympathetic fibers

Internal structure:

When viewed in transverse section, each kidney appears to consist of two layers. The dark brown outer layer is called cortex and the light brown inner layer is called medulla, which is composed of about one dozen renal pyramids. The layers are not discrete and there are projections form one to the other. The cortex extends into the medulla as renal columns. Similarly there are striations, known as medullary rays, which extend from the renal pyramids into the cortex.

The renal sinus, the space within the hilum, contains the renal pelvis that is the upper expanded end of the ureter. The pelvis is divided into 2 or 3 major calyces, each of which divides into 2 or 3 minor calyces. Each minor calyx is indented by the apex of the renal pyramid, called the renal papilla.

Important relations:

Right Kidney:

Anterior:

  • Suprarenal gland
  • Liver
  • Second part of duodenum
  • Right colic flexure

Posterior:

  • Diaphragm
  • Costodiaphragmatic recess of pleura
  • 12th rib
  • Psoas, quadratus lumborum and transversus abdominis muscles

Left kidney:

Anterior:

  • Suprarenal gland
  • Spleen
  • Stomach
  • Pancreas
  • Left colic flexure
  • Jejunum

Posterior:

  • Diaphragm
  • Costodiaphragmatic recess of pleura
  • Eleventh rib
  • Psoas, quadratus lumborum and transversus abdominis muscles

Blood supply:

Arterial:

Blood supply to the kidney comes from the renal artery, which arises from the abdominal aorta at the level of second lumbar vertebra. According to the most common arrangement, each renal artery divides into five segmental arteries. These segmental arteries enter the hilum, four in front and one behind the renal pelvis, and are distributed to different areas of the organ.

For each renal pyramid, a lobar artery arises from the segmental artery. Each of these lobar arteries gives rise to two or three interlobar arteries, which run towards the cortex.

At the junction of the cortex and medulla, the interlobar arteries give off the arcuate arteries, which arch over the bases of the pyramids. Interlobular arteries, which ascend into the cortex, arise from the arcuate arteries and give off the afferent glomerular arterioles.

The rather complex arterial system of the kidney can be summarized as:

Abdominal aorta -> Renal arteries -> Segmental arteries -> Lobar arteries -> Interlobar arteries -> Arcuate arteries -> Interlobular arteries -> Afferent glomerular arterioles.

Venous:

The venous blood from the kidney is drained by the renal vein, which itself drains into the inferior vena cava.

Lymph drainage:

Lymphatics from the kidney drain into the lateral aortic lymph nodes, which are present around the origin of the renal artery.

Nerve supply:

Each kidney receives its nerve supply from the corresponding renal sympathetic plexus. The afferent fibers from the kidney travel through the renal plexus and enter the spinal cord in the 10th, 11th and 12th thoracic segments.