Kidney (ren) is a paired organ that filters blood and forms urine. The kidneys lie at the back of the abdomen, on either side of the vertebral column, behind the peritoneum, the membrane lining the abdominal cavity. They also help regulate body water, salts and acid–base balance.
Where are the kidneys located, and what lies beside them?
The kidneys are retroperitoneal, meaning that they lie behind the peritoneum rather than within the cavity it encloses. They occupy the upper posterior abdomen. The right kidney usually lies lower than the left kidney because of the liver above it.
Behind each kidney are the diaphragm, the muscle separating the chest from the abdomen, and muscles of the posterior abdominal wall. These include psoas major near the vertebral column and quadratus lumborum farther laterally, towards the side. Typically, the eleventh and twelfth ribs lie behind the upper part of the left kidney, while the twelfth rib lies behind the upper part of the right kidney.
Their anterior relations, meaning structures in front of them, differ:
- The right kidney is related to the liver, duodenum, right colic flexure and small intestine. The duodenum is the first part of the small intestine; the right colic flexure is the bend between the ascending and transverse colon.
- The left kidney is related to the stomach, spleen, pancreas, left colic flexure and jejunum. The left colic flexure is the bend between the transverse and descending colon; the jejunum is the middle part of the small intestine.
- A suprarenal gland, a hormone-producing organ, lies above each kidney within the surrounding fascial compartment.
What are the kidney's surfaces, borders and coverings?
Each kidney has anterior and posterior surfaces, upper and lower poles, and medial and lateral borders. The poles are its rounded ends. The lateral border faces away from the midline and is convex, while the medial border faces towards the midline and is concave.
The hilum of kidney is the opening on the medial border through which vessels, nerves and the urine-collecting passage enter or leave. At the hilum, the usual arrangement from front to back is renal vein, renal artery and renal pelvis. The renal pelvis is the funnel-shaped collecting region that continues into the ureter, the tube carrying urine to the bladder.
The kidney capsule is a thin fibrous covering closely applied to the organ. Outside it are perirenal fat and renal fascia, a connective-tissue envelope around the kidney and its surrounding fat. Additional fat lies outside the fascia.
What are the internal parts of the kidney?
The cortex of kidney forms the outer region beneath the capsule. The renal medulla forms the inner region and is arranged into cone-shaped renal pyramids. Extensions of cortex between the pyramids are called renal columns.
Each pyramid ends in a renal papilla, its tip projecting into a minor calyx. A calyx is a cup-shaped urine-collecting space. Minor calyces join into major calyces, which open into the renal pelvis.
| Part | Position | Anatomical role |
|---|---|---|
| Kidney capsule | Closely surrounds the kidney | Forms its fibrous covering |
| Cortex of kidney | Outer region and between pyramids | Contains filtration structures and many renal tubules |
| Renal medulla | Inner region | Contains pyramids with tubules and collecting ducts |
| Renal pelvis | Central collecting region | Receives urine from major calyces and continues into the ureter |
The renal sinus is the internal space reached through the hilum. It contains the renal pelvis, calyces, blood vessels, nerves and fat. It is distinct from the cortex and medulla, which form the kidney's working tissue.
What is the kidney's blood supply?
The main renal arteries usually arise from the abdominal aorta, the main artery of the abdomen. Each divides into segmental arteries supplying territories within the kidney; additional renal arteries may also be present. Further branches pass between the pyramids, arch along the cortex–medulla boundary and extend into the cortex.
Blood reaches glomeruli, tufts of small blood vessels where filtration begins. After leaving these tufts, it passes through vessels around the renal tubules, the small tubes that modify the filtered fluid. Veins return blood to the renal vein, which drains into the inferior vena cava, the large vein returning blood from the lower body to the heart.
The left renal vein is longer than the right and crosses in front of the aorta. It passes behind the superior mesenteric artery, an artery supplying much of the intestine.
How is the kidney innervated, and where does lymph drain?
Nerves reach the kidney through the renal plexus, a network surrounding the renal artery. Its autonomic fibres belong to the nervous system that regulates involuntary activity. Sympathetic fibres, a division of this system, influence renal blood vessels and kidney activity; sensory fibres carry signals back towards the spinal cord.
Lymph, fluid collected from tissues, leaves through lymphatic vessels that accompany the renal vessels. It drains mainly to lumbar lymph nodes beside the abdominal aorta and inferior vena cava.
What does the kidney do?
The kidney forms urine through filtration followed by selective recovery and secretion. The nephron is its microscopic functional unit: it includes a filtration structure and a renal tubule. Tubules recover water and useful substances into the blood and transfer other substances into the fluid that becomes urine.
The kidneys regulate water, dissolved salts and acid–base balance. They also release renin, an enzyme involved in blood-pressure regulation, and erythropoietin, a hormone that stimulates red blood cell production. They activate vitamin D into a form involved in calcium balance.
How can you locate the kidney in the 3D model?
In the 3D model beside this article, rotate the body to a posterior view and locate the kidneys on either side of the vertebral column. Switch layers on and off or fade overlying structures to clarify their position. Select and isolate a kidney, then zoom and rotate it to examine its shape and the orientation of its medial border.