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The kidneys are remarkable retroperitoneal organs, located on the posterior abdominal wall on either side of the vertebral column, concealed behind the peritoneum. The intricate structure and multifaceted functions of the kidneys serve as a testament to the exquisite craftsmanship of our creator, which is evident not only in the kidneys but in every part of our body.

The kidneys play a vital role as a pair of excretory organs, responsible for eliminating waste products of metabolism, excess water, and salts from the blood, while also maintaining the body’s pH balance. If you’re curious about where the kidneys are located, they are positioned on the posterior abdominal wall and discussed in detail in the main paragraph of this article.

Where is the kidney located?

The kidneys, which are essential organs of the urinary system, are located in the abdominal cavity. They occupy four regions: the epigastric, hypochondriac, lumbar, and umbilical regions. The kidneys are positioned vertically, extending from the upper border of the twelfth thoracic vertebrae to the level of the third lumbar vertebrae.

The right kidney is slightly lower than the left kidney due to the position of other organs in the abdominal cavity, such as the liver. Additionally, the left kidney is slightly closer to the median plane of the body compared to the right kidney.

To further specify the location of the kidneys, the transpyloric plane can be used as a reference. This plane passes through the upper part of the hilus (the concave part) of the right kidney, and through the lower part of the hilus of the left kidney. The hilus is the region where the renal artery, renal vein, and ureter enter and exit the kidney.

Knowing the location of the kidneys is important for understanding their anatomical relationship with other organs in the abdominal cavity and for identifying potential sources of pain or discomfort in the renal region.

Anatomy of kidney

The anatomy of the kidney encompasses the study of its structure, shape, size, and location within the body. Understanding the anatomy of the kidney is crucial in order to accurately identify its location and comprehend its histology (microscopic structure). In fact, knowledge of kidney anatomy is fundamental to gaining a comprehensive understanding of this vital organ.

The anatomy of the kidney is complex and multifaceted, involving its external and internal structure. The external anatomy of the kidney involves its location within the abdominal cavity. As mentioned earlier, the kidneys are situated in the epigastric, hypochondriac, lumbar, and umbilical regions, extending vertically from the twelfth thoracic vertebrae to the third lumbar vertebrae. The right kidney is slightly lower than the left kidney, and the left kidney is positioned closer to the median plane of the body.

In terms of internal anatomy, the kidneys are composed of various structures, including the renal cortex, renal medulla, renal pelvis, and renal hilus. The renal cortex is the outermost layer of the kidney, while the renal medulla is located deeper within, consisting of renal pyramids. The renal pelvis is a funnel-shaped structure that collects urine from the renal pyramids and transports it to the ureter for elimination from the body. The renal hilus is the concave region on the medial side of the kidney where the renal artery, renal vein, and ureter enter and exit.

Understanding the anatomy of the kidney is essential for comprehending its function and for diagnosing and treating kidney-related conditions. A thorough knowledge of kidney anatomy forms the foundation for further study of its histology, physiology, and pathophysiology, which collectively contribute to a comprehensive understanding of this vital organ’s structure and function.

Shape, Size, Weight & Orientation

The kidneys are remarkable organs in terms of their size, shape, and appearance. An individual kidney is approximately 11 cm in length, 6 cm in breadth, and 3 cm in thickness. However, there may be slight variations in size between the left and right kidneys, with the left kidney being slightly longer and narrower compared to the right kidney. On average, the weight of a kidney is around 150 grams in males and 135 grams in females, and they exhibit a reddish-brown color.

The orientation of the kidneys within the abdominal cavity is also noteworthy. The long axis of the kidney is directed downward and laterally, with the upper poles being positioned closer to the median plane of the body compared to the lower poles. The transverse axis of the kidney is directed laterally and backward.

It’s interesting to note that in the fetal stage, the kidney is lobulated, meaning it is divided into approximately 12 lobules. However, after birth, these lobules gradually fuse together, resulting in a smooth and uniform appearance of the kidney in adults. Nonetheless, in some cases, remnants of fetal lobulation may persist even in adulthood, serving as evidence of the developmental history of the kidney.

Understanding the size, shape, and appearance of the kidneys is important in various clinical and anatomical contexts, such as in surgical procedures, radiological assessments, and pathological examinations. It allows healthcare professionals to accurately identify and assess the kidneys, and to recognize any deviations from normal anatomy that may be indicative of underlying conditions or diseases.

External features

Each kidney is bean-shaped. The kidney has upper and lower poles, medial and lateral borders, and anterior and posterior surfaces.

Two poles of the kidney

In addition to the unique orientation of the kidneys, the upper and lower poles of the kidneys also have distinct characteristics. The upper pole of the kidney is broad and is in close contact with the corresponding suprarenal gland, which is also known as the adrenal gland. The suprarenal gland is an important endocrine gland that produces hormones essential for various bodily functions, such as stress response and metabolism. The close proximity of the upper pole of the kidney to the suprarenal gland is noteworthy, as it reflects the interconnectedness and functional relationship between these two organs.

On the other hand, the lower pole of the kidney is pointed, forming the inferior aspect of the kidney. The lower pole is located relatively farther away from the suprarenal gland compared to the upper pole, and it typically extends towards the pelvic cavity. The pointed shape of the lower pole is a unique anatomical feature of the kidney and plays a role in the overall structure and function of this vital organ.

Two surfaces

The kidneys have two distinct surfaces that are important in understanding their anatomy. These surfaces are known as the anterior (ventral) and posterior (dorsal) surfaces.

1. Anterior (ventral) surface: The anterior surface of the kidney is positioned towards the front of the abdominal cavity and is in close proximity to other abdominal organs such as the liver, stomach, and intestines. It is generally more convex compared to the posterior surface, and it is covered by a fibrous layer called the renal fascia, which helps to protect and support the kidney.
2. Posterior (dorsal) surface: The posterior surface of the kidney faces towards the back of the abdominal cavity and is in contact with the muscles and bones of the back. It is generally more concave compared to the anterior surface, with a medial indentation known as the renal hilum. The renal hilum is an important anatomical feature that serves as the entry and exit point for blood vessels, nerves, and the ureter, which is the tube that carries urine from the kidneys to the bladder.

Both the anterior and posterior surfaces of the kidneys are important in understanding the overall anatomy and function of these organs. The anterior surface is in close proximity to other abdominal organs and structures, which can have clinical implications in terms of potential injuries or pathologies. The posterior surface, with its renal hilum, serves as the site for blood vessels and the ureter to enter and exit the kidney, and it is a critical area for surgical access and interventions.

Two Borders

The kidneys have two distinct borders, which are important in understanding their anatomy. These borders are known as the medial (inner) border and the lateral (outer) border.

1. Medial (inner) border: The medial border of the kidney is located towards the midline of the body and faces towards the vertebral column. It is generally concave in shape and has a smooth curve. The medial border is also known as the hilum, which is a prominent indentation on the medial aspect of the kidney. The renal hilum serves as the entry and exit point for blood vessels, nerves, and the ureter, which is the tube that carries urine from the kidneys to the bladder.
2. Lateral (outer) border: The lateral border of the kidney is located away from the midline of the body and faces towards the side of the body. It is generally convex in shape and has a more rounded appearance compared to the medial border. The lateral border is in close proximity to other organs and structures in the abdominal cavity, such as the peritoneum, ribs, and muscles of the back.

Both the medial and lateral borders of the kidneys are important in understanding the overall anatomy and function of these organs. The medial border serves as the site for blood vessels and the ureter to enter and exit the kidney, and it is a critical area for surgical access and interventions. The lateral border, on the other hand, is in close proximity to other abdominal organs and structures, which can have clinical implications in terms of potential injuries or pathologies.

Hilum

The hilum is a prominent anatomical feature of the kidney, located on the medial (inner) border of the organ. It is also known as the renal hilum or renal sinus. The hilum serves as the entry and exit point for blood vessels, nerves, and the ureter, which is the tube that carries urine from the kidneys to the bladder.

The renal hilum is a concave area on the medial border of the kidney, where the renal artery, renal vein, and ureter enter or exit the organ. The renal artery carries oxygenated blood from the heart to the kidney for filtration and processing, Commonly, one of the branches of the renal artery enters the hilus behind the renal pelvis, and a tributary of the renal vein may be found in the same plane, while the renal vein carries filtered blood back to the heart. The ureter, on the other hand, carries urine from the kidney to the bladder for excretion.

The renal hilum is surrounded by a fibrous layer called the renal sinus, which is a cavity within the kidney that contains fat and connective tissue. The renal sinus acts as a protective cushion for the blood vessels and ureter as they enter and exit the kidney, helping to prevent compression or damage to these structures.

The renal hilum is a crucial area for surgical access and interventions. Surgeons may approach the kidney through the hilum for procedures such as kidney transplantation, removal of kidney tumors, or placement of drainage tubes. Radiologists also use the hilum as a landmark in imaging studies to identify the location of the kidney and its surrounding structures.

Relations of the kidneys

The kidneys are retroperitoneal organs and are only partly covered by the peritoneum anteriorly.

Relations common to the two kidneys

• The upper pole of each kidney is related to the corresponding suprarenal gland (adrenal gland).
• The lower poles of the kidneys are positioned approximately 2.5 cm above the iliac crests.
• The medial border of each kidney is affiliated to the suprarenal gland, located above the hilum (the concave part of the kidney).
• The medial border of each kidney is also affiliated to the ureter, located below the hilum.
• The posterior surfaces of both kidneys are related to the diaphragm.
• The posterior surfaces of both kidneys are also related to the medial and lateral arcuate ligaments.
• The posterior surfaces of both kidneys are related to the psoas major muscle.
• The posterior surfaces of both kidneys are related to the quadratus lumborum muscle.
• The posterior surfaces of both kidneys are related to the transversus abdominis muscle.
• The posterior surfaces of both kidneys are related to the subcostal vessels.
• The posterior surfaces of both kidneys are related to the subcostal, iliohypogastric, and ilioinguinal nerves.
• The structures related to the hilum have been described earlier.

In addition, the right kidney is related to twelfth rib, and the left kidney to eleventh and twelveth ribs.

Other relations of the right kidney

Anterior Relations:

• Right suprarenal gland
• Liver
• Second part of the duodenum
• Hepatic flexure of the colon
• Small intestine

Note: The hepatic and intestinal surfaces are covered by peritoneum.

Lateral Relation:

• Right lobe of the liver
• Hepatic flexure of the colon

Other Relations of the left kidney

Anterior Relations:

• Left suprarenal glands
• Spleen
• Stomach
• Pancreas
• Splenic flexures and descending colon
• Jejunum

Note: The gastric, splenic, and jejunal surfaces are covered by peritoneum.

Lateral Relation:

• Spleen
• Descending colon

Nerve supply

• The kidney is supplied by the renal plexus, which is an offshoot of the coeliac plexus.
• The renal plexus contains sympathetic fibers (T10-L1) that are primarily vasomotor in function.
• The afferent nerves of the kidney belong to segments T10 to T12.

Exposure of the kidney from behind

In exposing the kidney from behind, the following layers have to be reflected one by one.

1. Skin
2. Superficial fascia
3. The posterior layer of thoracolumbar fascia with latissimus dorsi and serratus posterior inferior
4. Erector spinae, which can be removed for convenience
5. The middle layer of thoracolumbar fascia
6. Quadratus lumborum
7. The anterior layer of thoracolumbar fascia in which the related nerves are embedded.

Kidney Histology 

• The kidney is covered by a capsule.
• The cortex of the kidney, which is the outer layer, shows cut sections of glomeruli (where filtration of blood occurs), many sections of proximal convoluted tubules (where reabsorption of filtered substances occurs), some sections of distal convoluted tubules (where further reabsorption and secretion occur), and a few collecting ducts (which collect urine).
• Sections through the pyramid of the medulla, which is the inner layer of the kidney, show light-staining collecting ducts (which further concentrate urine), sections of the loop of Henle (which plays a role in concentrating urine), thick and thin segments of descending and ascending limbs of the loop of Henle (involved in water and ion reabsorption), capillaries (involved in blood supply to the kidney tissues), and connective tissue (which provides support and structure to the kidney).

Understanding the histology of the kidney is important in comprehending its structure and function at a microscopic level.

Nephron Anatomy

The nephron is the basic structural and functional unit of the kidney, responsible for filtering blood and forming urine. Here’s a brief overview of the anatomy of a nephron:

• Bowman’s capsule: It is a cup-like structure at the beginning of the nephron, surrounding the glomerulus (a network of tiny blood vessels). It collects the filtrate that is formed when blood is filtered through the glomerulus.
• Proximal convoluted tubule (PCT): It is a highly coiled tubule connected to Bowman’s capsule. It reabsorbs substances like glucose, ions, and water from the filtrate back into the bloodstream.
• Loop of Henle: It is a U-shaped loop that descends from the PCT into the medulla of the kidney and then ascends back towards the cortex. It plays a role in reabsorption of water and ions to concentrate urine.
• Distal convoluted tubule (DCT): It is another coiled tubule that comes after the Loop of Henle. It further regulates the reabsorption and secretion of ions and water, helping to fine-tune urine composition.
• Collecting duct: It is a larger tubule that receives filtrate from multiple nephrons and carries urine towards the renal pelvis for elimination from the body.

The nephron is a highly specialized structure with distinct regions that work together to filter blood, reabsorb important substances, and excrete waste products to form urine.

Conclusion

Kidneys are paired organs located on the posterior abdominal wall, and they are situated behind the peritoneum. They occupy the epigastric, hypochondriac, lumbar, and umbilical regions of the abdomen. Kidney histology is indeed important to understand the structural composition of the kidney, including the cortex, which contains glomeruli, proximal convoluted tubules, distal convoluted tubules, and collecting ducts. It’s important to have a good understanding of the anatomy and histology of the kidneys to appreciate their functions as excretory organs in the human body.