Kidney

The kidney (ren, Greek nephros) is a paired excretory organ that forms and excretes urine. The kidney is bean-shaped, dark red, and of a dense consistency. The dimensions of the kidney in an adult are as follows: length 10-12 cm, width 5-6 cm, thickness 4 cm. The weight of the kidney ranges from 120 to 200 g. The surface of the kidney in an adult is smooth. A more convex anterior surface (facies anterior) and a less convex posterior surface (facies posterior), an upper end (extremitas superior), as well as a convex lateral edge (margo lateralis) and a concave medial edge (margo medialis) are distinguished. In the middle section of the medial edge there is a depression - the renal hilum (hilum renalis). The renal artery and nerves enter the renal hilum, and the ureter, renal vein, and lymphatic vessels exit from them. The blood vessels and nerves form the so-called renal pedicle. The renal hilum passes into a large depression that projects into the substance of the kidney and is called the renal sinus (sinus renalis). The renal sinus contains the small and large calyces, the renal pelvis, blood and lymphatic vessels, nerves and fatty tissue.

The renal parenchyma consists of two distinct layers: the outer and the inner. The outer layer, the renal cortex (cortex renalis), is richly supplied with blood, which is why it has an intense red color. The cortex occupies the entire peripheral (outer) part of the organ; its thickness is 1 cm. The inner layer, known as the renal medulla (medulla renalis), is significantly inferior to the cortex in terms of blood supply. Structurally, the medulla lacks homogeneity. It is divided into 8-18 conical structures, known as renal pyramids (pyramides renales), arranged fan-shaped: their bases face the renal cortex, and the apex (renal papilla, papilla renalis) is located in the area of the renal hilum (hilum renale). Each of the pyramids is separated from the other by renal columns (columnae renales), which are sections of the renal cortex that extend into the medulla.

The renal pyramid and the part of the cortex above it are called the renal lobe. The medulla is divided into two zones - external and internal. The area of the external zone of the medulla, bordering the renal cortex, is called the corticomedullary junction. From it, the so-called medullary rays (radii medullares) extend into the renal cortex; these are the functional structures of the cortex - straight segments of the proximal tubules, thick ascending segments, and cortical collecting ducts. The internal zone (zona interna) of the medulla ends in the area of the renal papilla, free of structural elements of the nephron. The papillae are pierced by 10-25 small openings, which are the terminal segments of the renal collecting ducts (Bellini ducts). The mouths of these ducts are located around the apex of the papilla.

The renal papillae open into the minor renal calices (calices renales minores) - the first structural elements of the urinary tract, connecting the renal parenchyma with the urinary system, which is represented by the ureters, urinary bladder and urethra.

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Topography of the kidneys

The kidneys are located in the lumbar region (regio lumbalis) on both sides of the spinal column, on the inner surface of the posterior abdominal wall and lie retroperitoneally. The upper ends of the kidneys are brought closer to each other by up to 8 cm, and the lower ends are separated from each other by 11 cm. The longitudinal axis of the right and left kidneys intersect at the top at an angle open downwards. The left kidney is located slightly higher than the right, which lies directly under the liver. The upper end of the left kidney is at the level of the middle of the 11th thoracic vertebra, and the upper end of the right kidney corresponds to the lower edge of this vertebra. The lower end of the left kidney lies at the level of the upper edge of the 3rd lumbar vertebra, and the lower end of the right kidney is at the level of its middle. In relation to the ribs, the kidneys are located as follows: the 12th rib intersects the posterior surface of the left kidney almost in the middle of its length, and the right one - approximately on the border of its upper and middle thirds. There are individual features of the topography of the kidneys. There is a distinction between their high and low location. In 11% of women, the lower end of both kidneys touches the iliac crest.

The kidneys are in complex relationships with neighboring organs. The posterior surface of the kidney together with its membranes is adjacent to the lumbar part of the diaphragm, the quadratus lumborum muscle, the transverse abdominal muscle and the lumbar major muscle, which form a depression for the kidney - the renal bed. The upper end of the kidney is in contact with the adrenal gland. The anterior surface of the kidneys is covered over most of its length by a sheet of parietal peritoneum and is in contact with some internal organs. The liver is adjacent to the upper two-thirds of the anterior surface of the right kidney, and the right flexure of the colon is adjacent to the lower third. The descending part of the duodenum is adjacent to the medial edge of the right kidney. The anterior surface of the left kidney is in contact with the stomach in the upper third, with the pancreas in the middle third, and with the loops of the jejunum in the lower third. The lateral edge of the left kidney is adjacent to the spleen and the left flexure of the colon. The normal topographic position of the kidneys is ensured by their fixing apparatus, which includes the renal bed, the “renal pedicle”, and the renal membranes (especially the renal fascia).

Of great importance is the intra-abdominal pressure maintained by the contraction of the abdominal muscles.

Kidney membranes

The kidney has several membranes. On the outside, the kidney is covered with a thin connective tissue plate, the fibrous capsule (capsula fibrosa), which can be easily separated from the substance of the kidney. Outside the fibrous capsule, there is a fat capsule (capsula adiposa), which is quite thick and penetrates through the renal hilum into the renal sinus. The fat capsule is most pronounced on the back surface of the kidney, where a kind of fat pad is formed - the perirenal fat body (corpus adiposum pararenale). With a rapid decrease in the thickness of the fat capsule (with rapid weight loss), the kidney can become mobile (wandering kidney).

Outside the fatty capsule, the kidney is surrounded (in the form of a sac open downwards) by the renal fascia (fascia renalis), consisting of two sheets - prerenal and retrorenal. The prerenal sheet covers the left kidney, renal vessels, abdominal part of the aorta, inferior vena cava in front and continues in front of the spine to the right kidney. The retrorenal sheet of the renal fascia is attached to the lateral sections of the spinal column on the left and right. The lower edge of the pre- and retrorenal sheets of the renal fascia are not connected to each other. The renal fascia is connected to the fibrous capsule of the kidney by means of strands of fibrous connective tissue that penetrate the fatty capsule. The parietal sheet of the peritoneum is located in front of the prerenal sheet of the renal fascia.

X-ray anatomy of the kidney

On the radiograph, the contours of the kidney are smooth, have the appearance of arcuate lines; the shadow of the kidneys is uniform. The upper border of the shadow of the left kidney reaches the 11th rib and the middle of the body of the 11th thoracic vertebra, and the right one - the lower edge of the same vertebra. The shape and size of the kidney are revealed by introducing oxygen or gas into the retroperitoneal space - pneumoretroperitoneum. During pyelography (after the introduction of a contrast agent into the blood or retrogradely through the ureter), the shadow of the renal pelvis is at the level of the bodies of the 1st and 2nd lumbar vertebrae, the shadows of the renal calyces are visible. The state of the arterial bed of the kidney is revealed using arteriography.