The urinary system consists of kidneys that produce urine and the urinary tract.

The urinary tract includes the renal calices, pelvises, ureters, urinary bladder, and urethra.

In this note, we will consider the anatomy of the kidneys.

The kidney (ren) is a paired organ that is located in the retroperitoneal space, to the right and left of the lumbar spine.

The topography of the right and left kidney is different.

Skeletotopy

The right one is located slightly more inferior than the left one due to its proximity to the liver, at the level between the 12th thoracic and 3rd lumbar vertebrae. At the same time, the 12th rib crosses it at the border between the middle and superior thirds.

The left kidney is located at the level between the 11th thoracic and 2nd lumbar vertebrae. The 12th rib crosses the left kidney in the middle.

Holotopy

They are projected onto the lumbar region of the posterior abdominal wall. It projects over the lateral regions of the abdomen.

Syntopy

Similarly located structures are adjacent to both kidneys posteriorly, namely, the lumbar part of the diaphragm, the quadratus lumborum muscle, the transversus abdominis muscle, and the psoas major muscle.

The adrenal glands adhere to the superior pole of both kidneys.

Anteriorly, the right kidney borders the quadrate lobe of the liver, the descending part of the duodenum, the ascending colon, and the right colic flexure.

The left kidney with its anterior surface adheres to the posterior wall of the stomach, the tail of the pancreas, the spleen, the left colic flexure, and partially to the descending colon, and the jejunum.

The spleen adheres to the border of the left kidney.

In relation to the peritoneum, the kidneys are located retroperitoneally.

Each kidney has two poles:

The superior pole (extremitas superior)

The inferior pole (extremitas inferior)

On the medial border of the kidney, there is a concavity called the hilum of the kidney (hilum renale).

They lead to the small renal sinus (sinus renalis).

The hilum of the kidney contains some structures.

Listing them front-to-back:

1. The renal vein

2. The renal artery

3. The renal pelvis

A kidney is covered with a fibrous capsule (capsula fibrosa), which normally adheres loosely to the renal surface.

The fibrous capsule is surrounded on all sides by a fat capsule, which in turn is surrounded by a thin renal fascia (Gerota’s fascia) (fascia renalis).

This fascia has two sheaths: the pre-renal and the post-renal.

Let’s summarize and list the supporting structures of the kidney:

1. The renal fascia

2. The perirenal fat capsule

3. Adjacent muscles

4. Structures of the hilum of the kidney

5. The peritoneum and intra-abdominal pressure

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Anatomy of the kidney

Шорткод

The urinary system consists of kidneys that produce urine and the urinary tract.

The urinary tract includes the renal calices, pelvises, ureters, urinary bladder, and urethra.

In this note, we will consider the anatomy of the kidneys.

The kidney (ren) is a paired organ that is located in the retroperitoneal space, to the right and left of the lumbar spine.ШорткодШорткодШорткодШорткод

The topography of the right and left kidney is different.

Skeletotopy

The right one is located slightly more inferior than the left one due to its proximity to the liver, at the level between the 12th thoracic and 3rd lumbar vertebrae. At the same time, the 12th rib crosses it at the border between the middle and superior thirds.

The left kidney is located at the level between the 11th thoracic and 2nd lumbar vertebrae. The 12th rib crosses the left kidney in the middle.

Holotopy

They are projected onto the lumbar region of the posterior abdominal wall. It projects over the lateral regions of the abdomen.

Syntopy

Similarly located structures are adjacent to both kidneys posteriorly, namely, the lumbar part of the diaphragm, the quadratus lumborum muscle, the transversus abdominis muscle, and the psoas major muscle.

The adrenal glands adhere to the superior pole of both kidneys.

Anteriorly, the right kidney borders the quadrate lobe of the liver, the descending part of the duodenum, the ascending colon, and the right colic flexure.

The left kidney with its anterior surface adheres to the posterior wall of the stomach, the tail of the pancreas, the spleen, the left colic flexure, and partially to the descending colon, and the jejunum.

The spleen adheres to the border of the left kidney.

In relation to the peritoneum, the kidneys are located retroperitoneally.

Each kidney has two poles:

The superior pole (extremitas superior)Шорткод

The inferior pole (extremitas inferior)Шорткод

On the medial border of the kidney, there is a concavity called the hilum of the kidney (hilum renale).ШорткодШорткод

They lead to the small renal sinus (sinus renalis).ШорткодШорткод

The hilum of the kidney contains some structures.

Listing them front-to-back:

1. The renal veinШорткодШорткод

2. The renal arteryШорткодШорткод

3. The renal pelvisШорткодШорткод

A kidney is covered with a fibrous capsule (capsula fibrosa), which normally adheres loosely to the renal surface.ШорткодШорткод

The fibrous capsule is surrounded on all sides by a fat capsule, which in turn is surrounded by a thin renal fascia (Gerota’s fascia) (fascia renalis).ШорткодШорткод

This fascia has two sheaths: the pre-renal and the post-renal.

Let’s summarize and list the supporting structures of the kidney:

1. The renal fasciaШорткодШорткод

2. The perirenal fat capsuleШорткодШорткод

3. Adjacent muscles

4. Structures of the hilum of the kidney

Let’s move on to the internal features of the kidney.

It consists of the cortex (cortex renis) and the medulla (medulla renis).

In the cortex, there are a lighter radiant part and a darker folded part.

The cortex forms renal columns (columnae renales).

In the medulla, there are renal pyramids (pyramides renales).

At the top of the pyramid, there is a renal papilla (papilla renalis), on which the urinary papillary ducts open.

The substance of the kidney is divided into segments. Each segment combines 2-3 renal lobes (lobus renalis), each of which consists of a renal pyramid with a cortical substance adjacent to it.

The lobes are separated by interlobar arteries and veins.

Let’s list the renal segments:

1. The superior segment (segmentum superius), which corresponds to the superior pole of the kidney

2. The anterior superior segment (segmentum anterius superius)

3. The anterior inferior segment (segmentum anterius inferius)

4. The inferior segment (segmentum inferius), which corresponds to the inferior pole of the kidney

5. The posterior segment (segmentum posterius)

The renal cortex consists of glomeruli and, mainly, convoluted renal tubules. In the form of renal columns, it spreads to the wall of the pelvis.

The medulla consists of the renal tubules, collecting tubules, and blood vessels combined together in the form of renal pyramids.

The renal body (capsule) and related tubules are parts of the nephron, which is a structural and functional unit of the kidney. Normally, each kidney contains up to 1 million nephrons.

The structure of a nephron

The following structures are distinguished in the nephron:

1. The renal (or malpighian) body (corpusculum renis).

It consists of a vascular capillary glomerulus (glomerulus) and Bowman’s capsule (capsula glomeruli) surrounding it.

It has a double-layer wall and a cavity, into which primary urine flows.

2. The proximal convoluted tubule (tubulus contortus proximalis), which passes into the proximal straight tubule (tubulus rectus proximalis)

3. The loop of Henle (ansa nephroni), which is formed between the proximal and distal tubules.

It has a descending part, U-shaped section, and an ascending part

4. The distal straight tubule (tubulus rectus distalis), which continues into the distal convoluted tubule (tubulus contortus distalis)

And the latter, in turn, passes into the collecting tubule (tubulus renalis colligens)

The following types of nephrons are distinguished:

• Cortical (juxtaglomerular) nephrons
• Juxtamedullary nephrons, which have a long loop of Henle

The latter normally practically do not function but are involved only in extreme situations with the centralization of blood circulation.

The collecting tubule removes secondary urine from the nephron and sends it to the renal collecting system.

Initially, it flows into the collecting duct. The collecting ducts flow into the renal papillae (papillae renales), in which there are many small orifices.

Papillae open into minor calices (calices renales minores) through foramina (foramina papillaria).

Several small calices, connecting with each other, form a major calyx (calices renales majores)

And several major calices in the region of the sinus of the kidney combine into a renal pelvis (pelvis renalis).

The renal pelvis is the terminal structure of the kidney, and it passes into the ureter in the area of the hilum of the kidney.

Blood supply

In order to understand how the kidneys work, it is important to know the features of their blood supply.

The abdominal aorta (pars abdominalis aortae) supplies the kidney with blood from the branches of the renal artery (a. renalis)

It is divided into:

1. The anterior branch of the renal artery (r. anterior).

2. The posterior branch of the renal artery (r. posterior).

Subsequent branching occurs as follows:

The segmental artery (arteria segmentalis)

The interlobar artery (arteria interlobaris)

The arcuate arteries (arteria arcuata)

The interlobular artery (arteria interlobularis)

The interlobular arteries give rise to the afferent arterioles (vas afferens)

And those arterioles divide into small capillaries, forming a capillary glomerulus (glomerulus)

The efferent arteriole (vas efferens) arises from the glomerulus

This area (arteriole-glomerulus-arteriole) is called the rete mirabile, since, unlike the way it happens in other organs, an arteriole, not a venule, is formed from capillaries.

The efferent arteriole divides into small capillaries (or a secondary capillary network), which circumflex the renal tubules and at this point pass into venules.

Further branching of the venous plexus occurs as follows:

The interlobular vein (vena interlobularis)

The arcuate vein (vena arcuata)

The interlobar vein (vena interlobaris)

The renal vein (vena renalis)

The inferior vena cava (vena cava inferior)

This complex system provides not only supplies the kidney with blood but also performs its main function, namely the formation of urine.

Functions of the kidney

Three processes occur simultaneously in the kidneys:

• Glomerular filtration
• Tubular reabsorption
• Tubular secretion

Filtration is carried out through the capillary glomerulus into the lumen of Bowman’s capsule.

This is due to the fact that the diameter of the afferent arteriole is larger than the diameter of the efferent arteriole and, accordingly, a pressure gradient occurs.

The kidneys filtrate approximately 150–200 liters of primary urine per day.

Next, the primary urine enters the tubules, where reabsorption occurs (that is, the reverse absorption of water and some substances into the blood).

Simultaneously, various compounds are secreted into the lumen of the tubule.

This is how secondary urine is formed, in an amount of up to 1.5 liters per day.

In the zone between the afferent and efferent arterioles, there are structures called juxtaglomerular apparatus.

It consists of the macula densa, juxtavascular and juxtaglomerular cells.

These structures regulate the pressure between the afferent and efferent arterioles and affect the general hemodynamics, as well as water and salt metabolism.

This occurs by the secretion of renin, which triggers a cascade of reactions in the renin-angiotensin-aldosterone system.

Lymph drainage

The lymph drains from the kidneys to the lumbar lymph nodes, as well as the lateral lymph nodes of the aorta and inferior vena cava, inferior diaphragmatic, colic, and internal iliac lymph nodes.

Innervation

A kidney is innervated by the anterior branches of the inferior thoracic and superior lumbar spinal nerves and the fibers of the renal branches of the vagus nerve. Also, several nerve renal plexuses form around the kidney.

Dictionary

Anatomy of the kidney

Kidney

ren

Superior segment

segmentum superius

Anterior superior segment

segmentum superius anterius

Anterior inferior segment

segmentum inferius anterius

Inferior segment

segmentum inferius

Posterior segment

segmentum posterius

Hilum of the kidney

hilum renale

Fibrous capsule

capsula fibrosa

Renal fascia

fascia renalis

Cortex of the kidney

cortex renis

Medulla of the kidney

medulla renis

Renal columns

columnae renalis

Pyramids of the kidney

pyramides renalis

Renal corpuscle

corpusculum renis

Glomerulus

glomerulus

Capsule of the glomerulus

capsula glomeruli

Proximal convoluted tubule

tubulus renalis contorti proximalis

Proximal straight tubule

tubulus renalis rectus proximalis

Nephron loop

ansa nephroni

Distal straight tubule

tubulus renalis rectus distalis

Distal convoluted tubule

tubulus renalis contorti distalis

Collecting tubule

tubulus renalis colligeus

Renal papillae

papillae renales

Papillary foramina

foramina papillaria

Minor renal calices

calices renales minores

Major renal calices

calices renales majores

Renal pelvis

pelvis renalis

Renal artery

a. renalis

Anterior branch of the renal artery

r. anterior a. renalis

Posterior branch of the renal artery

r. posterior a. renalis

Superior and inferior segmental arteries

aa. segmenti superioris et inferioris

Posterior segmental arteries

aa. segmenti posteriores

Anterior segmental arteries

aa. segmenti anteriores

Interlobar arteries

aa. interlobares renis

Arcuate arteries

aa. arcuatae

Interlobular arteries

aa. interlobulares renis

Afferent arterioles

vas afferens

Renal glomerulus

glomerulus

Efferent arteriole

vas afferens

Renal vein

v. renalis

Inferior vena cava

v. cava inferior