Kidneys

History of development

The kidney belongs to the uropoetic system. It develops from a pre-kidney(pronephros), then becomes the primordial kidney(mesonephros) and then the post-kidney(metanephros).

(Graumann 2004)

• Pronephros:

The pronephros develops in the 3rd - 5th week. It already has a prerenal duct and prerenal tubules. However, it is functionless (Graumann 2004).

• Mesonephros:

The urinary kidney develops in the 4th week. It already has the ureteral duct, also known as the Wolff duct. It also has renal tubules and glomeruli originating from the aortic branches. The ureter is partially functional (Graumann 2004).

• Metanephros:

The after-kidney represents the final kidney as a functional excretory organ. It develops from the 5th SSW (Sigel 2013). It consists of 2 components: the epithelial ureteric bud and the mesenchymal metanephrogenic blasts. It begins to function as early as the 8th week of gestation. However, the first urine formation does not take place until the second half of fetal life (Graumann 2004).

Anatomy

The kidneys are bean-shaped and lie extraperitoneally in the kidney bed, the spatium retroperitoneale. The left kidney is slightly larger than the right. The weight is normally between 130 - 150 g, the kidneys are 10 - 12 cm long, 5 - 6 cm wide and about 4 cm thick. If one kidney is lost, the remaining kidney can hypertrophy to twice the size of the organ.

At the hilum renale there is a deep indentation through which the renal artery, renal vein and ureter enter or leave the kidney (Graumann 2004). The renal hilus leads into a larger bay, the renal sinus (Anhuber 2012).

The kidneys lie at the level of the 12th thoracic spine and extend to the 3rd lumbar spine, although usually the right kidney lies slightly lower because of the large hepatic lobe (Anhuber 2012).

The kidney is encased in a coarse capsule, the capsula fibrosa, which in turn is embedded in a layer of fat, the capsula adiposa, and encased in a connective tissue fascia, the gerota fascia (Manski 2019).

The interior of the kidney can be macroscopically divided into:

• Medulla renalis

In the renal medulla lie about 10 - 15 so-called medullary pyramids, which open into the renal papillae. A renal calyx belongs to each renal papilla.

• Cortex renalis

The cortex renalis is about 6 - 10 mm wide. Descending from the renal cortex are projections called columnae renalis, also known as Bertini's columns, which lie between adjacent pyramids (Anhuber 2012). The cortex renalis contains the loops of Henle, the collecting tubes and the radially arranged medullary rays (continuation of the pyramids). The medullary rays are surrounded by a parenchymal coat in which the renal corpuscles are located. From here, the primary urine is conducted via the corresponding medullary ray (Graumann 2004).

In the cranial part of the kidney lies the adrenal gland (Manski 2019).

Structure of the kidney

• Glomerulum:

There are between 1 million (Manski 2019) to 2.5 million glomerules in both kidneys (Anhuber 2012). The number of glomerules depends on birth weight (Manski 2019).

A glomerulum, together with Bowman's capsule, which is also known as Malpighi's capsule, forms a Malpighian renal corpuscle, which in turn, together with its associated tubule, forms a nephron (Anhuber 2012).

• Nephron:

In both kidneys there are as many nephrons as glomerulae. Each nephron consists of:

• a renal corpuscle (so-called glomerulum [Manski 2019])
• the associated urinary tract system
• with one proximal, one intermediate and one distal tubule each

[Anhuber 2012)

From there, the connecting piece leads to the collecting tube, which unites to form the papillary duct and opens into the renal calices (Anhuber 2012).

Vascular supply

The blood supply to the kidney is via the renal artery, which divides into the anterior and posterior branches to supply the individual segments. From the segmental arteries arise the Aa. interlobares, which pass into the Aa. arcuatae and then into the Aa. interlobulares.

The glomeruli are attached to the interlobular arteries like grapes. The glomeruli are connected by the arteriola glomerularis afferens and the arteriola glomerularis efferens (Graumann 2004).

The venous supply is via the V. renalis dextra et sinistra. The left renal vein is longer than the right vein because it passes below the superior mesenteric artery over the aorta. Both veins drain into the inferior vena cava (Manski 2019).

Role of the kidney

The kidneys are a paired, vital organ, which

• on the one hand, maintains the homeostasis of the organism via ion and osmoregulation
• on the other hand it produces important active substances such as renin, erythropoietin, etc.
• also metabolizes hormones by converting them into a water-soluble form, thus enabling their excretion
• is also the target organ of various hormones such as adiuretin = antidiuretic hormone (Graumann 2004).

The excretion of urinary substances occurs in a two-step process:

• 1. by ultrafiltration of the blood, a so-called primary urine is first formed
• 2. reabsorption leads to the formation of secondary urine, which is excreted via the urinary tract by micturition (Graumann 2004).

1. Anhuber F et al. (2012) Waldeyer Anatomy of Man. Textbook and atlas in one volume. De Gruyter Verlag 582 - 587
2. Graumann W et al (2004) Compact textbook anatomy in 4 volumes. Schattauer Publishers 207 - 209, 228 - 244.
3. Manski D (2019) The urology textbook. Dirk Manski Publishers 6 - 19
4. Sigel A et al (2013) Pediatric urology. Springer Verlag 1 - 6