Corpus: Kidney

The kidney is a paired, bean-shaped organ primarily responsible for the formation of urine through filtration, reabsorption, and concentration. The kidney plays a crucial role in regulating systemic functions such as:

• water and electrolyte balance
• acid-base balance

Additionally, the kidney functions as an endocrine organ, synthesizing and secreting renin and erythropoietin, which are involved in blood pressure regulation and erythropoiesis.

2.1. Location

In humans, the kidneys are located in the retroperitoneal space below the diaphragm, between the 12th thoracic vertebra and the 3rd lumbar vertebra. Due to the large right lobe of the liver, the right kidney is usually positioned slightly lower than the left. The kidneys are nestled in a renal bed formed by the quadratus lumborum and psoas major muscles, and are surrounded by a fibrous capsule (capsula fibrosa renis), an adipose capsule (capsula adiposa renis), and a renal fascia.

2.2. Dimensions

The kidneys are approximately 10-12 cm in length, 5-6 cm in width, and weigh between 120-200 grams.

2.3. Structure

Each kidney is composed of 6-9 renal lobes (lobi renales), which are divided into the renal cortex (cortex renalis) and renal medulla (medulla renalis). The medulla contains cone-shaped structures known as renal pyramids, whose tips, called renal papillae, project into the renal calyces. The cortex lies above the base of the pyramids and extends between them as renal columns (columnae renales). The cortex contains the renal corpuscles and convoluted tubules, while the medulla contains the straight parts of the tubules and the collecting ducts.

Human kidneys, default view from dorsal

2.4. Embryology

The kidneys develop from the intermediate mesoderm.

2.5. Vascular supply

The kidneys receive arterial blood from the renal arteries, which branch off from the abdominal aorta, and venous blood is drained by the renal veins into the inferior vena cava. The blood supply includes a series of capillary beds, known as a rete mirabile, which are essential for filtration and reabsorption processes.

2.5.1. Arteries

Renal arteries function as both vasa privata (for renal metabolism) and vasa publica (for filtration function). One left and one right renal artery arise at the level of L2 from the abdominal aorta. The right renal artery runs dorsal to the inferior vena cava and is slightly longer than the left renal artery. At the renal hilum, the renal artery divides into five smaller segmental arteries.

2.5.1.1. Branches of the Arteria renalis

• inferior suprarenal artery to the adrenal gland
• capuslar branches to the renal capsule
• ureteral branches to the ureter

2.5.1.2. Intrarenal Renal Vessels

The segmental arteries entering the renal hilum give rise to interlobar arteries, which run capsule-wards between two renal pyramids. From the interlobar arteries, the arcuate arteries (arteriae arcuatae) arise and run parallel to the renal surface at the base of the pyramids.

From there, the interlobar arteries arise, running perpendicular to the renal surface and supplying the afferent arterioles (arteriolae afferentes). These arterioles pass through the renal corpuscles (glomeruli) and continue as efferent arterioles (arteriolae efferentes), which still carry arterial blood. The glomerular capillary network is followed by the peritubular capillary bed, from which the blood then flows into the venous system. Thus, the kidney contains two successive capillary beds connected by an arteriole. This arrangement is referred to as a rete mirabile.

Venous drainage from the peritubular capillary bed occurs via the arcuate veins and interlobular veins.

A particular feature is the juxtamedullary nephrons, which are located closer to the corticomedullary border. After passing through the glomeruli, their blood continues into the vessels of the renal medulla, the vasa recta. These vessels have the structure of capillaries and descend straight down to the tip of the renal papilla. The blood eventually flows back through ascending vasa recta into the draining veins.

2.5.2. Venes

Venous outflow from the kidneys occurs through the renal sinus vein and the renal distal vein, which emerge at the renal hilum and drain into the inferior vena cava. The renal sinus vein, which lies in front of the abdominal aorta, receives tributaries from:

• the suprarenal sinus vein
• the left testicular vein in men (or the left ovarian vein in women)
• the left inferior phrenic vein.

2.5.3. Lymph drainage

Lymphatic drainage from the kidneys flows into the lumbar lymph nodes located around the aorta and inferior vena cava, eventually draining into the two lumbar trunks.

2.6. Smooth musculature

Within the kidney, a system of smooth muscle fibers is responsible for the transport of urine. This system includes:

• the musculus sphincter fornicis at the beginning of the renal calices
• the musculus sphincter calicis between the renal calyx and renal pelvis
• the musculus sphincter pelvicis within the renal pelvis.

2.7. Innervation

The kidneys are innervated by sympathetic fibers from the aorticorenal ganglia, forming the renal plexus. Parasympathetic innervation is provided by the vagus nerve, originating from spinal segments C1 and C2.

2.8. Topography

Dorsally, the kidney is in contact with several nerves, including:

• the subcostal nerve
• the iliohypogastric nerve
• the ilioinguinal nerve.

If the kidney becomes pathologically enlarged, it can exert pressure on these nerves, leading to pain that radiates ventrally into the inguinal region.

Additional dorsal anatomical relations to the kidneys include:

• the twelfth rib (and part of the eleventh rib on the left side)
• the diaphragm
• the costodiaphragmatic recess
• the quadratus lumborum muscle
• the psoas major muscle
• the tendon of the transversus abdominis muscle.

Ventrally, the kidneys have different relationships with surrounding structures:

• The right kidney is adjacent to the liver, the descending part of the duodenum (pars descendens/D2), and the right colic flexure (flexura coli dextra).
• The left kidney is in contact with the spleen, pancreas, stomach, left colic flexure (flexura coli sinistra), and jejunum.

The kidney consists of numerous smaller functional units known as nephrons, which are responsible for urine production. Each human kidney contains approximately 1 to 1.2 million nephrons. A nephron is composed of a renal corpuscle (glomerulus) and a tubular system. The structure of the nephron can be systematically divided into several sections, progressing from proximal to distal:

• Renal corpuscles: This includes Bowman's capsule and the glomerulus, where the initial filtration of blood takes place.
• Renal tubule:
  • Proximal tubule: It is further divided into the proximal convoluted tubule and pars recta straight part of the proximal tubule.
  • Intermediate tubule: This section is composed of the descending and ascending limbs, known as the pars descendens and pars ascendens.
  • Distal tubule: Like the proximal tubule, it is divided into a pars recta (straight part) and a pars convoluta (distal convoluted tubule).

The straight sections of the proximal and distal tubules, along with the intermediate tubule, are collectively referred to as the loop of Henle. The transition between the proximal and distal convoluted tubules, which are located in the renal cortex, and the loop of Henle, which extends into the medulla, creates a distinct medullary-cortical boundary in the renal parenchyma visible under a light microscope.

The collecting ducts, which can also be considered part of the nephron in a broader sense, represent the final section of the tubular system. These ducts collect fluid from multiple distal tubules, extend into the inner medulla of the medullary pyramids, and eventually converge to form the papillary ducts. The papillary ducts then release the secondary urine through the renal papilla into the renal calices, which lead to the renal pelvis and eventually to the ureter.

The kidney plays a crucial role in maintaining the body's water and electrolyte balance and excreting metabolic waste products. Due to these functions, it is one of the most heavily perfused organs in the human body, receiving 20-25% of the cardiac output, primarily for urine production.

5.1. Filtration of the primary urine

The first step in urine production is the filtration of primary urine, which occurs in the renal cortex. With an average fluid intake, approximately 180 liters of primary urine is produced each day.

In the glomerulus, a network of blood vessels within Bowman's capsule, blood plasma is filtered through the inner layer of Bowman's capsule due to a blood pressure of about 50 mmHg. This filtration process retains larger blood components, such as blood cells and macromolecules, within the blood vessel. However, this filtration is opposed by a counterpressure from the capsule space within Bowman's capsule, known as capsule pressure, which is approximately 17 mmHg. Additionally, large protein molecules in the blood create an oncotic or colloid osmotic pressure of around 25 mmHg, as they draw water back into the blood vessels.

The combined effects of capsule pressure and colloid osmotic pressure counteract the initial blood pressure, resulting in an effective filtration pressure of about 8 mmHg at Bowman's capsule. If blood pressure drops significantly, the filtration pressure also decreases, potentially leading to acute kidney failure.

To maintain filtration pressure, especially when blood pressure fluctuates, the kidneys have mechanisms to counter-regulate. The release of renin from the juxtaglomerular apparatus triggers the renin-angiotensin-aldosterone system (RAAS), which increases effective filtration pressure. Additionally, the kidneys can temporarily adjust the filtration pressure through the Bayliss effect, which involves vasoconstriction and vasodilation of the afferent arterioles, ensuring stable kidney function even during changes in blood pressure.

5.2. Modification of the primary urine

The second step in urine formation is the modification of primary urine, which primarily occurs in the proximal tubule. This process involves two key mechanisms: reabsorption and secretion.

Firstly, active reabsorption occurs, where electrolytes, glucose, and residual proteins are transported from the tubule back into the bloodstream. Water is also reabsorbed, primarily through a process known as the solvent-drag mechanism, where water follows the movement of solutes.

Secondly, active secretion takes place, where substances such as urea, uric acid, creatinine, amino acids, and additional electrolytes are actively transported from the blood into the tubule system for excretion. However, reabsorption has a transport maximum, meaning that if the concentration of a particular substance (such as glucose in the case of diabetes mellitus) exceeds a certain threshold (the renal threshold), it is not fully reabsorbed and instead is excreted in the urine.

These combined processes of reabsorption and secretion significantly reduce the volume of primary urine from approximately 180 liters to an average of 18-20 liters per day.

5.3. Urinary concentration

The final step in urine formation is the further concentration of urine, which occurs in the loop of Henle and the collecting ducts. This process relies on the countercurrent mechanism, which involves three key components:

• the thin, descending limb of the loop of Henle
• the thick, ascending limb of the loop of Henle
• the interstitial fluid surrounding these structures.

The thin, descending limb of the loop of Henle is permeable to water but not to solutes. As the filtrate moves down this limb, water is reabsorbed into the surrounding hypertonic interstitium, which has a high concentration of solutes. This movement of water out of the tubule into the interstitium concentrates the urine.

In contrast, the thick, ascending limb of the loop of Henle is impermeable to water but actively transports sodium ions out of the tubule and into the interstitium. This active transport of sodium increases the osmolarity of the interstitial fluid, creating a gradient that draws water out of the descending limb, further concentrating the urine.

The final product of this concentration process is secondary urine, which is much more concentrated than the primary urine. The volume of this secondary urine is reduced to about 1.5 liters per day, which is then excreted from the body.

5.4. Control of urine formation

Urine production is primarily regulated by two key hormones:

Aldosterone, which acts on the tubule cells, particularly in the distal tubules and collecting ducts, stimulates the expression of sodium and potassium channels. This leads to increased sodium reabsorption back into the bloodstream, while potassium is secreted into the urine. The reabsorption of sodium also promotes water retention, as water follows sodium, thereby reducing urine volume.

Adiuretin, also known as antidiuretic hormone (ADH), enhances water reabsorption in the distal tubules and collecting ducts. By making the tubule walls more permeable to water, ADH allows more water to be reabsorbed from the filtrate into the bloodstream, thus concentrating the urine and reducing the amount of water excreted from the body. This mechanism is crucial in maintaining the body's fluid balance, especially during dehydration or when fluid intake is low.

5.5. Other tasks of the kidney

• Regulation of Acid-Base Balance: The kidney plays a critical role in maintaining the body's acid-base balance. For more details, refer to the kidney's role in acid-base balance.
• Excretion of Foreign Substances: The kidney is responsible for the excretion of xenobiotics, which are foreign substances such as drugs and toxins.
• Hormone Production: Beyond its role in excretion, the kidney produces important hormones:
  • Erythropoietin: This hormone stimulates the production of red blood cells (erythrocytes).
  • Renin: An enzyme with hormone-like effects that is involved in blood pressure regulation.
• Conversion of Vitamin D: The kidney hydroxylates calcidiol to form calcitriol, the active form of vitamin D. Impaired formation of calcitriol, often seen in chronic kidney disease, can lead to low calcium levels (hypocalcemia).

6.1. Diagnostics:

• Physical Examination: Palpation of the kidney is performed during a clinical examination.
• Imaging Studies: The structure and morphology of the kidneys are evaluated using imaging techniques such as:
  • Ultrasound (Sonography)
  • Computed Tomography (CT)
  • Magnetic Resonance Imaging (MRI)
• Special Imaging Techniques: For specific diagnostic questions, an intravenous pyelogram (contrast-enhanced imaging of the kidneys) may be necessary.
• Kidney Function Tests:
  • Urine Analysis: Kidney function can be estimated by analyzing urine volume, concentration, and the levels of waste products (creatinine, urea, uric acid, potassium) in the blood.
  • Clearance Tests: More detailed information about kidney function can be obtained through clearance tests, which measure the kidney's ability to filter certain substances from the blood.
  • Urine Sediment and Test Strips: Additional information about kidney health can be gathered by examining urine sediment and using test strips to detect the presence of bacteria, protein, blood, or glucose.

6.2. Diseases

Some common kidney diseases include:

• Infections and Inflammation:
  • Pyelonephritis (kidney infection)
  • Glomerulonephritis (inflammation of the glomeruli)
• Kidney Stones:
  • Nephrolithiasis (formation of kidney stones)
• Kidney Tumors:
  • Hypernephroma (renal cell carcinoma)
  • Angiomyolipoma (benign kidney tumor)
• Congenital Malformations:
  • Double kidney
  • Polycystic kidney disease (cystic kidneys)
  • Horseshoe kidney
  • Ectopic kidneys (kidneys located in abnormal positions)
  • Pelvic kidney (kidney located in the pelvis)
• Kidney Failure:
  • Acute renal failure (sudden loss of kidney function), which can lead to anuria (absence of urine production)
  • Uremia (build-up of waste products in the blood due to kidney failure)
• Other Conditions:
  • Nephroptosis (kidney that descends from its normal position, also known as a "wandering" or "floating" kidney)
  • Arteriolonecrosis (death of small arteries within the kidney)
  • Arteriolosclerosis (hardening of small arteries within the kidney)

• Anderhuber et al, Waldeyer - Human Anatomy: Lehrbuch und Atlas in einem Band (19th updated edition), De Gruyter, 2012