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The Kidney

Image: “Internal Anatomy” by Phil Schatz. License: CC BY 4.0

Location of the Kidneys

The kidneys are located in the retroperitoneum beneath the diaphragm on each side of the spinal column (see image). They are embedded in a groove, the renal fossa, which consists of the Musculus psoas major and the Musculus quadratus lumborum. The left kidney lies between the level of the 11th rib and the 3rd lumbar vertebra.

Due to the size of the liver, the right kidney lies in a slightly lower position, beginning at the level of the 12th rib and extending to the lower edge of the 3rd lumbar vertebra. Physiologically, the kidneys have moving flexibility of about 3–4 cm, depending on the posture of the body and the movement of the diaphragm during breathing, because the kidneys are not firmly attached to the back of the abdominal wall.

Kidney Position in Abdomen

Image: “Kidneys” by Phil Schatz. License: CC BY 4.0

Shape and Areas of the Kidneys

The kidneys are bean-shaped and red-brown. Each kidney has a thickness of 4 cm, a width of 5–7 cm, and a length of approximately 11 cm (mnemonic: “4-7-11”). Each kidney weighs about 120–200 g.

The cranial and caudal edges are called the upper and lower poles (extremitas superior and extremitas inferior, respectively). The front surface of the kidney is called the facies anterior, and the back surface, the facies posterior. The kidney’s lateral edge (margo lateralis) has a convex shape. The margo medialis is concave. The renal hilum (hilum renale) with the kidney vessels is located on this medial edge. The renal vein (see image) lies ventrally to the renal artery, which is, in turn, anterior to the ureter (mnemonic: “VAU”). The renal hilum leads to the renal sinus and the renal pelvis.

The Kidney

Image: “Internal anatomy” by Phil Schatz. License: CC BY 4.0

Positional Relations to Other Organs

In relation to other organs:

  • The adrenal glands are located cranially to the upper kidney poles.
  • The subcostalis, Nervus iliohypogastricus, and Nervus ilioinguinalis run dorsally.

Note: Because of the kidney’s proximity to these nerves, pain originating from the kidney may spread to the groin.

The right hepatic lobe, pars descendens duodeni, and the right colonic flexure run ventrally to the right kidney. The root of the transverse mesocolon and the splenic flexure are situated anterior to the left kidney, stomach, spleen, and pancreas.

The Kidney’s Relation to the Peritoneum

Normal kidney size and its influencing factors

Image: “Normal kidney size and its influencing factors” by ncing factors. License: CC BY 2.0

The kidneys, the adrenal glands, and the ureters are retroperitoneal organs. This means that they are located behind the peritoneum.

The capsula fibrosa is a capsule of firm connective tissue surrounding the kidney. Another capsule of fat (capsula adiposa) encloses the kidney and the associated adrenal gland. The fascia renalis is another capsule of connective tissue.

Function of the Kidney

The entire blood volume of the body is flushed through the kidneys several times each day. This is how the kidneys can control and regulate, among other things, the blood pressure, the water-and-electrolyte balance, the elimination of metabolic end products and toxic substances, and the acid-base balance.

The kidneys are important in maintaining the internal environment:

  • Removing excess water, salt, and waste products
  • Maintaining the correct levels of nutrients and chemicals
  • Maintaining the acid-base balance
  • Maintaining blood pressure

The water-and-electrolyte balance

The hormone renin (see the table) is secreted by the kidneys as soon as there is a drop in blood pressure or volume. Renin acts as a protease to split angiotensinogen from the liver into angiotensin 1. Angiotensin 1 is then converted into angiotensin 2 through angiotensin-converting-enzyme (ACE) in the lungs. The result is, among other things, vasoconstriction involving an increase in blood pressure. Moreover, aldosterone is secreted by the adrenal gland. This leads to increased retention of water and sodium. This chain of reactions is called the renin-angiotensin-aldosterone-system (RAAS).

Atrial natriuretic peptide (ANP) is released if the heart’s right atrium has an increased blood volume. As a result, the kidneys excrete more water and sodium.

With ADH (also known as vasopressin), if the blood is too concentrated, osmoreceptors in the hypothalamus will react. Here, ADH is produced and later stored in the posterior lobe of the pituitary gland. As a result of this secretion, the kidneys transfer more water from the primary urine back to the blood, thus thinning the blood and increasing its volume.

The kidneys also regulate the blood content of potassium, bicarbonate, phosphate, magnesium, and calcium through the influences of hormones and specialized transporters.

Regulation of blood pressure

The kidneys regulate the blood pressure by changing the blood volume and the diameters of the blood vessels. Here, the renin-angiotensin-aldosterone system plays an important part.

Regulation of the acid-base balance

Changes in the pH level of the blood can have life-threatening consequences. The kidney has mechanisms for the excretion of protons and the reabsorption of bicarbonate.

The kidney’s hormones

The following table lists the hormones of the kidney:

Erythropoietin Secreted in cases of hypoxia Causes the proliferation of erythrocytes to increase the oxygen content in the blood
Renin Secreted due to changes in blood pressure or volume Activates RAAS (see above)
Calcitriol Influenced by parathormone, the concentration of calcium and phosphate, prolactin, and calcitriol Promotes bone mineralization and the absorption of calcium in the intestine

Synthesis of glucose

Glucose is produced in the kidneys by gluconeogenesis and glycogenolysis. This mainly happens through the formation of glucose-6-phosphate from substrates such as lactate, glycerol, and amino acids. Glucose is then released into the blood circulation. Therefore, the kidneys contribute to the regulation of blood sugar in the body.

Histology of the Kidneys

The kidney produces about 1.5 L of urine each day. The entire blood volume flows through the kidneys every 4–5 minutes. The kidney is a paired organ that is situated in the retroperitoneum and embedded in a firm capsule of connective tissue. Another protective capsule of fatty tissue is wrapped around this capsule. The kidney’s parenchyma is divided into the renal cortex (with a thickness of approximately 1 cm) and the renal medulla on the inside (see image).


Cortex. Image by Lecturio.

The renal cortex forms the renal columns, the columnae renales. They extend into the medulla and divide it into 8–15 pyramids. The inner medulla leads to the calyces of the renal pelvis. The renal pelvis, the calyces, the surrounding fatty tissue, the vessels, and the nerves form the renal sinus (sinus renalis). The pelvis narrows and ultimately forms the ureter that drains the urine from the hilum.

Histologically, the kidney can be divided into the interstitium and the nephron.

The Nephron: Functional Unit of the Kidney

The nephron is the functional unit of the kidney. It consists of the renal corpuscles as well as the tubules (tubuli renales) and forms an ingenious filter system (see image).

Blood Flow in the Nephron

Image: “Blood flow in the nephron” by Phil Schatz. License: CC BY 4.0

In each kidney, there are approximately 1–1.5 million renal corpuscles. A corpuscle is composed of the following components:

  • The glomerulus is a tuft of capillaries, woven together by anastomoses. They are supplied with fresh blood by the incoming vas afferens. The blood leaves the glomerulus by the vas efferens.
  • The Bowman’s capsule surrounds the glomerulus and consists of 2 layers. The parietal layer, formed by a single-layer epithelium lying on a basal lamina, is located on the outside. The visceral layer, formed by the so-called podocytes, is located on the inside. These are specialized cells with many processes lining the capillaries of the glomerulus. Both layers are connected at the vessel pole.
  • The mesangium consists of mesangium cells connected with gap junctions. They occupy the space between the capillaries in the nephron. These cells are responsible for the formation of the extracellular matrix (ECM) and the components of the glomerular basement membrane. They also have a phagocytic function and can contract when necessary to stabilize the capillary walls.

The blood-urine barrier: Filter in the nephron

This important filter consists of 3 layers. Each layer refines the structure of the filter.

  1. The capillary endothelium has small windows (100-nm wide) and a strong anionic glycocalyx.
  2. The glomerular basement membrane (GBM) is located in the center and is formed by the capillaries’ and podocytes’ basal lamina.
  3. The podocytes form processes that interdigitate, thereby forming gaps (40 nm). These pores are covered by a membrane consisting mostly of a protein essential for the functioning of the filtration barrier, which is called nephrin. Podocytes also have a strong anionic glycocalyx facing the urinary side.

Molecules up to a size of 4 nm are filtered through these structures and sorted out based on their electrical charge. In this way, the proteins are prevented from entering into the urine. The ratio of the pressure of capillaries to the capsule chamber (55 mm Hg: 15 mm Hg) also plays a role in filtration.

In general, renal corpuscles are only located in the renal cortex. Nephrons may, however, be called cortical, mediocortical, or juxtamedullary, depending on the position of the renal corpuscle. The blood that has to be filtered passes through the renal corpuscle, and then the blood-urine barrier selects the filtrate that enters the Bowman’s space. From here, the ultrafiltrate begins its process of water reabsorption and solute modification.

Schematic structure of the renal corpuscle:

Schematic structure of the renal corpuscle:

Image: “Schematic structure of the kidney body” by Michał Komorniczak. License: CC BY-SA 3.0

A: Renal corpuscle; B: Proximal tubule; C: Distal convoluted tube; D: Juxtaglomerular apparatus; 1: Basement membrane; 2: Bowman’s capsule, parietal layer; 3: Bowman’s capsule, visceral layer; 3a: Podocyte pedicels; 3b: Podocyte; 4: Bowman’s space (urinary space); 5a: Mesangium – intraglomerular cells; 5b: Mesangium – extraglomerular cells; 6: Juxtaglomerular cells; 7: Macula densa; 8: Myocytes (cells of smooth muscle); 9: Afferent arteriole; 10: Glomerulus capillaries; 11: Efferent arteriole.

The renal tubules (tubuli) of the nephron

The proximal tubule begins with the convoluted pars contorta and changes into the distal straight pars recta. In this part, 80% of the water, glucose, amino acids (Na+ symport), electrolytes, and urea contained in the urine are reabsorbed. This task is carried out by the cubic epithelium studded with a dense brush border.

The cells are connected and tightly sealed by tight junctions and the zonula adherens. The cells appear dark under the microscope. They are rich in mitochondria, vesicles for endocytosis, lysosomes, and peroxisomes. In this part of the renal tubules, calcidiol is converted to calcitriol under the influence of the parathyroid hormone (PTH).

The intermediary tubule, however, shows a flat epithelium that hardly features any vesicles or other of the aforementioned structures.

In the following distal tubule, mostly sodium chloride (NaCl) is absorbed, and only a little water. It is a flatter cubic epithelium with many Na/K-ATPases and many tight junctions. The distal tubule is also divided into the pars recta with the intermediary macula densa (which is next to the extraglomerular mesangium) and the pars contorta.

The loop of Henle sits between the proximal and distal tubules and refers to the part of the renal tubule that comprises the intermediate tubule and the straight parts of the proximal and the distal tubule.

The distal tubule merges into the junctional tubule and finally into the collecting tubule.

The collecting tubule consists of the following:

  • Principal cells: They are cubic, and the cytoplasm appears pale. Water can be reabsorbed from the urine under the influence of the antidiuretic hormone ADH. It is transported from cell to cell by aquaporins. The hormone aldosterone also works here to absorb sodium.
  • Intercalated cells – type A and B: These cells are used for the regulation of protons and potassium. Therefore, they have many H+-ATPases and H+/K+-ATPases. They are darker than the principal cells. Subsequently, several collecting tubules unite and form the ductus papillaris, which then runs to the kidney pelvis.

The juxtaglomerular apparatus is a distinctive structure in the kidney and acts as a local controller for the NaCl concentration and as a regulator for the blood pressure in the body. It consists of many different cells.

The juxtaglomerular cells lie between the endothelium and the media of the vas afferens. They react upon activation of the sympathetic system (e.g., hypovolemia) and when there is a drop in pressure that leads to the secretion of renin (renin → angiotensinogen → angiotensin 1 → angiotensin 2 (by ACE) → leads to an increase in aldosterone secretion in the adrenal glands and vasoconstriction → higher blood pressure).

The palisade-shaped cells of the macula densa belong to the cells of the distal tubule. They are located next to the vas afferens and continuously measure the concentration of intraluminal sodium. The vas afferens contracts when sodium is detected.

The extraglomerular mesangium cells (Goormaghtigh cells) are located between the juxtaglomerular cells and the macula densa cells.

The Interstitium

This structure consists of connective tissue and free cells as well as nerves, vessels, and peritubular fibroblasts. These cells produce erythropoietin, the growth hormone for erythrocytes when the oxygen in the blood is low.

In the clinic:

  • If the renal corpuscles and therefore the blood-urine barrier are damaged, the result is a higher permeability for proteins. This leads to hypoproteinemia, generalized edemas, and a higher vulnerability for infections.
  • The kidneys may reabsorb glucose up to a certain threshold value (a blood glucose level of 200 mg/dL). In patients with diabetes mellitus, exceeding this limit results in glucosuria.
  • There is a threshold value for hemoglobin as well. An excess (e.g., during increased hemolysis) leads to hemoglobinuria.

Diseases of the Kidney

For an overview of the most important pathologies of the kidney, next are the most relevant study examples for the categories malformations, inflammations, cysts, and kidney stones.

Malformations of the kidney: Renal agenesis and horseshoe kidney

Renal agenesis refers to the congenital absence of 1 (unilateral) or both (bilateral) kidneys. The absence of 1 kidney may be compensated for by the remaining kidney. Bilateral agenesis, however, is always fatal.

A horseshoe kidney is a frequently occurring abnormality of the kidney. The kidneys are fused together at their lower poles to form the shape of a horseshoe. This malformation is usually asymptomatic.

Renal inflammation

Acute pyelonephritis is a bacterial infection that is mostly caused by an ascending urinary tract infection. In 70% of the cases, the trigger is Escherichia coli – a common fact that appears in exams. Common symptoms include fever, dysuria (problems with urination), and pain on percussion in the flank.

Kidney cysts

“CT of patient with polycystic kidney disease

Image: “CT of a patient with polycystic kidney disease” by Sb2207. License: CC BY 3.0

The simple kidney cyst is the most common renal alteration. The Bosniak classification distinguishes between simple and complex cysts, depending on whether they have septa or calcifications.

The cystic kidney, in turn, refers to a pathology that is either congenital or acquired. The classification of this pathology follows the Potter system. The hereditary form is termed polycystic (see image), whereas cystic changes caused by dysplastic development are referred to as multicystic.

Kidney stones, or urolithiasis

Kidney Stone

Image: “Kidney stone” by Melensdad. License: CC BY 2.5

Kidney stones (see image) develop when there is an oversaturation of 1 substance in the urine. Depending on the urinary pH, the substance settles and forms stones.

There are many reasons for high concentrations of kidney stones in the urine. Too little fluid intake, high fluid loss, the state of being nutrition-rich in sodium chloride and calcium, immobilization during a long time, hormonal changes, gout, and many other things may be responsible for the formation of kidney stones.

Inorganic and organic stones are separate categories of kidney stones.

Inorganic stones:

  • Stones can be made of calcium oxalate (65%), calcium phosphate (10%), or magnesium-ammonium-phosphate (10% – also called struvite stones).

These stones are visible on radiographs without the application of a contrast medium.

Organic stones:

  • These include uric acid residues (10% – increased in gout) and cystine stones (1%).

These stones cannot be seen without the application of a contrast medium.

Symptoms occur when the urine flow is obstructed or the stone gets stuck in a narrow part. Common symptoms include pain in the flanks spreading to the lower abdomen and the back. They can even lead to colic, nausea, and vomiting, and sometimes to blood in the urine. As a result of kidney stones, complications may occur, leading to urinary obstruction and infection.

Small kidney stones often travel through the ureters and bladder by themselves. However, if passage of the stones is not possible, the following measures can be taken:

  • Extracorporeal shock wave lithotripsy: The kidney stone is placed at the intersection of 2 shock waves and decomposes into many fragments.
  • Pyelotomy: The patient undergoes surgery in which the renal pelvis is opened.
  • Flexible or semirigid ureteroscopy: The ureter is flushed, and the stone is crushed.

Renal cell carcinoma

The most common malignant renal cancer is renal cell carcinoma, formerly known as Grawitz’s tumor. It originates mainly in the epithelia of the proximal tubule. Under the microscope, cancer can be discerned as a clear cell tumor, which means the cytoplasm is full of glycogen and lipids.

Renal cell carcinoma can be detected early in an ultrasonic examination. Symptoms, such as pain in the flank or painless hematuria, occur later. The moment of detection is very crucial for the progression of the disease and prognosis. Men are affected twice as often as women are.

Kidney Transplantation

If there is a definitive kidney failure, a kidney transplant is the best treatment.

Note: The donor organ is inserted extraperitoneally into the fossa iliaca and anastomosed with the iliac vessels. Hereby, the typically performed skin incision resembles the form of a hockey stick.

Organs to be transplanted may be obtained either from a living donor or from a deceased donor (post-mortem donation). Living donation has some advantages: There is little or no waiting time and a longer functioning of the donor organ, and generally a higher life expectancy for the receiver.

Post-mortem donation means that the organ stems from a person who has been determined to be brain-dead. The criteria for a possible donor include the matching of the human leukocyte antigen (HLA) complex. The waiting time for such a donor organ is usually several years and is bridged by dialysis (“blood cleansing”).

Some complications may occur after a kidney transplant. In addition to the risk of wound infection, thrombosis, and other typical postoperative complications, there may be a rejection. To minimize the risk, the receiver will have to take immunosuppressants for their entire life.

Examination of the Kidney

The kidneys can be examined by palpation and radiographic methods, as shown in the following table:

Bimanual palpation In slim patients, the lower renal poles can be felt. In the supine position, 1 hand pushes onto the anterior abdominal wall. The other hand holds the back and pushes against the abdominal wall.
X-ray The renal pelvis is only visible in patients with radiopaque stones. Otherwise, contrast medium is needed: intravenous pyelography or retrograde pyelography.
Urinalysis This is performed on midstream urine. In the urine strip test, erythrocytes, leukocytes, pH, glucose, protein, nitrite, and ketone may be detected in the urine.
Sonography This is done in the supine position. Findings are standard size, extensions of the renal pelvis, kidney stones, cysts, and tumors.
Scintigraphy Assessment of the renal function.
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