What Is the Anatomy of a Kidney?

A frontal section through the kidney reveals the kidney's inner region, the medulla, and its outer region, the renal cortex.

• In the medulla, five to eight renal pyramids are separated by connective tissue renal columns.
• Each pyramid ends in a renal papilla and produces urine.
• Each renal papilla empties into a small pool known as a minor calyx.
• Major calyces are formed when multiple minor calyces come together, all major calyces connect to the single renal pelvis, which connects to the ureter.

The kidneys are paired and located on either side of the spine in the retroperitoneal region between the parietal peritoneum and posterior abdominal wall. These organs are well-protected by muscle, fat, and ribs. The right kidney is lower because of a minor displacement by the liver, whereas the left kidney is situated at about the T12 to L3 vertebrae.

The upper part of the kidneys is somewhat shielded by the 11th and 12th ribs.

• Male kidneys weigh 125 to 175 grams, whereas female kidneys weigh 115 to 155 grams. 
• They are directly covered by a fibrous capsule made of dense, irregular connective tissue, which helps maintain their shape and protects them.
• They are about 11 to 14 cm long, 6 cm wide and 4 cm thick.

This capsule is surrounded by a shock-absorbing layer, which is made up of adipose tissue, known as the renal fat pad, which in turn is encircled by the robust renal fascia. The kidneys are firmly attached to the posterior abdominal wall in a retroperitoneal position by the fascia and, to a smaller extent, by the peritoneum overlaying them.

The kidneys are adequately vascularized and receive roughly 25 percent of the cardiac output while at rest. Blood enters the kidneys through the paired renal arteries, which branch directly from the descending aorta and enter individually at the renal hila.

• Each renal artery splits into segmental arteries after entering the kidney, then branches again to generate interlobar arteries, which travel through the renal columns to reach the cortex.
• The interlobar arteries in turn divide into arcuate arteries, cortical radiate arteries, and then afferent arterioles.
• The afferent arterioles direct the blood into a modified capillary bed called the glomerulus, which is a component of the "functional unit" of the kidney, the nephron.

Each kidney has roughly 1.3 million nephrons, which are responsible for filtering blood. Renal veins return blood to the inferior vena cava after the nephrons have filtered it.

A portal system is formed when blood travels from the glomerulus to the efferent arteriole through the peritubular capillaries (including vasa recta), which encircle the proximal and distal convoluted tubules as well as the loop of Henle. This second capillary bed recovers most of the water and solutes. This filtrate is then purified and collected by collecting ducts that empty into minor calyces, which combine to create major calyces. The filtrate then travels to the renal pelvis and, finally to the ureters.

A nephron consists of a glomerulus and a sophisticated tubular system. The proximal convoluted tubule (PCT), the first part of the tubular system, and the glomerulus are situated in the renal cortex. The loop of Henle is a hairpin-like structure that emerges from the PCT, enters the medulla, travels back to the cortex, and then connects with the distal convoluted tubule (DCT). The nephron finally drains into the collecting duct by connecting tubules.

There are two different kinds of nephrons:

1. Superficial nephrons, which have short loops of Henle and glomeruli, are found near the cortical surface.
2. Juxtamedullary nephrons have long loops of Henle, which descend deeper into the renal medulla. These nephrons are located close to the corticomedullary junction.

The glomerulus filters large amounts of blood and the tubular system converts that blood into urine by reabsorbing and secreting free water and solutes from the blood.

The following are the five main parts of a nephron:

1. The glomerulus: Is formed by a tuft of capillaries encased in an impermeable capsule, which is known as Bowman's capsule. Two resistance arteries, the afferent and efferent arterioles, which control intraglomerular pressure, are located on either side of the glomerular capillaries. These capillaries may filter a lot of blood because of their unique characteristics. The filtration barrier is composed of three structures that provide the support and selective properties needed to form the primary glomerular filtrate, the ultrafiltrate.
   • Fenestrated endothelium of the glomerular capillaries
   • Glomerular basement membrane
   • Podocytes
2. The proximal convoluted tubule (PCT): Develops from Bowman's capsule and is situated next to the glomerulus in the renal cortex. The PCT originates from a simple cuboidal epithelium responsible for absorbing and transporting water, electrolytes, and other particles. These cells are distinguished by a brush border of microvilli (which increase the surface in contact with the glomerular ultrafiltrate), an abundance of long, thin mitochondria lining the basal pole of the cell, and numerous vesicles involved in the transcellular transport of 60 to 80 percent of the ultrafiltrate. The PCT is surrounded by peritubular capillaries. In addition to recovering the reabsorbed free water, ions, and other plasma components, such as amino acids and glucose, this capillary network supplies blood to the tubules.
3. The loop of Henle: The PCT leaves the renal cortex and changes into a thin descending limb of the loop of Henle, which pierces the renal medulla. The cells in the tubule become smaller and have fewer mitochondria and shorter microvilli. It is tough to observe these cells under light microscopy. The tubule then transforms into the thick ascending limb (TAL) and moves toward the cortex. The lining cells grow, develop additional microvilli and mitochondria, and actively transport sodium to dilute the urine.
4. The juxtaglomerular apparatus and the distal convoluted tubule: The juxtaglomerular system controls glomerular filtration via tubuloglomerular feedback. According to histology, this area is close to the glomerulus vascular pole. It is composed of the granular smooth muscle cells of the afferent arteriole of the glomerulus and the macula densa cells of the cortical TAL, which are physically and functionally linked by glomerular mesangial cells. The macula densa cells are tall, densely packed tubule cells, which are visually unique from other tubule cells.
   • The TAL turns into the DCT after returning to the renal cortex near its glomerulus of origin. The DCT is the portion of the nephron that lies between the macula densa and the cortical collecting tubule.
   • The DCT cells are tall cells distinguished by having the most mitochondria of any other type of cell in the nephron. The cells’ basal regions have a palisading look due to their substantial basolateral amplification, which encloses numerous mitochondria. In the DCT's last segment, intercalated cells form and persist throughout the connecting and collecting tubules.
5. Connecting and collecting tubules: The connecting tubules, the last component of the nephron, are where the last modification to the urine is made. There are two different types of cells in these tubules: intercalated cells and connecting tubule (CNT) cells. When seen under an electron microscope, the intercalated cells are dense and lack the basolateral amplification present in DCT cells. These cells control the secretion of bicarbonate and hydrogen. Although they have fewer mitochondria than DCT cells, the connecting tubule cells also feature basolateral amplification.
   • Principal cells signal the transition into the collecting tubules and the termination of the nephron. The CNT in cortical nephrons connects to the collecting tubule, which empties into a collecting duct. Juxtamedullary nephrons' connecting tubules unite to form an arcade that empties into a shared collecting duct.