General characteristics and functions of the kidney
Kidney (lat. ren, greek nephros) is a paired excretory organ that ensures blood purification and chemical homeostasis regulation through urine formation. The kidneys maintain the constancy of the body's internal environment by regulating water balance, blood ionic composition, osmotic pressure, and acid-base balance.
Main functions of the kidney:
- Homeostatic — maintaining the constancy of the internal environment.
- Metabolic — participation in protein, lipid, and carbohydrate metabolism.
- Incretory — secretion of renin and erythropoietin hormones that regulate the cardiovascular system and erythropoiesis.
- Hemostatic — production of humoral regulators of the blood coagulation and fibrinolysis system.
- Protective (excretory) — elimination of harmful substances from the body.
Renal topography
The kidneys are located in the lumbar region on both sides of the spinal column and lie retroperitoneally (behind the peritoneum).
Skeletotopy. The right kidney projects from the XII thoracic to the upper border of the IV lumbar vertebra; the left one lies one vertebra higher — from the upper pole of the XI thoracic to the upper border of the III lumbar vertebra. The lower position of the right kidney is associated with the adjacency of the liver.
Holotopy. The kidneys project onto the epigastric, hypochondriac, and lateral regions of the abdomen, and partially onto the umbilical region.
Syntopy of the right kidney: the anterior (more convex) surface is adjacent to the visceral surface of the right lobe of the liver (with the renal impression), the right colic flexure; the right suprarenal gland is adjacent to the upper pole.
Syntopy of the left kidney: the anterior surface contacts the posterior wall of the stomach, the tail of the pancreas, the spleen, and loops of the small intestine; the left suprarenal gland is adjacent to the upper pole.
The posterior (flatter) surface of both kidneys is adjacent to the muscles of the posterior abdominal wall: diaphragma, musculus psoas major, musculus quadratus lumborum, musculus transversus abdominis. These muscles form the renal muscular bed.
External structure and capsules
The kidney surface is smooth. There are: facies anterior (convex) and facies posterior (flattened); extremitas superior and extremitas inferior; margo lateralis (convex) and margo medialis (concave). On the medial margin, there is the renal hilum (hilum renalis), leading into the renal sinus (sinus renalis).
The renal artery and nerves enter, and the ureter, renal vein, and lymphatic vessels exit through the hilum — together they form the renal pedicle. The sinus contains minor and major renal calyces, the renal pelvis, vessels, nerves, and adipose tissue.
The kidney has three capsules (from inside out):
- Fibrous capsule (capsula fibrosa) — the proper membrane, dense connective tissue, easily separates from the renal parenchyma.
- Adipose capsule (capsula adiposa) — loose connective tissue with fat, prominent at the hilum and posteriorly.
- Renal fascia (fascia renalis) — the outer membrane consisting of anterior and posterior layers of dense connective tissue.
The renal fascia and adipose capsule are shared between the kidney and the suprarenal gland, whereas the fibrous capsule belongs exclusively to the kidney.
Fixation apparatus of the kidney
The position of the kidney is maintained by:
- intra-abdominal pressure created by the contraction of the abdominal wall muscles;
- the muscular bed (diaphragm, psoas major, quadratus lumborum, transversus abdominis);
- the renal pedicle — renal artery and vein;
- the renal fascia and adipose capsule connected to the fibrous capsule by connective tissue strands.
Internal structure: cortex and medulla
On section, a superficial cortex (cortex renalis) less than 1 cm thick and a deep medulla (medulla renalis) 2–2.5 cm thick are distinguished. The cortex extends between the pyramids as renal columns (columnae renales).
The medulla consists of 15–20 renal (Malpighian) pyramids (pyramides renales), the apices of which form renal papillae (papillae renales) with papillary orifices (foramina papillaria), constituting the area cribrosa.
Contents of the renal sinus: minor renal calyces (calices renales minores), major calyces (calices renales majores), and renal pelvis (pelvis renalis).
Vascular division: 5 segments (superior, anterosuperior, anteroinferior, inferior, posterior); a segment comprises 2–3 renal lobes (lobus renalis — a pyramid with the adjacent cortical area); each lobe contains about 600 cortical lobules.
Nephron — the structural and functional unit
The renal parenchyma is represented by renal tubules (tubuli renales), which together with capillaries form nephrons. Each kidney contains about 1 million nephrons: ~80% cortical and ~20% juxtamedullary.
The nephron consists of four parts:
- Renal corpuscle (Malpighian corpuscle) — glomerulus (glomerulus) of arterial capillaries and the glomerular capsule (Bowman's capsule).
- Proximal convoluted tubule (first-order tubule).
- Loop of nephron (loop of Henle) with descending and ascending limbs.
- Distal convoluted tubule (second-order tubule).
Distal tubules empty into collecting ducts, which merge into 12–15 papillary ducts opening at the apex of the papilla. In the cortex, the cortical labyrinth (straight tubules and the beginning of collecting ducts) and convoluted part (corpuscles and convoluted tubules) are distinguished; the radiata, surrounded by the convoluted part, forms the renal lobule.
Blood supply and the marvelous network
The arteria renalis branches off the abdominal aorta and divides into anterior and posterior branches at the hilum, and further into segmental arteries. Branching sequence:
- segmental arteries → interlobar (aa. interlobares);
- arcuate arteries (aa. arcuatae) — at the corticomedullary junction;
- interlobular arteries (aa. interlobulares);
- afferent arteriole (vas afferens), forming the glomerulus;
- efferent arteriole (vas efferens), entwining the tubules with a secondary capillary network.
A unique feature is the renal marvelous network (rete mirabile): an arteriole, not a venule, emerges from the glomerular capillary network and breaks up again into capillaries around the tubules. Venous drainage mirrors the arterial pathway: interlobular → arcuate → interlobar → renal vein emptying into the inferior vena cava.
Key points
- The kidney is a paired retroperitoneal organ with excretory, homeostatic, metabolic, incretory, and hemostatic functions.
- The right kidney lies at the level of Th XII–L IV, the left — one vertebra higher (Th XI–L III).
- Three capsules: fibrous capsule, adipose capsule, and renal fascia; the latter two are shared with the suprarenal gland.
- Fixation is provided by intra-abdominal pressure, the muscular bed, the renal pedicle, and the fascial-adipose apparatus.
- The parenchyma is divided into the cortex and medulla with 15–20 pyramids; each renal segment is supplied by a single segmental artery.
- The structural and functional unit is the nephron: corpuscle, proximal tubule, loop of Henle, distal tubule.
- There are about 1 million nephrons in the kidney; urine flows through papillary ducts into minor and major calyces and the renal pelvis.
- The renal marvelous network is the presence of two consecutive capillary networks between the afferent and efferent arterioles.