The structure of the kidney

Structure of the kidney. The substance of the kidney is not uniform in cross-section. It consists of a superficial layer 0.4 to 0.7 cm thick and a deep layer 2 to 2.5 cm thick, represented by pyramid-shaped areas. The superficial layer forms the cortex of the kidney, which is dark red in color and consists of renal corpuscles and proximal and distal tubules of the nephrons. The deep layer of the kidney is lighter, reddish in color, and is the medulla, which contains the descending and ascending parts of the tubules (nephrons), as well as collecting tubules and papillary tubules.

The renal cortex (cortex renalis), which forms its surface layer, is not homogeneous, but consists of alternating lighter and darker areas. The light areas are cone-shaped, and extend in the form of rays from the medulla into the cortex. The rays of the medulla (radii medullaris) form the radiant part (pars radiata), which contains the straight renal tubules, which continue into the medulla of the kidney, and the initial sections of the collecting ducts. The dark areas of the renal cortex are called the convoluted part (pars convoliita). They contain the renal corpuscles, proximal and distal sections of the convoluted renal tubules.

The renal medulla (medulla renalis), unlike the cortex, does not form a continuous layer, but on the frontal section of the organ looks like separate triangular sections, separated from each other by renal columns. The renal columns (columnae renalis) are narrow sections in which blood vessels pass, surrounded by connective tissue - the interlobar artery and vein. The triangular sections of the medulla are called renalpyramids (pyramides renales), there are from 10 to 15 of them in the kidney. Each renal pyramid has a base (basis pyramidis) facing the cortex, and an apex in the form of a renal papilla (papilla renalis), directed towards the renal sinus. The renal pyramid consists of straight tubules that form loops of nephrons, and collecting ducts passing through the medulla. These tubes gradually merge with each other and form 15-20 short papillary ducts (ductus papillares) in the area of the renal papilla. The latter open into the small renal cups on the surface of the papilla by papillary openings (foramina papillaria). Due to the presence of these openings, the top of the renal papilla has a kind of lattice structure and is called the cribriform area (area cribrosa).

The structural features of the kidney and its blood vessels allow dividing the renal substance into 5 segments: upper (segmentum superius), upper anterior (segmentum anterius superius), lower anterior (segmentum anterius inferius), lower (segmentum inferius) and posterior (segmentum posterius). Each segment unites 2-3 renal lobes. One renal lobe (lobus renalis) includes the renal pyramid with the adjacent cortex of the kidney and is limited by interlobular arteries and veins located in the renal columns. There are approximately 600 cortical lobules in the renal lobe. The cortical lobule (lobulus corticalis) consists of one radiant part surrounded by a folded part and is limited by adjacent interlobular arteries and veins.

The structural and functional unit of the kidney is the nephron. It consists of the glomerular capsule (capsula glomerularis; Shumlyansky-Bowman capsule), which has the shape of a double-walled goblet, and tubules. The capsule encloses the glomerular capillary network, resulting in the formation of the renal (Malpighian) corpuscle (corpusculum renale). The glomerular capsule continues into the proximal convoluted tubule (tubulus contortus proximalis) and passes into the nephron loop (ansa nephroni; loop of Henle), which has descending and ascending parts. The nephron loop passes into the distal convoluted tubule (tubulus contortus distalis), which flows into the renal collecting tubule (tubulus renalis colligens). The renal collecting tubules continue into the papillary ducts. Throughout their entire length, the nephron tubules are surrounded by adjacent blood capillaries.

About 80% of nephrons have renal corpuscles located in the cortex and a relatively short loop descending only into the outer part of the medulla. About 1% of nephrons are located entirely in the renal cortex. These are all cortical nephrons. The remaining 20% of nephrons have renal corpuscles, proximal and distal convoluted tubules located on the border with the medulla, and their long loops descend into the medulla - these are perimedullary (juxtamedullary) nephrons.

There are about a million nephrons in a kidney. The length of the tubules of one nephron varies from 20 to 50 mm, the total length of all the tubules in two kidneys is about 100 km.

The structure of the nephron is complex. The beginning of the nephron is its capsule, between the outer and inner walls of which there is a cavity of the nephron capsule. Inside the capsule is the glomerular capillary network (vascular glomerulus), formed by more than 50 hemocapillaries. The capsule of the nephron together with the vascular glomerulus make up the renal corpuscle with a diameter of about 20 μm. The endothelium of the blood capillaries of the vascular glomerulus has fenestrae up to 0.1 μm in size. The basal membrane is located outside the endothelium. On its outer side is the epithelium of the inner leaflet of the nephron capsule. The epithelial cells of this leaflet are large (up to 30 μm), irregular in shape and are called podocytes. Processes - cytopodia - extend from the podocytes and are attached to the basal membrane. Between the cytopodia there are narrow slits (pores) providing access to the basal membrane. The outer layer of the nephron capsule is represented by a single-layer cubic epithelium, also located on the basement membrane. The epithelium of the capillaries, the podocytes of the inner layer of the capsule and the basement membrane common to them form the filtration apparatus of the kidney. Through it, blood is filtered into the cavity of the capsule and primary urine is formed (more than 100 liters per day).

The proximal convoluted tubule of the nephron is a short tube with a narrow, irregularly shaped lumen with a diameter of about 60 μm. The walls of the tubule are formed by a single-layer cubic bordered epithelium. Epithelial cells on the apical surface have a brush border, the basal surface of the cells is characterized by striation. Lysosomes and mitochondria predominate among the organelles of the epithelial cells. At the level of this tubule, reverse absorption of proteins, glucose, electrolytes, and water from the primary urine into the blood capillaries that envelop the tubule (reabsorption) occurs.

The descending part of the nephron loop is thin (about 15 µm in diameter), lined with flat epithelial cells with light cytoplasm, poor in organelles. The ascending part of the loop is thick, has a diameter of about 30 µm. It is also lined with flat epithelial cells located on the basement membrane. At the level of the nephron loop, reabsorption of water, sodium and other substances occurs.

The distal convoluted tubules are short, with a diameter of 20-50 µm. The tubule walls are formed by a single layer of cubic cells lacking a brush border. The plasma membrane of most epithelial cells is folded due to the mitochondria located under the cytolemma. Further absorption of a large amount of water into the blood occurs through the walls of the distal convoluted tubules. The absorption process continues in the collecting tubules. As a result, the amount of final (secondary urine) decreases sharply. The concentration of urea, uric acid, and creatine (substances that are not subject to reabsorption) in the secondary urine increases.

The collecting renal tubules in the radiant part of the cortex are lined with a single-layer cuboidal epithelium, in their lower part (in the renal medulla) - with a single-layer low columnar epithelium. The epithelium of the collecting renal tubules includes light and dark cells. Light cells are poor in organelles, their cytoplasm forms internal folds. Dark cells are similar in ultrastructure to parietal cells of the gastric glands.

Each renal papilla at the top of the pyramid is surrounded by a funnel-shaped minor renal calyx (calix renalis minor). Sometimes several (2-3) renal papillae are directed into one minor renal calyx. The fusion of two or three minor renal calyces forms a major renal calyx (calix renalis major). When two or three major renal calyces merge, an expanded common cavity is formed - the renal pelvis (pelvis renalis), resembling a flattened funnel in shape. Gradually narrowing downwards, the renal pelvis passes into the ureter in the area of the renal hilum. The minor and major renal calyces, the renal pelvis and the ureter make up the urinary tract.

There are three stages of renal pelvis formation: embryonic, fetal and mature. At stage I, the large renal calyces are not expressed, so the small renal calyces flow directly into the renal pelvis. At stage II, the existing large renal calyces pass into the ureter, and the pelvis is not formed. At stage III, there is a normal number of small renal calyces, which flow into two large renal calyces; the latter pass into the renal pelvis, from where the ureter begins. According to the shape, the renal pelvis can be ampullar, tree-like and mixed.

The walls of the renal pelvis, major and minor calyces have the same structure. The walls are divided into mucous, muscular and external (adventitial) membranes. The walls of minor calyces in the area of the fornix (initial part) consist of smooth muscle cells that form a ring-shaped layer - the fornix (renal calyx) depressor. Nerve fibers, blood and lymphatic vessels approach this section of the wall. All this makes up the fornicate apparatus of the kidney, the role of which is to regulate the amount of urine excreted from the renal tubules into the minor calyces, to create an obstacle to the backflow of urine and to maintain intrapelvic pressure.

The kidneys are not only organs of excretion, they also perform an endocrine function. In the walls of the ascending tubule of the nephron loop at its transition to the distal convoluted tubule between the afferent and efferent glomerular arterioles, tall epithelial cells devoid of basal folding are located on a very thin basement membrane. This area of the distal tubule is called the macula densa. Presumably, it detects changes in the sodium content in the urine and affects the juxtaglomerular cells that secrete renin and renal erythropoietic factor. Juxtaglomerular cells are located under the endothelium in the walls of the afferent and efferent glomerular arterioles near the macula densa. In the stroma of the pyramids of the medulla, there are so-called interstitial cells that produce prostaglandins (biologically active substances with antihypertensive and other effects). The endocrine complex of the kidney is involved in the regulation of general and renal circulation, and through it affects urine formation.