The structure of the kidney

The structure of the kidney. The substance of the kidney on the incision is not uniform. It consists of a surface layer with a thickness of 0.4 to 0.7 cm and a deep layer 2 to 2.5 cm thick, represented by pyramid-shaped sections. The surface layer forms a crustal substance of a kidney of a dark red color, consisting of renal corpuscles, proximal and distal tubules of nephrons. The deep layer of the kidney is lighter, reddish, represents a brain substance in which the descending and ascending parts of the tubules (nephrons), as well as the collecting tubules and papillary tubules.

The cortex renalis, which forms its surface layer, is not homogeneous, but consists of alternating lighter and darker areas. Light areas are cone-shaped, they in the form of rays depart from the brain substance into the cortical one. The rays of the medullaris (radii medullaris) form the radiant part (pars radiata), in which the direct renal tubules, which extend into the medulla of the kidney, and the initial sections of the collecting tubules are located. The dark parts of the cortical substance of the kidney are called the folded part (pars convoliita). They contain renal corpuscles, proximal and distal sections of convoluted renal tubules.

The brain substance of the kidney (medulla renalis), unlike the cortical one, does not form a continuous layer, but has on the frontal section of the organ the appearance of separate triangular forms of the sections delimited from each other by the renal columns. The renal columns (columnae renalis) are narrow areas in which, in the environment of the connective tissue, blood vessels pass - the interlobar artery and the vein. The triangular portions are called medulla of kidney feast mid (pyramides renales), in their kidney from 10 to 15. Each kidney pyramid has a base (basis pyramidis), facing the cortex, and the tip of renal papilla (papilla renalis), directed in side of the renal sinus. The renal pyramid consists of direct tubules forming the nephron loops, and of the collecting tubules passing through the medulla. These tubes gradually merge with each other and form 15-20 short papillary ducts (ductus papillares) in the region of the renal papilla . The latter open into small kidney cups on the surface of the papilla of the papillae with holes (foramina papillaria). Due to the presence of these openings, the tip of the renal papilla has a lattice structure and is called the area cribrosa.

The peculiarities of the structure of the kidney and its blood vessels make it possible to divide the substance of the kidney into 5 segments: the upper segmentum (superficial), the upper anterior (segmentum anterius superius), the lower anterior (segmentum anterius inferius), the lower segmentum inferius and the posterum (segmentum posterius). Each segment unites 2-3 renal lobes. One renal fraction (lobus renalis) includes the renal pyramid with the cortical attached to it and is bounded by interlobular arteries and veins lying in the kidney poles. In the kidney lobe, there are about 600 cortical lobules. The cortical lobe (lobulus corticalis) consists of one radiant part surrounded by a folded part, and is bounded by adjacent interlobular arteries and veins.

The structural and functional unit of the kidney is the nephron (nephron). It consists of a capsule of the glomerulus (sarsula glomerularis; Shumlyansky-Bowman capsule), which has the form of a double-walled glass, and tubules. The capsule covers glomerular capillary network resulting in a kidney (Malpighian) body (corpusculum renale). The gland capsule continues into the proximal tubular contortus proximalis and passes into the nephron loop (ansa nephroni, Henle loop) in which the descending and ascending parts are distinguished. The nephron loop passes into the distal tubular contralus distalis, which flows into the collecting tubule (tubulus renalis colligens). The collecting kidney ducts continue into the papillary ducts. Throughout the tubulars of the nephron are surrounded by the adjacent blood capillaries.

About 80% of nephrons have renal corpuscles located in the cortex and a relatively short loop that descends only into the outer part of the medulla. Approximately 1% of nephrons are completely located in the cortical substance of the kidney. All these are cortical nephrons. In the remaining 20% of nephrons, renal corpuscles, proximal and distal convoluted tubules are located on the border with the medulla, and their long loops descend into the brain substance - they are near-cerebral (juxtamedullary) nephrons.

There are about a million nephrons in the 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 a glomerular capillary network (vascular glomerulus) formed by more than 50 hemocapillaries. The nephron capsule, together with the vascular glomerulus, forms a renal corpus about 20 μm in diameter. The endothelium of the circulatory capillaries of the vascular glomerulus has fenesters up to 0.1 μm in size. Outside the endothelium is the basal membrane. The epithelium of the inner sheet of the nephron capsule lies on its outer side. The epithelial cells of this leaf are large (up to 30 μm), irregular in shape and are called podocytes. From the podocytes depart the processes - the cytopodium, attached to the basal membrane. Between the cytopodia there are narrow slits (pores) opening the access to the basal membrane. The outer sheet of the nephron capsule is represented by a single-layered cubic epithelium, also located on the basal membrane. The epithelium of the capillaries, the podocytes of the inner layer of the capsule and the basement membrane common to them form a filtration apparatus of the kidney. Through it, the blood is filtered into the cavity of the capsule and the formation of primary urine (more than 100 liters per day).

The proximal convoluted tubule of the nephron is represented by 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-layered cubic epithelium. Epitheliocytes on the apical surface have a brush border, for the basal surface of cells is characterized by striation. Among the organelles of epithelial cells, lysosomes and mitochondria predominate. At the level of this tubule, there is a reverse absorption from the primary urine, proteins, glucose, electrolytes, water into the blood capillaries, braided tubule (reabsorption).

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

The distal convoluted tubules are short, their diameter is 20-50 μm. The walls of the tubules are formed by a single layer of cubic cells devoid of the brush border. The plasmatic membrane of most epithelial cells is folded by the mitochondria located under the cytolemma. Through the walls of the distal convoluted tubules, there is a further absorption of a large amount of water into the blood. The suction process continues in the collecting tubes. As a result, the amount of final (secondary urine) decreases sharply. The concentration in the secondary urine of urea, uric acid, creatine (substances not being reabsorbed) increases.

Collecting renal tubules in the radiant part of the cortex are lined with single-layered cubic epithelium, in its lower part (in the medulla of the kidney) - a single-layered low cylindrical epithelium. The composition of the epithelium of the collecting renal tubules distinguishes light and dark cells. Light cells are poor in organelles, their cytoplasm forms internal folds. Dark cells in the ultrastructure are close to the parietal cells of the glands of the stomach.

Each renal papilla on the top of the pyramid covers the funnel-shaped small renalis cup (calix renalis minor). Sometimes several (2-3) renal papillae are turned into one small kidney cup. From the junction of two or three small kidney cups, a large renal cup (calix renalis major) is formed. When two or three large renal cups merge with each other, an enlarged common cavity is formed - the renal pelvis (pelvis renalis), resembling a flattened funnel in shape. Gradually tapering downwards, the renal pelvis in the area of the kidney gate passes into the ureter. Small and large renal cups, renal pelvis and ureter make up the urinary tract.

There are three stages of the formation of the renal pelvis: embryonic, fetal and mature. At the first stage, large renal cups are not expressed, therefore small kidney cups directly fall into the renal pelvis. At stage II, the existing large kidney cups pass into the ureter, and the pelvis is not formed. At stage III, the usual number of small kidney cups is observed, which fall into two large kidney cups; the latter pass to the renal pelvis, from where the ureter begins. In the form of a renal pelvis is an ampullar, tree-like and mixed.

The walls of the pelvis, large and small kidney cups have the same structure. In the walls distinguish mucous, muscular and external (adventitial) membranes. The walls of the small kidney cups in the region of the arch (the initial part) consist of smooth muscle cells, which form an annular layer - the compressor of the vault (renal cup). Nerve fibers, blood and lymphatic vessels are suitable for this part of the wall. All this is a forniculum apparatus of the kidney, whose role is to regulate the amount of urine output from the renal tubules into small kidney cups, to create an obstacle to the return flow of urine and to maintain intra-venous pressure.

Kidneys are not only organs of excretion, they also perform endocrine function. In the walls of the ascending tubule of the nephron loop, when it passes into the distal convoluted tubule between the bringing and enduring glomerular arterioles on a very thin basal membrane, high epithelial cells are located, devoid of basal folding. This portion of the distal tubule is called a dense patch. Presumably, it catches changes in the sodium content in urine and affects the juxtaglomerular cells secreting renin and the renal erythropoietic factor. Yuxtaglomerular cells are located under the endothelium in the walls of the bringing and exiting glomerular arterioles near the dense spot. In the stroma of the pyramids of the medulla there are so-called interstitial cells that produce prostaglandins (biologically active substances of antihypertensive and other action). The endocrine complex of the kidney is involved in the regulation of the general and renal circulation, and through it affects the urination.