Kidney nephron

The nephron consists of a continuous tube of highly specialized heterogeneous cells that perform various functions. Each kidney contains between 800,000 and 1,300,000 nephrons. The total length of all nephrons in both kidneys is about 110 km. Most of the nephrons (85%) are located in the cortex (cortical nephrons), a smaller part (15%) is located at the border of the cortex and medulla in the so-called juxtamedullary zone (juxtamedullary nephrons). There are significant structural and functional differences between the nephrons: in cortical nephrons, the loop of Henle is short. It ends at the border of the outer and inner zones of the medulla, while the loop of Henle of juxtamedullary nephrons goes deep into the inner layer of the medulla.

Each nephron consists of several structural elements. According to the modern nomenclature, which was standardized in 1988, the composition of the nephron includes:

• renal glomerulus;
• proximal tubule (convoluted and straight part);
• descending thin segment;
• ascending thin segment;
• distal straight tubule (formerly thick ascending loop of Henle);
• distal convoluted tubule;
• connecting canal;
• cortical collecting duct;
• collecting duct of the outer zone of the medulla;
• collecting duct of the inner zone of the medulla.

The space between all structures of the nephron, both in the cortex and in the medulla, is filled with a dense connective tissue base, which is represented by interstitial cells located in the intercellular matrix.

Renal glomerulus

The renal glomerulus is the initial part of the nephron. It is a "network ball" of 7-20 capillary loops enclosed in Bowman's capsule. The glomerular capillaries are formed from the afferent glomerular arteriole and then join at the exit from the glomerulus into the efferent glomerular arteriole. There are anastomoses between the capillary loops. The central part of the glomerulus is occupied by mesangial cells surrounded by a mesangial matrix, which fix the capillary loops of the glomerulus to the vascular pole of the glomerulus - its handle - the place where the afferent arteriole enters and the efferent arteriole exits. Directly opposite in the glomerulus is the urinary pole - the place where the proximal tubule begins.

Renal capillaries participate in the formation of the glomerular filter, designed for the process of blood ultrafiltration - the first stage of urine formation, which consists of separating the liquid part of the blood flowing through them with substances dissolved in it. At the same time, formed elements of the blood and proteins should not get into the ultrafiltrate.

Structure of the glomerular filter

The glomerular filter consists of three layers - epithelium (podocytes), basement membrane and endothelial cells. Each of these layers is important in the filtration process.

Podocytes

They are represented by large, highly differentiated cells with a "body" from which large and small processes (podocyte legs) extend from the side of the glomerular capsule. These processes are closely intertwined with each other, envelop the surface of the glomerular capillaries from the outside and are immersed in the outer plate of the basement membrane. Between the small processes of the podocytes there are slit diaphragms, which are one of the variants of filtration pores. They prevent the penetration of proteins into the urine due to the small diameter of the pores (5-12 nm) and the electrochemical factor: the slit diaphragms are covered on the outside with a negatively charged glycocalyx (sialoprotein compounds), which prevents the penetration of proteins from the blood into the urine.

Thus, podocytes act as a structural support for the basement membrane and, in addition, create an anion barrier during biological ultrafiltration. It is suggested that podocytes have phagocytic and contractile activity.

Basement membrane of the glomerular capillaries

The basement membrane is three-layered: two thinner layers are located on the outer and inner sides of the membrane, and the inner layer, denser, is represented mainly by type IV collagen, laminin, as well as sialic acid and glycosaminoglycans, mainly heparan sulfate, which serve as a barrier to filtration of negatively charged macromolecules of blood plasma proteins through the basement membrane.

The basal membrane contains pores, the maximum size of which normally does not exceed the size of an albumin molecule. Finely dispersed proteins with a molecular weight lower than that of albumin can pass through them, but larger proteins cannot.

Thus, the second barrier to the passage of plasma proteins into the urine is the basement membrane of the glomerular capillaries due to the small size of the pores and the negative charge of the basement membrane.

Endothelial cells of the renal glomerular capillaries. These cells have similar structures that prevent protein from penetrating into the urine - pores and glycocalyx. The size of the pores of the endothelial lining is the largest (up to 100-150 nm). Anionic groups are located in the pore diaphragm, which limits the penetration of proteins into the urine.

Thus, the selectivity of filtration is ensured by the structures of the glomerular filter, which make it difficult for protein molecules larger than 1.8 nm to pass through the filter and completely block the passage of macromolecules larger than 4.5 nm, and the negative charge of the endothelium, podocytes and basement membrane, which makes it difficult to filter anionic macromolecules and facilitates the filtration of cationic macromolecules.

Mesangial matrix

Between the loops of the glomerular capillaries there is a mesangial matrix, the main components of which are collagen types IV and V, laminin and fibronectin. The multifunctionality of these cells has now been proven. Thus, mesangial cells perform several functions: they have contractility, which ensures their ability to control glomerular blood flow under the influence of biogenic amines and hormones, have phagocytic activity, participate in the reparation of the basement membrane, and can produce renin.

Renal tubules

Proximal tubule

The tubules are located only in the cortex and subcortical zones of the kidney. Anatomically, they are divided into a convoluted part and a shorter straight (descending) segment, which continues into the descending part of the loop of Henle.

A structural feature of the tubular epithelium is the presence of the so-called brush border in the cells - long and short cell outgrowths that increase the absorption surface by more than 40 times, due to which the reabsorption of filtered but necessary substances for the body occurs. In this section of the nephron, more than 60% of filtered electrolytes (sodium, potassium, chlorine, magnesium, phosphorus, calcium, etc.), more than 90% of bicarbonates and water are reabsorbed. In addition, amino acids, glucose, and finely dispersed proteins are reabsorbed.

There are several mechanisms of reabsorption:

• active transport against the electrochemical gradient, involved in the reabsorption of sodium and chlorine;
• passive transport of substances to restore osmotic balance (water transport);
• pinocytosis (reabsorption of finely dispersed proteins);
• sodium-dependent cotransport (reabsorption of glucose and amino acids);
• hormone-regulated transport (reabsorption of phosphorus under the influence of parathyroid hormone), and so on.

Loop of Henle

Anatomically, there are two variants of the loop of Henle: short and long loops. Short loops do not penetrate beyond the outer zone of the medulla; long loops of Henle penetrate into the inner zone of the medulla. Each loop of Henle consists of a descending thin segment, an ascending thin segment, and a distal straight tubule.

The distal straight tubule is often called the diluting segment because it is where dilution (reduction in osmotic concentration) of urine occurs due to the impermeability of this segment of the loop to water.

The ascending and descending segments are closely adjacent to the vasa recta, which pass through the medulla, and to the collecting ducts. This proximity of the structures creates a multidimensional network in which a countercurrent exchange of dissolved substances and water occurs, facilitating the performance of the loop's main function - dilution and concentration of urine.

Distal nephron

It includes the distal convoluted tubule and the connecting tube (connecting tubule), which connects the distal convoluted tubule with the cortical part of the collecting duct. The structure of the connecting tubule is represented by alternating epithelial cells of the distal convoluted tubule and collecting ducts. Functionally, it differs from them. In the distal nephron, reabsorption of ions and water occurs, but in much smaller quantities than in the proximal tubules. Almost all processes of electrolyte transport in the distal nephron are regulated by hormones (aldosterone, prostaglandins, antidiuretic hormone).

Collecting tubes

The last part of the tubular system does not formally belong to the nephron, since the collecting ducts have a different embryonic origin: they are formed from the outgrowth of the ureter. According to morphological and functional characteristics, they are divided into the cortical collecting duct, the collecting duct of the outer zone of the medulla, and the collecting duct of the inner zone of the medulla. In addition, papillary ducts are distinguished, flowing at the apex of the renal papilla into the minor renal calyx. No functional differences have been identified between the cortical and medulla sections of the collecting duct. The final urine is formed in these sections.

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