Kidney nephron

The nephron consists of a continuous tube of highly specialized heterogeneous cells performing various functions. Each kidney contains between 800,000 and 1,300,000 nephrons. The length of all nephrons in both kidneys is about 110 km. Most nephrons (85%) are located in the cortex (cortical nephrons), a smaller part (15%) is located on the border of the cortical and medullary substance in the so-called juxtammadullar zone (juxtamedullary nephrons). Between nephrons there are significant structural and functional differences: in cortical nephrons, the Henle loop is short. It ends at the border of the outer and inner zone of the medulla, while the Henle loop of juxtamendullary nephrons extends 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 following are distinguished in the nephron:

• renal glomerulus;
• proximal tubule (curved and straight part);
• downward slender segment;
• ascending thin segment;
• distal straight canaliculus (previously thick ascending loop segment of Henle);
• distal convoluted tubule;
• connecting canaliculus;
• cortical collecting tube;
• the collecting tube of the outer zone of the medulla;
• the collecting tube of the inner zone of the medulla.

The space between all structures of the nephron both in the cortex and in the brain substance 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 "tangle-net" of 7-20 capillary loops, which are enclosed in a Bowman capsule. The glomerular capillaries are formed from the glomerular arteriolus and then connect at the exit from the glomerulus into the glomerulus-bearing arteriolus. Between the capillary loops there are anastomoses. The central part of the glomerulus is occupied by mesangial cells surrounded by mesangial matrix, which fix the capillary loops of the glomerulus to the vascular pole of the glomerulus - to its handle - the place where the arteriolus is brought in and the outgoing arteriolus enters. Directly opposite in the glomerulus is the urinary pole - the place of the beginning of the proximal tubule.

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

The structure of the glomerular filter

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

Podocytes

They are represented by large, highly differentiated cells having a "body", from which large and small processes (legs of podocytes) leave the capsule of the glomerulus. These processes closely intertwine, enveloping the surface of the glomerular capillaries from the outside and plunging into the outer plate of the basal membrane. Between the small processes of the podocytes, there are slit diaphragms, which represent one of the variants of pore filtration. They prevent the penetration of proteins into the urine due to the small pore diameter (5-12 nm) and the electrochemical factor: the slit diaphragms on the outside are covered 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 basal membrane and, in addition, create an anion barrier in the process of biological ultrafiltration. It is suggested that the podocytes possess phagocytic and contractile activity.

Basal membrane of capillary glomeruli

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

The basement membrane contains pores, the maximum size of which does not exceed the size of the albumin molecule. Through them, finely dispersed proteins with a molecular weight lower than albumin can pass, and larger proteins do not pass.

Thus, the basal membrane of the glomerular capillaries acts as the second barrier for passage of plasma proteins into the urine due to the small pore size and the negative charge of the basal membrane.

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

Thus, filtering selectivity provides glomerular filter structures that make it difficult to pass protein molecules larger than 1.8 nm through the filter and completely block the passage of macromolecules larger than 4.5 nm, and a negative charge of the endothelium, podocytes and basal 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 is a mesangial matrix, the main components of which are Collagen IV and V types, laminin and fibronectin. At present, the multifunctionality of these cells has been proved. So, mesangial cells perform several functions: they have contractility, which ensures their ability to control glomerular blood flow under the action of biogenic amines and hormones, have phagocytic activity, participate in repair of the basal membrane, can produce renin.

Kidney canals

The proximal tubule

The tubules are located only in the cortical substance and the subcortical zones of the kidney. They are anatomically distinguished in them by a crimped part and a shorter straight (descending) segment, which extends into the descending part of the loop of Henle.

The structural feature of the tubular epithelium is the presence of the so-called brush border in the cells - long and short outgrowths of the cell, which increase the suction surface by more than 40 times, thereby reabsorbing the filtered but necessary substances for the organism. In this department of the nephron, more than 60% of the filtered electrolytes (sodium, potassium, chlorine, magnesium, phosphorus, calcium, etc.) are absorbed back, more than 90% of bicarbonates and water. In addition, there is a reabsorption of amino acids, glucose, finely divided proteins.

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 Henle

Anatomically, two variants of the Henle loop are distinguished: 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 canaliculus is often called the dilution segment due to the fact that the dilution (decrease in osmotic concentration) of urine occurs due to the impermeability of this segment of the water loop.

The ascending and descending segments closely adjoin the direct vessels passing through the brain substance, and to the collecting tubes. This closeness of structures creates a multidimensional network in which countercurrent exchange of dissolved substances and water occurs, contributing to the main function of the loop - dilution and concentration of urine.

Distal nephron

It includes a distal convoluted tubule and a connecting tube (connective canaliculus) that connects the distal convoluted tubule to the cortical part of the collecting tube. The structure of the connective tubule is represented by alternating epithelial cells of the distal convoluted tubule and collecting tubes. Functionally, it differs from them. In the distal nephron, there is a reabsorption of ions and water, but in a much smaller amount than in the proximal tubules. Almost all the processes of electrolyte transport in the distal nephron are regulated by hormones (aldosterone, prostaglandins, antidiuretic hormone).

Collection tubes

The last part of the tubular system does not formally belong to the nephron, since the collecting tubes have a different embryonic origin: they are formed from the ureteral outgrowth. According to their morphological and functional characteristics, they are divided into a cortical collecting tube, a collecting tube of the outer zone of the brain substance, and a collecting tube of the inner zone of the medulla. In addition, the papillary ducts that flow at the apex of the renal papilla are isolated into a small kidney cup. There were no functional differences between the cortical and cerebral divisions of the collecting tube. In these departments, the final urine is formed.

[1], [2], [3], [4], [5]