The nephron is the functional unit of the kidney, that is, the smallest structure capable of performing all the functions of the organ.
The kidneys typically have from one million to one and a half million nephrons each, thanks to which they are able to filter a total of 180 liters of plasma per day.
The knowledge of the nephrons from the anatomical point of view is essential to analyze the functions they are intended for. Each of them begins with Bowman's capsule, a hollow spherical structure with a blind bottom surrounding a spheroidal network of capillaries, the glomerulus (from glomus, gomitolo), fusing its own epithelium with the vascular one. In this way all the liquid filtered by the capillaries is directly collected in Bowman's capsule and from there directed to the successive traits of the nephron, respectively called proximal tubule, loop of Henle (with its two traits, descending and ascending) and distal tubule. The liquid present in the distal tubule - profoundly modified in volume and composition with respect to that contained in the first tract of the nephron - drains into a single larger tubule, the collecting duct, where the contents of more nephrons are poured (up to eight). The various collecting ducts, in turn, come together in increasingly larger ducts that form the renal pyramids; the tubes of each pyramid flow into the papillary collector channel, which flows into one of the smaller calyces to discharge its contents into the renal pelvis. From here the urine passes to the ureters, accumulating in the urinary bladder before being excreted through the urethra.
For educational purposes, in the image above the nephron appears to be unfolded, when in reality it turns back and folds itself several times (image below).
The fact that the renal tubule is folded back on itself causes the terminal portion of the ascending tract of the loop of Henle to pass between the afferent and efferent arterioles. This region, in which the tubular and arteriolar walls modify their structure, is called the juxtaglomerular apparatus and its function is to produce paracrine signals necessary for renal autoregulation (by controlling the glomerular filtration rate). In this area, the granular cells present in the wall of the efferent arteriole adjacent to the epithelium of the tubule (dense macula), secrete renin, a proteolytic enzyme involved in the synthesis of angiotensin starting from angiotensinogen, and therefore involved in the control mechanisms of arterial pressure.
Each portion of the nephron is specialized in a different functionality and therefore contains epithelial cells with a considerably variable structure, so as to allow selectivity in the secretion and reabsorption of the various substances. The high glomerular pressure leads to the continuous filtering of 20% of the blood that runs through the renal glomerulus, with consequent passage of preurine (ultrafiltered) in Bowman's capsule. At this point, the reabsorption processes that take place in subsequent sections of the nephron allow the recovery of a large quantity of useful substances, such as glucose and various mineral salts; vice versa, the secretion processes allow the body to eliminate those substances that are present in excess or more generally than waste. Even more particularly, in the proximal tract of the nephron, sugars, amino acids and other solutes are actively reabsorbed, but also water by osmosis; in the descending tract of the loop of Henle the reabsorption of water continues, while in the ascending tract the sodium chloride is reabsorbed. Finally, in the distal tubule and in the collecting duct, aldosterone and antidiuretic hormone act to adapt the volume and composition of urine (Na +, K +, urea) to the needs of the organism.
