The renal papillae are the anatomical structures of the renal parenchyma where the processing of the filtered tubular fluid in the glomeruli is completed. The fluid that leaves the papillae and enters the lesser calyces is the final urine, which will be conducted without modifications to the urinary bladder.
As the papillae are part of the renal parenchyma, it is necessary to know how the latter is organized. A section of the kidney along its long axis allows us to recognize two bands: a superficial one - called the cortex and a deeper one known as the medulla, of which the papillae are part.
Structure of the kidney of a mammal. Each of the "pyramids" drawn in the internal structure of the kidney corresponds to a renal papilla (Source: Davidson, AJ, Mouse kidney development (January 15, 2009), StemBook, ed. The Stem Cell Research Community, StemBook, doi / 10.3824 / stembook.1.34.1, http://www.stembook.org. Via Wikimedia Commons) The renal cortex is a superficial layer where the glomeruli are found and most of the tubular system that is associated with each of them to constitute a nephron: the proximal tubule, the loop of Henle, the distal tubules and the connecting ducts. Each kidney has a million nephrons
Within the cortex itself, a couple of thousand of these connecting ducts (nephrons) lead to a thicker duct called the cortical collector, which runs radially in depth and enters the renal medulla. This tube with the nephrons it receives is a renal lobule.
The renal medulla is not a continuous layer, but is organized as in masses of tissue in the form of pyramids or cones whose broad bases are oriented outwards, towards the cortex, with which they limit, while their vertices point radially inwards introducing in the minor calyces.
Each of these medullary pyramids represents a renal lobe and receives the collecting ducts of hundreds of lobules. The most superficial or external portion of each pyramid (1/3) is called the external medulla; the deepest (2/3) is the medulla medulla and it includes the papillary region.
Characteristics and histology
The most important components of the papillae are the papillary ducts of Bellini that give the final touches to the tubular fluid they receive. At the end of its journey through the papillary ducts, this liquid, already converted into urine, is poured into a smaller calyx and does not undergo further modifications.
The relatively thick papillary ducts are the terminal portions of the renal tubular system and are formed by the successive union of about seven collecting ducts, leaving the cortex and entering the pyramids, they have passed from cortical to medullary.
The mouth holes of the various Bellini ducts of a papilla give its mucous lining a perforated lamina appearance, which is why it is known as the lamina cribosa. Through this cribriform plate urine is poured into the calyx.
Anatomy of a human kidney (Source: Arcadian, via Wikimedia Commons)
In addition to the Bellini ducts, the ends of the long loops of Henle are also found in the papillae, members of those nephrons whose glomeruli are located in the cortex that immediately borders the medulla. Nephrons therefore called juxtamedullary.
Another additional component of the papillae are the so-called rectus vessels, which originate in the efferent arterioles of the juxtamedullary nephrons and descend directly towards the end of the papillae, and then rise straight back to the cortex.
Both the long loops of Henle and the straight vessels are ducts whose initial segments descend to the papillae, and there they curve to return to the cortex following an ascending path parallel to the descending one. The flow through both segments is said to be countercurrent.
Apart from the elements mentioned, the presence in the papillae of a set of cells without a precise histological organization and which is given the name of interstitial cells, of unknown function, but which could be precursors in tissue regeneration processes, is also described.
Hyperosmolar gradient in the renal medulla
One of the most outstanding characteristics of the renal medulla, which reaches its maximum expression in the papillae, is the existence of a hyperosmolar gradient in the interstitial fluid that bathes the structural elements described.
It should be noted that body fluids are generally in osmolar balance, and it is this balance that determines the distribution of water in the different compartments. Interstitial osmolarity, for example, is the same throughout the renal cortex and equal to that of plasma.
In the interstitium of the renal medulla, curiously, in the case of the same compartment, the osmolarity is not homogeneous, but increases progressively from about 300 mosmol / l near the cortex, to a value, in the human papilla, of around about 1200 mosmol / l.
The production and maintenance of this hyperosmolar gradient is, in large part, the result of the countercurrent organization already described for loops and straight vessels. The handles help form a countercurrent multiplier mechanism that creates the gradient.
If the vascular organization were like that of any other tissue, this gradient would dissipate because the blood stream would carry the solutes away. The straight glasses provide a countercurrent exchanger mechanism that prevents backwashing and helps preserve the gradient.
The existence of the hyperosmolar gradient is a fundamental characteristic that, as will be seen later, is added to other aspects that allow the production of urine with variable osmolarities and volumes adjusted to the physiological needs imposed by the circumstances.
Features
One of the functions of the papillae is to contribute to the formation of the hyperosmolar gradient and to determine the maximum osmolarity that can be achieved in its interstitium. Closely linked to this function is also that of helping to determine urinary volume and its osmolarity.
Both functions are associated with the degree of permeability that the papillary ducts offer to urea and water; permeability that is associated with the presence and plasma levels of antidiuretic hormone (ADH) or vasopressin.
At the level of the papillary interstitium, half of the osmolar concentration is NaCl (600 mosmol / l) and the other half corresponds to urea (600 mosmol / l). The concentration of urea at this site depends on the amount of this substance that is able to cross the wall of the papillary duct into the interstitium.
This is achieved because the urea concentration increases in the collecting ducts as the water is reabsorbed, so that when the liquid reaches the papillary ducts its concentration is so high that if the wall allows it, it diffuses through a chemical gradient into the interstitium.
If there is no ADH, the wall is impervious to urea. In this case, its interstitial concentration is low, and hyperosmolarity is low as well. ADH promotes the insertion of urea transporters that facilitate urea exit and its increase in the interstitium. The hyperosmolarity is then higher.
Interstitial hyperosmolarity is very important, because it represents the osmotic force that will allow the reabsorption of the water that is circulating through the collecting and papillary ducts. The water that is not reabsorbed in these final segments will eventually be excreted in the form of urine.
But for the water to be able to cross the wall of the ducts and be reabsorbed into the interstitium, the presence of aquaporins is required, which are produced in the cells of the tubular epithelium and are inserted into its membrane by the action of the antidiuretic hormone.
The papillary ducts, then, working in conjunction with ADH, contribute to the hyperosmolarity of the medulla and the production of urine of variable volumes and osmolarities. With maximum ADH, the urine volume is low and its osmolarity is high. Without ADH, the volume is high and the osmolarity is low.
References
- Ganong WF: Renal Function and Micturition, in Review of Medical Physiology, 25th ed. New York, McGraw-Hill Education, 2016.
- Guyton AC, Hall JE: The Urinary System, in Textbook of Medical Physiology, 13th ed, AC Guyton, JE Hall (eds). Philadelphia, Elsevier Inc., 2016.
- Koeppen BM and Stanton BA: Renal Transport Mechanisms: NaCl and Water reabsorption along the nephron, In: Renal Physiology 5th ed. Philadelphia, Elsevier Mosby, 2013.
- Lang F, Kurtz A: Niere, in Physiologie des Menschen mit Pathophysiologie, 31 th ed, RF Schmidt et al (eds). Heidelberg, Springer Medizin Verlag, 2010.
- Silbernagl S: Die function der nieren, in Physiologie, 6th ed; R Klinke et al (eds). Stuttgart, Georg Thieme Verlag, 2010.