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The medulla is made up of one or more renal pyramids, [8] forming papillae with their innermost parts. [9] Generally, urine produced by the cortex and medulla drains from the papillae into the calyces, and then into the renal pelvis, from which urine exits the kidney through the ureter.
The nephrons of the mammalian kidneys have loops of Henle, which are the most efficient way to reabsorb water and produce concentrated urine to conserve water in the body. [12] The mammalian kidneys combine both nephrons with a short and nephrons with a long loop of Henle. [77] The medulla is divided into outer and inner regions.
The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis.The collecting duct participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and vasopressin (antidiuretic hormone).
The renal medulla (Latin: medulla renis 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the segmental arteries which then branch to form interlobar arteries.
To achieve such balance between water and ions, the loop of Henle coordinates its function with the collecting duct to regulate the amount of water to reabsorb or to excrete. While the loop of Henle makes the medulla of the kidney salty, the collecting duct regulates the permeability of water that could be reabsorbed to such salty environment.
The hypertonicity of the medulla (and therefore concentration of urine) is determined in part by the size of the loops of Henle. [1]: 76 Unlike the descending limb, the thick ascending limb is impermeable to water, a critical feature of the countercurrent exchange mechanism employed by the loop. The ascending limb actively pumps sodium out of ...
Urea is usually excreted as a waste product from the kidneys. However, when plasma blood volume is low and ADH is released the aquaporins that are opened are also permeable to urea. This allows urea to leave the collecting duct into the medulla, creating a hyperosmotic solution that "attracts" water.
The descending limb of the loop of Henle is permeable to water but impermeable to solutes, due to the presence of aquaporin 1 in its tubular wall. Thus, water moves across the tubular wall into the medullary space, making the filtrate hypertonic (with a lower water potential). This is the filtrate that continues to the ascending limb. [2]