In humans, the excretory system consists of a %%pair of kidneys, one pair of ureters, a urinary bladder, and a urethra%%.
Kidneys are reddish brown, bean-shaped structures situated between the levels of the %%last thoracic and third lumbar vertebra%% close to the dorsal inner wall of the abdominal cavity.
In the majority of nephrons, the loop of Henle is too short and extends only very little into the medulla.
In some of the nephrons, the loop of Henle is very long and runs deep into the medulla.
The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the peritubular capillaries.
%%Vasa recta are absent or highly reduced in cortical nephrons.%%
This long duct extends from the cortex of the kidney to the inner parts of the medulla.
Mammals have the ability to produce %%concentrated urine.%%
The proximity between the Henle’s loop and vasa recta, as well as the counter-current in them, help in %%maintaining an increasing osmolarity%% towards the inner medullary interstitium, i.e., from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla.
Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the %%interstitium by the collecting tubule.%%
The above-described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the %%countercurrent mechanism.%%
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