This contributes to allowing the kidneys to produce dilute urine. Water, salts and urea can pass freely across the capillary walls of the vasa recta and therefore as the vessels reach deeper into the medulla where the interstitum surrounding them becomes more hypertonic, the blood inside them in turn becomes more hypertonic.
As the blood ascends back out of the medulla, the osmolarity reduces until it is only slightly higher than when it entered. The conclusion drawn from this is that the solutes which are reabsorbed from the fluid mainly remain in the surrounding tissue and maintain the concentration gradient. If the system was not arranged in loops then this would not be possible.
Although the blood leaving the medulla is always slighty more concentrated than when it entered upon reaching the cortex, if the urine being produced is very concentrated, it actually becomes hypo osmolar as more water than salt is being reabsorbed in the distal tubules which reside in there.
Overall the osmolarity of the blood decreases when concentrated urine is being produced and increased when dilute urine is being produced. The anatomy of the vasa recta can be found here. Use the flash card revision resource for this section to test yourself. From WikiVet English. Navigation menu Personal tools Create account Log in.
Possible causes include idiopathic, familial, trauma, tumor, infection or vascular lesions. Patients present with polyuria, usually beginning over a period of a few days. The diagnosis is made by showing that urinary concentration is impaired after water restriction but that there is a good response to exogenous vasopressin therapy. Nephrogenic diabetes insipidus can be identified by a patient's lack of response to AVP. Nephrogenic diabetes insipidus is caused by a familial defect, although milder forms can be acquired as a result of various forms of renal disease.
Pannabecker, , but for the purposes of the CICM exam, that level of detail is probably unnecessary. More important is the functional distinction between the different cell types in this part of the tubule, which can be oversimplified in the following manner:.
The image on the right is a 3D reconstruction of the loops of Henle long nephrons from Layton et al ; the green ascending segments were identified using an immunofluorescent marker specific for the CIC-K chloride channel, which only appears in the ion-permeable section of the tubule. The thin descending limb is highly water-permeable. When Gottschalk al punctured a descending limb and injected a droplet of water, they observed that the droplet disappeared very rapidly.
In case those numbers mean nothing to you, the authors helpfully measured the permeability of other segments of the tubule and reported their values:. So, in fact, the thin descending limb is even more water-permeable than a cortical collecting duct that has been soaked in vasopressin. This property is essential for the countercurrent mechanism responsible for the production of concentrated urine. At the beginning of the thin descending limb, tubular fluid closely resembles plasma in its osmolality and ionic composition.
At the end of the thin descending limb, as it enters the hairpin bend of the loop, the tubular fluid is now highly concentrated.
One might expect there to be a bit of inter-individual variability in this value, to say nothing of the variability between species, and that is in fact what is seen in comparative biology studies. Borrowing some data from Taniguchi et al , one is able to construct a graph that looks a little like this:. After the tubular fluid has concentrated maximally, it returns from the tip of the loop via the thin ascending limb.
A change in the epithelium here produces a marked difference in water permeability. Then, some reabsorption of sodium takes place, and so the osmolality of the fluid drops, i. This reabsorption seems to be entirely passive in this part of the tubule, in the sense that nobody has ever been able to demonstrate the existence of active transport mechanisms. When the investigators bathed the thin ascending limb in an extracellular electrolyte bath which was identical in composition to the fluid being perfused into the tubule, there was no change in the tubular electrolyte concentration.
This a fairly important part of the loop, as it contains several regions of interest. The other is the macula densa, a "plaque" of tubular cells which sits near the vascular pedicle of the glomerulus and which coordinates tubuloglomerular feedback , among its other functions. Chloride follows through its own basolateral channels, driven by its electrochemical gradient specifically, by the very negative intracellular potential.
This diagram and this discussion are mindless oversimplifications, the colouring book of physiological study. In case one has the appetite for greater detail, Mount describes this part of the tubule with extraordinary granularity.
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