Constant is maintained through flux: In continually returning to the blood most of the filtrate, the kidney helps maintain the body fluid and body electrolyte balance within a normal range.
The kidney filters the blood. Most of the ultra-filtrate is returned to the blood.
Only excess salt or excess water is removed.
Excess salt is removed by reducing the re-absorption of salts from the ultra-filtrate. The kidney can in that way make a dilute urine out of the ultra-filtrate.
The kidney can reabsorb much of the water. The kidney can that way form a concentrated urine out of the ultra-filtrate.
By adjustment of these two mechanisms, the kidney can concentrate or dilute the ultra-filtrate to formation (and excretion) of a dilute or concentrated urine.
The kidney can concentrate the ultra-filtrate from the 285 mOsmol (of the original plasma) to 1200 mOsmol, per Litre maximally concentrated urine. The minimum urine volume per day is then about 500 mL (obligate urine vol.). The daily solute load is 500-1000 mOsmol.
The kidney uses the counter-current exchange in the medulla to concentrate the urine. This is a mechanism derived of the complex of the loop of Henle tubule with the vasa rectae blood vessels. Together, these are a feature of those juxta-medullary nephrons. The juxta-medullary nephron, unlike the cortical nephron, is unusual in that it has its glomerulus sit at the junction with the medulla, its tubule looping deep into the medulla (the tubules of these nephrons forming much of the tissue of the renal pyramids [see ‘3’ in the image below]).
The solutes and water molecules are retained from the ultra-filtrate by their reabsorption through various channels, that differ along the tubule. These channels are selective to specific ions. The thin descending limb is permeable to water. Water passes passively (osmosis) through aquaporin-1 channels while only small amounts of ions and urea pass through, and these by paracellular means. The ascending limb is impermeable to water but permeable to ions. The tubule is convoluted to help maximise exchange with the plasma (vasa rectae) and at same time also with the renal interstitium. (That is, there are three fluids involved in the counter-current exchange mechanism: the tubular fluid; the plasma fluid; and the fluid of the interstitium.)
Think of renal ultra-filtrate as that portion of plasma by-passed through the medullary interstitium. In this way, ultra-filtrate is indirectly exposed to the electrolyte solution and the tonicity of the renal interstitium. The tubule and its epithelium is variously permeable to certain solutes but not to others along its length. The descending loop is permeable to water but relatively impermeable to ions. The ascending limb is permeable to ions (and urea) but impermeable to water. Active reabsorption of sodium ions occurs in the thick ascending limb.
The permeability of the tubule, whether to ions or to water, can be actively modified, enhanced by the insertion of channels into the tubular epithelium. In particular, distal tubule epithelial permeability to water and urea can be dramatically altered.
The Renal Medulla – Interstitium and Counter-current multiplier
The inner workings of the kidney reflect the anatomy of:
- medullary collecting ducts
- loops of Henle
- vasa recta
- interstitium
The counter-current multiplier is the renal concentrating mechanism. The kidney creates a concentration gradient along the medullary interstitium, from the cortico-medullary junction to the renal papilla. The ultra-filtrate is passed through this concentration gradient, drawing water out of the tubule and into the interstitium to help concentrate the urine. This is the main function of the loop of Henle. (The “thin” and “thick” loops are so named because of their simple squamous and simple columnar epithelium, respectively.)
How plasma (capillary), interstitium, and tubular fluid interact in the kidney medulla:
Because of the counter-current multiplier effect, an osmotic gradient is set up along the kidney’s cortico-papillary axis. The ultra-filtrate is passed through this gradient to effect concentration of the filtrate.
In the medulla, tubular loops are stacked side by side with little intervening interstitium, allowing for transfer of solutes between filtrate and interstitium.
The “thin” loop is lined by a simple squamous epithelium, the “thick” loop by a simple cuboidal epithelium. Capillaries intervene to allow for ion and fluid exchange.
The thin descending limb, thick ascending limb, and collecting duct: The descending limb is permeable to water by passive diffusion; the ascending limb to ions; and the collecting duct to water and urea.
The collecting duct is point of water salvage. The thin limb is 2.5 times more permeable to water than is the collecting duct, this the natural state for which there is passive diffusion of water by osmosis through aquaporin-1 channels (AQP-1). Much of the tubular fluid water will naturally be reabsorbed from the filtrate before reaching the thick ascending limb. But in the presence of anti-diuretic hormone (ADH), the collecting duct can quickly double its permeability to water (through aquaporin-2 (AQP-2), but also AQP-3, 4 ,6, and 8 channels).* Anti-diuretic Hormone acts to salvage water at the collecting duct, reflecting body volume status accordingly. A low volume status stimulates ADH release, and the collecting duct ramps up water reabsorption (CD water > AQP > vascular bed > venule) to concentrate the urine.
The kidney produces 180 L of ultra-filtrate per day, of which ~ 90% is reabsorbed in the proximal tubule and loop of Henle. The collecting duct will be responsible for absorbing (salvaging) most of the remaining 10%.
*humans express 13 AQPs
Works Cited
- Amerman, Erin C. (Ed.), “The Urinary System,” in Human Anatomy and Physiology, 2nd Edn. (Pearson, 2018), 944. https://www.pearson.com/content/dam/one-dot-com/one-dot-com/us/en/higher-ed/en/products-services/course-products/amerman-1e-info/pdf/amerman-sample-chapter24.pdf.
Bibliography
Leave a Reply