The glomerular filter of the kidney is one ofthe primary mechanisms behind urine production. In a single day, about 144 L ofultrafiltrate are filtered per day from the plasma which ends upproducing urine (Hausman et al. 2012). Through structure units calledglomeruli, they work to filter the blood of many components.(Hahm et al.
2017).A regular kidney contains about 1 millionindividual functioning nephrons, where each has a single glomerulus unit.Depending where in the kidney, a significant difference in size and rate offiltration is occurrent (Pollack et al., 2014). Theglomerulus is lined by fenestrated endothelia that serve as the glomerularbasement membrane and specialized visceral epithelial cells; this arrangement permitscontinuing filtration of the plasma (Pollack et al., 2014). The glomerular endothelium is the first layer in the filtrationbarrier and is in direct contact with the blood (Pollack et al.
, 2014). Themain determining factors of glomerular filtration at the nephron level are the transcapillaryhydraulic pressure difference, initial capillary oncotic, and the initialglomerular plasma flow rate (Pollack et al., 2014).Interestingly,glomeruli filter around 180 grams of D-glucose per day from plasma, from whichall is reabsorbed through glucose transporter located within the proximaltubules. Glucose appears in the urine if the capacity of thesetransporters is exceeded (Triplitt 2012).Another importantphysiological mechanism of urine production is reabsorption. The urineconcentrating mechanism takes part primarily in the outer medulla, where NaClis actively transported the loops of Henle into surrounding interstitium. Thisactive NaCl reabsorption increases the osmolality of interstitial fluid and encouragesosmotic-reabsorption of water from the tubular fluid and collecting ducts (Sands& Layton 2009) The most widely accepted mechanismremains the passive reabsorption of NaCl, in excess of solute secretion, fromthe thin ascending limbs of the loops of Henle (Sands & Layton 2009)NaCl reabsorption dilutes the thick ascending limbtubular fluid so the ascending limb fluid that enters the cortex is furtherdiluted by active NaCl reabsorption from cortical thick ascending limbs; endingup with osmolality less than the osmolality of blood plasma (Sands &Layton 2009).
The urine concentrating mechanism takes anotherpart this time in the physiological process of regulating water and sodiumexcretion. When intake of water is large enough that it dilutes blood plasma, urinewhich is more diluted that blood plasma is created; Whereas when water intakeis smaller and blood plasma is concentrated, urine which is more concentratedthat blood plasma is produced (Sands & Layton 2009). Water excretion requires the collecting duct system; thisstarts primarily in the cortex and ends at the papillary tip. When vasopressinis missing, collecting duct segments are near water impermeable. Dilute urineexcretion demands that minimal water be absorbed, and minimal solute besecreted along the collecting duct.
This is due to fluid that leaves the thickascending limb and enters the cortical collecting duct being dilute relative toplasma. (Sands & Layton 2009)