The NaCl is actively transported the loops of

The glomerular filter of the kidney is one of
the primary mechanisms behind urine production. In a single day, about 144 L of
ultrafiltrate are filtered per day from the plasma which ends up
producing urine (Hausman et al. 2012). Through structure units called
glomeruli, they work to filter the blood of many components.(Hahm et al. 2017).
A regular kidney contains about 1 million
individual functioning nephrons, where each has a single glomerulus unit.
Depending where in the kidney, a significant difference in size and rate of
filtration is occurrent (Pollack et al., 2014). The
glomerulus is lined by fenestrated endothelia that serve as the glomerular
basement membrane and specialized visceral epithelial cells; this arrangement permits
continuing filtration of the plasma (Pollack et al., 2014). The glomerular endothelium is the first layer in the filtration
barrier and is in direct contact with the blood (Pollack et al., 2014). The
main determining factors of glomerular filtration at the nephron level are the transcapillary
hydraulic pressure difference, initial capillary oncotic, and the initial
glomerular plasma flow rate  (Pollack et al., 2014).Interestingly,
glomeruli filter around 180 grams of D-glucose per day from plasma, from which
all is reabsorbed through glucose transporter located within the proximal
tubules. Glucose appears in the urine if the capacity of these
transporters is exceeded (Triplitt 2012).

Another important
physiological mechanism of urine production is reabsorption. The urine
concentrating mechanism takes part primarily in the outer medulla, where NaCl
is actively transported the loops of Henle into surrounding interstitium. This
active NaCl reabsorption increases the osmolality of interstitial fluid and encourages
osmotic-reabsorption of water from the tubular fluid and collecting ducts (Sands
& Layton 2009) The most widely accepted mechanism
remains the passive reabsorption of NaCl, in excess of solute secretion, from
the thin ascending limbs of the loops of Henle (Sands & Layton 2009)
NaCl reabsorption dilutes the thick ascending limb
tubular fluid so the ascending limb fluid that enters the cortex is further
diluted by active NaCl reabsorption from cortical thick ascending limbs; ending
up with osmolality less than the osmolality of blood plasma (Sands &
Layton 2009).

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The urine concentrating mechanism takes another
part this time in the physiological process of regulating water and sodium
excretion. When intake of water is large enough that it dilutes blood plasma, urine
which is more diluted that blood plasma is created; Whereas when water intake
is smaller and blood plasma is concentrated, urine which is more concentrated
that blood plasma is produced (Sands & Layton 2009). Water excretion requires the collecting duct system; this
starts primarily in the cortex and ends at the papillary tip. When vasopressin
is missing, collecting duct segments are near water impermeable. Dilute urine
excretion demands that minimal water be absorbed, and minimal solute be
secreted along the collecting duct. This is due to fluid that leaves the thick
ascending limb and enters the cortical collecting duct being dilute relative to
plasma. (Sands & Layton 2009)

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