I checked my anatomy notes and online figures about the kidney. There is no mention about dedicated arteries for feeding kidney cells. So it does not work the same way as the heart does, where there is a dedicated coronary artery. The kidneys use the same blood vessels for filtering and for nutrition/waste transport purposes too.
- Figure 1 - kidney anatomy - source
- Figure 2 - nephron anatomy - source
The urea is created from NH4+ and HCO3- in the liver (mostly) and the kidney because of blood pH regulation purposes. It neutralizes the HCO3- created by the lungs from CO2 and OH-.
The urea cycle (also known as the Ornithine cycle) is a cycle of
biochemical reactions occurring in many animals that produces urea
((NH2)2CO) from ammonia (NH3). This cycle was the first metabolic
cycle discovered (Hans Krebs and Kurt Henseleit, 1932), five years
before the discovery of the TCA cycle. In mammals, the urea cycle
takes place primarily in the liver, and to a lesser extent in the
In chemical terms, urea synthesis is an irreversible, energy driven
neutralization of the strong base HCO3- by the weak acid NH4+, and the
average daily excretion of 30 g of urea is equivalent to the disposal
of about 1 mol of HCO3- per day. Thus, a major function of hepatic
urea synthesis is to effect this neutralization, without which the
body would otherwise be confronted by a major load of alkali.
Urea is excreted by the kidney, and is normally present in plasma and
body fluids at a concentration of 3.0–6.5 mmol/L.
The kidney reabsorbs urea in order to concentrate the urine:
- Figure 3 - nephron with material transports - urea resorption at the end of the urine creation process - source
About 40% of the urea filtered is normally found in the final
urine, since there is more reabsorption than secretion along the
The kidney secretes the urea to the urine, but it absorbs more by the reabsorption than it secreted.
The kidney freely filters urea at the glomerulus, and then it both
reabsorbs and secretes it. Because the tubules reabsorb more urea than
they secrete, the amount of urea excreted in the urine is less than
the quantity filtered. In the example shown in Figure 36-1A (i.e.,
average urine flow), the kidneys excrete ∼40% of the filtered urea.
The primary sites for urea reabsorption are the proximal tubule and
the medullary collecting duct, whereas the primary sites for secretion
are the thin limbs of the loop of Henle.
So if we are talking about urea, then it is secreted to the urine. If we are talking about other waste products, then e.g. CO2 is certainly handled by the veins.