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The kidney is composed of tissues, and those tissues are made up of numerous cells - so how do these cells excrete their wastes?

Lastly, are there any wastes in the venous blood (renal vein), what makes the renal artery and vein different? - what substances do they each carry?

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  • $\begingroup$ Then there should be some selective resorption of water compared to urea somewhere along the Loop of Henle. That's how the kidney does all the amazing stuff it does. Read about the Loop of Henle and you'll have your answer to this and other great functions of the kidney, the autoregulatory functions, etc. It's amazing! $\endgroup$ – anongoodnurse Oct 21 '14 at 6:57
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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.

kidney

  • Figure 1 - kidney anatomy - source

nephron

  • 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 kidney.

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:

nephron transports

  • 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 nephron.

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.

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The kidneys keep the composition, or makeup, of the blood stable, which lets the body function.

Each kidney is made up of about a million filtering units called nephrons. The nephron includes a filter, called the glomerulus, and a tubule.

The nephrons work through a two-step process. The glomerulus lets fluid and waste products pass through it; however, it prevents blood cells and large molecules, mostly proteins, from passing. The filtered fluid then passes through the tubule, which sends needed minerals back to the bloodstream and removes wastes.

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  • $\begingroup$ Could you describe "if you talking about the areas were the actual waste is dumped" a bit more precisely. This is quite unclear as to what you mean. $\endgroup$ – jonsca Oct 21 '14 at 23:22
  • $\begingroup$ This is not an answer to the question $\endgroup$ – AliceD Nov 6 '14 at 5:27

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