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So azotemia generally means increased nitrogenous components in the body, which is usually measured in blood, which is usually measured through blood urea nitrogen and serum creatinine. It is because of the less excretion of the urine because of the oliguria. Nitrogenous components are pulled up in the blood and eventually in the urine, right?

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Great - the textbooks confirm and sharpen this nicely. Here is the full, corrected picture:

Azotemia - What's Actually Happening

Your understanding is largely correct, but the last part needs a small correction. Here is the full breakdown:

What azotemia is

Azotemia means the accumulation of nitrogenous byproducts of protein metabolism in the blood - primarily urea (measured as BUN) and creatinine. The definition is specifically about retention in the blood, not the urine. (Harrison's, Miller's Anesthesia)
The nitrogenous waste products you are referring to are byproducts of:
  • Protein/amino acid metabolism - produces urea (via the urea cycle in the liver)
  • Muscle creatine metabolism - produces creatinine at a relatively constant daily rate

How GFR connects to it

When GFR falls (due to reduced perfusion, intrinsic renal disease, or obstruction), the kidneys filter less plasma per minute. Because creatinine and urea are not being cleared fast enough, they back up and accumulate in the blood. So serum creatinine (PCr) rises, and BUN rises. (Harrison's Principles, 22e)

The oliguria connection - here is where your idea needs a small correction

You said: "nitrogenous components are pulled up in the blood and eventually in the urine" - this is where to be careful.
  • In oliguria, less urine is being formed, so less waste is being excreted - that is correct
  • What happens to the wastes that aren't excreted? They stay in the blood and accumulate there - that is azotemia
  • The small amount of urine that is produced in oliguric states is often highly concentrated (high urine osmolality, high urine urea), but the total amount of nitrogenous waste appearing in the urine is reduced, not increased
  • So the wastes pile up in the blood, and the urine output is insufficient to clear them
In other words: the nitrogenous components build up in the blood - they do not preferentially accumulate in urine. The urine that is formed may be concentrated, but there is too little of it.

Summary of causes (three categories)

Azotemia can result from:
  1. Prerenal - reduced renal perfusion (dehydration, heart failure, shock) -> reduced GFR -> less filtration -> BUN/Cr rise
  2. Intrinsic renal - glomerular, tubular, or vascular disease damaging the kidney directly
  3. Postrenal - obstruction downstream (e.g., bladder neck obstruction) causing back-pressure that reduces filtration
Oliguria is most commonly a feature of prerenal azotemia, where the kidney is responding to low perfusion by conserving water - hence scant, concentrated urine, and rising BUN and creatinine in the blood.
Short version of your correction: Nitrogenous components accumulate in the blood (that is what azotemia means). The urine output is low (oliguria), and the small volume of urine produced may be concentrated - but the wastes are backing up into the bloodstream, not flowing into the urine in greater amounts.

The above concept, if I want to make it like a sticky note, can you make it?

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