Postrenal acute kidney injury (acute renal failure) - causes, symptoms, & pathology
Acute Kidney Injury: Understanding the Basics
Definition of Acute Kidney Injury (AKI)
- AKI is characterized by a rapid decline in kidney function, typically occurring over a few days.
- Previously referred to as acute renal failure (ARF), AKI encompasses both significant and subtle decreases in kidney function.
Types of Acute Kidney Injury
- AKI can be categorized into three types:
- Prerenal AKI: Caused by factors before the kidneys.
- Postrenal AKI: Results from obstructions after the kidneys.
- Intrarenal AKI: Arises from issues within the kidneys themselves.
Kidney Function Overview
- The kidneys regulate blood composition by filtering waste, maintaining electrolyte balance, and controlling water levels. They also produce hormones.
- Blood enters the kidneys via the renal artery and is filtered through structures called glomeruli, with filtrate moving into renal tubules for further processing.
Mechanisms of Filtration and Reabsorption
Filtration Process
- The filtration process involves reabsorption (fluid/electrolytes moving back to blood) and secretion (movement from blood to filtrate). Waste products like urea and creatinine are also filtered out during this process.
Diagnostic Indicators
- The normal ratio of blood urea nitrogen (BUN) to creatinine is between 5:1 and 20:1, serving as an important diagnostic tool for assessing kidney function.
Postrenal Acute Kidney Injury Explained
Causes of Postrenal AKI
- Postrenal AKI occurs due to obstructions that impede urine flow from the kidneys:
- External compression from tumors or benign prostatic hyperplasia.
- Internal blockages such as kidney stones obstructing ureters or urethra.
Impact on Renal Function
- Unilateral obstruction may preserve renal function if one kidney remains unaffected; however, bilateral obstruction leads to postrenal AKI due to urine buildup causing pressure on renal tubules.
Consequences of Obstruction in Postrenal AKI
Effects on Glomerular Filtration Rate (GFR)
- Increased pressure in renal tubules reduces the pressure gradient necessary for filtration, leading to decreased GFR—resulting in less waste being filtered out and higher levels remaining in the bloodstream, causing azotemia and oliguria.
Changes in Urine Composition
- High tubular pressure causes increased reabsorption of sodium, water, and urea but minimal reabsorption of creatinine:
- BUN/creatinine ratio rises above 15:1.
- Urine sodium excretion typically falls below 20 mEq/L with FENa under 1%.
- Urine becomes concentrated (>500 mOsm/kg).
Progression Towards Tubular Damage
Long-term Effects on Tubule Functionality
- Prolonged high pressure damages epithelial cells responsible for reabsorption:
- Leads to decreased urea reabsorption resulting in a BUN/creatinine ratio dropping below 15:1.
- Increased sodium excretion with urine sodium rising above 40 mEq/L; FENa may exceed 2% in severe cases.
- Decreased water reabsorption results in diluted urine (<350 mOsm/kg).
Conclusion