There is a strong association between both acute and chronic dysfunction of the 
heart and kidneys with respect to morbidity and mortality. The complex 
interrelationships of longitudinal changes in both organ systems have been 
difficult to describe and fully understand due to a lack of categorization of 
the common clinical scenarios where these phenomena are encountered. Thus, 
cardiorenal syndromes (CRSs) have been subdivided into five syndromes which 
represent clinical vignettes in which both the heart and the kidney are involved 
in bidirectional injury and dysfunction via a final common pathway of 
cell-to-cell death and accelerated apoptosis mediated by oxidative stress. Types 
1 and 2 involve acute and chronic cardiovascular disease (CVD) scenarios leading 
to acute kidney injury (AKI) or accelerated chronic kidney disease (CKD). Types 
3 and 4 describe AKI and CKD, respectively, leading primarily to heart failure, 
although it is possible that acute coronary syndromes, stroke, and arrhythmias 
could be CVD outcomes in these forms of CRS. Finally, CRSs type 5 describe a 
systemic insult to both heart and the kidneys, such as sepsis, where both organs 
are injured simultaneously in persons with previously normal heart and kidney 
function at baseline. Both blood and urine biomarkers, including the assessment 
of catalytic iron, a critical element to the generation of oxygen-free radicals 
and oxidative stress, are reviewed in this paper.