It's seen as fairly basic/text-book knowledge, and finding a paper on it is pretty difficult. It was probably shown in the 19th century and isn't easily found

It's seen as fairly basic/text-book knowledge, and finding a paper on it is pretty difficult. It was probably shown in the 19th century and isn't easily found on google scholar! I did find one reference [1] from more recently. Part of our sepsis warning scores depend on the heart rate to rate the severity of the illness. I suspect even in mild colds your cardiac output goes up, but it might be difficult to notice just from measuring the pulse (stroke volume can change). Heart is just one example - our kidneys get worse as we get older too (see figure 4 of this paper - it's a straight line pretty much [2]). In practice, it doesn't cause us problems in day to day life, but give it a really big insult (like sepsis) and it can decompensate. The difference with kidneys is that we can support kidney failure with dialysis or filtering (if their heart can support that), whereas with cardiac failure this is not so simple. Brain failure is probably one of the worst forms of failure. Most causes have no cure, and they often require 24hr care. [1] http://ajpregu.physiology.org/content/246/3/R331.short http://ajpregu.physiology.org/content/246/3/R331.short [2] https://www.spandidos-publications.com/10.3892/mmr.2015.3292 https://www.spandidos-publications.com/10.3892/mmr.2015.3292