This is why blacks descended from slaves have health issues like hypertension-related diseases, renal issues and also (it's been theorized) prostate cancer.

To keep it short, slavery was a massive genetic bottleneck. The death rate from the point of capture to delivery at the ships was circa 10%. Confinement along the African coast killed off another 12%. Then, due to the unimaginably horrendous conditions during the "middle passage", an additional 12-15% did not survive. After that, up to 30% did not survive the first 3 years of slavery to become, as the slavers called it, "seasoned".

The majority of deaths for slaves were from exactly what you would expect. They were related to salt and water retention. It was mostly salt and water depleting illnesses and labor that was killing slaves on ships. Those who survived did so because of a renal "glitch"; an "enhanced ability to retain salt". Whereas their counterparts in Africa who were not captured have salt-retention levels similar to any other population, the slaves who actually survived that bottleneck had kidneys that were slower at releasing salt, which, while in the short term helped them survive the grueling work in horrid conditions, also led to a population of black Americans and Afro-Caribbean people with a tendency toward high blood pressure.

Prostate Cancer is being considered as another issue for descendants of slaves, and theories are that the SNPs responsible for their tendency toward prostate cancer provided short term benefits for surviving the conditions of slavery.

NOTE: I am not a geneticist or historian. I did this research after seeing the extent of my African heritage at Ancestry.com and after feeding my results into Promethease.