Enotes provides the interesting history of the slave trade during this period of time. Go to the link below and click on "Historical Context" on the right. I'll briefly summarize some of the most important aspects for you.
Even though it eventually became illegal to import slaves from Africa, it continued until the mid-1800s. It's estimated that around ten million Africans were taken and illegally sold into slavery. Not all slaves went to the U.S., but many of them did. The sale and trade of American-born slaves continued until the Civil War. By 1860, more than four million slaves lived in the southern part of the U.S.
The treatment of slaves on the ships was horrendous. All ships kept them chained in the hold (the lowest part) of the ship, only allowing them to come on deck once a day or once every two days to exercise and get some sunlight. Some captains were so afraid of revolt by the slaves, they didn't allow the slaves any time on deck. Sickness and death were prevalent since there were no sanitary facilities. The captain of a ship determined how many slaves would be packed into a ship. The greater the number of slaves crowded into the hold, the larger the number of deaths and the spread of disease that occurred. This is why some captains tried to cut down on death and sickness by travelling with fewer slaves.