How does the recessive sickle cell allele stay in the gene pool

Name the organs of humanelemintary canal.

The image below shows a common blood pressure gauge. what does this device do? a. measures the level of oxygen present in the blood b. measures the pressure of blood when the lungs expand and compress c. measures the electrical activity of the heart d. measures the pressure of blood when the heart contracts and relaxes

Several billion years ago which gas was the least prevailant in the atmosphere