What prevents the blood from flowing backwards

Why is it important to reduce the number of chromosomes in half when forming gametes for sexual reproduction?

Will someone me with this.. specialized cells which perform a particular function form: tissues organs or organism

Research cheetahs on the internet what has contributed to this animal becoming "endangered" or "threatened." what animal you have chosen? -cheetah how long has the animal been endangered or threatened? what has contributed to this animal’s endangered or threatened status? why is it important to save this animal from extinction? after researching and gathering facts, write a 350-word letter from the point of view of an animal rights' activist. be sure to include at least five facts that you learned from your research.