Help please I'm stuck
#life sciences​

hope this helps

I do think that they close to not allow too much of either water or air from entering.

the hollow tube allows water to flow easily.

https://www.bbc.co.uk/bitesize/guides/z28g7p3/revision/1

the menstrual cycle – higher tier. negative feedback mechanisms affect the production of hormones in the menstrual cycle. oestrogen also stimulates the release of lh (luteinising hormone) from the pituitary gland, which in turn controls the production of progesterone.