The inner leaflet of the human erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatidylserine. the outer leaflet predominantly of phosphatidylcholine and sphingomyelin. although the phospholipids components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. how?

-Flippases: membrane enzymes that move PE and PS to internal side against their gradient

-energetically difficult for flipping, hydrophilic head has to move through hydrophobic center segment of membrane

-Main reasons: energetic barrier, other enzymes work to maintain the sidedness

-PS on surface tags for apoptosis, want to avoid

these muscles maintain the blood pressure in the vessels. the smooth muscles are innervated by nerves of the autonomic nervous system. when the muscles contract, they cause vasoconstriction of the vessels and increase blood pressure. when the muscles relax, they enable vasodilation of the   blood vessels hence reduce blood pressure.