The problem of blood vessels

1.為什麼artery 的muscular wall 要厚??
Artery with thicker muscular wall is to withstand the high blood pressure caused by the pumping action of the heart that is the systolic pressure.

2.為什麼血管硬化會使血壓上升??
This is related to a term called compliance with the equation,
圖片參考：http://upload.wikimedia.org/math/7/1/9/719e4af636a17f8d07d6efcf3317d986.png
. This measure, the amount of the artery to be distended when 1mmHg blood pressure is applied. As you know, during the systolic period, the artery will be distended. And during this process, the kinetic energy of the blood will convert to the potential energy of the artery due to the increase in the length of the artery. But during the diastolic peroid, the artery, recoils. The PE will convert back to KE and keeps the blood moving along the artery. During this process, the artery absorbs some of the energy and acts as a buffer so that the blood pressure inside the artery will not increase sharply(當血管被distend時,血管壁會食了些血壓,所以比出一個緩衝作用). However, when a person grows old, the arteriosclerosis makes the artery not elastic enough, so that the buffer system is thus not as good as young one. Hence, the blood pressure, systolic pressure will be high than usual.(當血管被distend時,老人家的血管壁食的血壓會比較年輕時少得多,所以不能比出一個緩衝作用,所以老人的血壓會比年輕的人高一些)
3.如果artery 的muscular wall 過薄會否使artery破裂？？為什麼？？
NO, this depends on the position of the artery. As you mention in question4, when the arteries are far away form the heart, the blood pressure will drop sharply. As a result, even, the artery is thinner than usual, it will not burst. As you know, the blood capillary doesn't contain any elastic muscle. However, due to far away from the heart, it won't be burst either.

4.artery的friction會減緩血壓，因此離心臟愈遠的artery的血壓較低，但這應只會減緩''bu mb''血時的最高血壓，為什麼最低血壓和普通血壓也會被減緩？？
This is because, the blood has viscosity so that it is not an ideal fluid. As a result, the friction will also be generated during diastolic period.
P.S. actually, you don't need to know what complicance is at you level.
I hope you will find my writing helpful.

關於第1題lei~ 因為artery 係負責將由心臟pump出黎ge血運去身體各個部份, 所以佢會受heart ge rhymic contraction 同relaxation影響, 令佢要長期承受由li種運動引起ge high blood pressure. 所以為左可以withstand high blood pressure, artery ge muscular wall就要thick la~
正因為li個原因, 關於第三題, 我諗你ge假設係岩ge, muscular wall過薄會有機會爆血管a~
至於第四題, 如果你最高血壓被降低, 咁d blood 之後會遇到ge friction (in veins, capillaires等等)冇變ga ma, 所以blood pressure decrease ge幅度同之前一樣, 咁所以你ge最低血壓和普通血壓咪都會decrease lo~
第二題lei~ 我就唔肯定我ge答案一定岩la~ 我諗係因為elastic gei vessel wall 可以absorb shock, 將部份gei blood pressure absorb 左, 咁所以total blood pressure就可以decrease. 就好似你將個ball hit 落張床度, ball反彈ge 力就會細左. 但如果血管硬化, 就好似你將個波打去concrete wall度咁, 佢gei反彈力一樣強. 而硬化ge血管就好似li 個concrete wall 咁, 所以blood pressure就會increase la~