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    Biology Snab book states:

    "Contact between blood and walls of vessel causes peripheral resistance and slows down blood. capillaries offer a greater surface area, resisting flow more, slowing blood down and causing the blood pressure to fall"
    Then it goes on to say
    "if vessels contract the resistance increases and blood pressure is raised"

    i dont understand this, why would blood pressure increase when there is greater resistance? resistance slows down the flow of blood. Can someone please help me out.
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    please? anyone
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    (Original post by hupper12345)
    Biology Snab book states:

    "Contact between blood and walls of vessel causes peripheral resistance and slows down blood. capillaries offer a greater surface area, resisting flow more, slowing blood down and causing the blood pressure to fall"
    Then it goes on to say
    "if vessels contract the resistance increases and blood pressure is raised"

    i dont understand this, why would blood pressure increase when there is greater resistance? resistance slows down the flow of blood. Can someone please help me out.
    It is normally the arterioles which are the site of greatest resistance, they also tend to be more muscular in structure and are not as elastic as the large conducting arteries of the body (aorta).

    Normally in systole, blood is forced into the arteries and this causes stretch. The stretch recoils and pushes the blood in diastole (when the ventricles are relaxed). This stretch recoil determines diastolic blood pressure (the denominator in the blood pressure measurement). In arterioles, due to the muscular walls and narrow lumen, they cannot "stretch" and therefore their recoil is not sufficient enough to transmit the blood pressure. There is a degeneration of the pressure as most of it gets lost - thus being low especially in the capillaries.

    As for the second part of your question, the Poiseuille equation states that:

    Blood pressure = cardiac output x total peripheral resistance

    Thus, if the peripheral resistance was to increase, say to vasoconstrictors such as noradrenaline (narrowing the blood vessel lumen), then blood pressure will increase.

    Resistance is inversely proportional to radius; if the radius was to decrease (in contraction) then resistance would increase. If resistance increases, the "afterload" of the heart increases. Afterload is basically the amount of pressure the heart (particularly the left ventricle for the systemic circulation) must generate to overcome the pressures in the systemic circulation. The higher the resistance (and systemic pressure), the higher the pressure that must be generated by the heart (i.e. the blood pressure).

    It is true that increased resistance leads to a slight stasis or slowing down of the blood, but increases in blood pressure aim to prevent this by ejecting at stronger forces.

    Hope this has helped.
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    When the radius decreases the same volume of blood has to go through faster so the pressure rises (bernoulli principle)
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    (Original post by Doctor Tony)
    It is normally the arterioles which are the site of greatest resistance, they also tend to be more muscular in structure and are not as elastic as the large conducting arteries of the body (aorta).

    Normally in systole, blood is forced into the arteries and this causes stretch. The stretch recoils and pushes the blood in diastole (when the ventricles are relaxed). This stretch recoil determines diastolic blood pressure (the denominator in the blood pressure measurement). In arterioles, due to the muscular walls and narrow lumen, they cannot "stretch" and therefore their recoil is not sufficient enough to transmit the blood pressure. There is a degeneration of the pressure as most of it gets lost - thus being low especially in the capillaries.

    As for the second part of your question, the Poiseuille equation states that:

    Blood pressure = cardiac output x total peripheral resistance

    Thus, if the peripheral resistance was to increase, say to vasoconstrictors such as noradrenaline (narrowing the blood vessel lumen), then blood pressure will increase.

    Resistance is inversely proportional to radius; if the radius was to decrease (in contraction) then resistance would increase. If resistance increases, the "afterload" of the heart increases. Afterload is basically the amount of pressure the heart (particularly the left ventricle for the systemic circulation) must generate to overcome the pressures in the systemic circulation. The higher the resistance (and systemic pressure), the higher the pressure that must be generated by the heart (i.e. the blood pressure).

    It is true that increased resistance leads to a slight stasis or slowing down of the blood, but increases in blood pressure aim to prevent this by ejecting at stronger forces.

    Hope this has helped.
    Thank you much! I really appreciate it. I understand this concept much better now. Thanks again.
 
 
 
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