central venous pressure and mean arterial pressure

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Little Soldier

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    what? venous vasoconstriction increases blood flow to the right atrium. the veins have a higher tone and so less blood can pool in them

    edit: this is on top of the fact that its harder to drive blood into the veins due to their increased pressure.
     

    Rothbard

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      I don't think it's entirely your fault. The language used to describe a lot of this physiology is quite vague, and I had many of the same questions when I was taking physio. For example, in your thread about pressure = flow X resistance, people often neglect to point out that there are two pressures involved, upstream and downstream pressure. They just refer to an unspecified "pressure", which leads to confusion.

      The answer to your question here has to do with the Frank-Starling mechanism. The more pressure you have "inflating" the heart during diastole (that's CVP), the larger the end diastolic volume. The myocytes are more elongated, and this leads to greater contractile force during systole (this has to do with sarcomere length), which means increased stroke volume, arterial pressure, and cardiac output.

      If the cardiovascular system is in equilibrium, then all the above parameters and venous return will remain constant. But let's say we stop the heart. During this time, blood will drain out of the arteries into the veins (because pressure is higher in the arteries than in the veins). MAP goes down, CVP will go up as blood pools in the veins, and this will reduce venous return. If you stop the heart long enough, blood flow will completely stop (CO and venous return are zero), MAP will equal CVP (this is mean systemic filling pressure). Now start the heart up again. As the heart starts sucking blood out of the veins and pushing it into the arteries, CVP goes down, MAP goes up, and CO and venous return increase.

      The circulatory system is pretty much a closed circuit, and all blood flows through the heart. Any blood leaving the heart has to eventually return to it. Another way of saying this is that cardiac output has to equal venous return on average. If there's a discrepancy, then the system is out of equilibrium, and there are two possibilities (assuming there's no bleeding or anything like that).

      1. The pump (heart) is slowing down, CO/MAP are decreasing and CVP/VenousReturn are increasing. Blood is pooling in the veins.

      2. The pump is speeding up, CO/MAP are increasing and CVP/VenousReturn are decreasing. Blood is being sucked out of the veins and pushed into the arteries.

      Whether you have 1 or 2, eventually a new equilibrium will be reached and venous return will equal CO.
       

      0919mmk

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        no not true, higher CVP decreases venous return bcoz it will be more difficult to drive blood to rt atrium

        no the previous poster was right. Higher CVP means you have more pressure in your vena cava right? If you have more pressure in the vena cava, then you have more pressure driving blood in to the right atrium. That means the venous return is higher = more blood in heart = increased MAP.

        If you have...like... more pressure in a fire hose, you will have more water coming out, in this case, into the heart.
         

        Rothbard

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          no the previous poster was right. Higher CVP means you have more pressure in your vena cava right? If you have more pressure in the vena cava, then you have more pressure driving blood in to the right atrium. That means the venous return is higher = more blood in heart = increased MAP.

          If you have...like... more pressure in a fire hose, you will have more water coming out, in this case, into the heart.

          Venous return refers to how much blood is flowing through the veins to the heart. CVP is downstream pressure. High downstream pressure decreases the pressure gradient and reduces flow. If the heart is working properly, the increased CVP will increase CO, and this will eventually reduce CVP and increase venous return.
           

          SamarEsawy

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            Venous return refers to how much blood is flowing through the veins to the heart. CVP is downstream pressure. High downstream pressure decreases the pressure gradient and reduces flow. If the heart is working properly, the increased CVP will increase CO, and this will eventually reduce CVP and increase venous return.

            yes that's good thank you so much
             

            SamarEsawy

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            Feb 25, 2013
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              Venous return refers to how much blood is flowing through the veins to the heart. CVP is downstream pressure. High downstream pressure decreases the pressure gradient and reduces flow. If the heart is working properly, the increased CVP will increase CO, and this will eventually reduce CVP and increase venous return.

              yes that's good thank you so much
               

              Anastomoses

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                I don't think it's entirely your fault. The language used to describe a lot of this physiology is quite vague, and I had many of the same questions when I was taking physio. For example, in your thread about pressure = flow X resistance, people often neglect to point out that there are two pressures involved, upstream and downstream pressure. They just refer to an unspecified "pressure", which leads to confusion.

                The answer to your question here has to do with the Frank-Starling mechanism. The more pressure you have "inflating" the heart during diastole (that's CVP), the larger the end diastolic volume. The myocytes are more elongated, and this leads to greater contractile force during systole (this has to do with sarcomere length), which means increased stroke volume, arterial pressure, and cardiac output.

                If the cardiovascular system is in equilibrium, then all the above parameters and venous return will remain constant. But let's say we stop the heart. During this time, blood will drain out of the arteries into the veins (because pressure is higher in the arteries than in the veins). MAP goes down, CVP will go up as blood pools in the veins, and this will reduce venous return. If you stop the heart long enough, blood flow will completely stop (CO and venous return are zero), MAP will equal CVP (this is mean systemic filling pressure). Now start the heart up again. As the heart starts sucking blood out of the veins and pushing it into the arteries, CVP goes down, MAP goes up, and CO and venous return increase.

                The circulatory system is pretty much a closed circuit, and all blood flows through the heart. Any blood leaving the heart has to eventually return to it. Another way of saying this is that cardiac output has to equal venous return on average. If there's a discrepancy, then the system is out of equilibrium, and there are two possibilities (assuming there's no bleeding or anything like that).

                1. The pump (heart) is slowing down, CO/MAP are decreasing and CVP/VenousReturn are increasing. Blood is pooling in the veins.

                2. The pump is speeding up, CO/MAP are increasing and CVP/VenousReturn are decreasing. Blood is being sucked out of the veins and pushed into the arteries.

                Whether you have 1 or 2, eventually a new equilibrium will be reached and venous return will equal CO.

                Very nice. Even the patience to explain it. :)
                 
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