Attached is a list of the major factors which determine venous pressure, and hence right atrial pressure. Many of these factors are the same as those we discussed earlier regarding right atrial filling in the section on the cardiac cycle.
Blood volume is important because it effects cardiovascular filling pressure and more specifically, cardiac preload. Distribution of fluid within the various compartments is affected by blood volume through the Starling-Landis equation. Activity of the sympathetic nerves to the veins is another important factor, since norepinephrine released on smooth muscle of the veins controls venous tone.
Increased venous tone tends to move blood out of the venous volume reservoir and toward the heart. Respiratory movements of the chest wall, the diapraghm, and the abdomen effect venous pressure. An increasingly negative intrathoracic pressure draws blood toward the heart, while an increase in abdominal pressure pushes blood toward the heart, i.e. a push-pull mechanism.
Posture exerts a strong influence on venous return since orthostasis causes blood to settle in the legs and in the abdomen due to the effects of gravity; whereas, when lying supine gravity has no effect on body blood volume distribution. The downward movement of the tricuspid valve during ventricular systole expands atrial volume, producing a sucking action on the blood and causing it to enter.
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