Factors affecting venous return blood volume include mean corpuscular filling pressure, myocardial contractility, the squeezing effect of skeletal muscle, body position, and respiration. 1. Average filling pressure of body circulation: when blood volume increases or vasoconstriction occurs, the degree of filling in the blood vessels increases, and the venous return blood volume increases. 2. Myocardial contractility: when contractility is increased, ventricular ejection is increased and end-diastolic pressure in the atria is low, thus atrial pumping capacity is increased and the volume of blood returned to the heart is increased. 3. Skeletal muscle extrusion: when skeletal muscle contracts, it can extrude the veins inside the muscle, thus enhancing the venous return blood volume. 4. Body position: the volume of blood returned to the heart increases in the lying position compared with the upright position. 5. Breathing: the negative pressure in the pleural cavity increases during inhalation, which is conducive to the dilatation of the large veins and the right atrium, and the venous return blood volume increases.