Pressure in the Left Side is Higher than the Right Side
The normal pressure in the Left Ventricle is 120 mmHg. in an adult. This amount of pressure is needed to force the blood all the way from the left side of your heart to your toes and still have enough energy to bring the blood back to the right side of your heart. It is easy to see why the left side of your heart needs more pressure than the right side. In the meantime, the Right Ventricle has a much easier job to do. The normal pressure in the Right Ventricle is about 30 mmHg. which is enough to push the blood a short distance to the lungs and then back to the left side of the heart.

As a boy is lying down the blood is leaving his heart at a pressure of 100 mmHg. It travels down the arteries to the leg and by the time it reaches the capillaries in the foot the pressure is at about 95 mmHg. Then after it goes through the arterioles, which are the sites of high resistance, the pressure drops to 25 mmHg. By the time the blood returns to the veins it is at a pressure of only 5 mmHg. It flows back towards the heart, through the one-way veins and reaches the heart. The right side of the heart has a pressure of 0.

Blood Flows from High to Low
In order for blood to flow it has to have a pressure difference. Blood, like any other fluid, flows from the high pressure to the low pressure like a waterfall; the water comes down from high to low. So in the heart the highest pressure is in the Left Ventricle. The Left Ventricle pushes the blood down to the toes, which are at lower pressure, and the blood leaves the capillary beds in the toes with little pressure. This pressure is still higher than the 0 mmHg pressure in the Right Atrium, so the blood flows from the relatively higher pressure in the toe to the lower pressure in the Right Atrium.

Blood Pressure Sends Blood Against Gravity Back to the Heart
Now, when the person stands up the blood travels to the brain away from the ground (against gravity). Because it is going against gravity it will loose some of its force. However, the blood going down to the legs in the standing position is going with gravity via the arteries. So the pressure at the foot will increase to as much as 185 mmHg. This pressure is needed so that it will transmit to the vein pressures which now has 95 mmHg pressure. This is enough to work against gravity and to be able to propel the blood back to the heart.