Earth gravitational force pulls all the objects toward the centre of earth.But there are many instances which disobey the law of gravity. For example our blood flows from the veins situated in our legs to our heart which is against gravity and also the growth of plants is not downwards but upwards, the water vapours rising. All these are against gravity. Why is that so? Can anyone please give me an explanation?
There are other forces that act on the objects such that, when combined with gravity, produce a net force upwards. For example, water vapor rises up because it is less dense than air and therefore the upthrust, which acts upwards, exceeds the gravity, which acts downwards. So the net force is still upwards.
Everything obeys the law of gravity.
Our blood flowing from our legs up to our hearts is not disobeying the law of gravity, our heart pumps the blood through the body and this pumping action is strong enough to move the blood around the body, even in an upwards direction. In addition, our blood vessels contain valves. Movement of the leg squeezes the veins, which pushes the blood toward the heart. When the muscles contract the blood within the veins is squeezed up the vein and the valves open. When the muscle is at rest, the valves close helping to prevent the backward flow of blood. (Veins & Valves - Jobst)
The growth of plants - plants grow (often) upwards, they add cells as they grow and work against gravity to grow upwards.
Water vapours and gases that are "lighter than air" all rise against gravity because/when they have a lower density than the surrounding air and it is the air pressure that forces them to move upwards. They (the molecules) are also affected by gravity but in those cases the force of gravity is weaker than the force of the surrounding air pressure.
When we stand up, we are exerting ourselves, our muscles, to stand up against the force of gravity. We are always bound by the laws of gravity* and so we are not disobeying them, we are just doing extra work to overcome gravity.
* except in the case of Arthur Dent learning to fly.
If gravity doesnot affect blood flow, you will not feel any discomfort standing upside down for a long period of time.But normal people (without such training) feel blood rushing to their head(which is uncomfortable).So gravity affects blood flow.