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Transpiration.
The "little door" refers to the valves that are present throughout the venous blood vessles, and in the heart chambers. They prevent blood from flowing the wrong way in the vessels.
The blood in veins is not under direct pressure from the heart. It has little valves inside to prevent blood from flowing backwards away from the heart. These valves inside the veins prevent the backflow of blood in veins. They encourage the one-directional flow of blood to the heart.
veins contain valves that prevents blood flowing backwards between heartbeats.There are valves in the veins that stop the blood from flowing backwards.
In the cardiac cycle, blood flowing into the heart is called the systole.
Veins have valves which stop the blood from flowing backwards
Your heart keeps your blood "flowing"
Answer:Blood flowing to your lungs is full of carbon dioxide, which gets exchanged in your lungs and the blood flowing out of your lungs is clean/purified with oxygen in it.
There are valves in the heart - and the larger blood vessels that stop the blood flowing backwards.
There are two ways that blood is kept flowing in the same direction. The heart is pumping it in that direction, and blood vessels contain valves which prevent blood from flowing in the wrong direction.
Blood flowing into the left atrium comes from the two pulmonary veins.
The blood in veins is not under direct pressure from the heart. It has little valves inside to prevent blood from flowing backwards away from the heart. These valves inside the veins prevent the backflow of blood in veins. They encourage the one-directional flow of blood to the heart.