Flucuated between hot and cold periods
Answer:
The return of blood to the heart is assisted by the action of the skeletal- muscle pump. As muscles move, they squeeze the veins running through them. Veins contain a series of one-way valves, and they are squeezed, blood is pushed through the valves, which then close to prevent backflow.
This layer is called the cell wall.
The cell wall is located outside the cell membrane and provides the cells with structural support and protection. It gives cells rigidity and strength, offering protection against mechanical stress.
In most cells, it is flexible, meaning that it will bend rather than hold a fixed shape, but has a lot of tensile strength which is necessary to be able to withstand internal osmotic pressure.
Cell walls are found in plants, bacteria, fungi, algae and some archaea.
