The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. About 20% of the atmosphere's mass is contained in the stratosphere. The stratosphere is stratified in temperature, with warmer layers higher and cooler layers closer to the Earth. The increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet radiation by the ozone layer. This is in contrast to the troposphere, near the Earth's surface, where
Each side has a relatively thin-walled chamber that receives blood returning to the heart (atrium) and a muscular chamber that pumps blood out of the heart (ventricle). Blood Flow. The flow of blood through the heart is controlled by the opening and closing of valves and the contraction and relaxation of the myocardium. Heart valves are controlled by pressure changes within each chamber and contraction and relaxation are controlled by the heart's conduction system.
There are three different things that help blood get back to the heart. The three mechanisms are: venous valves, muscular pumps, and the respiratory pump. The blood in veins is at a lower pressure than it is in the arteries. The lower pressure makes for challenges getting that blood back to the heart. As there is less force (pressure) in the veins, blood would pool in the lower parts of the body if there were not mechanisms to prevent that.