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.
Chambers of the Heart. The heart has four chambers: two atria and two ventricles. 1  The right atrium receives oxygen-poor blood from the body and pumps it to the right ventricle. 2  The right ventricle pumps the oxygen-poor blood to the lungs.  The left atrium receives oxygen-rich blood from the lungs and pumps it to the left ventricle.