How does and enzyme speed up a chemical reaction

The enzyme speeds up the reaction by lowering the activation energy needed for the reaction to start.

not only is it colder, it also becomes darker, less salty, and more dense.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.