Which layer of the heart wall contracts and is composed primarily of cardiac muscle tissue?

Myocardium

Explanation: Myocardium is the middle layer of the heart wall. It is also the muscle tissue of the heart.

Myocardium is the thickest layer of the heart wall. It is madeup of cardiac muscle cells. These muscle cells possess contracting capability like other muscle cells.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.