The circulatory system is a closed system made up of the heart (an organ), blood vessels (including arteries, veins, and capillaries), and blood. The heart serves as a pump for the circulatory system, keeping blood flowing by altering fluid pressures throughout the body. This alteration of pressure is achieved by changing the rate at which the ventricles of the heart contract. Each time the heart beats, blood is pumped through the vascular system to all parts of the body. The lungs act as a pump too! When you breathe in, your lungs fill with air. This increases the pressure inside your chest, causing some of the muscles around the ribs to tighten. These muscles are called the intercostal muscles because they lie between each pair of ribs. As the muscles shorten, they pull the ribs toward the midline of your body, creating a vacuum that forces air out through your nose and mouth. This action moves the diaphragm up and down, expanding and contracting the lungs. The movement of the diaphragm causes increased blood flow to it from nearby arteries, which sends more oxygen-rich blood to the organs of the body. This is how you get energized during exercise - the lungs, heart, and other organs work together to keep you going! The skin also acts as a natural barrier against bacteria, viruses, and other harmful substances found in the environment.