Section 3: The Cardiovascular System
The cardiovascular system, or circulatory system, carries needed substances to cells and removes waste products from tissues. It’s made up of the heart, the blood vessels, and blood.
The heart is a hollow, muscular, powerful organ that pumps blood throughout the body. Each time your heart beats it pushes blood through the blood vessels. The heart has four chambers. The atriums are the upper two chambers, and the ventricles are the lower two chambers. The atrium is where blood first enters the heart, and then the ventricles pump blood out of it. The septum is the wall that separates the right side of the heart from the left side. Valves separate atria from a ventricle and prevent blood from flowing backward. They keep blood flowing in one direction. The “lub dub” sound is the opening and closing of those valves. In order to be effective and efficient, a heart must beat in an orderly way. A pacemaker is a group of cells located in the right atrium that sends out a signal to make the heart contract.
Blood flow is an important part of the cardiovascular system. Blood vessels carry blood all throughout the body and consist of arteries, capillaries, and veins. Arteries are blood vessels that move blood away from the heart towards organs and tissues. They carry oxygen-rich blood through their thick walls. Capillaries are microscopic and the most numerous of the blood vessels that connect arteries to veins. They have extremely thin walls. Veins return blood to the heart. They are more flexible than arteries and blood flows slowly and more smoothly through them. Without pressure, blood would cease to flow through the body. Blood pressure is the force of blood on the walls of the blood vessels.
Blood flows through the heart in two loops with the heart at the center. In loop one, blood travels from the heart to the lungs and then back to the heart. In loop two, blood is pumped from the heart throughout the body and then returns it again to the heart.