Section 3: The Cardiovascular System

The cardiovascular or circulatory system carries needed substances to cells and removes waste products from tissues.  It’s made up of the heart, 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 the ventricles pump blood out of it.  The septum is the wall separating the heart’s right side from the left side.  Valves separate the 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.  To be effective and efficient, a heart must beat in an orderly way. A pacemaker is a group of cells in the right atrium that sends a signal to make the heart contract.

Diagram showing blood flow of human heart

Blood flow is an integral part of the cardiovascular system.  Blood vessels carry blood 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 blood vessels connecting 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 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 back to the heart.  In loop two, blood is pumped from the heart throughout the body and then returns it to the heart.


  1. What is the purpose of the cardiovascular system?
  2. What is the difference between arteries and veins?
  3. Compare the two loops of blood flow.


Click here to go back to the Table of Contents