The circulatory system in humans is a closed system – blood travels through vessels in a loop from the tissues to the heart, with a second loop between the heart and the lungs. Some animals have open circulatory systems where the blood floods over the organs by filling the body cavity and then drains back to the heart. What makes the circulatory system work is the pump, specifically the heart. In humans, this organ has four chambers, contracting in sequence to push the blood around the entire body.
What does the circulatory system do?
The role of the circulatory system is to transport nutrients and oxygen to the tissues and carry waste products away from the tissues. It accomplishes this via the blood, a water-based fluid carrying cells and dissolved molecules throughout the body. The blood vessels are largest at the heart, branching off into smaller and smaller vessels in the tissues until they are only one red blood cell wide (the capillaries). This video by the MayoClinic offers a visual representation of this system.
The red blood cells carry oxygen, which diffuses across the thin capillary wall into the tissue. Carbon dioxide is then bound and carried back to the heart via the venous system. From the heart the blood goes to the lungs.
In the lungs, the carbon dioxide is exchanged for oxygen. The lungs contain small air sacs that increase the surface area available to the pulmonary capillaries, allowing the necessary gas exchange. The blood then travels back to the heart and then to the body.
The circulatory system achieves this circulation and accomplishes its role in the body because of the heart.
The heart as a pump
The heart and circulatory system make up the cardiovascular system. The heart is a unique organ. It is made of cardiac-striated muscle and an electrical system that allows it to contract and squeeze blood into the circulatory system. According to the MayoClinic, the heart beats 100,000 times per day. Each beat pumps several quarts of blood out into the aorta and pulmonary artery, pushing the blood in other parts of the body and the lungs into the capillaries and through the venous return. This diagram from the Texas Heart Institute shows the flow of blood to and from the circulatory system and heart.
Blood pressure: How the circulation works
The heart is able to pump blood out into the circulatory system because of the pressure it exerts on the blood, forcing it through valves. The blood vessel walls offer resistance that propels the blood forward, called blood pressure. Once the blood reaches the capillaries, the pressure decreases. Oxygen exchange occurs because of various pressures in the capillaries and tissue spaces. The pressure is even less in the veins, but the force of blood coming out of the arteries and the pressure from skeletal muscles squeezing the veins forces the blood through that half of the circulatory system, called venous return. Essentially, pressure is how and why the circulatory system works.
The human circulatory system in a nutshell
The two loops of the human circulatory system allow the blood to flow to every tissue, bringing oxygen to every cell. Without the heart, this flow would not be possible.