The cardiovascular and respiratory systems are intimately related. Together, they act to deliver oxygen to all cells in the body -- without oxygen, the cells cannot survive. Understanding how the respiratory and cardiovascular systems interact first requires knowledge of how each system functions separately, then how they work together.
The cardiovascular system consists of the heart and blood vessels. There are 3 main types of blood vessels:
-- arteries, which move blood from the heart
-- veins, which return blood back to the heart
-- capillaries, which are very tiny vessels connecting the arteries and veins
The heart is divided into 4 chambers, called the right atrium, right ventricle, left atrium and left ventricle. Oxygen-rich blood is pumped from the left ventricle to arteries, which carry the blood to the body. The arteries divide into smaller arteries that eventually lead to capillaries. The capillaries are tiny blood vessels in close contact with the body's cells. Oxygen and other nutrients in the blood travel across the capillary wall to enter the cells. At the same time, carbon dioxide and other waste products move in the opposite direction, from the cells to the capillaries. Blood low in oxygen and high in carbon dioxide then travels from the capillaries to veins, which bring the blood to the right atrium of the heart.
The respiratory system consists of the airway, lungs and 2 types of muscles. The airway is a system of passageways through which air moves to and from the lungs. Beginning with the nose and mouth, the airway continues down through the throat and windpipe, or trachea, to the bronchi, which enter the lungs. In the lungs, the bronchi divide into smaller tubes that eventually come to a dead end as sacs called alveoli. Capillaries surround each alveoli. Oxygen in the air within the alveoli moves across the alveolar wall to enter the blood in the capillaries, and carbon dioxide travels in the other direction, from the capillaries into the alveoli.
Intercostal muscles -- located between the ribs -- and the diaphragm -- the large muscle below the lungs separating the chest from the abdomen -- cause the chest to become larger and smaller as a person breathes in and out. When the chest gets larger, the lungs expand, allowing air to enter them. As the chest gets smaller, the lungs are compressed, forcing air out.
The cardiovascular and respiratory systems work together toward the same goal: bringing oxygen to and removing carbon dioxide from the body's cells. They act in sequence and are often referred to as the cardiorespiratory system, a combination name that emphasizes their intimate relationship. After oxygen and carbon dioxide move across the alveolar wall in the lungs, the oxygen-rich, carbon dioxide-poor blood travels through pulmonary veins to the left atrium of the heart. From here, it passes into the left ventricle and then to the rest of the body.
Poorly oxygenated blood from the body's cells enters the right atrium of the heart. It then passes into the right ventricle, which pumps the blood through pulmonary arteries to the lungs. The pulmonary arteries bring the blood to the capillaries surrounding the alveoli, where the oxygen and carbon dioxide exchange occurs. Blood high in oxygen and low in carbon dioxide then returns to the left atrium via pulmonary veins, continuing the cycle.
Reviewed and revised by: Mary D. Daley, M.D.