The human body needs oxygen to form the chemical reactions used to produce energy for vital functions. Oxygen enters the body when the diaphragm relaxes and allows the lungs to expand and take in air. When oxygen enters the lungs, it gets picked up by the blood vessels near the lungs and spread throughout the body. The lungs and circulatory system work together to ensure that the body has a steady supply of oxygen. Without adequate oxygen, vital functions gradually shut down.
Mammalian lungs are elastic sacs that expand to allow air to flow into them through the two bronchial tubes. Once each bronchial tube enters the lung, it splits into multiple bronchi and gets smaller until it ends in the alveoli. These small sacs are attached to microscopic blood vessels that connect to the heart. Oxygen transfers from the alveoli into the oxygen-poor blood, and carbon dioxide moves to the lungs and is expelled on the exhalation when the lungs contract. The lungs and circulatory system are connected via the pulmonary blood vessels.
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The heart pumps deoxygenated blood from the right atrium into the lower chamber, the right ventricle. The blood travels from the right ventricle into the pulmonary artery, which divides to deliver blood to each lung. When the pulmonary artery connects with the alveoli in the lungs, the gas exchange occurs and the newly-oxygenated blood travels through the left ventricle. The blood moves into the left atrium and exits the heart through the aorta, the largest artery in the human body. The lungs and circulatory system start the process over again when blood re-enters the heart from its trip spreading oxygen through tissues.
One-way valves in the heart keep blood flowing in one direction through the circulatory system. Oxygen-rich blood flows through the arteries and away from the heart to the rest of the body. Veins direct oxygen-poor blood back to the heart, where it goes to the lungs to pick up a fresh supply of oxygen. Valves also exist in the veins to prevent carbon dioxide from mixing with oxygen and causing problems.
Some of the most fatal health conditions occur when the lungs and circulatory system are damaged because of an unhealthy lifestyle, especially because the two are so interlinked. For example, if the lungs become clogged with smoke and tar from cigarette smoke, oxygen cannot pass through the alveoli effectively. Similarly, a high-fat diet can cause fatty deposits to accumulate on the insides of the arteries, thus raising blood pressure and potentially blocking the artery. If the artery is completely blocked, it can cause cardiac arrest because the circulatory system is damaged.