Every cell and tissue of our bodies requires oxygen for its functions and produces carbon dioxide as waste. We breathe in oxygen to the lungs where it is exchanged in the alveoli for carbon dioxide in the blood. Red blood cells carry oxygen throughout the body. In the capillaries, small blood vessels in the tissues, the oxygen is exchanged for carbon dioxide in the cells.
The respiratory system is responsible for bringing in oxygen by inhalation and releasing carbon dioxide by exhalation. This system consists of the upper and lower respiratory tracts, including the lungs, which are located on the left and right side of the chest cavity.
Air, which is mostly nitrogen but contains oxygen as well, is pulled in through the mouth and nose by pressure manipulations inside the respiratory tract. This is accomplished by various muscles that contract and expand the lungs.
The diaphragm is a muscle underneath the lungs that pushes up against them and aids in their contraction and relaxation. Muscles lining the ribcage, the intercostals, also help this process by squeezing the lungs.
The Upper Respiratory Tract
The upper respiratory tract includes the nasal passages, pharynx, and larynx. The pharynx is the throat and is divided into the opening behind the oral cavity, or mouth, (oropharynx) and where the nasal passages extend upward (nasopharynx) through the nose and nostrils. Extending downward from the pharynx is the windpipe and esophagus. The oropharynx is the only anatomical structure to handle both food and air, besides the mouth itself.
The larynx is also known as the voicebox. It is made of tissue folds, called vocal cords, and cartilage. The folds allow sounds and speech to be formed as air passes through it. A flap called the epiglottis covers the larynx during swallowing to prevent food from entering the respiratory tract.
The Lower Respiratory Tract
The lower respiratory tract consists of the trachea, or windpipe, bronchial tree, and the lungs. The windpipe is held open by incomplete rings of cartilage. It is capable of partially collapsing when the esophagus transports food after swallowing. The trachea is lined with goblet cells, cells that produce mucus. This mucus protects the respiratory tract from foreign particles being breathed in. The size of a particle will determine how far it gets into the respiratory system.
The bronchial tree is a series of increasingly smaller branches off of the windpipe. The left bronchus branches off into the left lung, and the right bronchus branches off into the right lung. The bronchi undergo further branching, as bronchioles, to transport air into the lungs for gas exchange. The bronchioles terminate at the alveolar sacs, which contain small, thin-walled air pouches called alveoli. Alveolar ducts contain blood vessels and the close proximity of the air sacs and blood allow for efficient gas exchange.
The lungs are made up of lobes. The right lung has three, the left lung has two. The left side is smaller because the heart takes up the space of the third lobe. The lung tissue is made up of the alveoli. The lungs inflate and deflate as air is transported into and out of the air sacs. The pulmonary artery and pulmonary vein transport blood to and from the lungs for gas exchange.