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The Lungs

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The Lungs
The Lungs

The Lungs

Credit: National Cancer Institute

The Lungs

The lungs are organs of the respiratory system that allow us to take in and expel air. In the breathing process, the lungs take in oxygen from the air through inhalation. Carbon dioxide produced by cellular respiration is in turn released through exhalation. The lungs are also closely associated with the cardiovascular system as they are the sites for gas exchange between the air and the blood.

Lung Anatomy

The body contains two lungs, of which one is positioned on the left side of the chest cavity and the other on the right side. The right lung is separated into three divisions or lobes, while the left lung contains two lobes. Each lung is surrounded by a two-layered membrane lining (pleura) that attaches the lungs to the chest cavity. The membrane layers of the pleura are separated by a space filled with fluid. Since the lungs are enclosed and contained within the chest cavity, they must use special passages or airways to connect with the outside environment. The following are structures that assist in the transportation of air to the lungs.
  • Nose and Mouth - openings that allow outside air to flow into the lungs.

  • Pharynx (throat) - directs air from the nose and mouth to the larynx.

  • Larynx (voice box) - directs air to the windpipe and contains vocal cords for vocalization.

  • Trachea (windpipe) - splits into left and right bronchial tubes which direct air to the left and right lungs.

  • Bronchioles - smaller bronchial tubes that direct air to small air sacs known as alveoli.

  • Alveoli - bronchiole terminal sacs that are surrounded by capillaries and are the respiratory surfaces of the lungs.

The Lungs and Circulation

The lungs work in conjunction with the heart to circulate oxygen throughout the body. As the heart circulates blood via the cardiac cycle, oxygen depleted blood returning to the heart is pumped to the lungs. The pulmonary artery transports blood from the heart to the lungs. This artery extends from the right ventricle of the heart and branches into left and right pulmonary arteries. The left pulmonary artery extends to the left lung and the right pulmonary artery to the right lung. The pulmonary arteries form smaller blood vessels called arterioles which direct blood flow to the capillaries surrounding lung alveoli.

The alveoli in the lungs are coated with a moist film that dissolves air in the lungs. Oxygen diffuses across the thin epithelium of the alveoli sacs into the blood within the surrounding capillaries. Carbon dioxide also diffuses from the blood in the capillaries to the alveoli air sacs. The now oxygen rich blood is returned to the heart via the pulmonary veins. Carbon dioxide is expelled from the lungs by exhalation.

The Lungs and Breathing

Air is supplied to the lungs through the process of breathing. The diaphragm plays a key role in breathing. The diaphragm is a muscular partition that separates the chest cavity from the abdominal cavity. When relaxed, the diaphragm is shaped like a dome. This shape limits space in the chest cavity. When the diaphragm contracts, it moves downward toward the abdominal area causing the chest cavity to expand. This lowers the air pressure in the lungs causing air in the environment to be pulled into the lungs through air passages. This process is called inhalation. As the diaphragm relaxes, space in the chest cavity is reduced forcing air out of the lungs. This is called exhalation. Regulation of breathing is a function of the autonomic nervous system. Breathing is controlled by a region of the brain called the medulla oblongata. Neurons in this brain region send signals to the diaphragm and the muscles between the ribs to regulate the contractions which initiate the breathing process.

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