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Anatomy and physiology of the lung
The lungs are located in the chest and are part of the respiratory system.
The lungs take up most of the space inside the chest. The lungs are surrounded by the chest wall. The chest wall is made up of the ribs and the muscles between the ribs. The lungs are separated by the mediastinummediastinumThe space in the chest between the lungs, breastbone and spine that contains the heart, great blood vessels, thymus, trachea (windpipe), esophagus and lymph nodes., which contains the heart and other organs. Below the lungs is the diaphragm, a thin muscle that separates the chest cavity from the abdomen.
Each lung is divided into lobes (sections):
- The left lung has 2 lobes.
- The heart sits in a groove (cardiac notch) in the lower lobe.
- The right lung has 3 lobes and is slightly larger than the left lung.
The trachea (windpipe) is the tube-shaped airway in the neck and chest. It divides into 2 tubes or branches called the main bronchi. One bronchus goes to each lung. The area where each bronchus enters the lung is called the hilum.
The pleura is a thin membrane that covers the lungs and lines the chest wall. It protects and cushions the lungs and produces a fluid that acts like a lubricant so the lungs can move smoothly in the chest cavity. The pleura is made up of 2 layers:
- inner (visceral) pleura – the layer next to the lung
- outer (parietal) pleura – the layer that lines the chest wall
The area between the 2 layers is called the pleural space.
Each of the main bronchi divides or branches into smaller bronchi (which have small glands and cartilage in their walls). These smaller bronchi eventually divide into even smaller tubes called bronchioles (which have no glands or cartilage). At the end of the bronchioles are millions of tiny sacs called alveoli. Surrounding the alveoli are very tiny blood vessels (capillaries).
The bronchi are lined with cells that have very fine hair-like projections called cilia.
The lungs produce a mixture of fats and proteins called lung or pulmonary surfactant. The surfactant coats the surfaces of the alveoli, making it easier for them to expand and deflate with each breath.
Different groups of lymph nodeslymph nodesA small, bean-shaped mass of lymphatic tissue along lymph vessels (tubes through which lymph fluid travels in the body). Lymph nodes store lymphocytes (a type of white blood cell that fights germs, foreign substances or cancer cells) and filters bacteria and foreign substances (including cancer cell, which are part of the lymphatic systemlymphatic systemThe group of tissues and organs that produce and store cells that fight infection and diseases., drain fluid normally produced in the lung:
- bronchial nodes – lymph nodes around the main bronchi
- hilar nodes – lymph nodes in the area where the trachea divides into the main bronchi
- upper (superior) mediastinal nodes – lymph nodes at the top of the mediastinum
- subcarinal mediastinal nodes – lymph nodes just below the trachea where it divides into the main bronchi
- lower (inferior) mediastinal nodes – lymph nodes at the bottom of the mediastinum
The main functions of the lungs are to transfer oxygen from the air to the blood and to release carbon dioxide from the blood to the air.
Air enters the mouth or nose and travels through the trachea (windpipe), bronchi and bronchioles to the alveoli. The exchange of oxygen and carbon dioxide takes place in the alveoli.
- The alveoli absorb oxygen from the air and pass it into the blood, which circulates the oxygen around the body.
- Carbon dioxide, which is a waste product of the body’s cells, passes from the blood into the alveoli and is breathed out.
The lungs also play a role in the body’s defences against harmful substances in the air, such as smoke, pollution, bacteria or viruses. These substances can pass through the nose and become trapped in the lungs. The lungs produce a thick, slippery fluid (mucus), which can trap and partly destroy these materials. The cilia move rapidly to push the mucus up through the bronchi, where it is removed by coughing or swallowing.