Lungs

The lungs are a pair of organs in the chest that assist in gas exchange, especially taking oxygen in and expelling carbon dioxide. These are essential for respiration and are placed either side of the heart within the rib cage in the region called the thoracic cavity. They appear conical and have a large surface area for effective gas exchange.

Appearance
Upon birth, the lungs are a pink colour but this darkens during adulthood and can appear to be a mottled black colour. This is due to the build up of cabonacaeous substances that collect throughout life time. The surface of the lungs is smooth and has a number of lobules on the outer surface.

Structure
The lungs are made of a spongy texture which floats when in water due to the presence of air. The lungs are also highly elastic for efficient breathing and are made of a serous coat, a subserous alveolar tissue and parenchyma.

The serous coat and the subserous alveolar tissue cover the entirity of the lungs. The serous coat is extremely thin and transparent in colour whereas the tissue is made up of many elastic fibres. Areloar tissue connects the bronchus tube, pulmonary nerves and the lymphatic system within the lungs.

Bronchus
The main airway in and out of the lungs is via the bronchus. This tube is made of cartilage rings and is the main opening passage to the lungs. it divides throughout the lungs creating a network for the air to flow. It subdivides into seconadary bronchi. The cartilage around these is present in plates and is much less abundant than around the bronchus. These then subdivide further into tertiary bronchi and bronchioles which then lead into alevoli where the majority of gas exchange takes place. The bronchioles do not have any surrounding cartilage but elastic fibres do stop them from collapsing.

Alveoli
Alveoli occur in clusters of sacs within the lungs and are connected to each other via pores which allows air to flow between them. They are made of a single layer of epithelial cells and this coupled with the large amount of aleveoli in the lungs provides efficient gas exchange creating a highly effective respiratory system. It is estimated that the surface area of the alveoli is approximately the size of a tennis court.

Pulmonary system
In order to remove carbon dioxide from the body, the lungs must get a constant supply of blood. There are a number of blood vessels involved in carrying deoxygenated blood to the lungs and oxygenated blood to the rest of the body.

Pulmonary Artery
The pulmonary artery takes blood to the lungs and can be found on the front and top of the bronchi. It divides like the bronchi to provide sufficient coverage across the lungs.

Pulmonary Capillaries
The capillaries lie below a mucus membrane in the walls of the Alveoli. They are involved in gas exchange and are extremely thin to allow diffusion of oxygen into the blood stream and carbon dioxide out.

Pulmonary Veins
Large branches sprout from the capillaries and eventually reach the same network as the arteries and bronchial tubes. They carry the blood back to the heart and are the only vein to carry oxygenated blood.

Bronchial Arteries
These arteries are important in providing nutrition for the lungs themselves. They supply the bronchial tubes and form with capillaries in order to provide nutrition for the lungs.

Process of breathing
In order for the lungs to take in air, they must change in shape. During inhalation, muscles contract in order to create more volume in the chest cavity to take in as much air as possible for efficient gas exchange. The control of breathing is unconscious which means that individuals are not aware of controlling their breathing. During inhalation the external intercostal muscles contract. These muscles are located in between the ribs in the ribcage. The contraction is caused by a release of acetylcholine and causes the diaphragm to also contract and push downwards. Therefore, the chest wall and lungs can expand as the ribs move upwards and outwards, creating a larger area inside the lungs for air to flow in. When an individual exhales, the external intercostal muscles relax. This is classed as passive breathing as the diaphragm relaxes and the chest cavity return to their resting positions. This involves no contraction of muscles and therefore is a passive process. Exhalation can also be active and this does involve contraction of muscles. As the external intercostal muscles relax, the internal intercostal muscles and the abdominal muscles contract to create a more rapid return to resting position. This movement forces the air out of the lungs in a quick manner for a short exhalation.

Lung Disease
The lungs are a moist environment and are linked to the main airway of the body. Therefore, they attract a number of bacteria which can cause some serious medical conditions. One such condition is Tuberculosiscaused by the bacterium Mycobacterium Tuberculosis. The bacterium attacks the lung tissue within the lungs and causes it to try to repair the damage which results in large amounts of scar tissue which obstructs the lungs. This makes it difficult to breathe and symptoms are often coughing up blood and mucous from the lungs. Other bacteria and some viruses can cause a condition called pneumonia which can be fatal in young children and the elderly. This affects the function of the lungs and patients struggle to breathe and display flu like symptoms. Other lung diseases include emphysema where the lungs have lost their elasticity. Therefore, they cannot expand sufficiently in order to take in enough oxygen and patients have trouble breathing. This can be caused from excessive smoking throughout an individuals lifetime. Lung cancer is also a common disease of the lungs and often fatal.