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Gas exchange The human gas exchange system links the circulatory system with the atmosphere. It is adapted to: Clean and warm the air that enters during breathing. Maximize the surface area for diffusion of oxygen and carbon dioxide between the blood and atmosphere. Minimize the distance for this diffusion. Maintain adequate gradients for this diffusion.

Most organisms need a supply of oxygen for respiration. In single-celled organisms the oxygen simply diffuses from the fluid outside the cell, through the cell surface membrane and into the cytoplasm. In a multicellular organism such as a human, however, most of the cells are a considerable distance away from the external environment from which the oxygen is obtained. Multicellular organisms therefore usually have a specialized gas exchange surface where oxygen from the external environment can diffuse into the body, and carbon dioxide can diffuse out.

In humans the gas exchange surface is the alveoli (singular: alveolus) in the lungs.

This means that a large number of oxygen and carbon dioxide molecules can diffuse through the surface at any one moment to give us a high rate of gas exchange.

LungsThe lungs are in the thoracic (chest) cavity surrounded by the pleural membranes which enclose an airtight space. This space contains a small quantity of fluid to allow friction-free movement as the lungs are ventilated by the movement of the diaphragm and ribs.

Trachea, bronchi and bronchioles

The lungs are ventilated with air that passes through a branching system of airways. Leading from the throat to the lungs is the trachea. At the base of the trachea are two bronchi (singular: bronchus), which subdivide and branch extensively forming a bronchial tree in each lung. Each bronchus divides many times to form smaller bronchioles. Terminal bronchioles divide to form even narrower respiratory bronchioles that supply the alveoli with air.

Cartilage in the trachea and bronchi keeps these airways open and air resistance low, and prevents them from collapsing or bursting as the air pressure changes during breathing. In the trachea there is a regular arrangement of C-shaped rings of cartilage.In the bronchi there are irregular blocks of cartilage.

Bronchioles are surrounded by smooth muscle, which can contract or relax to adjust the diameter of these tiny airways. During exercise, the muscles relax to allow a greater flow of air to the alveoli. The absence of cartilage makes these adjustments possible.

The structure of the airways from the trachea to the alveoli

The lining is comprised of ciliated epithelium which rests on a basement membrane made of protein fibers. In between the ciliated cells are goblet cells (here stained blue). Beneath the epithelium is an area of loose tissue with blood vessels and mucous glands. The trachea as a whole is supported by C-shaped rings of cartilage, a portion of which appears as the thick layer running across the bottom of the picture. A light micrograph of part of the trachea in transverse section (TS) ( 65).

Ciliated epithelium

A light micrograph of a small bronchiole in TS (135). Surrounding the epithelium is smooth muscle. There is no cartilage. Around the bronchiole are some alveoli.

Warming and cleaning the air As air flows through the nose and the trachea, it is warmed to body temperature and moistened by evaporation from the lining, so protecting the delicate surfaces inside the lungs from desiccation (drying out). Protection is also needed against the suspended matter carried in the air, which may include dust, sand, pollen, fungal spores, bacteria and viruses. All are potential threats to the proper functioning of the lungs. Particles larger than about 510 m are caught on the hairs inside the nose and the mucus lining the nasal passages and other airways.

In the trachea and bronchi, the mucus is produced by the goblet cells of the ciliated epithelium. The upper part of each goblet cell is swollen with MUCIN droplets which have been secreted by the cell. Mucus is a slimy solution of mucin, which is composed of glycoproteins with many carbohydrate chains that make them sticky and able to trap inhaled particles. The rest of the goblet cell, which contains the nucleus, is quite slender like the stem of a goblet. Mucus is also made by mucous glands beneath the epithelium. Some chemical pollutants, such as sulfur dioxide and nitrogen dioxide, can dissolve in mucus to form an acidic solution that irritates the lining of the airways.

Alveoli

At the end of the pathway between the atmosphere and the bloodstream are the alveoli.Alveolar walls contain elastic fibers which stretch during inspiration and recoil during expiration to help force out air. This elasticity allows alveoli to expand according to the volume of air breathed in. When the alveoli are fully expanded during exercise, the surface area available for diffusion increases, and the air is expelled efficiently when the elastic fibers recoil.

The alveoli have extremely thin walls, each consisting of a single layer of squamous epithelial cells no more than 0.5 m thick. Pressed closely against the alveoli walls are blood capillaries, also with very thin single-celled walls. Oxygen and carbon dioxide molecules diffuse quickly between the air and the blood because the distance is very small.

Diffusion is the net movement of molecules or ions down a concentration gradient. So, for gas exchange to take place rapidly a steep concentration gradient must be maintained. This is done by breathing and by the movement of the blood. Breathing brings supplies of fresh air into the lungs with a relatively high oxygen concentration and a relatively low carbon dioxide concentration. Blood is brought to the lungs with a lower concentration of oxygen and a higher concentration of carbon dioxide than the air in the alveoli. Oxygen therefore diffuses down its concentration gradient from the air in the alveoli to the blood and carbon dioxide diffuses down its concentration gradient in the opposite direction. The blood is constantly flowing through and out of the lungs, so, as the oxygenated blood leaves, more deoxygenated blood enters to maintain the concentration gradient with each new breath.

Smoking

The World Health Organization (WHO) considers smoking to be a disease.

Until the end of the 19th century, tobacco was smoked almost exclusively by men and in pipes and cigars, involving little inhalation. Then the manufacture of cigarettes began. Smoking cigarettes became fashionable for European men during the First World War and in the 1940s women started smoking in large numbers too.

The numbers of smokers in many countries, such as the UK and the USA, has decreased sharply in recent decades. Meanwhile, there has been a huge rise in the number of people smoking in countries such as China and Pakistan. It is estimated that in Pakistan as many as 40% of men and 8% of women are regular smokers.

The smoking of a flavored tobacco called shisha has become fashionable among young people across the world, with many thinking that as it is smoked through a water pipe it is safer than smoking cigarettes.

Tobacco smoke

The tobacco companies do not declare the ingredients in their products, but it is known by analysis that there are over 4000 different chemicals in cigarette smoke, many of which are toxic. Tobacco smoke is composed of mainstream smoke (from the filter or mouth end) and side stream smoke (from the burning tip). When a person smokes about 85% of the smoke that is released is side stream smoke. Many of the toxic ingredients are in a higher concentration in side stream than in mainstream smoke, and any other people in the vicinity are also exposed to them. Breathing in someone elses cigarette smoke is called passive smoking.

The main components of cigarette smoke pose a threat to human health. These are: Tar which contains carcinogens (cancer-causing compounds) Carbon monoxide Nicotine. In general tar and carcinogens damage the gas exchange system, carbon monoxide and nicotine damage the cardiovascular system

Tar is a mixture of compounds that settles on the lining of the airways in the lungs and stimulates a series of changes that may lead to obstructive lung diseases and lung cancer. Carcinogens are cancer-causing compounds. These cause mutations in the genes that control cell division.

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Lung diseases Lung diseases are a major cause of illness and death worldwide. Air pollution, smoking and allergic reactions are the causes of almost all cases. The gas exchange system is naturally efficient and adaptable. Healthy people breathe with little conscious effort; for people with lung disease, every breath may be a struggle.

The lungs large surface area of delicate tissue is constantly exposed to moving streams of air that may carry potentially harmful gases and particles. Despite the filtering system in the airways, very small particles (less than 2 m in diameter) can reach the alveoli and stay there. The air that flows down the trachea with each breath fills the huge volume of tiny airways. The small particles settle out easily because the air flow in the depths of the lungs is very slow. Such deposits make the lungs susceptible to airborne infections such as influenza and pneumonia and, in some people, can cause an allergic reaction leading to asthma.

Persistent dry cough that often gets worse at nightLow-grade feverShortness of breathFatigue or tirednessChest pain that gets worse when you take a deep breath or coughLoss of appetitePneumonia

Every disease has a characteristic set of signs and symptoms. Signs are the visible expression of a disease which a doctor could find by examining a patient for example, a high temperature. Symptoms cannot be detected by an examination and can only be reported by the patient for example, a headache or nausea. Often, however, the word symptom is used more loosely to cover both signs and symptoms. Chronic (long-term) obstructive pulmonary diseases (COPD) such as asthma, chronic bronchitis and emphysema are now common in many countries. Atmospheric pollution from vehicle and industrial emissions and tobacco smoke are linked with these diseases.

Chronic bronchitis

Tar in cigarette smoke stimulates Goblet Cells and mucous glands to enlarge and secrete more mucus. Tar also inhibits the cleaning action of the ciliated epithelium that lines the airways. It destroys many cilia and weakens the sweeping action of those that remain. As a result, mucus accumulates in the bronchioles and the smallest of these may become obstructed. As mucus is not moved or at best only moved slowly dirt, bacteria and viruses collect and block the bronchioles. This stimulates smokers cough which is an attempt to move the mucus up the airways.

With time the damaged epithelia are replaced by scar tissue and the smooth muscle surrounding the bronchioles and bronchi becomes thicker. This thickening of the airways causes them to narrow and makes it difficult to breathe.

Infections such as pneumonia easily develop in the accumulated mucus. When there is an infection in the lungs, the linings become inflamed and this further narrows the airways. This damage and obstruction of the airways is chronic bronchitis. Sufferers have a severe cough, producing large quantities of phlegm, which is a mixture of mucus, bacteria and some white blood cells.

EmphysemaThe inflammation of the constantly infected lungs causes phagocytes to leave the blood and line the airways. Phagocytes are white blood cells that remove bacteria from the body.To reach the lining of the lungs from the capillaries, phagocytes release the protein digesting enzyme elastase. This enzyme destroys elastin in the walls of the alveoli, so making a pathway for the phagocytes to reach the surface and remove bacteria.

Elastin is responsible for the recoil of the alveoli when we breathe out. With much smaller quantities of elastin in the alveolar walls, the alveoli do not stretch and recoil when breathing in and out. As a result, the bronchioles collapse during expiration, trapping air inthe alveoli, which often burst. Large spaces appear where thealveoli have burst, and this reduces the surface area for gas exchange. The number of capillaries also decreases, so less oxygen is absorbed into the blood. This condition is called Emphysema.

As the disease progresses, the blood vessels in the lungs become more resistant to the flow of blood. To compensate for this increased resistance, the blood pressure in the pulmonary artery increases and, over time, the right side of the heart enlarges. As lung function deteriorates, wheezing occurs and breathlessness becomes progressively worse. It may becomeso bad in some people that they cannot get out of bed.People with severe emphysema often need a continuous supply of oxygen through a face mask to stay alive.

Chronic bronchitis and emphysema often occur togetherand constitute a serious risk to health. The gradual onset of breathlessness only becomes troublesome when about half of the lungs is destroyed. Only in very rare circumstances is it reversible. If smoking is given up when still young, lung function can improve. In older people recovery from COPD (chronic obstructive pulmonary disease) is not possible. Over 60 million people worldwide have COPD. The WHO predicts that COPD will become the third leading cause of death worldwide by 2030.There are legal controls in many countries on emissions of pollutants from industrial, domestic and transport fuels and on conditions in work places.Many countries have also banned smoking in public places, such as shops, restaurants and on public transport.

Lung cancer

Tar in tobacco smoke contains several substances that are carcinogens. These react directly or via breakdown products, with DNA in epithelial cells to produce mutations which are the first in a series of changes that lead to the development of a mass of cells, known as a tumor. As the cancer develops it spreads through the bronchial epithelium and enters the lymphatic tissues in the lung. Cells may break away and spread to other organs, so that secondary tumors become established.

Lung cancer takes 2030 years to develop. Most of the growth of a tumor occurs before there are any symptoms. The most common symptom of lung cancer is coughing up blood, as a result of tissue damage. People with lung cancer also have chest pain and find it difficult to breathe. It is rare for a cancer to be diagnosed before it reaches 1 cm in diameter.

A tumor like this is known as a malignant tumor.

A scanning electron micrograph of a bronchial carcinoma a cancer in a bronchus. Cancers often develop at the base of the trachea where it divides into the bronchi as this is where most of the tar is deposited. The disorganized malignant tumor cells at the bottom right are invading the normal tissue of the ciliated epithelium ( 1000).

Tumors in the lungs are located by one of three methods: Bronchoscopy using an endoscope to allow a direct view of the lining of the bronchi

Chest X-ray

CT scan A computerized tomography (CT) scan combines a series of X-ray images taken from different angles and uses computer processing to create cross-sectional images, or slices.

By the time most lung cancers are discovered, they are well advanced.Treatment involves surgery, radiotherapy andchemotherapy and is dependent on the type of lung cancer, how far it has developed and whether it has spreadinto other areas of the body. Ifthe cancer is small and in one lung, then either a part orall of the lung is removed. However, metastasis has usually happened by the time of the diagnosis so, if thereare secondary tumors, surgery will not cure the disease. Thisis why smoking is linked to so many different cancers. Chemotherapy with anti-cancer drugs orradiotherapy with X-rays is used.

Nicotine

As a result, heart rate and blood pressure increase and there is a decrease in blood supply to the extremities of the body, such as hands and feet, reducing their supply of oxygen. Nicotine also increases the risk of blood clotting. Nicotine is a highly addictive drug that influences reward centers in the brain. It stimulates nerve endings in the brain to release the transmitter substance dopamine, which is associated with reinforcing pleasurable experiences. This makes it very hard to give up smoking. Nicotine is the drug in tobacco. It is absorbed very readily by the blood and travels to the brain within a few seconds. It stimulates the nervous system to reduce the diameter of the arterioles and to release the hormone ADRENALINE from the adrenal glands.

CARBON MONOXIDECarbon monoxide diffuses across the walls of the alveoli and into the blood in the lungs. It diffuses into red blood cells where it combines with hemoglobin to form the stable compound CARBOXY HEMOGLOBIN . This means that hemoglobin does not become fully oxygenated. The quantity of oxygen transported in the blood may be 510% less in a smoker than in a non-smoker. Less oxygen is supplied to the heart muscle, putting a strain on it especially when the heart rate increases during exercise. Carbon monoxide may also damage the lining of the arteries.

These short-term effects are readily reversible in people who have not smoked for very long. Long-term smokers put the health of their cardiovascular system at risk. Damage to the walls of arteries may lead to the build-up of fatty tissue and the reduction of blood flow. Coronary heart disease (CHD) and stroke may be the result. These diseases are a major cause of death and disability. They are responsible for 20% of all deaths worldwide and up to 50% of deaths in developed countries. Cardiovascular diseases are multifactorial, meaning that many factors contribute to the development of these diseases. Smoking is just one among several risk factors that increase the chances of developing one of the cardiovascular diseases, such as CHD and stroke.