U5 Gas Exchange and Smoking PDF

Gas exchange As it was discussed earlier, large volume organisms will have a relatively small surface area. Thus the surface area of these organisms will not be enough for gas exchange; also large organism has a high metabolic rate which need more surface area for more supply of...

Gas exchange As it was discussed earlier, large volume organisms will have a relatively small surface area. Thus the surface area of these organisms will not be enough for gas exchange; also large organism has a high metabolic rate which need more surface area for more supply of oxygen. Insects and mammals rely on an internal specialized system of gas exchange providing large surface area. The internal system of gas exchange also decreases water loss from the gas exchange surface. Insects are adapted to provide an efficient gas exchange system as follows with less water loss: 1) They have an impermeable exoskeleton and internal gas exchange system in order to prevent water loss (dessication). This is an adaptation to their terrestrial (land) lifestyle. Insects are found in some of the driest places in the world 2) Insects have a network of tracheae, the finer branches (tracheoles) extend to all parts of the body and may become functionally intracellular in muscle fibers. Oxygen moves down tracheoles, diffuses directly into each cell. Thus oxygen is carried in the gas phase directly to its sites of utilization. i.e. an internal gas exchange system as in mammals 3) Spiracles closed by valves: They are mainly closed when the insect is at rest which helps to protect gas exchange system and prevent water loss, allowing insects to live in some of the driest places on earth. 138 Gas exchange system in human The gaseous exchange system links the circulatory system with the atmosphere. It consists of the nose, pharynx, larynx, trachea which divides into 2 bronchi. Each bronchus enters a lung where it branches into many smaller bronchioles. Terminal bronchioles divide to form even narrower respiratory bronchioles that supply the alveoli in the lungs with air. The lungs are in the thoracic cavity surrounded by 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. 139 The system is to provide large surface area for gas exchange. It also cleans, moistens and warmth the air while passing through the system. The system provides a close contact between the air and the blood for the gas exchange. It also maintains adequate gradient for diffusion. Transverse section in the trachea: diameter 1.8 cm The inner most layer is the mucosa. It is formed of pseudostratified columnar ciliated epithelium resting on a basement membrane made of protein fibres with goblet cells. The goblet cells secrete mucus. Mucus is a sticky substance. It traps dust and microbes. The cilia continuously sweep the dirty mucus out of the trachea preventing it from reaching the lung. Submucosa: It is a connective tissue layer with mucus secreting glands and blood capillaries. Mucus secreting glands add more mucus in the lumen and the blood capillaries supply oxygen and nutrients especially important as the epithelium has no blood supply. 140 Cartilage: It forms C-shaped incomplete rings of cartilage. It gives support to the wall. It prevents collapse or rupture of the wall due to changes in the pressure. It keeps the trachea open Section in the trachea all the time decreasing the resistance to air flow. The ring is incomplete to allow the oesophagus running in the back of trachea to expand during the passage of the bolus. Smooth muscle completes the ring of the cartilage. The muscle contracts and relaxes changing the diameter of the trachea. Transverse section in the bronchus: the diameter is 1.2 cm It is narrower than the trachea. It is lined by ciliated epithelium with goblet cells (mucosa). The epithelial cells are shorter than those lining the trachea and goblet cells are fewer. The submucosa is formed of connective tissue with blood vessels. The smooth muscle forms a complete ring. The cartilage forms blocks all around. T.S in a bronchus Transverse section through a bronchiole: diameter is about 1.0 mm Bronchioles are similar to the bronchi but without goblet cells or cartilage. Respiratory bronchioles are about 0.5mm in diameter and have few cilia 141 T.S. in a bronchiole Alveoli: The wall of the alveolus is the gas exchange surface. Alveoli are adapted to gas exchange. The adaptation of the alveoli for gas exchange because: *They provide a large surface area. There are about 700 million alveoli in the two lungs. *The wall of the alveolus is made of one cell thick of flat squamous epithelium to decrease the distance for gases to diffuse. *The wall of the alveolus is moist to prevent dryness of the cells. *They contain cells which secrete surfactant (phospholipid). Surfactant lowers the surface tension of water, thus prevents alveoli from collapse. *The alveoli are surrounded by a network of capillaries in close contact to decrease the distance gases have to diffuse. Blood flows continuously in the capillaries carrying away the gases to keep steep concentration gradient for more diffusion. *The walls of the alveoli are ventilated to maintain high concentration of O2 and low concentration of CO2 inside. *Walls of the alveoli are elastic. Elastic fibres stretch during inhalation and recoil during exhalation. The stretching increases the volume of the lung and surface area for gas 142 diffusion. Stretching also prevents rupture of the wall during inhalation. The elasticity allows alveoli to expand according to the volume of air breathed in. The passive elastic recoil during exhalation helps passive expelling of the air out. In addition, the alveoli contain macrophages to engulf and digest foreign particles. In severe infection phagocytes from the blood join them. Gas exchange between the air in the alveoli and the blood: Gas exchange occurs by diffusion from area of high concentration to area of low concentration (down concentration gradient). Oxygen diffuses from the air in the alveoli (higher concentration) to the red blood cells in the capillaries (lower concentration). Oxygen first dissolves in the layer of moisture, and then it crosses the alveolar epithelium. It then crosses the endothelium of the capillaries. Finally, oxygen crosses the plasma membrane of the red blood cell to combine with haemoglobin. The distance is very short, it is around 1 µm. Carbon dioxide diffuses in the opposite way i.e. from the blood, crossing the endothelium, then the alveolar epithelium. CO2 dissolves in the layer of the moisture to the air. Effects of tar and carcinogens in tobacco smoke on the gas exchange system: Tobacco smoke contains many harmful substances. The most well known harmful substances on the gas exchange system are tar which contains carcinogens. Tar settles on the lining of the airways in the lungs and stimulates a series of changes that may lead to chronic obstructive pulmonary disease (COPD) and cancer. Chronic obstructive pulmonary disease includes chronic bronchitis and emphysema. 143 Despite the filtering system in the airways, very small particles (less than 2 µm in diameter) can reach the alveoli and stay there. These 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 allergic reactions leading to asthma. Chronic bronchitis: *Tar paralyses and then destructs the cilia *It causes enlargement of the goblet cells and mucus secreting gland. As a result, the mucus accumulates collecting dirt, bacteria and viruses. This dirty mucus blocks the bronchioles leading to difficulty in breathing and less gas exchange *The individual coughs to expel the excess mucus. Cough will lead to: *Enlargement of the smooth muscles which leads to narrowing of the bronchioles, also *Rupture of the epithelium and its replacement with scar tissue. *The mucus traps dust and bacteria. Mucus provides nutrients to them. Bacteria start inflammation of the lining resulting in swelling adding more to the obstruction. Smoking weakens the immune system and so the body cannot defend itself against pathogens. 144 *tar settles on the lining decreasing gas diffusion. The individual suffers from chronic cough with large quantities of phlegm, together with difficulty in breathing and easy fatigue as a result of decreased diffusion of oxygen. Emphysema With increased infection, macrophages become not enough. Phagocytes from the blood join the macrophage. To make a path, phagocytes secrete elastase to digest elastin in the elastic fibres. This leads to loss of lung elasticity. Normally elastic recoil is responsible for passive exhalation. With loss of elastic recoil, air is trapped inside the alveoli. Trapping of air together with narrowed bronchioles during expiration lead to rupture of the walls of the alveoli (emphysema). Large air spaces appear where the alveoli have burst. Rupture of the wall of the alveoli decreases the surface area for gas exchange. In addition, there are fewer capillaries. These lead to decrease in oxygen diffusion and the individual suffers from breathlessness (when about half of the lung is destroyed). This is only reversible in very rare circumstances. In addition, air is not refreshed, and so oxygen concentration inside the alveoli decreases, a factor adding to the decrease in oxygen diffusion. Normally the passive elastic recoil is responsible for passive exhalation. With loss of elastic recoil, the patient suffers from difficulty in exhalation or conscious exhalation (non-smoker can force out about 4 dm3 of air after deep breath; someone with emphysema can force out only about 1.3 dm3). 145 As the disease progresses, the blood vessels in the lungs become more resistant to the flow of the 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 a result of all of the above, the patient suffers, in addition to cough with phlegm, from breathlessness, easy fatigue, wheezing and difficulty in expiration. Recovery from COPD in old people is not possible. Lung cancer: It usually occurs at the base of the trachea. Tar and other carcinogen cause mutation in the gene controlling cell division in epithelial cells. The cells divide uncontrollably making a mass of cells called tumour. The cells of the tumour divide rapidly draining nutrients from adjacent cells. Cells from the tumour detach and spread in the blood and lymph to appear in other areas where metastasis appears. Lung cancer takes 20-30 years to develop. Most of the growth of a tumour occurs before there are any symptoms. The patient complains from cough with blood as a result of tissue damage, pain in the chest, breathlessness, easy fatigue with loss of weight. Lung cancer can be diagnosed by bronchoscopy to view the lining of the bronchi, chest X-ray and CT scan. 146 Effects of particulates from vehicle exhausts on the mammalian gas exchange system Exhaust gases Combustion of certain fuels, such as diesel, petrol, natural gas, fuel oil etc leads car exhaust fumes which contain certain poisonous chemicals, including carbon monoxide, sulfur dioxide, nitrogen oxides, formaldehyde, benzene and soot, The exhaust gas of a vehicle is removed from the vehicle and released in the environment through what is known as the exhaust pipe or propelling nozzle. 147 Carbon monoxide Carbon monoxide is a colorless, tasteless and odorless gas in itself, and is one of the chief gases that make exhaust fumes hazardous to human health, as it binds to the hemoglobin in our blood, which results in suffocation. If exposed to even a minuscule amount (0.0035%) of carbon monoxide constantly for 6-8 hours, one will start experiencing the initial symptoms of carbon monoxide poisoning, which include lightheadedness, confusion, dizziness, and headache. It increasingly becomes worse as the concentration of the gas in the air rises. Hydrocarbons (benzene) It has dangerous consequences to our health both immediately and over the long term. As a well-known carcinogen (something that causes cancer) such as leukemia, benzene is known to severely impact bone marrow, which could lead to a drop in the number of red blood cells, leading to anemia. Sulfur dioxide A colorless gas with a sharp, pungent smell, sulfur dioxide irritates the organs of the respiratory tract, including the nose and throat, causing wheezing, coughing and shortness of breath. In the long run, prolonged exposure to sulfur dioxide has been associated with asthma and other similar conditions. Soot Soot is that powdery stuff that makes exhaust fumes black, it is the mass that is left behind as a result of the incomplete combustion of hydrocarbons. The detrimental effects of soot are too many to list, but they include influenza, asthma and even cancer. It’s also associated with acute vascular dysfunction and an increased risk of coronary artery disease. 148 Gas exchange in plants Plants rely on a large surface area to volume ratio for - Photosynthesis - Gas exchange through their leaf surface 149 Plants that live in conditions with limited water supply are called xerophytes Xerophytes are adapted to reduce water loss as follows: Leaves have a smaller surface area to volume ratio Thicker cuticle Rolled leaves- Especially downwards over the lower epidermis where there are more stomata thus - Trap air within the rolled leaf - This air becomes saturated with vapour water, thus have high water potential. - Reduction of water potential gradient between the inside of the leaf and the trapped air reduces water loss by evaporation Fine hairs around the stomata reduce air movement, so humidity builds up reducing transpiration. Cacti are succulent, i.e. they store water in their fleshy tissues. The stomata of many cacti are closed during the day when temperature is high and open at night when evaporation is at minimum. For photosynthesis to occur, CO2 diffuses at night and combines with an acid. During the day time CO2 is released from the acid. Adaptations to living in water: 150 Plants adapted to living in water are called hydrophytes. They are adapted in many ways: *Water lily has leaves with large air spaces to make them buoyant, so they float near the surface to gain light for photosynthesis. *The lower epidermis lacks stomata to prevent water entering the air spaces. Stomata are present in the upper surface for gas exchange. *Roots of hydrophytes are poorly developed and have air spaces to get oxygen from mud *Stems lack much support as water provides buoyancy for the plant Importance of water to plants: It comprises about 70-90% of the body or even more on fresh weight basis, (i) Water helps in the germination of seeds (ii) Water helps in the process of photosynthesis by which plants prepare their food (iii) Water helps in the transport of nutrients and minerals from the soil to the plants. (iv) water gives turgidity to the plant for support (v) water is important for cell elongation 151