Respiratory System: Gas Exchange, Alveoli, and Breathing

RESPIRATORY SYSTEM / BREATHING SYSTEM Gaseous exchange in humans In humans, carbon dioxide and oxygen gas are exchanged between the blood and air in the lungs. The oxygen diffuses from the alveoli into the blood and carbon dioxide from the blood into the alveoli-the carbon dioxide is then exhaled....

RESPIRATORY SYSTEM / BREATHING SYSTEM Gaseous exchange in humans In humans, carbon dioxide and oxygen gas are exchanged between the blood and air in the lungs. The oxygen diffuses from the alveoli into the blood and carbon dioxide from the blood into the alveoli-the carbon dioxide is then exhaled. Thorax is the part of your body that lies between your neck and the abdomen (around the stomach). Parts of respiratory system: The organs which make up the air passage starting from nostrils to the lungs are known as respiratory system. 1. Nostrils/ Nosal cavity: Air enters through two nostrils. Nostrils have fine hair. Breathing through the nose allows the air to become warm, moist and filtered. It has the following advantages. (i) Inner layer of the nose has goblet cells which produce mucus. So mucus moistens the air. Air is also warmed as it passes through the nasal passage. (ii) Inner lining also has the tiny hair which are always moving. Dust and foreign particles (bacteria) get trapped in the hair and mucus. (iii)There are small sensory cells in the mucus membrane that can detect harmful chemicals in the inhaled air. 2. Pharynx: The air in the nasal passages enters the pharynx (oral cavity). It has a lid like structure called epiglottis which covers the trachea during swallowing food, to prevent food from entering the trachea. 2 3. Larynx: From pharynx air then moves to the larynx (the voice box). Larynx is a flap like membrane which vibrates to produce sound. 4. Trachea: The air enters into trachea. The trachea has goblet cells which secrete mucus. Mucus trap bacteria and dust from inhaled air. This prevents bacteria and dust from entering the lungs where they might cause infection. The trachea is lined with a layer of cilia. Cilia beat backward and forward to sweep the mucus and trapped bacteria upwards. This prevents the mucus from reaching the lungs. The trachea also has rings of cartilage to keep it open through out. 5. Bronchi: Tubes branching off the trachea with one bronchus (singular) for each lung. 6. Bronchiole: Each bronchus divides repeatedly into very small, fine branches called bronchioles in the lungs connected to the alveoli. 7. Alveoli: Each bronchiole end in a cluster of air sacs called alveoli (singular-alveolus). Thousands of alveoli are found in the lungs, providing a very large surface area for gas exchange. 8. Lungs: Each lung lies in the pleural cavity, within which the lung expands. The pleural cavity has two transparent elastic membranes called pleural membranes. The inner membrane covers the lung. The outer membrane is in contact with the ribs and the diaphragm. Between the two pleural membranes there is a thin layer of lubricating fluid called pleural fluid which allows the two membranes to glide over each other easily when the lungs expand and contract during breathing. This reduces friction. 9. Diaphragm: The diaphragm is a dome-shaped sheet of muscle and has elastic tissue. It separates the thorax from the abdomen. When the diaphragm muscles contract, it flattens downwards. When the diaphragm muscles relax, it becomes dome shaped (arches upwards again). 10. Ribs: The chest wall is supported by the ribs. They are attached to the backbone on the dorsal side. The ribs are attached to the chest bone (sternum) on the ventral side. There are two sets of muscles (the external and internal intercostal muscles). They are found between the ribs. They are antagonistic muscles. When the external intercostal muscles contract, the internal intercostal muscles relax and vice versa. Ventilation (breathing) –Ventilation is the movement of air in and out of the lungs. Ventilation is brought about by the action of the ribs, intercostal muscles and diaphragm. Breathing in and Breathing out 3 Breathing in (Inhalation) Breathing out (Exhalation) i) External intercostal muscles contract while internal intercostal muscles relax and pull the ribcage upwards and outwards. i) External Intercostal muscles relax while internal intercostal muscles contract and ribcage moves downwards and inwards. ii) The diaphragm muscle contracts and flattens. ii) The diaphragm muscles relax and diaphragm becomes dome shaped. iii) The volume of the thoracic cavity increases. iii) The volume of the thorax decreases. iv) The air pressure in lungs decreases. Atmospheric pressure is higher than pressure inside the lungs. iv) The pressure in the lungs increases. v) Air enters in the lungs. v) Air is pushed out of the lungs. The Alveoli At the end of each bronchiole, there is a small air sacs called alveoli. These are mass of thin walled, pouch like structures where gas exchange takes place. Capillaries are closed wrapped around the outside of the alveoli. O2 diffuses across the wall of the alveoli into the blood and CO2 diffuses the other way. Note: The alveoli are the respiratory surface in humans. Adaptations of Alveoli The walls of alveoli have several features which makes them an efficient gas exchange surface. 1. Alveoli are one cell thick. This makes the diffusion distance shorter and makes the diffusion of gases faster. 4 2. Each alveolus is surrounded by a dense network of capillaries. This also allows the quick exchange of gases between alveoli and blood by increasing concentration gradient. 3. The alveoli wall has thin film of water (moist) in which gases dissolve during diffusion. This makes the gas exchange faster. 4. They are many. This makes them have large surface area so that a lot of gas can diffuse across at the same time. The concentration of oxygen is very high inside the alveolus and very low in the blood, so oxygen molecules diffuse from the alveolus into the red blood cells and combine with haemoglobin. At the very same time, carbon dioxide diffuses from the blood plasma into the alveolus because the concentration of CO2 is very high in the blood and low in the alveolus. 5 Composition of Inspired and Expired Air Gas Inspired Air Expired Air Oxygen 21% 16% Carbon Dioxide 0.04% 4% Nitrogen 79% 79% Water Vapour Variable High Comparing the Inhaled air (Atmospheric air) with Exhaled air The Exhaled air contains: less oxygen more carbon dioxide warm (high temperature) high moisture 6 https://www.youtube.com/watch?v=Y18Vz51Nkos Biological consequences of smoking on gas exchange system Cigarette (tobacco) smoke contains: 1. Nicotine 2. Tar 3. Carbon monoxide Nicotine- Is a stimulant. It makes you feel alert and active. It increases heart rate and blood pressure. Is a poisonous substance and is absorbed into the blood. It’s addictive. Tar It is a carcinogenic substance. These are substances that can lead to the production of cancerous cells and cause cancer. Tar can lead to lung cancer. Carbon monoxide- CO is absorbed in the blood. It combines rapidly with haemoglobin and forms Carboxyhaemoglobin which is a stable compound. CO prevents oxygen from combining with haemoglobin. As a result, less oxygen is carried by the blood. Due to lack of oxygen person feels tired. Above components can lead to the following effects to the lungs; 1. Chronic Bronchitis The chemicals in the cigarette and smoke destroy Cilia. The reduced numbers of cilia mean that the mucus is not swept away from the lungs, but remains to block the air passages. The smoke irritates the lining of the airways stimulating the goblet cells to secrete a lot of mucus resulting into strong cough. Infection from bacteria in mucus can lead to bronchitis. Bronchitis blocks normal air flow so the sufferer has difficulty breathing properly. The layer of excess mucus lining the walls of the alveoli increase the diffusion distance of gases making gas exchange slower. 2. Emphysema Smoke damages the wall of the alveoli which break down and fuse together again forming enlarged, irregular air spaces. This greatly reduces the surface area for gas exchange resulting into short breathes and sounds while breathing. The blood of a person with emphysema carries less oxygen. This leads to the sufferer being unable to carry out even mild exercise such as walking. 7 3. Lung Cancer When tar reaches the lungs, it is absorbed by cells of the bronchi, bronchioles and the lungs. The tar causes excessive division and reproduction of these cells which develops into cancer. The cancer can be spread to other organs too. 4. Carbon monoxide poisoning Carbon monoxide prevents oxygen from combining with haemoglobin. As a result, less oxygen is carried by the blood. Due to lack of oxygen person feels tired. Pregnant women who smoke give birth to underweight babies because less oxygen results into anaerobic respiration leading to little energy for cell division. Effects of smoking on the circulatory system Nicotine and Carbon monoxide in smoke increases the blood pressure. This increases the chances of blood clotting. This can lead to blockage of coronary artery which can lead to coronary heart disease hence heart attack. Less oxygen is delivered to heart muscle cells, this can cause angina followed by a heart attack or failure can take place leading to death Effects of exercise on breathing rate. Procedure. 1. Let a group of five students sit quietly for five minutes, making sure that they are completely relaxed. The students should be of the same gender, age and mass. 2. Let them count the number of breaths they take in one minute by placing a thumb on their wrist and for one minute. 3. Record this in a table. 8 4. Repeat this until they get a steady resting value for the resting rate. 5. The students then carry out some similar vigorous exercise such as running on the spot for three minutes. Immediately after finishing the exercise, they sit down and record the breathing rate per minute. 6. They should continue to record their breaths per minute, every minute until the rate returns to the normal resting rate. Results Breathing rate increases during the exercise and takes some time before it returns to normal after the exercise. During exercise both the breathing rate and heart rate increases to supply the muscles with more oxygen and glucose to allow the muscles to respire aerobically hence have sufficient energy to contract. The breathing rate takes time to return to normal after the exercise because enough oxygen is needed to break down all the lactic acid that built up in the muscles into carbon dioxide and water. The lesser the time the heart rate takes to return to normal, the more fit an individual is. Regular exercise is important to keep the heart muscle in good tone (strong enough). This enables the heart to pump more blood per beat resulting into fewer beats providing more oxygen hence more aerobic respiration. The heart is more efficient in maintaining blood pressure and this lowers the risk of coronary heart disease and stroke.