Gas exchange in humans -In organisms, there are special areas where the oxygen enters and carbon dioxide leaves. One gas is entering, and the other leaving, so these are surfaces for gas exchange. -The alveolus is the gas exchange surface in humans. There are many tiny air sacs or alveoli at the end of each bronchiole. - The walls of the alveoli have several features which make them an efficient gas exchange surface: They are very thin. They are only one cell thick. The capillary walls are also only one cell thick. An oxygen molecule only has to diffuse across this small thickness to get into the blood. They have a good blood supply to take gases to and from the exchange surface. Blood is constantly pumped to the lungs along the pulmonary artery. This branches into thousands of capillaries which take blood to all parts of the lungs. Carbon dioxide in the blood can diffuse out into the air spaces in the alveoli and oxygen can diffuse into the blood. The blood is then taken back to the heart in the pulmonary vein, ready to be pumped to the rest of the body They have a large surface area, to allow faster diffusion of gases across the surface. The total surface area of all the alveoli in your lungs is over 100m2. They have a good ventilation with air so that the diffusion gradient can be maintained. The breathing movements keep the lungs well supplied with oxygen. This is called ventilation. 1 Alveoli in the lungs, and the blood vessels that are associated with them The breathing system Structures in the human breathing system 2 Structure Ribs Intercostal muscle Diaphragm Trachea Larynx Bronchi Bronchioles Alveoli Function Bone structure that protects the internal organs such as the lungs Muscles between the ribs which control the movement of the ribs causing inhalation or exhalation Sheet of connective tissue and muscle at the bottom of the thorax that helps change the volume of the thorax to allow inhalation and exhalation Connect the mouth and nose to the lungs Also known as the voice box, when air pass across sound is produced large tubes blanching off the trachea with one bronchus in each lung Bronchi split to form smaller tubes called bronchioles in the lungs connected to the alveoli Tiny air sacs where gas exchange takes place Goblet cells, Mucus and ciliated cells -Air can enter the body through either the nose or mouth. - Hairs in the nose trap dust particles in the air. -In the nose and air way passages are goblet cells. The goblet cells secrete mucus -The mucus traps dust, bacteria and particles and prevents them from entering the lungs and damaging them -Ciliated epithelial cells are also found in the passage of the airways. The cilia beats and sweeps the mucus, containing bacteria and dust particles, up to the nose and back of the throat, so that it can be removed and does not block the lungs. Goblet cell and ciliated epithelial cell The trachea -From the nose or mouth, the air then passes into the trachea. -The trachea has rings of cartilage around it. As a person breathes in and out, the pressure of the air in the trachea increases and decreases. -The cartilage helps to prevent the trachea collapsing at times when the air pressure inside is lower than the pressure of the air outside it. 3 The intercostal muscles -muscles are only able to pull on bones, not push them -this means there must be two sets of intercostal muscles: one to pull the rib cage up and another set to pull it down -the external intercostal muscles are found on the outside of the rib cage and the internal intercostal muscles are found on the inside of the rib cage There are 2 sets of intercostal muscles: the external, on the outside of the rib cage, and the internal, on the inside of the rib cage Ventilation in the lungs -The diaphragm is a thin sheet of muscle that separates the chest cavity from the abdomen and is ultimately responsible for controlling ventilation (breathing in or out) in the lungs -The external and internal intercostal muscles work as antagonistic pairs (meaning they work in different directions from each other) During inhalation -The diaphragm contracts and flattens. - This increases the volume of the thorax (chest cavity) -which leads to a decrease in air pressure inside the lungs relative to outside the body drawing air in. The external set of intercostal muscles contract - Rib cage moves up and out. This also increases the volume of the thorax decreasing air pressure and drawing air in 4 During exhalation -The diaphragm relaxes and moves upwards in a dome shape. -This decreases he volume of the thorax -which leads to an increase in air pressure inside the lungs relative to the outside and forcing the air out -The external set of intercostal muscles relax so the ribs drop down and in. - This decreases the volume of the thorax and increases air pressure forcing air out Summary When there is need to increase the rate of gas exchange (for example during strenuous activity) the internal intercostal muscles will also work to pull the ribs down and in to decrease the volume of the thorax more, forcing air out more forcefully and quickly-this is called forced exhalation -There is a greater need to rid the body of increased levels of carbon dioxide -This allows a greater volume of gases to be exchanges 5 Differences in inspired and expired air Gas Inspired air 21% Expired air 16% Carbon dioxide 0.04% 4% Water vapour Lower Higher Oxygen Nitrogen 78% 78% Reason for difference Oxygen is removed from blood by respiring cells and used in aerobic respiration. So blood returning to lungs has a lower oxygen concentration than the air in the alveoli. Oxygen diffuses into the blood from lungs Carbon dioxide is produced by respiration and diffuses into blood from respiring cells. The blood transports the carbon dioxide to lungs. The carbon dioxide diffuses into alveolus as it is in higher concentration that the air in the alveolus water vapour evaporates from the moist lining of the alveoli into the expired air as a result of the warmth of the body Nitrogen gas is very stable and so cannot be used by the body for this reason its concentration does not cha ge in inspired or expired air Using limewater to test for carbon dioxide in exhaled air -when breathing in the air is drawn through Tube A -when breathing out the air is blown into boiling tube B -Lime water is clear but becomes cloudy (or milky) when carbon dioxide is bubbled through it -The lime water in boiling tube A will remain clear but the limewater in boiling tube B will become cloudy -This shows that the percentage of carbon dioxide in exhaled air is higher than in inhaled air 6 Effect of exercise on breathing Explaining the effect of exercise on breathing -Breathing rate and depth of breathing increases during exercise because muscles are contracting much harder and frequently so they require more energy. -The muscle cells need more oxygen to be delivered to them (and carbon dioxide removed) for aerobic respiration. -The cells in the muscles combine oxygen with glucose as fast as they can, to release extra energy for muscle contraction. - The person breathes deeper and faster to get more oxygen into the blood. The heart beats faster to get the oxygen to the muscles as quickly as possible -Eventually a limit is reached. The heart and lungs cannot supply oxygen to the muscles any faster. But the muscles continue to contract quickly, requiring more energy. (How can that extra energy be found?) -Extra energy can be provided by anaerobic respiration. The muscle cells continue to respire aerobically with the oxygen they have, but they also break down some glucose without combining it with oxygen (anaerobically) -after exercise has finished, the lactic acid that has built up in muscles needs to be removed as it lowers the pH of cells and can denature enzymes catalysing cell reactions -it can only be broken down in the liver by combining it with oxygen known as repaying oxygen debt. This is done by aerobic respiration in the liver cells. -Even though extra energy is no longer needed, breathing rate remains high and more deeply than normal, and the heart rate continues to be high. -Extra oxygen is transported to break down the lactic acid. The faster heart rate also helps to transport lactic acid as quickly as possible from the muscles to the liver. Investigating the effect of exercise on breathing -exercise increases the frequency and depth of breathing -this can be investigated by counting the breaths taken during one minute at rest and measuring the average chest expansion over 5 breaths using a tape measure held around the chest -exercise for a set time (at least 3 minutes) -immediately after exercising, count the number of breaths taken in one minute and measure the average chest expansion over 5 breaths -following exercise, the number of breaths per minute will have increased and the chest expansion will also have increased 7
