Gas Exchange in Humans PDF

7 Gas exchange in humans Loud snoring can be an indication of sleep apnoea Sleep apnoea Think about…...

7 Gas exchange in humans Loud snoring can be an indication of sleep apnoea Sleep apnoea Think about… 1 After air has been inhaled Sleep apnoea* is a disorder in which a person’s breathing is briefly through the nose, by what path interrupted repeatedly during sleep. It can occur when the muscles at does the air flow to reach the the back of the throat fail to keep the airway open. As the person tries lungs? to breathe, they snore loudly. Less oxygen is transported in their blood. 2 How is oxygen transported in the blood? Watch more (Answers on p. 31) sleep apnoea 睡眠窒息症 Acknowledgements and Important Notice: All questions from the HKDSE, HKCEE and HKALE are reproduced by permission of the HKEAA. Unauthorized use of the aforementioned questions in this electronic version is prohibited. II Organisms and Environment 7.1 Human breathing system A Why do humans need a breathing system? Organisms need energy to carry out life processes. Energy is released from food during respiration, a chemical process that consumes oxygen and produces carbon dioxide as a waste product. To ensure a continuous supply of oxygen for respiration in cells and the removal of carbon dioxide produced in the process, gas exchange* between an organism and the external environment is necessary. Gas exchange always takes body surface place by diffusion across a moist respiratory surface*. Unicellular organisms (e.g. Amoeba) have a large oxygen Cross-link surface area to volume ratio. Revise the concept of surface Their body surface acts as the area to volume ratio in carbon dioxide Bk 1A, Ch 5. respiratory surface for efficient gas exchange (Fig 7.1). water Fig 7.1 Gas exchange in Amoeba (×120) However, large multicellular organisms (e.g. humans) have a smaller surface area to volume ratio. The diffusion of gases across the body surface cannot satisfy the needs of these organisms. For efficient gas exchange, they have: A breathing system is also a breathing system* which provides a larger respiratory surface and called a respiratory system. allows gases to move into and out of the body. a transport system* which carries gases between the respiratory surface and body cells. DSE B Structure of human breathing system 12(IA)Q22, 14(IB)Q2, 17(IB)Q5b Fig 7.2 on p. 3 shows the human breathing system. It includes a pair of lungs inside which the respiratory surface is located. The lungs are connected with the external environment via the respiratory tract*. Some muscles and other structures help air flow into and out of the lungs along the respiratory tract (Fig 7.3 on p. 3). breathing system 呼吸系統 gas exchange 氣體交換 respiratory surface 呼吸表面 respiratory tract 呼吸道 transport system 運送系統 7– 2  7 Gas exchange in humans 3D model 7.1 nose nostril* nasal cavity* epiglottis* pharynx* larynx* trachea* (windpipe) inner pleural membrane* respiratory tract outer pleural membrane* bronchus* bronchiole* rib* air sac* intercostal muscles* lung diaphragm* Fig 7.2 The human breathing system nasal cavity pharynx larynx airflow when airflow when trachea we inhale we exhale bronchi bronchioles (in lungs) air sacs (in lungs) Fig 7.3 Directions of airflow along the respiratory tract air sac 氣囊 bronchiole 小支氣管 bronchus 支氣管 diaphragm 橫膈膜 epiglottis 會厭 inner pleural membrane 內胸膜 intercostal muscle 肋間肌 larynx 喉 nasal cavity 鼻腔 nostril 鼻孔 outer pleural membrane 外胸膜 pharynx 咽 rib 肋骨 trachea 氣管 7– 3 II Organisms and Environment 7.1 Examination of the mammalian breathing system Procedure 1 Your teacher will show you the breathing system of a dissected rat. 2 With the help of the photographs below, identify the different structures of the breathing system. lung trachea outer pleural membrane intercostal muscles rib rib right lung left lung diaphragm Fig 7.4 The lungs Fig 7.5 The pleural membrane* and intercostal muscles nose nostril pharynx epiglottis larynx trachea bronchus diaphragm Fig 7.6 The respiratory tract and diaphragm pleural membrane 胸膜 7– 4  7 Gas exchange in humans 1 Nostrils and nasal cavity of nose When we inhale through our nose, air flows through the nostrils and enters the nasal cavity. Our nose has special features to clean, moisten and warm the inhaled air. i) Hairs inside the nostrils filter nose larger dust particles from the hairs inhaled air (Fig 7.7). nostrils Fig 7.7 Nostrils The nasal cavity is also lined ii) Mucus-secreting cells in the lining of the nasal cavity secrete with olfactory cells*. They mucus (Fig 7.8). The mucus traps dust particles and microorganisms are involved in the sense of smell. in the inhaled air. Cilia of ciliated epithelial cells in the lining beat to sweep the mucus towards the pharynx (Fig 7.9). The mucus is then coughed up, or swallowed into the stomach where most of the Watch the beating of cilia trapped microorganisms will be killed by hydrochloric acid. at: https://www.youtube.com/ iii) The mucus also moistens the inhaled air to prevent the moist watch?v=6IORN0jtKD0 respiratory surface inside the lungs from drying out. iv) The lining of the nasal cavity has a rich supply of blood vessels. The blood in the capillaries* warms the inhaled air. This avoids irritation of the lungs. ciliated epithelial cell* cilia* direction of dust particle or mucus flow microorganism to pharynx mucus* cilia mucus-secreting cell blood vessels mucus-secreting cell ciliated epithelial cell Fig 7.8 The lining of the nasal cavity (×500) Fig 7.9 The beating of cilia sweeps the mucus towards the pharynx capillary 微血管 ciliated epithelial cell 纖毛上皮細胞 cilium 纖毛 mucus 黏液 olfactory cell 嗅細胞 7– 5 II Organisms and Environment 2 Pharynx and Larynx nasal cavity pharynx The pharynx is the common passage for air and food. It leads to the larynx and the oesophagus (Fig 7.10). During swallowing, the larynx is raised and presses against the epiglottis. This covers the opening of the larynx and prevents the bolus from entering the respiratory tract. In this way, choking is prevented. The boxlike larynx is mainly made up of cartilage (Fig 7.11a). The epiglottis larynx cartilage prevents the larynx from collapsing due to pressure changes oesophagus during breathing. In the larynx there are two folds of tissues called the Fig 7.10 The pharynx and vocal cords (Fig 7.11b). Sounds are produced when air passes between larynx the vocal cords and causes them to vibrate. epiglottis epiglottis bone cartilage* larynx vocal vocal cords* cords trachea trachea (a) Vertical section, as viewed from the back (b) Viewed from the top The larynx is also called the Fig 7.11 The larynx voice box. 3 Trachea, bronchi and bronchioles Air from the larynx passes to the trachea, which branches into the right and left bronchi at its lower end. One bronchus enters each lung and branches repeatedly into many bronchioles. The structure of the trachea and bronchi is similar (Fig 7.12 on p. 7). i) The walls contain cartilage which prevents the trachea and bronchi from collapsing. The cartilage of the trachea is C-shaped. The open part of the cartilage lies next to the oesophagus. This allows the oesophagus to expand when food is swallowed. ii) The walls also contain smooth muscles and elastic fibres. The contraction and relaxation of the smooth muscles helps regulate the diameter of the airway and hence airflow. iii) The inner walls are lined with mucus-secreting cells and ciliated epithelial cells. The walls also have a rich supply of blood vessels. These features help clean, moisten and warm the inhaled air as in the nasal cavity. cartilage 軟骨 vocal cord 聲帶 7– 6  7 Gas exchange in humans ? Which part is mostly Unlike the trachea and bronchi, bronchioles have no cartilage. Their affected by the walls are mostly made up of smooth muscles and elastic fibres contraction and relaxation of the (Fig 7.12). Larger bronchioles are lined with mucus-secreting cells and smooth muscles: the ciliated epithelial cells. trachea, the bronchus or the bronchiole? Why? Trachea oesophagus smooth muscles* C-shaped cartilage ciliated trachea epithelium lumen of the trachea (×10) left bronchus Bronchus smooth muscles cartilage plates ciliated epithelium lumen of the bronchus (×20) Larger bronchiole bronchiole smooth muscles ciliated epithelium lumen of the bronchiole (×60) (a) Branching from the trachea (b) Simplified diagrams (left) and photomicrographs (right) of cross sections Fig 7.12 The trachea, bronchi and bronchioles smooth muscle 平滑肌 7– 7 II Organisms and Environment 4 Lungs and air sacs Each lung is divided into lobes. trachea The right lung has three lobes whereas the left lung has two lobes (Fig 7.13). The left lung is left lung smaller in size. This leaves space for the heart. right lung Fig 7.13 A pair of human lungs In each lung, the bronchioles end in a large number of tiny cup-like structures called the air sacs (Fig 7.14 and 7.15). The wall of each air sac is made up of an epithelium* which is only one-celled thick. It secretes a watery fluid which keeps the inner surface of the air sac moist. The air sacs together provide the respiratory surface where gas exchange takes place. The air sacs are surrounded by networks of capillaries. The capillaries receive deoxygenated blood (low in oxygen concentration and high in carbon dioxide concentration) from the heart via the pulmonary artery. After gas exchange with the air in the air sacs, the blood in the capillaries becomes high in oxygen concentration and low in carbon dioxide concentration. This oxygenated blood is carried away from the lungs to the heart via the pulmonary veins. oxygenated blood* to heart deoxygenated blood* from heart bronchiole branch of pulmonary vein* bronchiole one air sac branch of pulmonary artery* capillary air sac wall of air sac (one-celled thick and moist) Fig 7.14 Air sacs Fig 7.15 Lung tissues (×150) The capillaries make the lung tissue pink in colour. The large amount of air-filled space makes the lung tissue spongy. The lung tissue is Cross-link also elastic because it contains elastic fibres. When we inhale, the The mechanism of the elastic fibres stretch to allow the lungs to inflate. When we exhale, the breathing actions will be recoiling* of the stretched elastic fibres helps force air out of the lungs. discussed in Section 7.4. deoxygenated blood 缺氧血 epithelium 上皮 oxygenated blood 含氧血 pulmonary artery 肺動脈 pulmonary vein 肺靜脈 recoiling 反衝 7– 8  7 Gas exchange in humans 5 Structures around the lungs The lungs are protected by the rib cage* which consists of 12 pairs of ribs, the sternum and the vertebral column (Fig 7.16). Between each pair of ribs are the intercostal muscles. A sheet of muscles called the In its relaxed state, the diaphragm lies beneath the lungs. diaphragm is in dome shape. intercostal muscle rib sternum* cartilage vertebral column* Fig 7.16 Rib cage The chamber enclosed by the rib cage and the diaphragm is the thoracic cavity*. Within the thoracic cavity, each lung is surrounded by two layers of pleural membranes (Fig 7.17). The inner pleural membrane adheres to the outer surface of the lung. The outer pleural membrane adheres to the inner wall of the thoracic cavity and the upper surface of the diaphragm. The space between the two pleural membranes is the pleural cavity. It contains a film of pleural fluid secreted by the pleural membranes. The pleural membranes and The fluid holds the two layers of pleural membranes together. The the pleural fluid are like fluid also acts as a lubricant* to reduce friction between the pleural two pieces of microscope slides with a film of water membranes during breathing movement. in between. The slides are difficult to pull apart but can move alongside each other easily. rib intercostal muscle lung outer pleural membrane inner pleural membrane pleural fluid* in diaphragm pleural cavity* Fig 7.17 Pleural membranes surrounding a lung lubricant 潤滑劑 pleural cavity 胸膜腔 pleural fluid 胸膜液 rib cage 肋骨籃 sternum 胸骨 thoracic cavity 胸腔 vertebral column 脊柱 7– 9 II Organisms and Environment 7.2 Examination of the pig lungs Procedure 1 Examine the pig lungs. Identify the larynx, larynx epiglottis, trachea, bronchi and lungs. trachea 2 Note the colour of the lungs and count the number of lobes in each lung. 3 Using a pair of forceps, feel the hardness of the trachea and the lung tissue. cut right lung 4 Pump air into the lungs for a few seconds left using an air pump. Note the change in the lung Cover any exposed Fig 7.18 Pig lungs wounds with sterile volume of the lungs. dressings. 5 Cut a piece of lung tissue. Put it into water and Wear disposable gloves. observe if it floats or sinks. Pneumothorax The pressure inside the pleural cavity is normally negative (i.e. lower than the atmospheric pressure). If the pleural membrane of a lung is ruptured, e.g. by a fractured rib, air leaks into the pleural cavity and the negative pressure cannot be maintained. The lung collapses due to its own elasticity. This condition is called pneumothorax*. Severe pneumothorax can be life-threatening. sternum air leaked into the pleural cavity heart collapsed normal left lung right lung Why is the other lung ? not affected? rib Fig 7.19 A medical image showing pneumothorax pneumothorax 氣胸 7– 10  7 Gas exchange in humans 1 By what paths does the air flow between the nasal cavity and air sacs when we inhale and exhale? airflow when we inhale nasal bronchioles air sacs pharynx larynx trachea bronchi cavity (in lungs) (in lungs) airflow when we exhale 2 What are the adaptive features of the respiratory tract for cleaning, moistening and warming the inhaled air? a Hairs inside the nostrils filter larger dust particles. b Mucus-secreting cells in the inner lining secrete mucus. The mucus traps dust particles and microorganisms. Cilia of ciliated epithelial cells beat to sweep the mucus towards the pharynx. The mucus is then coughed up, or swallowed into the stomach where most of the trapped microorganisms will be killed by hydrochloric acid. c The mucus moistens the inhaled air. d The blood in the capillaries warms the inhaled air. Level 1 Level 2 Questions 1 and 2: State whether the 4 The photomicrograph shows some cells in statements are true or false. the inner wall of the trachea. 1 Gas exchange across the body surface is X more efficient in smaller organisms than in larger organisms. p. 2 2 Air inhaled through the nose is cleaner than air inhaled through the mouth. p. 5 3 Which of the following parts of the (×320) respiratory tract is not supported by What is the function of structure X? cartilage? A to trap large dust particles A larynx B to trap microorganisms B bronchiole C to sweep dust particles towards the C trachea pharynx D bronchus p. 7 D to sweep mucus towards the pharynx p. 5, 6 7– 11 II Organisms and Environment 7.2 Gas exchange in the air sacs DSE A How does gas exchange take place? 13(IA)Q1, 15(IA)Q13, 19(IB)Q5 Exchange of respiratory gases (i.e. oxygen and carbon dioxide) takes place in air sacs (Fig 7.20). Oxygen diffuses from the air in the air sacs to the blood in capillaries. Carbon dioxide diffuses in the opposite direction. Uptake of oxygen ❶ ❸ Removal of deoxygenated blood carbon dioxide from pulmonary artery 1 Air is inhaled. (low in oxygen, high in 1 Carbon dioxide carbon dioxide) oxygenated blood to diffuses out of pulmonary veins the blood. (high in oxygen, low in carbon dioxide) 2 Oxygen in the 2 Carbon dioxide air dissolves in diffuses into water film. the air in the air sac. red blood cell 3 Oxygen diffuses capillary wall water film into red blood 3 Air is exhaled. cells. ❷ ❷ wall of air sac ❸ ❶ Fig 7.20 Gas exchange between the air in an air sac and the blood in a capillary ? How many layers of cells separate the air 1 Uptake of oxygen by the blood in an air sac from the When we inhale, atmospheric air flows into air sacs. Oxygen in blood in a capillary? inhaled air dissolves in the water film lining the air sacs. Since the oxygen concentration of the water film is higher than that of the deoxygenated blood, dissolved oxygen in the water film diffuses across the walls of the air sacs and the capillaries into the blood. 2 Removal of carbon dioxide from the blood Since the carbon dioxide concentration of the deoxygenated blood is higher than that of the water film, carbon dioxide in the blood diffuses across the walls of the capillaries and the air sacs into the air in the air sacs. Carbon dioxide is then removed from the body when we exhale. Animation 7.1 After gas exchange, the oxygen content of the blood increases and the carbon dioxide content decreases. The blood becomes oxygenated. It is carried away from the lungs to the heart via the pulmonary veins. 7– 12  7 Gas exchange in humans DSE B Adaptive features of air sacs for 18(IA)Q8, 19(IB)Q5 gas exchange Air sacs are specially adapted for gas exchange. 1 There are a large number of air sacs The left and right lungs have several hundred million air sacs. This In the lungs of an adult, the provides a very large surface area for the diffusion of gases. air sacs collectively make up an area about the size of a badminton court! 2 The walls of air sacs are very thin The epithelia making up the walls of the air sacs are only one-celled thick (Fig 7.21). This reduces the diffusion distance of gases. 3 The inner surfaces of air sacs are moist Oxygen in inhaled air dissolves in the water film lining the inner surfaces of the air sacs before diffusion takes place. 4 Air sacs have a rich supply of capillaries There are numerous capillaries surrounding the air sacs (Fig 7.22). This allows rapid transport of gases to and away from the air sacs. Therefore, a steep concentration gradient of gases between the air sacs and the blood can be maintained for efficient diffusion of gases. 5 The walls of air sacs are in close contact with capillaries The walls of the air sacs are very close to the capillaries surrounding the air sacs (Fig 7.22). This reduces the diffusion distance of gases. air sac air sac epithelial cell of air sac nucleus cell of capillary wall capillary red blood cell capillary Fig 7.21 A simplified diagram showing the section of an Fig 7.22 Capillaries surrounding an air sac air sac (×1700) 7– 13 II Organisms and Environment 7.3 Examination of the mammalian air sacs Procedure air sac 1 Examine the photomicrograph or prepared slide of mammalian lungs with a microscope under larger blood lower-power magnification. Identify the air sacs. vessel 2 Draw a labelled diagram of the air sacs. epithelial cell of air sac red blood cell in a capillary Fig 7.23 T.S. of human lung tissues (×200) C Differences in compositions between inhaled air and exhaled air As gas exchange takes place in the lungs, inhaled air and exhaled air have different compositions. Comparison of the oxygen content of inhaled air and 7.4 exhaled air Procedure 1 Collect a jar of inhaled air (atmospheric air). Collect a jar of exhaled air as Practical 7.4 shown. deflagrating exhaled air spoon blow burning candle water trough rubber tubing water insulating mat 2 Lower a burning candle quickly into each jar. Close the jar and record how long the candle burns for. Results and discussion The candle in exhaled air burns for a shorter time. The results show that exhaled air contains less oxygen than inhaled air. 7– 14  7 Gas exchange in humans Comparison of the carbon dioxide content of inhaled air 7.5 and exhaled air Procedure mouthpiece 1 Breathe deeply and slowly through the mouthpiece of the apparatus shown. Inhaled air passes through tube A while exhaled air passes through tube B. 2 Observe any colour change of the indicator. Practical 7.5 hydrogencarbonate indicator A B Results and discussion The colour of the hydrogencarbonate indicator changes with the concentration of carbon dioxide. Concentration > 0.04% ~ 0.04% < 0.04% Colour Yellow Red Purple The hydrogencarbonate indicator in B turns yellow while that in A remains red. This shows that exhaled air contains more carbon dioxide than inhaled air. The table below compares the compositions of inhaled air and exhaled air. Inhaled air Exhaled air Explanation for the difference Oxygen 21% 16% Oxygen in inhaled air diffuses from the air sacs into the blood during gas exchange. It is then used by body cells for respiration. Carbon 0.04% 4% Carbon dioxide produced by body cells during respiration dioxide diffuses from the blood into the air sacs during gas exchange. Nitrogen 78% 78% Nitrogen is not used nor produced by body cells. Water vapour Variable Saturated Exhaled air is moistened by the water film lining the air sacs and the mucus lining the respiratory tract. Other gases 1% 1% These gases are not used nor produced by body cells. Can you suggest a test for the presence of ? water vapour in exhaled air? 7– 15 II Organisms and Environment Besides the differences in their compositions, exhaled air is also warmer than inhaled air. This is because exhaled air is warmed by blood in the capillaries near the inner surface of the respiratory tract. We can feel the warmth of exhaled air if we breathe onto our hands. Exhaled air contains more carbon dioxide than oxygen. Although exhaled air has a higher percentage of carbon dioxide than inhaled air, its absolute amount is still smaller than that of oxygen. Respiratory diseases Asthma*, emphysema* and severe acute respiratory syndrome* (SARS) are respiratory diseases. They reduce the efficiency of gas exchange and result in a lower oxygen content in the blood. During an asthma attack, the bronchioles constrict (Fig 7.24b). This reduces the amount of atmospheric air reaching the air sacs. The concentration gradient of gases between the air sacs and the blood is less steep. Thus, gas exchange is less efficient. In emphysema, the walls of the air sacs break down (Fig 7.24c). This reduces the area of the respiratory surface for gas exchange. In SARS, fluid accumulates in the air sacs (Fig 7.24d). This increases the diffusion distance of gases. Thus, gas exchange is less efficient. bronchiole bronchiole constricts air sacs capillary normal oxygen content in blood lower oxygen content in blood (a) Normal (b) Asthma attack walls fluid break down accumulates lower oxygen content in blood lower oxygen content in blood (c) Emphysema (d) SARS Fig 7.24 Appearance of bronchioles and air sacs in different conditions asthma 哮喘 emphysema 肺氣腫 severe acute respiratory syndrome (SARS) 嚴重急性呼吸系統綜合症 7– 16  7 Gas exchange in humans Electronic cigarettes were first developed as a tool for quitting smoking. Smoking and our breathing system However, both their Cigarette smoke contains many harmful chemicals which affect the effectiveness and safety are in doubt. Learn about breathing system. their harmful effects at: Some chemicals destroy the cilia of the airway. Mucus cannot be https://www.taco.gov. removed and the trapped microorganisms may cause lung infection. hk/t/english/infostation/ Some chemicals stimulate mucus secretion. The airway becomes infostation_ec.html narrower and airflow is reduced. Tar* deposits on the surface of the air sacs. This reduces the area of the respiratory surface for gas exchange. Some chemicals cause lung cancer and emphysema (Fig 7.25). smaller air sac larger air sac How does the number ? of air sacs differ in (a) Non-smoker (×80) (b) Smoker with emphysema (×80) the non-smoker and the smoker with Fig 7.25 Lung tissues of a non-smoker and a smoker with emphysema emphysema? Level 1 X Y Z Questions 1 and 2: State whether the Oxygen content statements are true or false. 122 106 160 (arbitrary unit) 1 Oxygen in air sacs must dissolve in the Carbon dioxide water film before diffusing across the walls content (arbitrary 27 41 4 of the air sacs and the capillaries. p. 12 unit) 2 Inhaled air consists mainly of oxygen and exhaled air consists mainly of carbon Which of the following combinations dioxide. p. 15 correctly identifies the air samples? Level 2 Inhaled air Exhaled air Air in air sacs 3 The table shows the oxygen content and A X Y Z carbon dioxide content in three air samples B Y X Z (X, Y and Z). C Z X Y D Z Y X p. 12, 15 tar 焦油 7– 17 II Organisms and Environment 1 How does gas exchange take place in the air sacs? Uptake of oxygen by the blood: Oxygen in inhaled air dissolves in the water film lining the air sacs. Since the oxygen concentration of the water film is higher than that of the deoxygenated blood, dissolved oxygen in the water film diffuses across the walls of the air sacs and the capillaries into the blood. Removal of carbon dioxide from the blood: Since the carbon dioxide concentration of the deoxygenated blood is higher than that of the water film, carbon dioxide in the blood diffuses across the walls of the capillaries and the air sacs into the air in the air sacs. 2 What are the adaptive features of air sacs for gas exchange? Adaptive feature Explanation a Large in number Provides a very large surface area for diffusion b Very thin walls Reduce the diffusion distance (one-celled thick) c Moist inner surfaces Allow oxygen in inhaled air to dissolve in the water film before diffusion d Rich supply of Allows rapid transport of gases to capillaries and away from the air sacs, so that a steep concentration gradient of gases can be maintained for diffusion e Close contact with Reduces the diffusion distance capillaries 3 What are the differences between inhaled air and exhaled air? Inhaled air Exhaled air Oxygen content Higher Lower Carbon dioxide Lower Higher content Water vapour Variable Saturated content Temperature Lower Higher 7– 18  7 Gas exchange in humans 7.3 Transport of respiratory gases Most of our body cells are not close to the air sacs of the lungs. Oxygen and carbon dioxide are transported between them by blood. A Red blood cells and haemoglobin Oxygen is not very soluble in water. Only a very small proportion of oxygen is transported in its dissolved form in plasma* (the watery component) of blood. Most oxygen is transported in red blood cells. These cells are specially adapted for carrying oxygen. 1 Red blood cells are packed with haemoglobin Haemoglobin* is an iron-containing protein. It is an excellent oxygen carrier as it binds readily to oxygen and this binding is reversible. Under high oxygen concentration (as in the lungs), oxygen binds to haemoglobin to form oxyhaemoglobin*. Under low oxygen concentration (as in body tissues), oxyhaemoglobin breaks down into haemoglobin and oxygen. high oxygen concentration (in lungs) haemoglobin + oxygen oxyhaemoglobin low oxygen concentration (in body tissues) These properties allow haemoglobin to bind to oxygen when The presence of haemoglobin haemoglobin travels past the lungs. Oxygen is released to body cells allows blood to carry about 70 times more oxygen than when oxyhaemoglobin travels past the body tissues. Haemoglobin in red plasma. blood cells greatly increases the oxygen-carrying capacity of blood. 2 Mature red blood cells have no nucleus The absence of a nucleus provides space for more haemoglobin. 3 Red blood cells have a biconcave disc shape Their biconcave disc shape* provides a large surface area to volume ratio and a short distance for the diffusion of oxygen (Fig 7.26). This Fig 7.26 Electron micrograph of red allows oxygen to reach and leave haemoglobin in the red blood cells blood cells (×2200) rapidly. biconcave disc shape 雙凹圓盤狀 haemoglobin 血紅蛋白 oxyhaemoglobin 氧合血紅蛋白 plasma 血漿 7– 19 II Organisms and Environment B How is oxygen transported? 1 Uptake of oxygen from the air sacs In the air sacs of the lungs, oxygen concentration is high due to the continuous replacement of air from the external environment. About 98% of oxygen is Most of this oxygen diffuses into red blood cells, where it binds to transported in the form of haemoglobin to form oxyhaemoglobin. The blood becomes oxygenated oxyhaemoglobin. Only about 2% is transported in its and is carried via the pulmonary veins to the heart. The heart then dissolved form in plasma. pumps the blood to different body tissues. 2 Release of oxygen to the body cells In body tissues, oxygen concentration is low because oxygen is consumed continuously by body cells during respiration. Oxyhaemoglobin gives blood Oxyhaemoglobin in red blood cells breaks down into haemoglobin a bright red colour. When and oxygen. The oxygen then diffuses into the body cells. The blood oxyhaemoglobin breaks down, blood becomes becomes deoxygenated and is carried back to the heart, and then to the purplish red. lungs via the pulmonary artery. Key: lungs 1 In lungs (high oxygen concentration) oxygenated blood inhaled air deoxygenated blood direction of transport air sac oxygen of oxygen red blood cell capillary haemoglobin + oxygen pulmonary artery capillaries oxyhaemoglobin blood flow direction pulmonary vein 2 In body tissues (low oxygen concentration) heart body cell body tissues haemoglobin + oxygen oxyhaemoglobin capillaries blood flow direction Fig 7.27 The uptake of oxygen from the air sacs and the release of oxygen to the body cells 7– 20  7 Gas exchange in humans C How is carbon dioxide transported? 1 Uptake of carbon dioxide from the body cells In body tissues, carbon dioxide concentration is high because carbon dioxide is produced continuously by body cells during respiration. About 70% of carbon Most of this carbon dioxide diffuses into red blood cells, where it dioxide is transported in the reacts with water (H2O) to form hydrogencarbonate ions* (HCO3−) form of hydrogencarbonate ions. About 23% binds and hydrogen ions (H+). This reaction is catalysed by an enzyme. The to haemoglobin (at a hydrogencarbonate ions then diffuse out of the red blood cells and are site different from that carried by plasma to the air sacs of the lungs. for oxygen) and 7% is transported in its dissolved form in plasma. 2 Release of carbon dioxide to the air sacs In the air sacs of the lungs, carbon dioxide concentration is low because carbon dioxide is removed continuously when we exhale. Hydrogencarbonate ions in plasma diffuse into red blood cells, where hydrogencarbonate ions and hydrogen ions react to form carbon dioxide and water. This reaction is catalysed by the same enzyme. The carbon dioxide diffuses into plasma and then into the air sacs for removal. Key: lungs 2 In lungs (low CO2 concentration) oxygenated blood exhaled air deoxygenated blood direction of transport air sac CO2 of carbon dioxide (CO2) CO2 + H2O enzyme pulmonary artery capillaries HCO3– HCO3– + H+ blood flow direction pulmonary vein 1 In body tissues (high CO2 concentration) heart body cell capillary body tissues plasma red blood CO2 + H2O cell enzyme HCO3– HCO3– + H+ capillaries blood flow direction Fig 7.28 The uptake of carbon dioxide from the body cells and the release of carbon dioxide to the air sacs hydrogencarbonate ion 碳酸氫鹽離子 7– 21 II Organisms and Environment Below shows the reversible reaction between carbon dioxide and water to form hydrogencarbonate ions and hydrogen ions in red blood cells. high CO2 concentration (in body tissues) CO2 + H2O enzyme H+ + HCO3− low CO2 concentration (in lungs) An invisible killer: carbon monoxide Carbon monoxide* is a poisonous gas. Compared with oxygen, carbon monoxide binds to haemoglobin about 200 times more readily. This makes haemoglobin unavailable for oxygen-binding. A trace amount of carbon monoxide can significantly reduce the oxygen-carrying capacity of blood. Inhaling a high concentration of carbon monoxide can cause death in just a few minutes. Carbon monoxide is colourless and Fig 7.29 Car exhaust fumes odourless. Sources of the gas include car contain carbon exhaust fumes, smoke from fires, town gas monoxide and cigarette smoke. Level 1 Level 2 Questions 1 to 3: State whether the statements 4 The biconcave disc shape of red blood cells are true or false. (1) increases the respiratory surface for gas 1 Haemoglobin increases the solubility of exchange. oxygen in the plasma. p. 19 (2) reduces the diffusion distance for carbon dioxide. 2 When the oxygen concentration is low, oxygen binds to haemoglobin to form (3) allows the cells to hold more oxyhaemoglobin. p. 19 haemoglobin. A (2) only B (3) only 3 The formation of hydrogencarbonate ions C (1) and (2) only D (1) and (3) only from carbon dioxide and water is catalysed p. 19 by an enzyme. p. 21 carbon monoxide 一氧化碳 7– 22  7 Gas exchange in humans 1 What are the adaptive features of red blood cells for carrying oxygen? Adaptive feature Explanation a Packed with Greatly increases the oxygen-carrying haemoglobin capacity of blood b No nucleus when Provides space for more mature haemoglobin c Biconcave disc Provides a large surface area to shape volume ratio and a short distance for the diffusion of oxygen 2 How is oxygen transported from the air sacs to the body cells? In the air sacs, where oxygen concentration is high, most of the oxygen diffuses into red blood cells and binds to haemoglobin to form oxyhaemoglobin. The blood becomes oxygenated and is carried to the heart and then to different body tissues. In body tissues, where oxygen concentration is low, oxyhaemoglobin in red blood cells breaks down into haemoglobin and oxygen. This oxygen then diffuses into the body cells. 3 How is carbon dioxide transported from the body cells to the air sacs? In body tissues, where carbon dioxide concentration is high, most of the carbon dioxide diffuses into red blood cells and reacts with water to form hydrogencarbonate ions and hydrogen ions. The hydrogencarbonate ions then diffuse out of the red blood cells and are carried by plasma to the air sacs. In the air sacs, where carbon dioxide concentration is low, hydrogencarbonate ions in plasma diffuse into red blood cells and react with hydrogen ions to form carbon dioxide and water. This carbon dioxide diffuses into plasma and then into the air sacs. 7– 23 II Organisms and Environment DSE 13(IA)Q30, 14(IA)Q27, 7.4 Ventilation 15(IA)Q15, 17(IB)Q5a We learnt that the rapid transport of respiratory gases by blood helps maintain a steep concentration gradient between air sacs and the Watch this to prepare for blood in surrounding capillaries. This in turn facilitates gas exchange. your class and answer the Ventilation* is the flow of air into and out of our lungs. It also questions. helps maintain a steep concentration gradient as air in the air sacs is Video & continuously replaced from the external environment. questions Ventilation is brought about by breathing actions, which include inhalation* and exhalation*. A Inhalation Inhalation is also called inspiration. It brings air into the lungs. The mechanism works as follows (Fig 7.30). 1a The intercostal muscles contract. This causes the rib cage to move upwards and outwards. 1b The diaphragm muscles contract. This causes the diaphragm to become flattened. 2 The above actions increase the volume of the thoracic cavity. This in turn causes an increase in the volume of the lungs because the inner wall of the thoracic cavity and the outer surfaces of the lungs are held together by pleural membranes and pleural fluid. 3 The air pressure in the lungs decreases and becomes lower than the atmospheric pressure. 4 Air rushes into the lungs through the respiratory tract. 4 Air rushes in. 1a Intercostal muscles contract. Rib cage 2 Volume of thoracic moves upwards cavity increases, and outwards. and hence the volume of the lungs increases. pleural membranes 3 Air pressure in the lungs decreases. Animation 7.2 1b Diaphragm muscles contract, causing the diaphragm to become flattened. Fig 7.30 How inhalation works exhalation 呼氣 inhalation 吸氣 ventilation 換氣 7– 24  7 Gas exchange in humans B Exhalation Exhalation is also called expiration. Air is forced out of the lungs in the process. The mechanism works as follows (Fig 7.31). 1a The intercostal muscles relax. This causes the rib cage to move downwards and inwards. 1b The diaphragm muscles relax. This causes the diaphragm to return to its dome shape. 2 The above actions decrease the volume of the thoracic cavity. The elastic fibres of the lungs that have been stretched during inhalation recoil, resulting in a decrease in the volume of the lungs. 3 The air pressure in the lungs increases and becomes higher than the atmospheric pressure. 4 Air is forced out of the lungs through the respiratory tract. 4 Air is forced out. 1a Intercostal muscles relax. Rib cage 2 Volume of thoracic moves downwards cavity decreases, and and inwards. hence the volume of the lungs decreases. pleural membranes 3 Air pressure in the lungs increases. Feel the movement of your rib cage by putting your hand on your chest during 1b Diaphragm muscles relax, causing the diaphragm to return to its dome shape. inhalation and exhalation. Fig 7.31 How exhalation works Choking treatment Choking occurs when a piece of food accidentally gets stuck in the airway. Sometimes, the airway may be blocked completely, causing suffocation*. food expelled A rescuer can give thrusts to the person’s upper abdomen to push the diaphragm upwards (Fig 7.32). This upward and compresses the lungs and forces air out. inward thrusts In this way, the food may be expelled Fig 7.32 How to treat choking from the airway to restore airflow. suffocation 窒息 7– 25 II Organisms and Environment C Models demonstrating the mechanisms of inhalation and exhalation 1 Rib-cage model A rib-cage model is often used to show the movement of the ribs during inhalation and exhalation (Fig 7.33). Inhalation Exhalation 2 Ribs move rubber band 2 Ribs move upwards and (intercostal muscle) downwards outwards. and inwards. 3 Volumes of the thoracic cavity and 3 Volumes of the 1 Intercostal the lungs increase. thoracic cavity and muscles the lungs decrease. contract (rubber band shortens). vertical rod 1 1 Intercostal (sternum) muscles relax (rubber band horizontal vertical rod 2 lengthens). rod (rib) (vertebral column) Fig 7.33 Rib-cage model showing the movement of ribs during inhalation and exhalation 2 Bell-jar model A bell-jar model is often used to show how the movement of the diaphragm brings about inhalation and exhalation (Fig 7.34). Inhalation Exhalation 4 Air rushes into 4 Air is forced out. the lungs. glass tube (trachea) wall of bell jar (thoracic wall) 3 Volume of the 3 Volume of the lungs increases. lungs decreases. balloons 2 Volume of the 2 Volume of the (lungs) thoracic cavity thoracic cavity increases and decreases and the air pressure the air pressure rubber sheet inside decreases. 1 Diaphragm is inside increases. (diaphragm) pushed upwards. 1 Diaphragm is pulled down. Fig 7.34 Bell-jar model showing how the movement of the diaphragm brings about inhalation and exhalation 7– 26  7 Gas exchange in humans Making a lung model A lung model can help us better understand how inhalation and exhalation work. Follow the steps in the video below to make your own lung model. https://www.youtube.com/ watch?v=H62wTF9vKPQ Using the model that you have made, explain to your class how air flows into and out of the lungs. Fig 7.35 A lung model During ventilation, a change in the air pressure in our lungs causes a change in the volume of our lungs. During ventilation, a change in the volume of our lungs causes a change in the air pressure in our lungs. Crying or laughing? Breathing actions not only bring about ventilation, they also allow us to express emotions such as crying and laughing. Crying and laughing usually go along with different facial expressions, but both are basically an inhalation followed by short bursts of exhalation. Sometimes it is difficult to tell if a person is crying or laughing. Fig 7.36 Crying and laughing are brought about by the same breathing actions 7– 27 II Organisms and Environment Learning through examples Skill builder Skill practice The graph below shows the change in air pressure in the lungs of an adult at rest. 762 air pressure in the lungs (mmHg) 0 760 time (s) 1 2 3 4 atmospheric pressure = 760 mmHg 758 a Calculate the rate of breathing of the adult. Express your answer in breaths per minute and show your working. (3 marks) b State the time when the volume of the lungs is the largest. (1 mark) c i During which time interval does exhalation occur? (1 mark) ii Describe how the actions of the intercostal muscles and diaphragm muscles cause air to flow out of the lungs during exhalation. (4 marks) Suggested answers a From the graph, one breath takes 4 s. 1 One breath consists of one inhalation and Rate of breathing: one exhalation. (60 ÷ 4) breaths per minute 1 = 15 breaths per minute 1 b 2s 1 c i 2 to 4 s 1 Interpreting a graph of air pressure in the ii The intercostal muscles relax. This causes the rib cage to move lungs against time downwards and inwards. 1 Refer to p. 29. The diaphragm muscles relax. This causes the diaphragm to return to its dome shape. 1 Online tutorial 7.1 These actions decrease the volume of the thoracic cavity. 1 The air pressure in the lungs increases and becomes higher than the atmospheric pressure. Air is forced out of the lungs. 1 7– 28  7 Gas exchange in humans Learning through examples Skill builder Skill practice Interpreting a graph of air pressure in the lungs against time When reading a graph of air pressure in the lungs against time (see the graph below), note the following: ❶ From 0 to 2 s, the air pressure in the lungs is lower than the atmospheric pressure. Thus, air is flowing into the lungs. Inhalation occurs. ❷ From 2 to 4 s, the air pressure in the lungs is higher than the atmospheric pressure. Thus, air is flowing out of the lungs. Exhalation occurs. ❸ At 0, 2 and 4 s, the air pressure in the lungs is equal to the atmospheric pressure. There is no airflow into or out of the lungs. This is the time between inhalation or exhalation. ❹ At 2 s, inhalation ends and exhalation is about to begin. The lung volume at this time is the largest (see the graph of volume of the lungs against time). ❺ At 0 or 4 s, exhalation ends and inhalation is about to begin. The lung volume at this time is the smallest (see the graph of volume of the lungs against time). Change in air pressure in the lungs during inhalation and exhalation Inhalation Exhalation air pressure in the lungs (mmHg) 762 ❸ ❹ ❺ 760 time (s) 0 1 2 3 4 atmospheric pressure = 760 mmHg 758 ❶ ❷ Change in volume of the lungs during inhalation and exhalation Inhalation Exhalation volume of the lungs (arbitrary unit) 0 time (s) 1 2 3 4 7– 29 II Organisms and Environment Learning through examples Skill builder Skill practice The graph below shows the change in air pressure in the lungs of a person. 762 the lungs (mmHg) air pressure in 760 time (s) P Q R S T U 758 atmospheric pressure = 760 mmHg a During which time interval(s) does inhalation occur? (1 mark) b During which time interval(s) does exhalation occur? (1 mark) c If the breathing rate of the person is 12 breaths per minute, what is the duration of the time interval RT? Show your working. (2 marks) d Sketch a graph to show the corresponding change in the lung volume of the person during the time interval PU. (3 marks) Q19, 20 (p. 37), Q23 (p. 38) Level 1 Level 2 1 The following are some of the events that 2 Which of the following is a correct occur during exhalation. cause-and-effect in inhalation? (1) Air flows out of the lungs. A Airflow into the lungs causes the lungs (2) The intercostal muscles and diaphragm to inflate. muscles relax. B Relaxation of the diaphragm muscles (3) The volume of the lungs decreases. causes the diaphragm to return to its dome shape. (4) The air pressure in the lungs increases. C Downward and inward movement of The correct sequence of the events is the rib cage causes an increase in the A (2) ➔ (1) ➔ (3) ➔ (4) volume of the thoracic cavity. B (2) ➔ (3) ➔ (4) ➔ (1) D The increase in the volume of the C (2) ➔ (4) ➔ (1) ➔ (3) thoracic cavity causes an increase in the D (2) ➔ (4) ➔ (3) ➔ (1) p. 25 volume of the lungs. p. 24 7– 30  7 Gas exchange in humans 1 What is ventilation and how is it brought about? Ventilation is the flow of air into and out of the lungs. Ventilation is brought about by breathing actions, which include inhalation and exhalation. 2 What are the differences between inhalation and exhalation? Inhalation Exhalation Intercostal muscles Contract Relax Movement of rib cage Upwards and Downwards and outwards inwards Diaphragm muscles Contract Relax Shape of diaphragm Flattened Dome shape Volume of thoracic Increases Decreases cavity Volume of the lungs Increases Decreases Air pressure in the lungs Lower than Higher than atmospheric atmospheric pressure pressure Direction of airflow Into the lungs Out of the lungs Recall Think about... (p. 1) 1 The air inhaled passes through the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and finally reaches the air sacs of the lungs. 2 Most of the oxygen diffuses into the red blood cells and binds to haemoglobin to form oxyhaemoglobin. Suggested answers to ? p. 7 The bronchiole because it has no cartilage to restrict the contraction and relaxation of the smooth muscles. p. 10 Each lung is surrounded by its own pleural membranes. p. 12 Two p. 15 Breathe onto a cold mirror. Rub a piece of dry cobalt chloride paper onto the condensation. The paper turns from blue to pink if the condensation is made up of water. p. 17 The smoker has a smaller number of air sacs. 7– 31 II Organisms and Environment Key terms 1 air sac 氣囊 11 inhalation 吸氣 2 breathing system 呼吸系統 12 intercostal muscle 肋間肌 3 bronchiole