Respiratory Physiology Gaseous Exchange Session Objectives. What you will cover Basic anatomy of the lung including airways Breathing movements Lung volumes and capacities Compliance and Resistance in relation to work of breathing Gaseous Exchange Control of Breathing Rate Your objectives are State the function of support structures and epithelia of the bronchial tree Describe the role of surfactant in terms of airway patency Describe the structure of an alveolus State properties of alveoli that adapt them to gaseous exchange State the percentage composition of gases in the atmosphere State air pressure at sea level Explain what is meant by partial pressure Explain how partial pressures contribute to gaseous exchange Calculate the partial pressure of gases State the function of the diaphragm and accessory muscles involved in breathing movements Describe the physical events involved in inhalation and exhalation Explain how transpulmonary pressures facilitate breathing Explain the role of pleural membranes in breathing Define and identify lung volumes, capacities and flow rates. Name the centres in the brain stem that regulate normal breathing Explain how these centres control breathing rate Describe how carbon dioxide regulates breathing rate Suggested reading: Tortora, Ch 23 Marieb Ch 23 36
The Lungs and Breathing Body cells need a continuous and plentiful supply of oxygen. This is because we are not adapted to store this gas. Cells need oxygen for cellular respiration and the ultimate generation of ATP. The circulatory system and the respiratory system are closely involved in getting and delivering oxygen to cells and eliminating waste carbon dioxide from the body. The respiratory system also plays an important role in maintaining the acid-base balance of the blood [i.e. blood ph]. Respiratory Epithelium Upper Respiratory Tract What are the functions of the mucus produced by the goblet cells in the respiratory tract? What is the function of cilia? 37
Alveolar Sacs, Alveoli and Respiratory Membranes Respiratory Bronchiole Pulmonary Artery Complete the missing words. Use the diagram to help you. bronchioles are connected to alveoli along regions called. Alveolar are common chambers that connect to multiple individual alveoli. The alveoli give the lungs a appearance. Associated with each alveolus is an extensive network of. There is also elastic tissue surrounding alveoli that can and help push air out of the lungs. Alveolar epithelium is simple epithelium and the cells also known as alveolar cells. Type II alveolar cells secrete a detergent-like molecule that the surface tension of the fluid lining the lungs. This helps to keep the alveoli open and prevents them from. If no surfactant Each lung is covered in a double layer of membranes. The space between the two membranes is filled with fluid. This provides and reduces friction between the membranes as we breathe. spongy alveolar ducts collapsing pleural recoil squamous sacs reduces lubrication Type I 38
respiratory capillaries surfactant 39
How does air get into the lungs? One thing that you need to be very clear about is that breathing is not a strictly passive process. Air does not just rush into and fill our lungs spontaneously. We need to generate pressure gradients to draw air into or force air out of our lungs. The role of the diaphragm and intercostal muscles. What is the diaphragm made of and where is it located? Made of Located What does the diaphragm allow the chest cavity to generate? Where are the intercostal muscles located, and what do they facilitate? Gaseous Exchange The surface of alveoli is the site where oxygen gas and carbon dioxide gas are exchanged. There are approximately 300 million of these tiny structures, with a surface area of roughly 85 m 2. List four properties of alveoli that make them ideally adapted to gaseous exchange. 1. 2. 3. 4. 40
Air composition and Partial Pressures Before we go to consider the mechanisms and concepts behind gas exchange, you need to be familiar with the composition of air. Complete the table describing the percentage composition of inhaled and exhaled air. Use the values given. 79% 0.03% 16% 4% 21% 79% Gas Inhaled Air (%) Exhaled Air (%) Nitrogen Oxygen Carbon dioxide Air Pressure The air around you now is pushing against you, even if you don t realise it. Air exerts pressure on us. Air pressure comes from the mixture of the gases in the air. Air pressure comes from the molecules of oxygen, carbon dioxide, and nitrogen, and from water vapour bashing against one another and us. Air pressure is the sum of the component pressures of the gases in the mixture. Standard air pressure measurements for physiological purposes are taken from sea level. Like blood pressure, air pressure is measured in mm Hg. At sea level, air pressure is 760 mm Hg. 41
Partial pressures Now, you know that the main gases in the air around us are nitrogen and oxygen. And you know the representative percentages of these gases in the air as 79% and 21% respectively. You also know that these gases, together, exert pressure. The nitrogen and oxygen, together, make air pressure at sea level 760 mm Hg. We can work out what contribution of pressure to the total 760 mm Hg of total air pressure that nitrogen and oxygen make by using their percentage representations. This is called the Partial Pressure of the gas and is done this way Partial pressure = percentage concentration x total pressure of the gas mixture So, for the partial pressure of Nitrogen, we work it out like Partial pressure = 79% x 760 mm Hg = 0.79 x 760 mm Hg = 600.4 mm Hg. So, in air at sea level, where the total air pressure is 760 mm Hg, around 600 mm Hg of pressure comes from nitrogen. This means that roughly 160 mm Hg of pressure comes from oxygen [the balance of the sum when you take 600 from 760]. Work it out. Calculate the partial pressure of atmospheric oxygen when ambient air pressure is 760 mm Hg at sea level and the percentage of oxygen is 20.9%. Partial Pressure of Oxygen = 42
Lung Volumes and Capacities. The volume of air that moves in or out of the lungs is called pulmonary ventilation. It is the product of the amount of air inspired in each breath and the number of breaths per minute. Respiratory volumes are measured with a spirometer. The subject breathes through a tube that is connected to a set of bellows. These move down or up with the breathing movement. This displacement is recorded onto graph paper. The diagram below shows a typical spirograph recording. The changes in air volumes at different stages of breathing are shown. Note that some of the measures are volumes, while others are capacities. Knowledge of these volumes and capacities can assist in the diagnosis and monitoring of disease. 43
. List the volumes and capacities separately. Volume Capacity Complete the definitions of the lung volumes and capacities in the table. Use the diagram to help you find the volumes for males and females. Volume/Capacity Definition Typical Male Volume [ml] Typical Female Volume [ml] Tidal Volume Inspiratory Reserve Volume Expiratory Reserve Volume Residual Volume Total Lung Capacity Vital Capacity Functional Residual Capacity 44
Transpulmonary Air Pressures The images below show what happens to air pressure in the lungs during inhalation and exhalation. Essentially it is the differences between the pressures inside the lungs and the pleural membranes that drive the movement of air. Complete the missing words. Use the words provided to complete the blanks. Intrapulmonary pressure is the air pressure found in the alveoli. At rest intrapulmonary pressure is about mm Hg. On this pressure drops by about 1mm Hg to. This difference allows approximately 500 ml of air to enter the lungs. This is roughly the volume. When you breathe heavily the size of the pressure gradient so that air enters the lungs. There is also a pressure exerted between the pleural membranes. This is approximately 756mm Hg That is mm Hg less than intrapulmonary pressure. This is because the natural elastic recoil of lungs pulls on the pleural membranes, then lowers pressure. In fact, intrapleural pressure is lower than intrapulmonary pressure at all times. If the pleural membranes are damaged or burst in any way, this causes the intrapleural pressure to equalise with intrapulmonary pressure. The result of this is that the lungs, a condition known as. To treat this, as much air as possible is removed from the space, which causes the air pressure to be less than that of the lungs. 760 mm Hg increases four collapse 759 mm Hg pressure more atelectasis tidal intrapleural Controlling Breathing Rate 45
Respiratory Rate This is used clinically as a baseline measure or general observation of patient health. What is a normal breathing rate for an adult in breaths per minute? State meaning of the following terms tachypnoea bradypnoea? Dyspnoea Control of Respiratory Rate We do have some limited control over our breathing rate this control allows us to sing and speak or play wind instruments. This control is exerted by the cerebral cortex. There is also very tightly regulated involuntary control of our breathing rate. Contrary to popular belief, it is not driven by a need for oxygen. If we think in terms of homeostasis, most systems work by using negative feedback controls. It is therefore sensible for the body to regulate itself by being driven by levels of potential toxins. Carbon dioxide and the hydrogen ions produced by the body s attempts to balance blood ph are what drive our breathing rate. 46
Where is the respiratory centre located? Where are the apneustic and pneumotaxic areas and what is their function? Apneustic location Function Pneumotaxic location Function Name the two regulatory areas of the medullary rhythmicity centre and give their functions Web Pages [accessed August 2004] http://www.lungusa.org/site/ This has internal link to cartoon animation of breathing http://www.physiologyeducation.org/materials/simres.html Simulations of breathing mechanics http://www.brit-thoracic.org.uk/ The British Thoracic Society home page http://biomedia.bio.purdue.edu/genbiolm/gbrespiration/html/anatomy.html 47