Today we are covering from the specification: Key words & Definitions Ventilation - Scientific term for “breathing” – where air is constantly moving in and out of the lungs. Inspiration - Also known as “inhalation”, occurs when air pressure in the atmosphere is greater than that of the lungs; forcing air into the alveoli. Expiration - Also known as “exhalation”, occurs when air pressure in the lungs is greater than that in the atmosphere; forcing air out of the alveoli. How does ventilation happen? • Ventilation requires pressure changes to occur in the lungs, so that air can be forced in and out of the alveoli. • The pressure changes are brought about by the movement of two sets of muscles.... How does ventilation happen? 1. The diaphragm muscle, which is a sheet of tissue that separates the thorax (chest) from the abdomen. 2. The intercostal muscles which lie between the ribs. There are two sets of intercostal muscles.... The intercostal muscles – an antagonistic pair • • The external intercostal muscles, whose contraction leads to inspiration. The internal intercostal muscles, whose contraction leads to expiration (N.B. these are not in use during normal breathing). The pleural cavity Each of the lungs is enclosed in a double membrane known as the pleural membrane. The space between the two membranes is called the pleural cavity, and is filled with a small amount of pleural fluid. lung This fluid lubricates the lungs. It also adheres to the outer walls of the lungs to the thoracic (chest) cavity by water cohesion, so that the lungs expand with the chest while breathing. pleural membranes 7 of 28 © Boardworks Ltd 2008 The mechanism of ventilation 8 of 28 © Boardworks Ltd 2008 Ventilation Ventilation Task – 15 minutes • Draw a table like the one below.... Inspiration Expiration External intercostal muscles Internal intercostal muscles Diaphragm Air pressure in lungs Air movement along pressure gradient Lung volume • Fill in what happens to the things on the left (i.e diaphragm, air pressure in lungs...) during inspiration and expiration. Answers, please! Inspiration External intercostal muscles Internal intercostal muscles Diaphragm Air pressure in lungs Air movement along pressure gradient Lung volume Expiration Answers, please! Inspiration Expiration External intercostal muscles Contract – pulling ribs upwards and outwards. Relax – permitting rib cage to move downwards and outwards. Internal intercostal muscles Are relaxed. Contract – moving the ribs downwards and decreasing the volume of the thorax (forced expiration only). Diaphragm Contracts – moves downwards from domed position. Relaxes – elasticity returns it to domed position. Air pressure in lungs Decreases Increases Air movement along pressure gradient Into lungs Out of lungs Lung volume Increases Decreases Maintaining the structure of the alveoli During inhalation, the chest cavity increases in volume, lowering the pressure in the lungs to draw in fresh air. This decrease in pressure leads to a tendency for the lungs to collapse. Cartilage keeps the trachea and bronchi open, but the alveoli lack this structural support. Lung surfactant is a phospholipid that coats the surfaces of the lungs. Without it, the watery lining of the alveoli would create a surface tension, which would cause them to collapse. 16 of 28 alveoli surfactant © Boardworks Ltd 2008 Composition of inhaled/exhaled air composition (%) In one breathing cycle, the air in the lungs loses only some of its oxygen content. This is why mouth-to-mouth resuscitation can be effective. 90 78% 78% 80 70 60 50 40 30 20 10 0 N2 17 of 28 inhaled air exhaled air 21% 15% 0.04% 4% O2 CO2 <1% 3% <1% <1% H 2O other © Boardworks Ltd 2008 Spirometry 18 of 28 © Boardworks Ltd 2008 Spirometric terms 19 of 28 © Boardworks Ltd 2008 Describe the trace Say what’s happening and why (what must the person be doing at each time?). But how do we measure tidal volume? We can use a spirometer Pulmonary Ventilation • Pulmonary ventilation is the total volume of air that is moved into the lungs during one minute (dm3min-1). To calculate it we multiply together two factors.... Pulmonary Ventilation 1. Tidal volume – the volume of air normal taken at each breath when the body is at rest. This is usually around 0.5dm3 (half a litre). 2. Ventilation (breathing) rate – i.e the number of breaths taken in one minute. This is normally 12-20 in a healthy adult. Pulmonary Ventilation – to summarise Pulmonary ventilation (dm3min-1) = tidal volume x ventilation rate (dm3) (breaths min-1)