Question

paste a picture of the model of lungs you built at home or took from the internet and label what equivalent part of the respiratory system each of the parts of your model represents. Then briefly describe how your model can be used to explain ventilation referring to the volume and pressure changes required and mimicked in your model. State the gas law(s) that apply to ventilation. If you did not build your own model – insert a picture from either a website that explains how to build the lung model (reference the web site!), or use the picture provided in prac manual 3 to label the components of the model.

Answer 1

The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi; on the inferior surface, the lungs are bordered by the diaphragm. The diaphragm is the flat, dome-shaped muscle located at the base of the lungs and thoracic cavity. The lungs are enclosed by the pleurae, which are attached to the mediastinum. The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right.

: Boyle’s law is a gas law that describes the relationship between the pressure and volume of gas for a mass and temperature. This law is the mechanism by which the human respiratory system functions. Boyle’s law is equivalent to PV = K (P is pressure, V is volume, K is a constant), or one may state that pressure is inversely proportional to the volume.

The lungs do not follow Boyle’s law at all volumes. In a resting state with a normal tidal volume, when the alveoli are not collapsed nor are the lungs at maximal capacity, the lungs follow proportional changes of volume and pressure in accordance to Boyle’s law. At low lung volumes, it takes a large pressure change to make small changes in the volume (low compliance of lung tissue). At high volumes within the lung, it takes a more negative pressure to expand the tissue, once again not in compliance with a direct relationship as Boyle’s law dictates. At low and high volumes, the lung has low compliance meaning that the ability of the tissue to expand or its elasticity decreases (compliance = [change in volume]/[change in pressure]).

Organ Systems Involved
The primary organ system involved in the usage of Boyle’s law is the respiratory system. The human body brings air into the lungs by negative pressure. At baseline, the thoracic cavity is in static equilibrium with an intrapleural pressure near -5cmH2O. During inspiration, there is a contraction of inspiratory muscles (diaphragm, external intercostal muscles; additional muscles such as the scalene and sternocleidomastoid can take part under specific circumstances) that increases intrathoracic volume. Due to the combined motion of the lungs and the chest wall, the lungs will begin to expand as the thorax expands during inspiration. According to Boyle’s law, as the volume increases the pressure must decrease, therefore as the intrapleural volume increases, the intrapleural pressure decreases to about -8cm H2O occurs at end inspiration

Volume-Controlled Ventilation
All of these first-generation ICU ventilators only provided VCV, and until the 1970s, it was without the option for patient-triggered breaths. Pressure-limited ventilators (eg, Bird and Puritan-Bennett machines) have been available since the 1950s, but they were not designed for continuous ventilatory support.

With VCV, the variable that is constant during each breath is tidal volume. With this approach, there is a variable inspiratory pressure. With changes in respiratory mechanics (eg, resistance, compliance) and patient's effort, airway pressure varies because volume delivery is constant. With VCV, the clinician sets tidal volume (VT), flow pattern, peak inspiratory flow, rate, and trigger sensitivity. In some ventilators, inspiratory time, minute volume, and I:E ratio are set instead of VT and flow. In other ventilators, both inspiratory time and flow are set; if the inspiratory time is longer than the time required to deliver the VT, an end-inspiratory pause will result. In practice, modern ventilators provide VCV by controlling the inspiratory flow. VCV can be applied as CMV or as synchronized IMV (CMV-IMV).

Pressure-Controlled Ventilation
With pressure-controlled ventilation (PCV), a fixed pressure is applied during the inspiratory phase. In addition, inspiratory time, or I:E ratio, and trigger sensitivity are set. As respiratory mechanics (eg, resistance, compliance) and patient's effort change, VT must vary because pressure is constant. As noted in Table 7-1, the primary difference between these VCV and PCV is a fixed VT or a fixed peak inspiratory pressure (PIP), respectively. In newer generation ventilators, the clinician may also set the rise time, which is the time required for the set pressure to be reached. This occurs by varying the slope of the flow increase from baseline to peak flow.