Cardiorespiratory Mechanics Flashcards
Describe the pressure changes during rapid ejection
Opening of the aortic & pulmonary valves mark the start of this phase. As ventricles contract pressure within them exceeds pressure in aorta and pulmonary arteries. Semilunar valves open, blood pumped out and the volumes of ventricles decrease. No heart sounds for this phase.
Describe the relationship between volume change and pressure in the lungs
Following expiration, there are few molecules of air in the lungs and these are relatively compressed so both pressure and volume are low. However, as inspiration begins and lung space increases, pressure initially decreases as the same small number of molecules inhabit a larger space. However, once this pressure is below atmospheric pressure, air molecules enter the lungs, increasing pressure due to a greater number of molecules. During expiration, lung volume begins to decrease which initially causes an increase in pressure as there is littler space for the same number of molecules but once this equals that of atmospheric pressure and exceeds it, air moves out of the lungs and pressure falls.
Describe pressure flow relationship
Pressure generates flow. Flow rate graph (L/s) resembles the pressure change graph as a fall in pressure results in a fall in flow. The pressure graph also tracks the alveolar pressure and hence, Palv is the same as that. The intrapleural pressure is the difference between atmospheric pressure and alveolar pressure, which is equal to alveolar pressure as ATM always zero.
How does dead space impact ability to ventilate?
Dead space reflects the volume of inspired air that does not participate in gas exchange. If this is greater than tidal volume, person simply cannot exchange any air at alevolar level.
What does Poiseuille’s Law indicate about diameter and resistance to flow?
States that resistance is inversely proportional to the 4th power of the radius. Hence, if radius is halved, resistance increases 16 times. In terms of airways, initially, there is an increase in resistance as the airway generations increase but following the 4th generation, there is a logarithmic decrease in resistance as number of airways exponentially grows.
Why is there an initial increase up to generation 4 of the airways in terms of resistance?
The upper airways such as trachea, bronchi and primary bronchioles have a cartilaginous support surrounding them and hence cannot constrict and dilate as effectively with changing flow and volume. Moreover, the diameter continues to decrease from trachea to bronchi and hence resistance increases. However, as the number of terminal bronchioles increase, resistance decreases because firstly these can dilate with increase in lung volume and secondly, the sheer number of them means total surface area for air movement is much larger.
