Frogs versus Mammals and The Mechanism of Inspiration Help
Introduction to Frogs Versus Mammals and The Mechanism of Inspiration
One obvious question we could ask is, “Okay, but how do we breathe, in the first place? How do we inhale air into our lungs?” The answer to this question involves a discussion of the mechanism of inspiration.
Bulk Flow Of Air
Air circulates through the respiratory tree in much the same way that blood circulates through the cardiovascular system. As we learned in Chapter 17, blood flows down a blood pressure (BP) gradient, from a place where the BP is higher, towards another place where the BP is lower. In general, we can call this a bulk flow process. Bulk flow is the pressure-driven movement of some fluid substance (such as blood or air) from an area of greater pressure towards an area of lower pressure. In other words, bulk flow occurs down a pressure gradient.
For ventilation in lung-breathing animals, such as frogs and mammals, then, the problem becomes one of creating an air pressure gradient – a difference in air pressure – between the air in the atmosphere and the air within the alveoli (tiny lung air sacs). With such a gradient, there will be a bulk flow of air from the atmosphere, and into the lung alveoli.
Equal Gas Pressures
There are two total gas pressures to consider. The atmospheric pressure is the pressure created by all of the gases in the atmosphere. (The atmosphere is the approximately 1 mile-thick blanket of air covering the surface of the Earth.) At sea level, the atmospheric pressure pushes with a total force of about 760 mmHg. The atmospheric pressure also pushes with this force upon the lips and nostrils. So when a person opens his mouth, the atmospheric pressure tends to push air down into his lung alveoli (see Figure 18.4, A).
Conversely, the intra-alveolar ( in -trah-al- VEE -oh-lar) pressure is the total pressure exerted by all of the gas molecules within the alveoli. When a person opens his mouth, the intra-alveolar pressure tends to push air out of the alveoli, and out of the nose and mouth.
The atmospheric pressure (tending to push air into the alveoli) and the intra-alveolar pressure (tending to push air out of the alveoli) are thus two opposing pressures. Between breaths, we have an equality of these two pressures:
Atmospheric pressure = Intra-alveolar pressure and
No air pressure gradient, therefore no bulk flow of air into or out of the lungs
Inspiration in frogs and little bears. (A) Atmospheric pressure versus intra-alveolar pressure.
Positive and Negative-Pressure Breathing
In order for the frog or mammal to breathe, the equality between the atmospheric and intra-alveolar pressures must be broken. In positive-pressure breathing , an air pressure gradient is created by doing something “positive”: that is, by increasing one of the pressures to make it higher than the other one. Consider the case of the frog and most other kinds of amphibians (Figure 18.4, B). The frog opens its nostrils, lowers the floor of its mouth, and gulps air into its oral cavity. The next step is the one that creates a positive-pressure breathing effect: the frog closes its nostrils and raises the floor of its mouth. This action creates a pushing force, thereby increasing the atmospheric pressure within the frog’s mouth. Inspiration occurs because air from the mouth is then pushed down through the pharynx and trachea, and into the lung alveoli, which have a lower pressure.
Fig. 18.4 (B) Positive-pressure breathing. (C) Negative-pressure breathing.