Breathing is all about creating a difference in air pressure between the lungs and the environment. Air will then flow down its pressure gradient because gases (like air) always move from areas of high pressure to areas of lower pressure. Both frogs and mammals take advantage of this in different ways to move air into their lungs. Frogs and mammals create this pressure gradient in different ways. Frogs actively create a higher pressure in their mouths (positive pressure breathing) whereas mammals use their diaphragm to create a low pressure within their lungs (negative pressure breathing). Whatever the mechanism, the end result is the same. A pressure gradient is created (where the lungs are at a lower pressure) and air flows into the lungs.
As discussed briefly in a previous post, a frog gulps air into its mouth, closes its mouth and nostrils, and moves the bottom of its mouth upwards to make its mouth cavity very small. This causes an increase in pressure (because the same amount of air now has less space) in the mouth cavity and the air moves into the frogs’ lungs (which are at lower pressure) because air always moves from a high to a low pressure. This is called positive pressure breathing because the frog actively creates a positive (increased) pressure which is greater than atmospheric pressure in its mouth to establish the air pressure gradient. Breathing out is (usually) passive (i.e. doesn’t require energy) in frogs (although it can be active in some amphibians) and the high elasticity of the body wall helps push air out of the lungs once the mouth is re-opened.
Although a similar pressure gradient is created when mammals (like us) breathe, it is created by moving the diaphragm downwards and the ribs upwards and out (using the intercostal muscles). The frog (and other amphibians) can’t breathe this way because they don’t have ribs! This causes the lungs to expand and decreases the air pressure inside them (because the same amount of air now has more space). Air then moves from outside the body where it is at higher pressure, to inside the lungs where a low pressure has been created. It’s kid of like a suction pump. Because a negative pressure that is lower than atmospheric pressure is created in the lungs, this type of breathing is called negative-pressure breathing. Again, breathing out is usually passive. The diaphragm and rib (intercostal) muscles relax and everything that happened to allow the mammal to breathe in reverses. Birds and reptiles also use negative pressure breathing and I will discuss some of the cool quirks of their respiratory systems in future blogs.
Interestingly, humans can actually be taught to breathe like frogs using positive pressure breathing. Frog breathing can be taught to humans whose breathing muscles become paralysed due to e.g. polio or stroke, allowing them to breathe on their own instead of with mechanical ventilation. And although frogs can’t be taught to breathe like humans, they are actually bimodal (i.e. two way) breathers with ~50% of total gas exchange (~80% carbon dioxide release and ~20% oxygen uptake) happening across the skin, and as tadpoles they have gills and breathe underwater.