Here’s a structure and function question for your students to think about: how can a sponge soak up so much water? The anatomy and physiology answer: the same way your lungs can exchange blood gases so quickly. Structurally, a sponge has a large surface–every pore increases the amount of contact between the porous cellulose fibers and water molecules. Similarly, alveoli in the lungs increase the surface area for the uptake of oxygen from the environment. Then capillaries next to the alveoli allow gas exchange to occur between the internal and external environment.
Another question to pose to your students: what happens to a sponge when the liquid absorbed is spilled coffee instead of water? What happens to the larger molecules that get stuck in the pores? They stain the sponge, and the sponge must be rinsed out. What’s an analogous process in your lungs?
One more thing to consider: what if that spilled coffee is the last cup in the pot and has solid coffee grounds in it? The solid particles on the counter can be removed by the sponge, but now the sponge must be washed thoroughly, maybe even bleached, and some grounds may have to be picked out manually.
Our lungs perform a similar function with every breath we take. Rarely is the air we breathe perfectly proportioned atmospheric gases. Our respiratory system adapts to ranges in oxygen saturation with changes in altitude, changes in humidity (desert to rain forest or winter to summer), and changes in particulate matter (from dust to pollen to pathogens). It also adapts to the occasional guzzle of hot coffee that goes down the wrong way. It’s an amazing system!
Additional Reading: It Takes Guts to Teach A&P
To help students understand the respiratory system, take a look at our infographic explaining the structure of lungs and how the lungs and diaphragm work in tandem, allowing us to inhale and exhale.
Explore the mechanics of breathing and the physical laws that explain lung function with a reading that explains gas laws in relation to inhaling and exhaling.
To supplement the reading, students can simulate lung function using the miniature lung function model. This model illustrates how the movement of the diaphragm affects pressure and volume during human respiration. They can also assess their personal vital capacity, tidal volume, and expiratory reserve with a spirometer and total lung capacity with the lung volume bag set. Some absorb and release a high volume of water and some very little.
Breathing is an essential physiological function. Through breathing, our bodies obtain oxygen for our cells and eliminate carbon dioxide, a cellular waste product. Breathing is involuntary—the autonomic nervous system regulates our breathing without the need for conscious effort. However, if we want to, we can take temporary control of our breathing function, inhaling or exhaling as forcefully as we like.
The mechanics of breathing can be explained in terms of gas laws. Boyle’s law states that the pressure and the volume of a gas in a closed container are inversely proportional—as the volume of the container decreases, the pressure of the contained gas increases. As the volume increases, the pressure decreases.
Mathematically, Boyle’s law can be stated this way:
pV = k
If the volume in a closed container is changed, the pressure changes along with the volume. The product of the two numbers remains the same value before and after the change:
p1V1 = p2V2
Many textbooks use the example of a cylinder and piston when introducing this law. As seen in the figure below, as the piston moves downward, the volume decreases. The gas molecules inside the cylinder have less space to move and therefore strike the walls of the cylinder more often. As a result, the pressure increases. (Note: the pressure and volume of a gas also depend on temperature, and Boyle’s law applies when temperature is constant.)
The key concept to understand is that the movement of air depends on a difference in pressure, or a pressure gradient. Even when you take a deep breath, you are not “sucking” air into your lungs. Instead, air moves due to a pressure gradient created by your respiratory system. Gases, such as air, travel along a pressure gradient from high pressure to low pressure.
The structure of the human respiratory system functions well to create the pressure gradient needed for inhaling and exhaling. During inhalation, air is drawn in from the surroundings by a change in the volume of the lungs. This change in lung volume is due to a change in the volume of a closed container—the thoracic cavity. The thoracic cavity (also known as the chest cavity) is separated from the abdominal cavity by a domed sheet of skeletal muscle called the diaphragm. As the diaphragm contracts and is pulled down, the ribs expand, increasing the volume of the thoracic cavity.
As explained by Boyle’s law, when volume increases, the pressure in the thoracic cavity decreases. The lungs, which connect to the outside via the bronchi, trachea, nose, and mouth, provide the means for the internal and external pressure to equalize. Air rushes from the higher pressure of the surroundings into the mouth or nose and then to the lungs. Once pressure equilibrates, inhalation ends.
The diaphragm relaxes, and the volume of the thoracic cavity and lungs decreases. Another pressure gradient forms, this time with higher pressure inside of the body (rather than outside of it). Air moves from the lungs out of the mouth or nose and into the surroundings during exhalation. Breathing depends on this back-and-forth pressure gradient caused by volume changes due to contraction and relaxation of the diaphragm.
Essential Question: What is the main function of the respiratory system?
Objective: Review respiratory system structure and function for test preparation.