Final answer:
Closing your hand around the sponge expelled air due to decreased volume, while opening it allowed air to refill the sponge. Breathing in causes your diaphragm and intercostal muscles to expand your chest cavity, decreasing lung pressure and drawing in air, making the chest expand and fingers move apart.
Step-by-step explanation:
When you closed your hand around the sponge, you decreased the volume of the space inside your hand, causing air to be pushed out of the sponge. Upon opening your hand, the volume increased again, creating lower pressure inside your hand relative to the outside air pressure, which allowed air to fill the sponge again.
During respiration, when you breathed in, your diaphragm and intercostal muscles contracted, expanding your chest cavity. This expansion increased the volume of your lungs, decreased the pressure, and resulted in air flowing into your lungs, causing your fingers to move apart. However, the movement of your fingers was primarily due to the expansion of your chest cavity while taking a deep breath. Yes, air filled your lungs as a result of this process, which is an application of Boyle's Law in respiration.
Breathing is a complex process that involves changing volumes and pressures within the lungs, using the stretchy and spongy properties of lung tissue, and the mechanics of the muscles of respiration. The average density of your body decreases when you take a deep breath due to the lower density of air compared to your body's density before breathing in.