Respiratory System:

Respiration means getting oxygen into your body and removing carbon dioxide. Different organisms use different body parts for this job, called respiratory surfaces. Your skin can exchange gases, which is why amphibians like frogs breathe through their moist skin—this is cutaneous respiration. Fish use gills, which are feathery structures that extract oxygen dissolved in water as it flows over them. You and other land mammals use lungs, which are sac-like organs inside your chest.

All respiratory surfaces share important characteristics: they must be moist because gases dissolve in water, they need a good blood supply to carry oxygen around, and they must have a large surface area for efficient exchange. The lining must also be thin so gases pass through easily. Think of how a Nigerian lungfish survives in dried-up swamps by breathing air through its lungs when water disappears.

Gaseous exchange is simply how oxygen enters your body and carbon dioxide leaves it. Think of your lungs as having millions of tiny air sacs called alveoli. When you breathe in, oxygen diffuses across the thin alveolar walls into your blood through a process called simple diffusion. At the same time, carbon dioxide waste from your blood diffuses in the opposite direction into the alveoli so you can breathe it out.

Consider a Lagos resident climbing stairs—their muscles need extra oxygen quickly, so they breathe faster. This increases gaseous exchange because more air reaches the alveoli. The alveolar walls are moist and very thin, making diffusion very efficient. Your red blood cells pick up oxygen molecules and transport them throughout your body while releasing unwanted carbon dioxide.