Lungs, gills, and skin all solve the same gas problem
A respiratory system is simply the organ set that moves gases for animals and plants. Land vertebrates tuck exchange surfaces into lungs; fish bathe gills in water; insects use simple tubes; amphibians even breathe through skin. Humans split the job into upper and lower airways that branch like a tree.
Every breathing organism needs a large, thin, well-supplied surface where gases can cross. Land animals fold that surface inside the body as lung linings made of millions of microscopic sacs, called alveoli in mammals and reptiles and atria in birds, each wrapped in blood vessels. Most fish instead use gills, whose flat filaments and lamellae expose a big area of vascular tissue to passing water. Insects get by with very simple anatomy, amphibians exchange gas partly through the skin, and plants use pores called stomata, sometimes moving gases in the opposite direction to animals.
In people the airway splits into an upper tract, running from the nose and sinuses through the pharynx to the larynx above the vocal folds, and a lower tract that continues down through the trachea, bronchi and bronchioles to the alveoli. The lower tract branches like an upturned tree roughly 23 times in an adult, against about 13 in a mouse. The first sixteen or so generations merely conduct air; only the last few, from the respiratory bronchioles onward, actually trade gases. The trachea is about 1.8 cm across, and the two main bronchi 1 to 1.4 cm.
Because the alveoli are dead ends, air must leave the way it came. About 150 ml of stale air sits in the airways after each exhalation and gets drawn back in first, so each resting breath of roughly 500 ml delivers less fresh air than its size suggests. Even a forced exhalation leaves 1 to 1.5 litres behind, and total lung capacity is around 6 litres; neither figure can be measured with an ordinary spirometer.
At rest, inhaling is mostly the work of the diaphragm, a domed sheet of muscle that flattens as it contracts, while the intercostal muscles lift the ribs in a pump-handle motion. Falling pressure in the chest lets the elastic lungs expand. Breathing out at rest is passive and lasts about twice as long, and a healthy adult repeats the cycle 12 to 16 times a minute. During exercise, accessory muscles running from the neck and skull to the upper ribs join in.
Source: Respiratory system
