Gas ExchangeThe actual exchanging of the gases is dependent upon important structures such as lungs or gills, and the principle of diffusion. Diffusion says that the molecules or particles will move from an area where they are very concentrated into an area where they are less concentrated.
As shown below in a liquid model, the red dots start with a high concentration on the left, but after time they have spread out into the area on the right. This is relevant during respiration because oxygen and carbon dioxide are often highly concentrated in opposite places, and simultaneous diffusion is how gas exchange occurs.
A diagram demonstrating diffusion. The jar on the left is separated by a semipermeable membrane, which allows the red dots to move from one side of the jar to the other slowly. In the jar on the left, the red dots were added to the left side of the jar. The jar on the right is the same jar at a later time. The red dots have spread out to the right side on the jar until there is an equal amount on both sides. This is the process of diffusion when a substance moves from an area of high concentration into an area of low concentration until both areas are at an equal concentration.
Gas exchange takes place in the capillaries for animals with a closed circulatory system such as birds, mammals, reptiles, and some amphibians. Remember that the capillaries are the smallest blood vessel, and they can be found near every cell in the body. With trillions of cells, that is a lot of capillaries.
InsectsThe respiratory apparatus in insects consists of a system of tubes, called tracheae, which directly ventilate the tissues. When an animal actively moves air to the site of gas exchange, it is called ventilation. The tubes act similarly to a closed circulatory system of blood vessels which divide and branch out into smaller and smaller tubes extending into all parts of the insect like plumbing pipes.
The insect has openings called spiracles scattered throughout its body, which are the openings to the tracheae. In small insects, gas exchange occurs by diffusion only. Larger insects will actively pump air into the tubes.
Aquatic insects must seal their spiracles when they are under the water to prevent flooding their tubes. Amazingly, some aquatic insects even have specialized spiracles that can puncture underwater plants and access their plants' oxygen storage centers. Think of it like an underwater vampire bug that sucks oxygen.
MammalsThe chief organ in mammalian respiration is the lungs. The lungs are actively ventilated via a suction-pump mechanism of inhalation and exhalation. Breathing is dependent upon the rib muscles and the diaphragm, which is a structure located just beneath the lungs like a dome-shaped floor (or a dome-shaped roof for the intestinal cavity). Check this site out for how to make a lung model.
Inhalation happens when the rib cage opens up and the diaphragm flattens and moves downward. The lungs can then expand into the larger space that causes the air pressure inside them to decrease, and the drop in air pressure inside the lung makes the outside air rush inside.
Exhalation is the opposite process. The diaphragm and the rib muscles relax to their neutral state that causes the lungs to contract. The squashing of the lungs increases their air pressure and forces the air to flow out.
A diagram of ventilation in most mammals. The left image shows inhalation with a flattened diaphragm. The right side shows the dome shaped diaphragm forcing the air out during exhalation.
In most mammals, the first place that air enters upon inhalation is the nose. It gets warmed, moistened, and filtered by cilia and mucus membranes which can trap dust and pathogens. Air then reaches the epiglottis, which is the tiny leaf shaped flap at the back of the throat. The epiglottis regulates air going into the windpipe and closes upon swallowing to prevent food from being inhaled. It is the gatekeeper to the lungs. If the epiglottis is the gatekeeper, who's the key master?
Diagram of structures of the lungs.
The trachea is a long structure of soft tissue surrounded by c-shaped rings of cartilage. In humans the trachea splits into two bronchi branches that lead to each lung. Each bronchi divides into increasingly smaller branches, until they form a massive tree of tubes. The smallest branches are called the bronchioles, and each bronchiole ends with a tiny air sac (no larger than a grain of sand) called an alveolus.
The tiny alveoli (alveoli is the plural of alveolus) are crucial because they increase the surface area that can be used for gas exchange. If the lungs were just empty sacs the only area available for gas exchange would be the walls of the lungs, which in humans is approximately 0.01 meters squared. In contrast, the alveoli structures provide 75 square meters of surface area where oxygen absorption can take place. That is the size of half a volleyball court.
Diagram of an alveolus near a capillary and the gas exchange process in the lungs.
As discussed above, gas exchange takes place in the capillaries, so the alveoli are closely aligned with the network of capillaries. This brings the blood carrying waste products into close enough proximity with fresh air for diffusion to take place. The waste is removed and the oxygen is taken up by the blood.
The blood is able to carry the fresh oxygen in red blood cells because of the hemoglobin protein, which can attach oxygen molecules. Think of hemoglobin like a bus that carries oxygen passengers. Each hemoglobin protein can carry four passengers of oxygen at one time.
When red blood cells are oxygen rich they are bright red, and when they are deoxygenated they are a deep purple. When the blood reaches the systemic capillaries near the cells, the carbon dioxide and oxygen diffuse in opposite directions.
After circulating through the heart, the blood arrives at the capillaries near the lungs. Water vapor and carbon dioxide are exhaled, and the process begins again with inhalation.
Just as the heart beats on its own, following sinoatrial node signals, breathing is done without conscious effort. There are sections of the brain, called the medulla and pons, that regulate respiration. They decide how fast respiration needs to take place by monitoring the level of carbon dioxide in the blood. In times of excitement or during exercise, the cells require more oxygen than normal. Respiration speeds up. Additionally, the heartbeat increases because the circulatory system is required for the respiration system to function.
Tidal volume is the amount of air breathed in or out during a respiratory cycle. The tidal volume and respiratory frequency vary amongst species and can also be affected by age, pregnancy, exercise, excitement, temperature, and body size. Horses have an average respiration of 12 times per minute, but pigs breathe an average of 40 times per minute.
Horses are obligate nasal breathers, which means that they must breathe through their noses. Humans and many other mammals can breathe through either their mouths or their nasal passages. A horse cannot breathe through its mouth. It is thought that this modification allows horses to graze with their heads down while separate nasal passages breath in air and sniff for potential predators.
Marine mammals breathe oxygen with lungs just like their terrestrial brethren, but with a few differences. First of all, to prevent water from getting into their airway they have adapted muscles or cartilaginous flaps to seal their tracheas when under the water. Additionally, they exchange up to 90% of their gases in a single breath, which helps them gather as much oxygen as possible. A sperm whale can last for 138 minutes on a single breath.
Lastly, it can be dangerous for diving mammals to have air in their lungs when they dive to great depths. For this reason, many marine mammals will prepare for a deep dive by taking a breath, exchanging gases in the blood, and exhaling to empty their lungs.
Reptiles and AmphibiansReptiles and amphibians both have lungs and exchange gases in the capillaries like mammals, but there are some differences in how they ventilate their respiratory systems. Reptiles do not typically breathe the same way as mammals since many reptiles lack a diaphragm. Without it they rely on muscles used in locomotion to ventilate their lungs.
Amphibians are capable of buccal pumping to push air into the lungs. This begins by muscles pulling air through the mouth or nose into a buccal cavity. Throat muscles then pump and move the floor of the mouth up in a way that Is visible from the outside. This forces air out of the mouth and into the lungs. Look at this frog's throat constantly moving.
Apart from their capillaries, amphibians can also perform gas exchange directly through their highly vascularized skin. This means that their skin has lots of blood vessels going through it. Since the blood vessels are close to their permeable skin surface, diffusion can take place right through the skin. In fact, some salamanders have no lungs at all, and they get all of their oxygen through their skin. The take home message is never get in a breath holding contest with a salamander. We wouldn't recommend a staring contest, either.
BirdsThe respiratory system of birds is similar to that of mammals. Air is pulled in using a suction-type pull. Gases are exchanged in the capillaries. The major difference is the route of airflow through the bird. Birds have air sacs that collect air. They then force the air through their lungs like bellows stoking a fire.
When a bird inhales, air is brought into the posterior air sacs, which expand. Upon exhalation, the air is forced from the posterior air sacs into the lungs. This is where gas exchange takes place. A second inhalation will move the air from the lungs to the anterior air sac. A second exhalation will push the air out of the body.
This progression of air through the bird means that the lungs are compressed during inhalation and expand during exhalation. It also takes two full inhalations and exhalations to move one gulp of air through the bird. That's a lot of gulps.
Diagram of the ventilation process in avian respiration showing the air going into an air sac before it reaches the lungs and again after it passes through the lungs.
The unidirectional flow of air through the lungs allows all the air flowing through the lungs to be fresh air with maximal oxygen to be collected. In humans, this is not the case since there is only one pathway to the lungs and it is used for both entry and exit. During flight, air sacs and lungs are continuously filled with oxygen rich air which provides maximal air to be absorbed into the blood stream, which is necessary for the high metabolism needed for flight.
Aquatic RespirationIn fish, respiration takes place in their gills. Gills can collect dissolved oxygen from the water and release carbon dioxide. Gills are much more complex than just a slit in the cheek of a fish.
Gills are comprised of gill arches with hundreds of gill filaments extending from them. Each filament is lined with rows of lamellae, and the gas exchange takes place as water flows through them. The frills and flaps increase the surface area to allow more gas exchange to take place, just as the alveoli do in the lungs.
Fish utilize a countercurrent exchange pathway (except for cartilaginous fish), which means that their arteries are arranged so that blood flows in the opposite direction of water movement against the gills. By having their respiration pathway in this orientation, maximum gas exchange can take place.
If the blood and the water were moving in the same direction, the blood would always be next to the same bit of water which would soon be depleted of oxygen. By setting up a countercurrent pathway, the blood is always passing water that still has oxygen. This allows the blood to gather as much oxygen as it can hold.
Since water must be flowing over the gills to provide a continual source of oxygen, fish have developed several ways to keep them ventilated. Some fish swim with their mouths open almost all of the time. Other fish have a special flap called an operculum, which is used to force water across the gills.
The exception to all fish having gills is the lungfish, which has working lungs. It can survive when its water habitat dries up from seasonal drought. Aptly named fish. Similarly, there are also certain land crabs that use gills to breathe outside of the water.
The lungfish is a unique animal which has gills and lungs. Image from here.