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Every cell in an animal requires oxygen to perform cellular respiration. Cellular respiration is the process by which animals take in oxygen and exchange it for carbon dioxide and water as waste products. Animals have specialized systems that help them do this successfully and efficiently. Even a fish will drown if it can’t breathe underwater.
The actual exchanging of gases is dependent upon important structures such as lungs or gills, and the principle of diffusion. Diffusion is the process where molecules or particles move from an area where they are very concentrated into an area where they are less concentrated.
Below is an illustration showing the process of diffusion. A container is separated with a semipermeable membrane, dividing an area with a high concentration of a molecule (red dots) from an area with lower concentration. The membrane allows the molecules to move from one side to the other. Over time, the molecules will move from an area of higher concentration to an area of lower concentration. The molecules will continue to migrate across the membrane until there is an equal amount on both sides. This is called equilibrium, or when both sides of a membrane have equal concentrations.
This is relevant during respiration because oxygen and carbon dioxide are often highly concentrated on opposite sides of a cell membrane. Diffusion allows gas exchange to occur.
A diagram demonstrating diffusion. Image from here.
In animals with a closed circulatory system (such as birds, mammals, reptiles, and some amphibians), gas exchange takes place across the capillaries. Remember that the capillaries are the smallest blood vessel and can be found near every cell in the body. With trillions of cells, that’s a lot of capillaries.
The respiratory system in insects consists of a network of tubes, called tracheae, which directly ventilate the tissues. Actively moving air to the site of gas exchange is called ventilation. The tubes divide and branch out into smaller and smaller tubes extending into all parts of the insect, similar to the way arteries branch out into tiny capillaries in a closed circulatory system.
Insects have openings scattered throughout its body called spiracles. Spiracles are openings to the tracheae. In small insects, gas exchange occurs by diffusion only. Larger insects will actively breathe to pump air into the tubes.
Aquatic insects must seal their spiracles when they are under water to prevent flooding their tubes. Amazingly, some aquatic insects even have specialized spiracles that can puncture underwater plants and access those plants' oxygen storage centers. Think of it like an underwater vampire bug that sucks oxygen.
The 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, a structure shaped like a dome-shaped floor just beneath the lungs.
Inhalation happens when the rib cage opens up and the diaphragm flattens and moves downward. The lungs expand into the larger space, causing the air pressure inside to decrease. The drop in air pressure inside the lung makes the outside air rush in.
Exhalation is the opposite process. The diaphragm and the rib muscles relax to their neutral state, causing 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.
Most mammals are nose breathers. Inhaling through the nose warms and moistens the air. The air is filtered by cilia and mucus membranes, which trap dust and pathogens. Air then reaches the epiglottis, 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’s the gatekeeper to the lungs.
Diagram of structures of the lungs. Image from here.
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 (plural of alveolus) are crucial because they increase the surface area used for gas exchange. If the lungs were just empty sacs, then only area available for gas exchange would be the walls of the lungs. In humans, that comes out to an area of approximately 0.01 m2. The alveoli, though, provide a whopping 75m2 of surface area where oxygen absorption can take place. That’s the size of half a volleyball court and it's all inside of you.
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 have a close working relationship with the network of capillaries. This brings the blood-carrying waste products close enough to the fresh air for diffusion to take place. The waste is removed and the oxygen is taken up by the blood. The hemoglobin in blood attaches oxygen molecules, kind of like a bus carrying passengers. Each hemoglobin protein can carry four passengers of oxygen at one time. Oxygen is delivered to the cells and carbon dioxide is removed. Water vapor and carbon dioxide are exhaled, and the process begins again with inhalation.
Just as the heart beats on its own, breathing is done without conscious effort. There are sections of the brain, called the medulla and pons that regulate respiration. They control the rate of respiration by monitoring carbon dioxide levels in the blood. In times of excitement or during exercise, the cells require more oxygen than normal and respiration speeds up.
Tidal volume is the amount of air breathed in or out during a respiratory cycle. The tidal volume and respiratory frequency, or the number of inhalations over a period of time (such as breaths per minute), vary amongst species, and it’s affected by age, pregnancy, exercise, excitement, temperature, and body size. For instance, 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, meaning that they can only breathe through their noses and are unable to breathe through their mouths. Humans and many other mammals can breathe through either their mouths or their noses. It’s thought that this modification allows horses to graze with their heads down while separate nasal passages breathe in air and sniff for potential predators.
Marine mammals breathe oxygen with lungs just like their terrestrial brethren, but with a few differences. To prevent water from getting into their airway, they have adapted muscles or cartilaginous flaps to seal their tracheas when under the water. We wish our cartilaginous flaps did that, but our ancestors skipped out on a couple million years of pruning up in the ocean, so we're out of luck. Marine mammals also exchange up to 90% of their gases in a single breath, which helps them gather as much oxygen and expel as much waste as possible. A sperm whale can last for 138 minutes on a single breath.
It can be dangerous for diving mammals to have air in their lungs when they dive to great depths. The water pressure would exert too much force on the air in their lungs, causing them to burst. 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 amphibians have lungs and exchange gases in the capillaries like mammals, but there are some differences in how they ventilate their respiratory systems. Reptiles don’t typically breathe the same way as mammals, since many reptiles lack a diaphragm. Reptiles use their axial muscles, the ones attached to their ribs, to expand their ribcage for breathing. During periods of intense activity, reptiles might be forced to hold their breath, as they use those muscles for running away.
Some reptiles get around this by buccal pumping while they run. Buccal pumping is when an animal uses the muscles of the mouth and throat to pull air into the lungs. Muscles pull 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’s visible from the outside. This forces air out of the mouth and into the lungs. This is what amphibians do, by puffing up their chinny-chin-chins to get the air in. Look at this frog's constantly moving throat .
Apart from their capillaries, amphibians perform gas exchange directly through their skin. This works for them because their skin has lots of blood vessels very close to the 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 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 body. 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. Then the bird exhales and the air is forced from the posterior air sacs into the lungs, where gas exchange occurs. The bird inhales a second time, moving the air from the lungs to the anterior air sac. A second exhalation pushes 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. It shows the air going into an air sac before it reaches the lungs and again after it passes through the lungs. Image from here.
The unidirectional flow 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’s only one pathway to the lungs and it’s used for both entry and exit. During flight, air sacs and lungs are continuously filled with oxygen rich air. This provides maximal oxygen to be absorbed into the blood stream, which is necessary for the high metabolism needed for flight.
In fish, respiration takes place in the gills. Gills collect dissolved oxygen from the water and release carbon dioxide. Gills are much more complex than just a slit in the cheeks 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.
Image from here.
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 is still oxygenated. This allows the blood to gather as much oxygen as possible.
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.
Like all good rules, there’s an exception. While all fish have gills, one fish also has lungs. The lungfish can survive when its water habitat dries up from seasonal drought. What an aptly named fish. There’s also certain land crabs that have both lungs and gills, and can breathe both under the sea and on land.
The lungfish is a unique animal which has gills and lungs. Image from here.
Did you know that in mammals, fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin does? Makes sense, because the fetus has to collect oxygen through the placental wall. In order to compete with the mother's blood, fetuses have a special form of hemoglobin that binds oxygen better and can compensate for the disadvantage of the placental barrier. The fetal hemoglobin lasts in the infant until approximately six months after birth.