The Respiratory system of Reptiles
The famous examples of Reptiles include snakes, lizards, crocodiles, turtles, and tortoises.
All reptiles are known to respire through their lungs. Some that are aquatic and remain most of the time inside water can also respire using their papillae.
Unlike the amphibians, the lungs in reptiles are very well developed. Their lungs are powerful, and muscular with more surface area for gas exchange.
Their respiratory system includes a pair of external nares, nasal chambers, internal nares, glottis, larynx, trachea, bronchi, and lungs. The external nares lie a little in front of the eyes.
The external nares lead into nasal passages or chambers, which open into the room of the buccal cavity.
The glottis is located behind the tongue and it opens posteriorly into a short chamber, the larynx.
The glottis is actually the part of the larynx that affects voice modulation and also enables them to hold large prey items while breathing without any disturbance.
The larynx is the windpipe that opens into a narrow, elongated cylindrical tube, the trachea. The trachea is supported by many cartilaginous tracheal rings.
The trachea after reaching the thorax portion of the reptiles’ body gets divided into two narrow tubes called the bronchi (singular: bronchus). Unlike mammals, they lack the bronchi tree.
Of the two bronchi, each bronchus is connected to a lung on each half of the body. The lungs looks elastic, elongated sacs, and the right lung is slightly larger than the left one.
The lungs has numerous alveoli present in it. The alveoli are the little sacs across which gaseous exchange takes place by diffusion between blood and the alveoli cells.
Thus, giving the reptiles the ability to have more surface area for gas exchange in their lungs.
How do Reptiles breathe?
As already said, reptiles breathe air only through their lungs. And yes, the reptiles have various ways of moving air into and out of their lungs.
The most common way of inhaling and exhaling air is by taking the air into and out of their lungs through their external nares.
In this common way of breathing and respiring, the oxygenated air in the reptilian respiratory system flows in through the Nares to the Nasal chamber, while passing between the nasal cavity and the nasopharynx to the Glottis at the base of the tongue.
Laryngeal muscles can adjust the size of the glottic opening, and the role of the glottis here is to control the opening and closing of the larynx (the windpipe) during inhaling and exhaling the air.
Next from the Glottis the air passes into the Trachea. Then from the trachea, the oxygenated air enters the Bronchi and then moves towards the Lungs.
Now, the lungs have numerous alveoli (air sacs) that make up the surface of the lung and its structure almost.
These alveoli are the regions where the gaseous exchange between the blood and the lungs takes place.
These alveolar regions are saturated by numerous blood capillaries concentrated throughout the lungs.
The oxygen from the oxygenated air that is inhaled gets imported to the blood cells, while in the other case, the carbon dioxide from the blood gets exported to the lungs. All these happens between the alveoli and blood capillaries by the simple diffusion method.
Do reptiles have lungs or gills to breathe?
All reptiles have lungs to breathe. They don’t have gills, and instead of gills, they do have papillae that do the same function as gills when they are inside water for a long time.
All reptiles breathe through their lungs. The reptiles’ lung has a much greater surface area for the exchange of gases than the lungs of amphibians.
Now it is to be noted that, lung ventilation is done differently in each main reptile group.
Just like, in those category of squamates that includes scaled reptiles such as lizards and snakes, the lungs are ventilated almost exclusively by the muscles of the chest wall.
Apart from the lungs, some reptiles like turtles, etc. can also exchange gases between the water and the body through cloacal respiration.
Turtles get oxygen from the water by moving the water over their body surfaces covered in blood vessels.
So, it is very sure and it has been seen that all reptiles have lungs, but neither one have gills.
In fact, gills are seen during the larval stages of chordates like Amphibians, but in the case of reptiles the development is direct without any larval stage, and so they don’t have any kind of gills.
All of the adults and young ones have lungs that they use for breathing when they are on land. And, in water they use the papillae for gaseous exchange.
How do aquatic reptiles breathe?
All reptiles, let it be aquatic, semi-aquatic, or terrestrial ones, all have lungs to breathe and respire.
But, in the case of aquatic ones that remain most of the time inside water are known to breathe using their papillae when they are deep inside the water. But, when they are on land they use their lungs to breathe.
The external layer of the lining of the cloaca is lined by papillae which are like small tissue projections that are rich in blood capillaries.
The type of respiration done by the papillae of the cloaca when the reptile is underwater is called ‘cloacal respiration’. On the other case, when they breathe using lungs is called ‘pulmonary respiration’.
Through cloacal respiration, aquatic reptiles like turtles get oxygen from the water that enters through the cloaca covered in blood vessels and then by moving the water over their body surfaces.
Turtles have a cloaca, which is essentially their butt, that has a lot of blood vessels, so the most efficient way to get oxygen is through the cloaca, hence the term cloacal respiration.
The papillae of the cloaca as already said is rich in blood capillaries. The capillary network takes the dissolved oxygen from the water where it enters the bloodstream through the process of diffusion.
The carbon dioxide is removed out of the blood to the open water by the blood capillaries as well through the diffusion process.
It has to be also noted that aquatic reptiles rise above to the surface of the water and take out its nose to intake oxygen from the air and fill it in their mouth for pulmonary resiration.
Some reptiles like most of the turtles and aquatic snakes can intake air from the land or after rising to the water surface. They can store that intaken air in the reserviors of the lungs, reduce their metabolic rate, after that they will soon submerge underwater.
Now, for how long they can store that intaken air roughly depends from species to species. This time range can depend from about half an hour to about several hours.
Just for instance, a resting or sleeping turtle can remain underwater for 4 to 7 hours. On the other case, crocodiles can remain underwater for only about 30 minutes or a little more than that, and then they again need to come to the surface of water to intake air.
Some aquatic reptiles like many turtles are known to aestivate or hibernate and so can dig themselves deep into the mud at the bottom of the water surface and can remain inactive for days to months. During that time, they lower and pause down their body metabolic rate a lot.
During such aestivating or hibernating days, the turtles get the limited oxygen they need through their papillae of the cloaca in a process called cloacal respiration by intaking oxygen and releasing carbon dioxide directly in and from the water.
Overview: Physiology of breathing and respiration in Reptiles
Reptiles can perform pulmonary respiration, that is the process by which oxygen enters and carbon dioxide exits the alveoli of the lungs.
Ventilation of oxygenated air in and carbon dioxide out of the body occurs via. expansion and contraction of the ribs.
The makeover of the lungs and the whole respiratory system being coordinately working with the circulatory system allows the reptiles to breathe and respire.
The reptiles use their external nares to inhale oxygenated air into the body. The next, air enters into the nasal chamber. Then passing through the glottis it enters the larynx or the windpipe.
From the larynx, the oxygenated air enter the lungs passing through the bronchi. There in the lungs, there are numerous alveoli.
Alveoli are the sites that are rich in minute blood capillaries. The surface wall of the alveoli is very thin that helps in the transfer of oxygen from the oxygenated air into the blood flowing through the blood capillaries by a simple diffusion method.
And, the carbon dioxide gas passing through the carbonated blood gets passed out into the alveoli from the blood flowing through the capillaries.
Now carbon dioxide along with water vapour (byproduct of respiration) gets out of the body passing back from the alveoli to the bronchi to the larynx to the nasal passage and is then exhaled out into the open air by the external nares.
There’s a difference between respiration and breathing. Breathing is actually a biological process of inhaling and exhaling of the gases between the body surface and the environment.
Whereas, Respiration, on the other hand, is a chemical process that takes place in the cell. This involves the production of energy typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.
Now, why the oxygen gets in the blood and carbon dioxide out? The reason is diffusion of oxygen and carbon dioxide taking place between alveoli and blood and between blood and the body cells.
Therefore, the circulatory system has the role to play and so it works with proper coordination with the respiratory system to transport oxygen in and carbon dioxide out of the body.
In the respiratory surfaces of the alveoli in the lungs, oxygenated air when found in higher concentration gets combined with haemoglobin of the blood to form oxyhaemoglobin by passing through the wall of the alveoli to the blood inside the capillaries.
Now the oxygen is easily transported by blood to the different tissues of the body where it dissociates due to the less concentrations of oxygen in the tissues and cells, and thus, gives up its oxygen to the tissues.
Now, the resultant Carbon dioxide formed due to a series of chemical reactions that want to come out of the tissue and cell is in high concentration, therefore it passes into the blood and binds with haemoglobin.
Carbon dioxide taken by the blood is passed out to the alveoli of the lungs, where they are released out through the nasal passage.
Can reptiles breathe without oxygen?
Yes, some (but not all) reptiles have been seen to breathe even without oxygen. But, they are not able to stay without oxygen for a very long time.
It’s because during the absence of oxygen that is during anaerobic respiration, the end products that comes out are Lactic acid or ethanol and ATP molecules.
When there’s too much of Lactic acid build up inside the body it may result in a condition known as acidosis, it can be fatal to an animal.
Acidosis is a process causing increased acidity in the blood and other body tissues
This condition can be seen in reptiles during those times when the reptiles tend to remain inside the water more than usual, may be due to the external fear from predators or due to other external pressures.
Some turtles can remain underwater for several days, and during hibernation they can remain underwater for several months which is more than the time they can stay inside water.
During those hibernating harsh times they use very little oxygen by taking it from water by clocal respiration.
But, what happens when the ground and water freeze in winter? Turtles survive the cold by hibernating, and they may burrow in the muddy bottoms of ponds or streams.
So, during those times when the oxygen availability in water decreases, they start to perform a kind of anaerobic respiration, thus producing Lactic acid as byproduct.
Thus, in simple words, when muscle is starved for oxygen, the body starts to produces lactic acid, which causes muscle cramps and weakness.
Although turtles have built-in mechanism inside their body to avoid fatal acidosis, by exploiting large buffer reserves inside their large mineralized shell.
This helps them stay without oxygen for a much longer time, but not forever.