In the lungs of animals, oxygen and carbon dioxide are exchanged for gas exchange through concentration gradient diffusion, a process that expels carbon dioxide, a metabolic waste product, and restores depleted oxygen levels in the blood.
It was during the Cambrian period that animals found this solution, which led to the subsequent "big explosion" of terrestrial animals.
Through the aerobic process of the lungs, animals greatly improve the efficiency of oxygen acquisition and produce high energy, which is essential for the development, maintenance and repair of complex cells, tissues and muscles.
The horse's lungs take up a considerable amount of space
And when we talk about lungs, we have to mention horses, which have relatively unique lungs.
Equine animals evolved about 55 million years ago, and they were originally the size of modern small and medium-sized pet dogs and had three hooves.
There are now only seven species left in this family, all of which are single-hoofed and all of them are large in size, as well as extremely athletic, thanks to their lungs.
The picture below is the horse's lungs, which look huge when fully inflated, and look half the height of a veterinarian!
Of all animals that breathe with their lungs, the proportion of lung weight to body weight in horses is very large, with their lungs weighing an average of 7 kg (lungs are usually very light), which is about 1.5% of their body weight and 1.5 times the size of the lungs of cows of the same size.
With such huge lungs, they can inhale 60 liters of air per breath, which gives them super athletic ability and can have a large body size.
Among the land large mammals, only human lungs rival horses (in equal function and proportion), so you'll find that humans are also super athletic.
Schematic diagram of the alveoli
However, horses have more than 50 times more alveoli in their lungs than humans, and the alveoli are the exchange sites for oxygen and carbon dioxide, and more alveoli mean more ability to breathe.
The horse's super breathing ability is not reflected in the rest, when most of the lung capacity is actually superfluous.
Some data show that when breathing normally, the utilization rate of air in the horse lungs is only about 40%, and the other 60% of the air is exhaled without contact with the alveoli, while in comparison, the utilization rate of air in humans can reach 70% every time they breathe.
Horses' lung capacity is to prepare them for running, and once they run fast, their muscles need more oxygen, and to ensure the oxygen supply while running, they have a very unique adaptive feature: spleen contraction.
Simply put, horses can contract their spleen by releasing adrenaline, which can then release all the red blood cells into the bloodstream so that they can capture more oxygen and carry it to their muscles to keep them working!
In this way, their hematocrit (the percentage of red blood cells that carry oxygen out of the total blood volume) soars from 30-40% to 60-70%, increasing oxygen utilization.
Some cheating human athletes will do similar things by injecting erythropoietin synthesis drugs (a type of stimulant) in order to improve their performance, and horses are born with this ability.
Although horses have strong lungs and the adaptive traits that go with them, they also have significant drawbacks.
Pictured: Horses can only breathe through their nostrils, but fortunately they have large nostrils
Horses do not have the aid of breathing with their mouths like humans, they can only breathe through their noses because horses have a longer soft palate that closes the connection between the nose and the digestive tract.
This type of breathing reduces the rate at which air enters the lungs, and horses have a unique adaptive feature in order to be able to run quickly: when running fast, they switch to passive breathing, and the breathing rate is completely coordinated by the movement of the limbs.
Here's how they breathe when they run fast:
During the hind leg suspension phase, when the muscles in the abdomen pull the hind legs forward, the inhalation is carried out by the organs in the horse's abdominal cavity from the diaphragm to bring air into the lungs; In the hind leg lengthening phase, which is when the hind leg moves backwards, exhale as the contents are pushed forward into the diaphragm, forcing air out of the lungs.
The problem with this approach is that when they exercise vigorously, they can get air at a much faster rate than expected, which creates pressure around the lungs, and studies have shown that the horse's breathing during a normal run can put five times more pressure on the lungs than usual.
This pressure also increases if a human breeder tries to get a horse to run faster, causing damage to the trachea or even the entire lungs – usually bleeding from the lungs.
Bleeding from the lungs is a common condition in horses in captivity, caused by running too fast and breathing too quickly – extreme pressure can cause capillaries close to the alveoli to rupture.
The horse's respiratory system makes horses one of the most athletic large land mammals, but it also makes them one of the most susceptible species of all mammals to serious respiratory diseases.