In the lungs of animals, oxygen and carbon dioxide undergo gas exchange through concentration gradients, a process that eliminates metabolic waste carbon dioxide and restores depleted oxygen levels in the blood.
Animals found this solution during the Cambrian period, leading to the subsequent "explosion" of terrestrial animals.
Through the aerobic process in the lungs, animals significantly enhance oxygen acquisition efficiency and generate high energy, crucial for the development, maintenance, and repair of complex cells, tissues, and muscles, making life increasingly complex thanks to the presence of lungs.
Image: The lungs of a horse take up a considerable amount of space.
When we talk about lungs, we have to mention horses, as they have relatively unique lungs.
Horses, descendants of animals from about 55 million years ago, initially were only the size of small to medium-sized pet dogs today, and they had three hooves.
The current era is home to only 7 species of this animal, all of which are ungulates with large bodies and exceptional athletic abilities, thanks to their lungs.
The image below shows the lung of a horse, which appears incredibly massive when fully inflated, almost half the height of a veterinarian!
Among all animals that breathe with lungs, horses have a relatively large lung capacity compared to their body weight. Their lungs weigh an average of 7 kilograms (lungs are typically quite light), about 1.5% of their body weight, making them 1.5 times larger than those of cows of similar size.
With such huge lungs, horses can inhale 60 liters of air with each breath, giving them exceptional athletic abilities and allowing for their larger body size.
Among large land mammals, only humans have lungs comparable to horses in both function and proportion, which is why humans also possess exceptional athletic capabilities.
Alveoli Diagram
However, a horse's lungs have over 50 times more alveoli than humans. Alveoli are where the exchange of oxygen and carbon dioxide takes place, and having more alveoli means a stronger respiratory capacity.
The exceptional breathing ability of horses is not evident when they are at rest; in fact, most of their lung capacity is actually surplus during rest.
Data shows that during normal breathing, horses only utilize about 40% of the air in their lungs, with the remaining 60% of air not coming into contact with the alveoli and being exhaled. In contrast, humans can utilize around 70% of the air with each breath.
Horses' lung capacity is geared towards running. When they sprint, their muscles require more oxygen. To ensure an adequate oxygen supply during running, horses have a unique adaptive feature: spleen contraction.
In simple terms, horses can contract their spleen by releasing adrenaline, allowing the spleen to release all red blood cells into the bloodstream. This enables them to capture more oxygen and deliver it to the muscles to sustain their activity!
Through this mechanism, the blood's hematocrit (the percentage of red blood cells that transport oxygen in the total blood volume) can increase from 30-40% to 60-70%, thereby enhancing oxygen utilization.
Some unethical human athletes may use erythropoietin synthetic drugs (a type of stimulant) to boost their performance by increasing red blood cell production, a capability that horses naturally possess.
Despite having powerful lungs and adaptive features to support them, horses also have notable weaknesses.
Image: Horses can only breathe through their nostrils, but luckily, their nostrils are quite large.
Unlike humans, horses cannot use their mouths to assist in breathing. They can only breathe through their noses because of their long soft palate, which closes off the connection between the nose and the digestive tract.
This breathing method reduces the rate at which air enters the lungs. To be able to run quickly, horses have developed a unique adaptive feature: when running fast, they switch to passive breathing, with the breathing rate entirely coordinated by the movement of their limbs.
When they are running fast, their breathing goes something like this:
During the suspension phase of the hind legs - when the muscles of the abdomen pull the hind legs forward, they inhale. This is because the organs in the horse's abdominal cavity are pushed backward by the diaphragm, causing air to enter the lungs. And during the extension phase of the hind legs - when the hind legs move backward, they exhale. This is because the contents are pushed forward into the diaphragm, forcing air out of the lungs.
This method poses some issues. During intense exercise, the rate at which air flows in can be much higher than expected, creating pressure around the lungs. Research has shown that during regular running, a horse's breathing can put five times more pressure on the lungs than usual.
If a human caretaker tries to make a horse run faster, the pressure on the horse's lungs can increase, leading to damage to the airways and even the entire lungs - often resulting in pulmonary bleeding.
In domesticated horses, pulmonary bleeding is a common ailment caused by running too fast and breathing too rapidly - the immense pressure can cause the tiny blood vessels near the lung alveoli to rupture.
The respiratory system of horses makes them one of the most powerful land mammals in terms of athletic ability, but it also makes them one of the most susceptible species among all mammals to serious respiratory diseases.
