* Muscles: Horses have highly efficient muscles, particularly in their legs and hindquarters. They have a high percentage of slow-twitch muscle fibers, which are specialized for endurance and don't fatigue as quickly as fast-twitch fibers.
* Respiratory System: Horses have large lungs and a high respiratory rate, allowing them to take in and utilize large amounts of oxygen. Their nostrils are designed to open wide, maximizing airflow.
* Circulatory System: Horses have a powerful heart that pumps blood efficiently. They have a high blood volume and a complex network of blood vessels, ensuring oxygen and nutrients reach muscles quickly.
* Digestive System: Horses have evolved a digestive system that efficiently extracts energy from forage. Their large intestines are adapted for fermenting fiber, providing a sustained energy source.
* Skeletal System: Horses have strong, lightweight bones and tendons that allow them to move with grace and efficiency. Their long legs provide a long stride, and their hoof structure is adapted for running.
While not technically an internal organ, it's important to mention the hooves, which are a critical adaptation for efficient running.
It's also worth noting that horses have developed these adaptations over millions of years. Their long legs, powerful hearts, and efficient respiratory system are all part of a complex, interconnected system that allows them to excel at long-distance running.