How does the horse's body respond to increased demands for oxygen from work?

When a horse is subjected to increased demands for oxygen during physical activity, one of the primary responses is the production of more red blood cells. Red blood cells are crucial for transporting oxygen from the lungs to the muscles, where it is used for energy production.

As the horse exercises more intensively, the demand for oxygen increases, and the body compensates by enhancing the number of red blood cells. This process is called erythropoiesis, which helps improve the horse's endurance and overall performance. Enhanced oxygen delivery to the muscles enables better efficiency during strenuous activities.

In contrast, simply decreasing activity would not meet the oxygen demands required for work. Increasing heart size can occur over time with consistent training but is not an immediate response to increased work demands. Losing weight does not directly relate to oxygen delivery; instead, it can impact a horse’s overall health and performance. Thus, producing more red blood cells best aligns with the horse's physiological adaptation to increased oxygen demands during work.