A horse has a 14.5 gallon (55 liters) lung capacity, and they inhale around 1.3 gallons (5 liters) with each breath while they’re relaxed or resting. The resting breathing rate for horses is approximately 12 breaths per minute, and they can breathe up to 16 gallons (60 liters) of air every 60 seconds.  During exercise, a horse's respiration rate increases, and at full gallop, some horses will take up to 120 breaths per minute. The average air intake during exercise is around 4 gallons (15 liters) for each breath, so that's 475 (1800 liters) per minute or 15 - 26 gallons (60 - 100 liters) per second.  A healthy, athletic horse can absorb oxygen from the lungs better than a retired, inactive one, but you can't increase a horse's lung capacity.