Muscle cramps:
Muscle cramps are involuntary contractions or spasms in one or more muscles. They are characterized by sudden, intense pain and a tight, knotted feeling in the affected muscle. Cramps can occur in any muscle but are commonly experienced in the legs, feet, and calves. They can last anywhere from a few seconds to several minutes and can be quite uncomfortable.

Role of lactic acid:
Lactic acid is a byproduct of anaerobic metabolism – the process by which the body produces energy in the absence of sufficient oxygen. During intense exercise or physical activity, the muscles may not receive enough oxygen to meet the increased demand for energy. In this situation, the body switches to anaerobic metabolism, leading to the accumulation of lactic acid in the muscles.

Contrary to popular belief, lactic acid does not directly cause muscle cramps. However, it can contribute to the sensation of muscle fatigue and discomfort during and after exercise. The accumulation of lactic acid in the muscles may make them more prone to cramping, especially if the individual is dehydrated or has inadequate nutrient levels.

External respiration:
External respiration refers to the exchange of gases between the environment and the body. It involves the inhalation of oxygen (O2) from the atmosphere and the exhalation of carbon dioxide (CO2) back into the air. This process occurs in the respiratory system, specifically in the lungs.

During external respiration, oxygen-rich air is inhaled through the nose or mouth, passes through the trachea, and enters the lungs. In the lungs, the air moves into tiny air sacs called alveoli, which are surrounded by blood vessels called capillaries. Oxygen molecules diffuse across the thin walls of the alveoli and into the bloodstream, where they bind to red blood cells.

At the same time, carbon dioxide, a waste product of cellular respiration, moves from the bloodstream into the alveoli. It then travels back up the respiratory tract and is exhaled from the body. This exchange of gases ensures that oxygen is delivered to the body's tissues for cellular respiration, while carbon dioxide is efficiently removed.

During external respiration, several factors such as the concentration gradient of gases, diffusion, and the structure of the respiratory system facilitate the exchange of oxygen and carbon dioxide between the air and the blood. This process is essential for maintaining adequate oxygen levels in the body and removing waste carbon dioxide.