Which type of muscle is responsible for moving the skeleton?

A. To filter blood and form urine: The kidneys filter blood to remove metabolic wastes, excess electrolytes, and water, producing urine. They also play a key role in regulating blood volume, blood pressure, and maintaining electrolyte and acid-base balance in the body.

B. To produce bile: Bile is a digestive fluid produced by the liver that emulsifies fats in the small intestine. The kidneys do not participate in digestion or bile production but focus on waste elimination and homeostasis.

C. To regulate heart rate: Heart rate is controlled by the autonomic nervous system and the pacemaker activity of the sinoatrial node in the heart. The kidneys can indirectly influence heart rate through blood pressure regulation, but they do not directly control cardiac rhythm.

D. To digest food: Digestion involves the breakdown of nutrients in the gastrointestinal tract through mechanical and chemical processes. The kidneys are involved in filtration, excretion, and homeostasis rather than digestion.

A. Increased ATP levels: During exercise, ATP is consumed rapidly to fuel muscle contractions. Increased ATP levels are not a cause of fatigue; rather, ATP depletion can contribute to reduced muscle performance.

B. Depletion of oxygen: Oxygen depletion can limit aerobic metabolism, but muscles often continue generating energy anaerobically. Oxygen shortage alone is not the primary cause of fatigue in prolonged activity.

C. Lack of glycogen stores: Glycogen is a key energy source, and its depletion can reduce endurance. However, fatigue is more directly related to metabolic byproducts affecting muscle contraction.

D. Accumulation of lactic acid: During intense or prolonged exercise, anaerobic metabolism produces lactic acid, which lowers pH in muscle cells. This acidic environment interferes with enzyme activity and muscle contraction, leading to fatigue.