Which muscles are primarily responsible for the process of breathing?

The intercostal muscles and diaphragm are primarily responsible for the process of breathing, making this choice the correct answer. The diaphragm is a dome-shaped muscle located at the base of the thoracic cavity. When it contracts, it moves downward, increasing the volume of the thoracic cavity, which causes air to be drawn in through the lungs. The intercostal muscles, located between the ribs, assist in this process by expanding and contracting the rib cage, which also contributes to the increase and decrease in thoracic volume during inhalation and exhalation. Together, these muscles effectively control the mechanics of breathing under normal conditions.

The other options do not directly contribute to the act of breathing. Smooth muscles and cardiac muscles are involved in various involuntary functions, like managing blood flow and heart activity, but they do not play a role in respiration. Abdominal muscles, while they can assist in forceful exhalation, are not the primary muscles for breathing during normal, quiet respiration. Veins and arteries are part of the circulatory system and do not have any direct function in the mechanics of breathing itself.