Breathing

The diaphragm is a skeletal muscle forming the floor of the thoracic cavity that contracts during inhalation, moving downward to increase thoracic volume. This expansion creates more room for the lungs to expand and reduces internal pressure, allowing air to flow in from the atmosphere.

During inhalation, the diaphragm and intercostal muscles contract, increasing thoracic volume and decreasing pressure below atmospheric levels. This pressure gradient pulls air into the lungs. During exhalation, these muscles relax, volume decreases, pressure rises above atmospheric levels, and air is pushed out.

Surfactant is a mixture of phospholipids and lipoproteins lining alveolar surfaces that reduces surface tension of alveolar fluid. This prevents alveoli from collapsing and makes inflation easier. Premature infants lacking sufficient surfactant develop respiratory distress syndrome, requiring significant effort to breathe.

Each lung is surrounded by two pleural membranes separated by fluid that creates an adhesive force. As the thoracic cavity expands during inhalation, this adhesive connection causes the lungs to stretch and expand along with the chest wall, maintaining coordination between structures.

Restrictive diseases like pulmonary fibrosis prevent lung expansion by scarring lung tissue. Obstructive diseases like asthma and emphysema block airways, limiting gas exchange. Both categories reduce gas flow to and from the lungs but through different mechanisms affecting breathing efficiency.

Bronchi and bronchioles have stiff walls that do not expand, though they can become blocked or inflamed. Alveoli, tiny air sacs in the lungs, are flexible and allow the lungs to increase in volume during inhalation. This structural difference enables effective gas exchange and lung inflation.

Boyle's law states that pressure is inversely proportional to volume in a closed space. When the diaphragm contracts and thoracic volume increases, pressure inside the lungs drops below atmospheric pressure. This pressure gradient moves air from higher-pressure atmosphere into lower-pressure lungs.