Physiology of Respiration I: Functions of the Respiratory System

During inhalation, the diaphragm and external intercostal muscles contract, expanding the chest cavity and reducing intrathoracic pressure. This pressure drop allows air to flow into the lungs. The scalene muscles also assist in this expansion, particularly during deep breathing or exercise.

Oxygen moves from the alveoli to the arterial blood through diffusion across the alveolar-capillary membrane. Hemoglobin in red blood cells binds oxygen to form oxyhemoglobin, which transports it throughout the body. A small portion of oxygen also dissolves directly in plasma for transport.

Airway diameter directly affects resistance to airflow during breathing. Bronchoconstriction, which narrows airways, increases resistance and makes breathing more difficult. Conversely, bronchodilation widens airways and decreases resistance, facilitating easier airflow through the respiratory passages. Understanding these dynamics helps identify breathing complications.

Lung compliance reflects the lungs' ability to expand during inhalation, determined by the elasticity of lung tissues and chest wall flexibility. High compliance means the lungs expand easily with minimal effort, while low compliance indicates stiffness and requires greater muscular effort to breathe effectively.

Carbon dioxide is transported in three forms: dissolved in plasma, bound to hemoglobin as carbaminohemoglobin, and as bicarbonate ions in red blood cells. This multi-form transport system is crucial for maintaining acid-base balance and efficiently removing metabolic waste from tissues to the lungs.

Exhalation is driven by the elastic recoil of the lungs and chest wall, which raises intrathoracic pressure and expels air from the lungs. This passive process occurs when the diaphragm and intercostal muscles relax, allowing the respiratory structures to return to their resting position.

Oxygenation efficiency is influenced by partial pressure of oxygen, pH, and temperature. These factors affect how readily hemoglobin binds oxygen in the lungs and releases it to tissues. Optimal conditions ensure adequate oxygen delivery to support cellular metabolism and overall body function.