Oxygen Transport to Muscles: From Inhalation to Cellular Respiration

Overview

Oxygen delivery to working muscles is a multi-step physiological process involving the respiratory, circulatory, and muscular systems.

1. Inhalation & Pulmonary Ventilation

During inhalation, the diaphragm contracts and the intercostal muscles elevate the rib cage, expanding the thoracic cavity. This creates negative intrapleural pressure, drawing air into the alveoli.

2. Alveolar–Capillary Diffusion

Oxygen diffuses from the alveolar air (high PO₂) across the alveolar–capillary membrane into pulmonary capillary blood (low PO₂), driven by the partial pressure gradient.

3. Binding to Hemoglobin & Transport in Blood

• O₂ binds reversibly to hemoglobin (Hb) in red blood cells; each Hb molecule carries up to four O₂ molecules.
• Oxygen-rich blood travels via pulmonary veins → left atrium → left ventricle → aorta → systemic arteries → muscle capillaries.

4. Capillary–Muscle Fiber Diffusion

At the muscle capillary bed, O₂ dissociates from hemoglobin, diffuses through plasma and interstitial fluid, crosses the muscle fiber membrane (sarcolemma), and enters the mitochondria for oxidative phosphorylation.

Key Points

• Hemoglobin is the primary O₂ carrier; dissolved O₂ in plasma is minimal.
• The Bohr effect (lower pH, higher CO₂, higher temperature) enhances O₂ unloading in active muscles.
• Myoglobin in muscle fibers acts as an intracellular O₂ reserve.

This integrated process ensures that oxygen is continuously supplied to meet the energy demands of exercising muscle.