Bloodgas interface

Gas exchange takes place at the blood-gas interface, which exists where the alveoli and the pulmonary capillaries come together. The alveoli are the smallest airways in the lungs; the pulmonary capillaries are found in the walls of the alveoli. Inspired oxygen moves from the alveoli into the capillaries for eventual transport to tissues. Entering the lungs by way of the pulmonary circulation, carbon dioxide moves from the capillaries into the alveoli for elimination by expiration. Oxygen and carbon dioxide move across the blood-gas interface by way of simple diffusion from an area of high concentration to an area of low concentration.

According to Pick's law of diffusion, the amount of gas that moves across the blood-gas interface is proportional to the surface area of the interface and inversely proportional to thickness of the interface. In other words, gas exchange in the lungs is promoted when the surface area for diffusion is maximized and the thickness of the barrier to diffusion is minimized. In fact, anatomically, the lungs are ideally suited for the function of gas exchange. There are 300 million alveoli in the lungs. Furthermore, the walls of each alveolus are completely lined with capillaries. There are as many as 280 billion pulmonary capillaries or almost 1000 capillaries per alveolus, resulting in a vast surface area for gas exchange of approximately 70 m2.

More specifically, the blood-gas interface consists of the alveolar epithelium, capillary endothelium, and interstitium. The alveolar wall is made up of a single layer of flattened type I alveolar cells. The capillaries surrounding the alveoli also consist of a single layer of cells — endothelial cells. In between the alveolar epithelium and capillary endothelium is a very small amount of interstitium. Taken together, only 0.5 mm separates the air in the alveoli from the blood in the capillaries. The extreme thinness of the blood-gas interface further facilitates gas exchange by way of diffusion.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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