UPSC Biology Respiratory System / श्वसन प्रणाली The Respiratory System

The Respiratory System

Respiration in Single Cell Animals

Single-celled organisms exchange gasses directly across their cell membrane. However, the slow diffusion rate of oxygen relative to carbon dioxide limits the size of single celled organisms. Simple animals that lack specialized exchange surfaces have flattened, tubular, or thin shaped body plans, which are the most efficient for gas exchange. However these simple animals are rather small in size.

Respiration in multicellular animals

Large animals cannot maintain gas exchange by diffusion across their outer surface. They developed a variety of respiratory surfaces that all increase the surface area for exchange, thus allowing for larger bodies. A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only cross cell membranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist.

Respiratory System Principles

1. Movement of an oxygen containing medium so it contacts a moist membrane overlying blood vessels.

2. Diffusion of oxygen from the medium into the blood.

3. Transport of oxygen to the tissues and cells of the body.

4. Diffusion of oxygen from the blood into cells.

5. Carbon dioxide follows a reverse path.