INTRODUCTION

When the respiratory system is mentioned, people generally think of breathing, but breathing is only one of the activities of the respiratory system. The body cells need a continuous supply of oxygen for the metabolic processes that are neccessary to maintain life.

The respiratory system also works with the circulatory system to provide this oxygen and to remove the waste products of the metabolism. It also helps to regulate the pH of the blood.

EXTERNAL RESPIRATIOn

Respiration is the mechanical process of breathing, that is, unconscious exchange of air between the lungs and the external environment. This exchange of air at the lungs is called external respiration.

In external respiration, oxygen is inhaled (air inhaled contains about 21 per cent oxygen) into the air spaces (sacs) of the lungs and immediately passes into tiny capillary blood vessesl sorrounding the air spaces. Simultaneously, carbon dioxide, a gas produced when oxygen and food combine in cells, passes from the capillary blood vessels into the air spaces of the lungs to be exhaled (exhaled air contains about 16 per cent oxygen).

INTERNAL RESPIRATION

While external respiration occurs between the outside environment and the capillary bloodstream of the lungs, another form of respiration is occurring simultaneoulsy between the individual body cells and the iny capillary blood vessels that surround them. This process is called internal (cellular) respiration.

Internal respiration is the exchange of gases not at the lungs, but at the cells within all the organs of the body. In this process, oxygen passes out of the bloodstream and into the tissue cells. At the same time, carbon dioxide passes from the issue cells into the bloodstream and is carried by the blood back to the lungs to be exhaled.