Review of Medical Physiology - William F. Ganong 2002

Respiration
Pulmonary Functions
Introduction

The term respiration is used to describe two distinct processes: external respiration, which involves the absorption of O₂ and the elimination of CO₂ from the body, and internal respiration, which encompasses cellular utilization of O₂, production of CO₂, and gas exchange between Cells and their fluid environment. The specifics of O₂ utilization and CO₂ production are detailed in Chapter 17. This section focuses on the Functions of the Respiratory system in external respiration—namely, the processes responsible for transporting O₂ to the Lungs and eliminating CO₂ from them. Chapter 35 covers The transport of O₂ and CO₂ to and from the Tissues. The respiratory system consists of the gas-exchange organ (the lungs) and the ventilatory apparatus. This apparatus comprises the chest wall; the Respiratory Muscles, which alter the volume of the thoracic cavity; Brain centers that regulate breathing; and the Neural Pathways and nerves connecting the brain to the muscles. Under normal resting conditions, the human respiratory rate is 12–15 breaths per minute. Approximately 500 mL of air per breath, or 6–8 L/min, is ventilated during inspiration and expiration. Inhaled air mixes with the gas residing in the alveoli, and through simple diffusion, O₂ enters the pulmonary capillary Blood, while CO₂ is released into the alveoli. Consequently, every minute, 250 mL of O₂ enters The Human Body and 200 mL of CO₂ is eliminated.

Trace amounts of other gases, such as intestinal methane, are also present in exhaled air. Alcohol and acetone are likewise eliminated in the breath when present in the body at certain concentrations. In total, as many as 250 different volatile Organic compounds can be detected in human exhaled breath.