Subject - General Science
Chapter : The Respiratory System

Respiration in Single Cell Animals

Single-celled organisms exchange gases 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

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

• Diffusion of oxygen from the medium into the blood.

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

• Diffusion of oxygen from the blood into cells.

• Carbon dioxide follows a reverse path.

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The Human Respiratory System

The Pathway

• Air enters the nostrils

• passes through the nasopharynx,

• the oral pharynx

• through the glottis

• into the trachea

• into the right and left bronchi, which branches and rebranches into

• bronchioles, each of which terminates in a cluster of alveloi

• Only in the alveoli does actual gas exchange takes place. There are some 300 million alveoli in two adult lungs. These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin!).

Breathing

• In mammals, the diaphragm divides the body cavity into the-

• abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the

• thoracic cavity, which contains the heart and lungs.

Central Control of Breathing

The rate of cellular respiration (oxygen consumption and carbon dioxide production) varies with level of activity. Vigorous exercise can increase by 20-25 times the demand of the tissues for oxygen. This is met by increasing the rate and depth of breathing.

Local Control of Breathing

The smooth muscle in the walls of the bronchioles is very sensitive to the concentration of carbon dioxide. A rising level of CO2 causes the bronchioles to dilate. This lowers the resistance in the airways and thus increases the flow of air in and out.

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